Download Electrophilic Selenium Catalysis with Electrophilic N

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Review
Electrophilic Selenium Catalysis with Electrophilic
N-F Reagents as the Oxidants
Ruizhi Guo, Lihao Liao and Xiaodan Zhao *
Institute of Organic Chemistry & MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of
Chemistry, Sun Yat-Sen University, Guangzhou 510275, China; [email protected] (R.G.);
[email protected] (L.L.)
* Correspondence: [email protected]; Tel.: +86-20-8411-0418
Academic Editors: Claudio Santi and Luana Bagnoli
Received: 10 April 2017; Accepted: 16 May 2017; Published: 19 May 2017
Abstract: A suitable oxidative system is crucial to electrophilic selenium catalysis (ESC). This short
review offers the overview of recent development in ESC with electrophilic N-F reagents as the oxidants.
Several highly selective transformations of alkenes such as allylic or vinylic imidation, pyridination,
syn-dichlorination, oxidative cyclization and asymmetric cyclization have been described.
Keywords: alkenes; electrophilic fluorinating reagents; selenium catalysis; regioselectivity;
stereoselectivity
1. Introduction
Functionalization of alkenes is a perpetual goal in organic synthesis. One of the attractive
routes to elaborate the carbon–carbon double bond of alkenes is through electrophilic selenium
reagent-promoted selenofunctionalization. In this context, several electrophilic organoselenium
reagents ArSeX (X = Cl, Br, OTf, etc.) have been developed and widely applied in routine synthesis [1–9].
In general, the introduced selenium moiety was further modified via oxidative or reductive manner
leading to the formation of non-selenium-containing products. This process was not green and wasted
stoichiometric selenium reagents. From the view of atom economy, accomplishing the transformation
with a catalytic amount of organoselenium compounds is environment-friendly and highly desirable.
A selenenylation–deselenenylation process, namely electrophilic selenium catalysis (ESC), met
the requirement. The implementation of this innovation was first documented by Sharpless and
co-workers in 1979 [10,11]. In the transformation, PhSeSePh was employed as the pre-catalyst, and
underwent the secession of the Se-Se bond to generate an electrophilic species PhSeCl in the presence
of N-chlorosuccinimide (NCS). Subsequent chloroselenenylation–deselenenylation process afforded
allylic chloride and regenerated PhSeCl. After this seminal work, several oxidative systems such as
PhSeSePh/persulfate [12–19], PhSeSePh/H2 O2 [20–25], PhSeSePh/hypervalent iodide [26–29] and so
on [30–33] have been developed in this realm [34–36]. Although considerable achievements have been
made, limited transformations and rare effective asymmetric conversion necessitate a more fruitful
oxidative system. Recently, a new oxidative system, the combination of electrophilic N-F reagents
(Scheme 1) [37–42] and organoselenium compounds, has been applied in ESC process. This discovery
promoted the development of impressive transformations and even asymmetric conversion over the
past four years. The versatile reactivity, mild conditions, and excellent regio- and stereoselectivity of
this catalytic system have attracted more and more attention of organic chemists. Consequently, this
review is summarized to make a profile of recent development in ESC with electrophilic N-F reagents
as the oxidants and reaction mechanisms are discussed concomitantly.
Molecules 2017, 22, 835; doi:10.3390/molecules22050835
www.mdpi.com/journal/molecules
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Me
Molecules 2017, 22, 835
OO
S
S
N
O
O
O
F O
S
NFSIN S
O V O
-0.78
F
Scheme 1.
Me
N
F
Me
N
F OTf
TMPyF-OTf
Me
Me
N
-0.73
F VOTf
Me
TMPyF-OTf
NFSI
Electrophilic
-0.78 VN-F reagents -0.73
V
applied
N
N
OTf
PyF-OTf
N
OTf
-0.47
V
F
in
FN
F
2 of 13
Cl
Cl2BF4
NSelectfluor
2BF4
-0.04 V
PyF-OTf
Selectfluor
potentials
vs. SCE).
-0.04
V
-0.47
V
ESC
process
(redox
potentials
vs.
Scheme 1. Electrophilic N-F reagents applied in ESC process (redox potentials vs. SCE).
2. ESC with N-Fluorobenzenesulfonimide (NFSI) as the Oxidant
2. ESC with N-Fluorobenzenesulfonimide (NFSI) as the Oxidant
2.1. Allylic or Vinylic Imidation of Alkenes
2.1. Allylic or Vinylic Imidation of Alkenes
Numerous biologically active compounds contain nitrogen atoms.
atoms. Transition-metal catalyzed
Numerous
biologically
active
compounds
contain
nitrogen
atoms.
Transition-metal
catalyzed
direct amination of alkenes is an efficient route for the synthesis of these
compounds
among the
direct
amination
of
alkenes
is
an
efficient
route
for
the
synthesis
of
these
compounds
among
the as a
developed methods. The
The transformations
transformations generally
generally went
went through
through the
the formation
formation of
of C-N bond
developed methods. The transformations generally went through the formation of C-N bond as a
key step. However, there still existed some problems such as regio- and stereoselectivity, functional
key step. However, there still existed some problems such as regio- and stereoselectivity, functional
group tolerance
substrate
scope.
To address
these issues,
development
of efficientofamination
toleranceand
and
substrate
scope.
To address
these the
issues,
the development
efficient
group tolerance and substrate scope. To address these issues, the development of efficient
methods
is
desirable.
amination
methods
is
desirable.
amination methods is desirable.
In 2013,
Breder
andand
co-workers
effectiveroute
routetoto
synthesize
allylic
vinylic
In 2013,
Breder
co-workersdisclosed
disclosed an
an effective
synthesize
allylic
and and
vinylic
imidation
products
with
olefins
(1) by(1)
electrophilic
seleniumselenium
catalysis.catalysis.
N-Fluorobenzene-sulfonimide
products
with
selenium
catalysis.
N-Fluorobenzeneimidation
products
witholefins
olefins
(1) by
by electrophilic
electrophilic
N-Fluorobenzene(NFSI)
was utilized
as
the
terminal
and nitrogen
(Scheme
2) [43].
This work
sulfonimide
(NFSI)
was
utilizedasoxidant
asthe
theterminal
terminal
oxidantsource
and
source
(Scheme
2) [43].
This This
sulfonimide
(NFSI)
was
utilized
oxidant
andnitrogen
nitrogen
source
(Scheme
2)represents
[43].
work
represents
the
first
application
of
N-F
reagents
in
organoselenium
catalysis.
As
it
is
the
first
application
N-Fapplication
reagents in organoselenium
As it is wellcatalysis.
known, halogenation
work
represents
theoffirst
of N-F reagents catalysis.
in organoselenium
As itwell
is well
known,
halogenation
products
are
formed
in
general
when
olefins
react
with
N-X
(X
=
Cl,
products
are formed inproducts
general when
olefins react
with N-X
(X =olefins
Cl, Br,react
I) reagents
in the(Xpresence
of I)a
known, halogenation
are formed
in general
when
with N-X
=Br,
Cl,I)Br,
reagents
in
the
presence
of
a
catalytic
amount
of
diphenyl
diselenide
(PhSeSePh)
[10,11,44,45].
catalytic
amount
of
diphenyl
diselenide
(PhSeSePh)
[10,11,44,45].
Surprisingly,
the
authors
found
that,
reagents in the presence of a catalytic amount of diphenyl diselenide (PhSeSePh) [10,11,44,45].
Surprisingly, the authors found that, when linear olefins tethered with an electron-withdrawing
when
linear olefins
tetheredfound
with an
electron-withdrawing
reacted
with
in the presence of
Surprisingly,
the authors
that,
when linear olefinsgroup
tethered
with
anNFSI
electron-withdrawing
group reacted with NFSI in the presence of PhSeSePh, allylic imides (2) were generated smoothly
PhSeSePh,
allylic
imides
generatedof
smoothly
instead
of
allylic
fluorides
[46].
By means
of this
group reacted
with
NFSI(2)inwere
the presence
PhSeSePh,
allylic
imides
(2)
were
generated
smoothly
instead
of allylic fluorides [46]. By means of this method, γ4-amino acid derivatives, an important
4
4
method,
γ allylic
-amino
acid
derivatives,
important
structure
widely
found
in derivatives,
biological
peptides,
instead
of
fluorides
[46].
By an
means
of this
method,
γ -amino
acid
an important
structure
widely
found
in
biological
peptides,
could
be synthesized
facilely.
It should be
point
outcould
be
synthesized
facilely.
It
should
be
point
out
that
this
protocol
was
compatible
with
cyclic
olefins.
structure
widely
found
in
biological
peptides,
could
be
synthesized
facilely.
It
should
be
point
out
that this protocol was compatible with cyclic olefins. However, vinylic imides (3) were obtained
However,
vinylic
imides
(3)
were
obtained
along
with
allylic
imides
in
some
cases
[43].
that this
protocol
was
compatible
with
cyclic
olefins.
However,
vinylic
imides
(3)
were
obtained
along with allylic imides in some cases [43].
along with allylic imides in some cases [43].
H
NR2
PhSeSePh (5 mol%)
1
R2
NR
EWG
2
R1
NFSI (1 equiv)
PhSeSePh
mol%)
4 Å MS,(5THF,
rt
EWG
NFSI (1 equiv)
4 Å MS, THF, rt
2 NR
2
Me
NR2
Me
NR2
CO2Bn
84%
R = SOCO
2Ph2Bn
Me
Me
NR2
85%
NR2
R1 H
EWG
R2
PhSeSePh (5 mol%)
NFSI (1 equiv)
R1
NR2
2
R
PhSeSePh
(5 mol%)
1
or Dioxane,
rt
R1 1
EWG THF
3R
2
NFSI
(1
equiv)
R
R2
THF or Dioxane, rt NR
NR
Br
2
2
NR2
1
3
Me
SO2Ph
SO2Ph
O S
O
NR2
79%
O S
O
Br
95%
NR2
NR2
70%
Me
85%
79%
84%
95%
70%
Scheme
R = SO
Ph 2. Organoselenium-catalyzed allylic and vinylic imidation of alkenes.
2
Scheme
2. Organoselenium-catalyzed allylic and vinylic imidation of alkenes.
No desired product was detected when the imidation proceeded without PhSeSePh under
Scheme 2. Organoselenium-catalyzed allylic and vinylic imidation of alkenes.
No
desired
product
was detected
when
proceeded
without
under
otherwise
identical
conditions.
This ruled
out the
the imidation
possibility of
background
reaction.PhSeSePh
Subsequent
mechanistic
studies
revealed that
Se-Se
was crucial of
forbackground
this transformation.
Under
the
otherwise
identical
conditions.
Thisthe
ruled
outbond
the possibility
reaction.
Subsequent
No desired product was detected when the imidation proceeded without PhSeSePh under
conditionstudies
of PhSeBr
as the catalyst,
imidated
product
was generated.
it was Under
found the
mechanistic
revealed
that thenoSe-Se
bond
was crucial
for thisFurthermore,
transformation.
otherwise
identical
conditions.
This
ruled
out
the
possibility
of
background
reaction.
Subsequent
that both
NFSI and
PhSeSePh
thegenerated.
NMR experimental
studies
when
condition
of PhSeBr
as the
catalyst,decomposed
no imidatedgradually
product in
was
Furthermore,
it was
found
mechanistic
studies
revealed
that the
Se-Se bond
wasnot
crucial
for
this transformation.
Under
the
PhSeSePh
was
mixed
with
NFSI.
However,
NFSI
did
further
decompose
after
PhSeSePh
was
that both NFSI and PhSeSePh decomposed gradually in the NMR experimental studies when PhSeSePh
condition
of
PhSeBr
as
the
catalyst,
no
imidated
product
was
generated.
Furthermore,
it
was
found
consumed completely. Thus, the authors speculated that the Se-Se bond did not break to form
was mixed with NFSI. However, NFSI did not further decompose after PhSeSePh was consumed
that PhSeX
both NFSI
in the
NMR reaction,
experimental
studies awhen
(X = and
F or PhSeSePh
N(SO2Ph)2) decomposed
species in thegradually
initial of the
catalyzed
and proposed
completely. Thus, the authors speculated that the Se-Se bond did not break to form PhSeX (X = F
mechanism
follows
(Scheme
[47]. The reaction
initiated
by the formation
of the
ionicPhSeSePh
species 4. was
PhSeSePh
was as
mixed
with
NFSI.3) However,
NFSI isdid
not further
decompose
after
or N(SO
in theof
initial
ofaffords
the catalyzed
reaction,
and proposed a mechanism
as the
follows
2 Ph)
2 ) species attack
Then,
electrophilic
olefin
intermediate
5. Subsequent
rise to
consumed
completely.
Thus, the
authors
speculated
that
the Se-Seelimination
bond did gives
not break
to form
(Scheme
3)
[47].
The
reaction
is
initiated
by
the
formation
of
the
ionic
species
4.
Then,
electrophilic
desired
and the
catalyst [43].
PhSeX
(X =product
F or N(SO
2Ph)2) species in the initial of the catalyzed reaction, and proposed a
attack of olefin affords intermediate 5. Subsequent elimination gives rise to the desired product and
mechanism as follows (Scheme 3) [47]. The reaction is initiated by the formation of the ionic species 4.
the catalyst [43].
Then, electrophilic attack of olefin affords intermediate 5. Subsequent elimination gives rise to the
desired product and the catalyst [43].
Molecules 2017, 22, 835
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2 + HF
PhSeSePh
NFSI
2 + HF
PhSeSePh
NFSI
NR2 F
1
EWG
R NR
2
Se F Ph
Ph
Se
EWG
R1
5 Se
Ph
Ph
Se
5
1
3 of 13
NR2
Ph
Se
Ph NR Se
2
4
F
Ph
Se
Ph
Se
4
F
Scheme3.3.Proposed
Proposedmechanism
mechanism of
of allylic
allylic
1 and
Scheme
and vinylic
vinylicimidation
imidationofofalkenes.
alkenes.
Scheme 3. Proposed mechanism of allylic and vinylic imidation of alkenes.
Successively, Zhao and co-workers reported an efficient route to synthesize 3-amino allylic
Successively,
Zhao and co-workers reported an efficient route to synthesize 3-amino allylic
alcohols through NFSI/PhSeSePh/base system (Scheme 4) [48]. 3-Amino allylic alcohols are valuable
Successively,
Zhao and co-workers
reported
an efficient
to synthesize
3-amino
allylic
alcohols
through
NFSI/PhSeSePh/base
system
(Scheme
[48]. route
3-Amino
allylic
alcohols
synthetic
intermediates
and generally obtained
through4)
multi-step
synthesis.
Inspired
byare
thevaluable
effect
alcohols
through
NFSI/PhSeSePh/base
system
(Scheme
4)
[48].
3-Amino
allylic
alcohols
are
valuable
synthetic
intermediates
generallythe
obtained
multi-step
synthesis.
Inspired
by the
effect
of neighboring
groupand
assistance,
authorsthrough
proposed
that 3-amino
allylic
alcohols
could
be of
synthetic
intermediates
and
generally
obtained
through
multi-step
synthesis.
Inspired
by
the
effect
neighboring
group
assistance,
authors proposed
that 3-amino
allylic alcohols
could
be synthesized
synthesized
conveniently
viathe
organoselenium
catalyzed
direct imidation
of allylic
alcohols.
It was
of neighboring
group assistance,
the authors
proposed
that
3-amino
allylic
alcohols
could
be
conveniently
organoselenium
catalyzed
imidation
of allylic
alcohols.
was
found
that regiofound that via
regioand stereoselectivity
of direct
the reaction
could
be controlled
byIthydroxyl
group
on
synthesized conveniently via organoselenium catalyzed direct imidation of allylic alcohols. It was
and
stereoselectivity
the reaction
could be(6)
controlled
by hydroxyl
group
on presence
substratesof[49–53].
When
substrates
[49–53]. of
When
allylic alcohols
were treated
with NFSI
in the
base and
a
found that regio- and stereoselectivity of the reaction could be controlled by hydroxyl group on
allylic
alcohols
(6) were
treated with
in the
presence
of base
and
a catalytic amount
of PhSeSePh,
catalytic
amount
of PhSeSePh,
onlyNFSI
3-amino
allylic
alcohols
(7), an
anti-Markovnikov
product,
were
substrates [49–53]. When allylic alcohols (6) were treated with NFSI in the presence of base and a
only
3-amino
alcohols
(7),
an1,2-disubstituted
anti-Markovnikov
product,
were formed.
Both terminal
alkenes
formed.
Bothallylic
terminal
alkenes
and
alkenes
also underwent
the selective
imidation
to
catalytic amount of PhSeSePh, only 3-amino allylic alcohols (7), an anti-Markovnikov product, were
give
the desired products
under
similar conditions.
Control
experiments
indicated
that hydroxyl
and
1,2-disubstituted
alkenes
alsothe
underwent
the selective
imidation
to give
the desired
products
formed. Both terminal alkenes and 1,2-disubstituted alkenes also underwent the selective imidation to
group was
responsible
for theControl
excellentexperiments
selectivity. When
reaction of hydroxyl
simple alkene
without
hydroxyl
under
similar
conditions.
indicated
group
was
give the
the desired
products
under the similar
conditions.
Controlthat
experiments indicated
that responsible
hydroxyl
group
was performed
under
the reaction
standardofconditions,
a mixture
of hydroxyl
allylic andgroup
vinylicwas
imides
was
forgroup
the excellent
selectivity.
When
simple
alkene
without
performed
was responsible for the excellent selectivity. When reaction of simple alkene without hydroxyl
formed
with
erosion
of
stereoselectivity
[48].
It
is
worth
mentioning
that,
when
allylic
alcohols
under
thewas
standard
conditions,
a mixture
allylic anda vinylic
wasand
formed
erosion
group
performed
under the
standardofconditions,
mixtureimides
of allylic
vinylicwith
imides
was of
underwent the [48].
imidation
without
base, α,β-unsaturated
aldehydes
(8) were
obtainedthe
dueimidation
to the
stereoselectivity
It
is
worth
mentioning
that,
when
allylic
alcohols
underwent
formed with erosion of stereoselectivity [48]. It is worth mentioning that, when allylic alcohols
decomposition
of 3-amino allylic
alcohols
in acidic
conditions
This is a newofand
practical
without
base, α,β-unsaturated
aldehydes
were
obtained
due
to[54,55].
the decomposition
3-amino
underwent
the imidation without
base,(8)
α,β-unsaturated
aldehydes
(8) were obtained
due to allylic
the
method
synthesize
α,β-unsaturated
derivatives
with
easily available
allylic alcohols.
alcohols
into
acidic
conditions
Thisaldehyde
is ainnew
andconditions
practical
method
synthesize
decomposition
of
3-amino [54,55].
allylic alcohols
acidic
[54,55]. to
This
is a newα,β-unsaturated
and practical
aldehyde
with
easily available
allylicderivatives
alcohols. with easily available allylic alcohols.
method derivatives
to synthesize
α,β-unsaturated
aldehyde
Scheme 4. Organoselenium-catalyzed regio- and stereoselective imidation of terminal alkenes.
Scheme 4. Organoselenium-catalyzed regio- and stereoselective imidation of terminal alkenes.
Scheme of
4. Isobenzofuranones
Organoselenium-catalyzed
and stereoselective
imidation
terminal alkenes.
2.2. Synthesis
and IndoleregioDerivatives
via Intramolecular
C-H of
Functionalization
By usingofNFSI
as the oxidantand
in ESC
process,
several
heterocycles
have been synthesized
2.2. Synthesis
Isobenzofuranones
Indole
Derivatives
viavaluable
Intramolecular
C-H Functionalization
2.2. Synthesis of Isobenzofuranones and Indole Derivatives via Intramolecular C-H Functionalization
via oxidative cyclization. In 2015, Breder and co-workers developed a direct allylic acyloxylation
By using NFSI as the oxidant in ESC process, several valuable heterocycles have been synthesized
protocol
for NFSI
the construction
of isobenzofuranones
[56]. When
2-cinnamylbenzoic
acid
analogues
(9)
By using
as the oxidant
in ESC process, several
valuable
heterocycles have
been
synthesized
via oxidative cyclization. In 2015, Breder and co-workers developed a direct allylic acyloxylation
were
treated
with
NFSI
in
the
presence
of
PhSeSePh,
a
variety
of
the
corresponding
viaprotocol
oxidative
In 2015,
Breder and co-workers
developed
a direct allylic
acyloxylation
for cyclization.
the construction
of isobenzofuranones
[56]. When
2-cinnamylbenzoic
acid analogues
(9)
isobenzofuranones
(10) were
generated
in moderate
to When
good yields
(Scheme 5). However,
the aryl (9)
protocol
for
the
construction
of
isobenzofuranones
[56].
2-cinnamylbenzoic
acid
analogues
were treated with NFSI in the presence of PhSeSePh, a variety of the corresponding
groups
connecting
tointhe
bond
were necessary for of
thethe
successful
implementation
of this
were
treated
with NFSI
thedouble
presence
of PhSeSePh,
corresponding
isobenzofuranones
isobenzofuranones
(10)
were
generated
in moderatea variety
to good yields
(Scheme 5). However,
the aryl
(10)
were generated
to bond
good were
yieldsnecessary
(Scheme for
5). However,
the aryl
groups connecting
groups
connectingin
tomoderate
the double
the successful
implementation
of this to
Molecules 2017, 22, 835
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Molecules 2017, 22, 835
4 of 13
the double bond were necessary for the successful implementation of this transformation. When aryl
2017, 22, 835
of in
13
groupMolecules
was replaced
by alkyl
6-exo-trig
product
was
afforded
in product
78% yield
under
the4standard
transformation.
When
arylone,
group
was replaced
by 11
alkyl
one,
6-exo-trig
11 was
afforded
conditions
[56].under the standard conditions [56].
78% yield
transformation. When aryl group was replaced by alkyl one, 6-exo-trig product 11 was afforded in
78% yield under the standard conditions
[56].
O
O
O
R
R
O
OH
PhSeSePh (10 mol%)
NFSI (1 equiv)
4 Å MS, toluene,
rt
PhSeSePh
(10 mol%)
NFSI (1 equiv)
(het)Ar
4 Å MS, toluene, rt
9
9
O
(het)Ar
OO
81%
O
44%
81%
O
F
O
O
10
F3C
(het)Ar
(het)Ar O
OO
MeO
OO
F3C
39%
S
O
O
39%
S
O
O
O
11, 78%
10
O
O
44%
F
R
MeO
OO
OO
R
OH
Ph
49%
Ph
49%
O
Ph
Ph
OH
OH
Et
n-Pr
Et
11, 78%
n-Pr
Scheme 5. Organoselenium-catalyzed synthesis of isobenzofuranones.
Scheme
5. Organoselenium-catalyzed synthesis of isobenzofuranones.
Scheme 5. Organoselenium-catalyzed
synthesis
of isobenzofuranones.
The role of diselenide
in this transformation was
appealing
owing to the rarity of selenium
3)-H functionalization
The
role of
diselenide
in this transformation
was appealing
to[57,58].
the rarity
of selenium
catalyzed
C(sp
compared to hypervalent
iodineowing
catalysis
According
to
The role
of diselenide in this transformation was appealing owing to the rarity of selenium
catalyzed
C(sp
functionalization
compared
hypervalent
catalysis
[57,58].
According
a series
of3 )-H
mechanistic
studies, the
authors to
rationalized
thatiodine
the reaction
underwent
allylic
catalyzed C(sp3)-H functionalization compared to hypervalent iodine catalysis [57,58]. According to
N2’ displacement
(Schemerationalized
6). Initially, diselenide
oxidized underwent
to electrophilic
to a selenenylation-S
series of mechanistic
studies,process
the authors
that theisreaction
allylic
a series of mechanistic studies, the authors rationalized that the reaction underwent allylic
species
12.
Then,
it
reacts
with
olefinic
acid
9
to
form
allylic
selenium
cationic
13.
Subsequent
S
N2’
selenenylation-S
2’
displacement
process
(Scheme
6).
Initially,
diselenide
is
oxidized
to
electrophilic
N N2’ displacement process (Scheme 6). Initially, diselenide is oxidized to electrophilic
selenenylation-S
displacement
of
selenium
moiety
releases the
desired
product
10
and regenerates
the catalyst
[56]. S 2’
species
12. Then,
it reacts
with
olefinic
allylicselenium
selenium
cationic
Subsequent
species
12. Then,
it reacts
with
olefinicacid
acid99to
to form
form allylic
cationic
13. 13.
Subsequent
SN2’ N
displacement
of selenium
moiety
releases
desired
product
10
and
regenerates
the
catalyst
[56].
1the
displacement
of selenium
moiety
releases
the
desired
product
10
and
regenerates
the
catalyst
[56].
10 + HZ
PhSeSePh
NFSI
10 + HZ1
PhSeSePh
NFSI
O
O
Z1
13
Z1
13
OH
Ph
OH
Se
Se
Ph
Ar Ph
Se
Se
Ar Ph
HZ2
Z1
Ph
Se
Ph
Se
Z12
Z
Se 12Ph
Ph
Se
Z2
12
9
Z1 = N(SO2Ph)22, if Z2 = F or vice versa
9
HZ
2of direct allylic acyloxylation.
Scheme 6. Proposed
mechanism
Z1 = N(SO
2Ph)2, if Z = F or vice versa
Scheme 6. Proposed
mechanism
of direct
allylicfunctionalization,
acyloxylation.
In further investigation
NFSI/PhSeSePh
system
in C-H
Scheme 6. of
Proposed
mechanism
of direct
allylic acyloxylation. Breder and Zhao
independently disclosed an effective route to synthesize indole derivatives via ESC process
In further investigation of NFSI/PhSeSePh system in C-H functionalization, Breder and Zhao
(Scheme 7) [59,60]. According to this procedure, 2-alkenylaniline (14) underwent the intramolecular
In
further
investigation
NFSI/PhSeSePh
system inindole
C-H derivatives
functionalization,
independently
disclosed anofeffective
route to synthesize
via ESC Breder
process and
C(sp2)-H amination in the presence of NFSI and PhSeSePh. Under the conditions, N-Ts-indole (15)
7) [59,60].disclosed
Accordingan
to this
procedure,
underwent
the intramolecular
Zhao(Scheme
independently
effective
route2-alkenylaniline
to synthesize (14)
indole
derivatives
via ESC process
derivative was obtained efficiently, but no intermolecular imidated product from NFSI was
2)-H amination in the presence of NFSI and PhSeSePh. Under the conditions, N-Ts-indole (15)
C(sp7)
(Scheme
[59,60].
According
to
this
procedure,
2-alkenylaniline
(14)
underwent
the
intramolecular
detected. In the case of tosyl, nosyl, and mesyl protected anilines, the corresponding products
were
2
derivative
was
obtained
efficiently,
but
no
intermolecular
imidated
product
from
NFSI was
C(sp afforded
)-H amination
in the
presence
of NFSI
and PhSeSePh.
Under
thewhen
conditions,
in moderate
yields.
However,
no desired
product was
formed
Cbz, Ac,N-Ts-indole
Boc and so (15)
detected.
Inobtained
the case of
tosyl, nosyl, and
protected anilines,
the corresponding
products
were
derivative
was
nomesyl
intermolecular
imidated
from NFSI
detected.
on were
utilized
as theefficiently,
protectingbut
groups.
This finding indicated
thatproduct
the nucleophilicity
ofwas
nitrogen
afforded in moderate yields. However, no desired product was formed when Cbz, Ac, Boc and so
key tonosyl,
the success
of thisprotected
transformation.
Thisthe
method
is general, products
both 2-arylwere
and 2-alkyl
In theatom
caseisofatosyl,
and mesyl
anilines,
corresponding
afforded in
on were utilized as the protecting groups. This finding indicated that the nucleophilicity of nitrogen
indoles
evenHowever,
azaindoleno
derivatives
could be
in good
yields.
when
moderate
yields.
desired product
wassynthesized
formed when
Cbz, Ac,
Boc Furthermore,
and so on were
utilized
atom is a key to the success of this transformation. This method is general, both 2-aryl and 2-alkyl
trisubstituted
alkene
was
treated
with
NFSI
under
the
standard
conditions,
a
2,3-disubstituted
as theindoles
protecting
groups.
This
finding
indicated
that
the
nucleophilicity
of
nitrogen
atom
is
a
key
even azaindole derivatives could be synthesized in good yields. Furthermore, whento the
success
of
this
transformation.
This method
is general,
bothstandard
2-aryl and
2-alkyl indoles
even azaindole
trisubstituted
alkene was treated
with NFSI
under the
conditions,
a 2,3-disubstituted
derivatives could be synthesized in good yields. Furthermore, when trisubstituted alkene was treated
Molecules 2017, 22, 835
5 of 13
Molecules 2017, 22, 835
5 of 13
with
NFSI under the standard conditions, a 2,3-disubstituted N-Ts-indole 16 was formed in 99%
yield
via a 1,2-phenyl migration process [61–63]. In order to elucidate the mechanism, a cross experiment
N-Ts-indole 16 was formed in 99% yield via a 1,2-phenyl migration process [61–63]. In order to elucidate
by using PhSeBr and tolylSeBr as the catalysts have been conducted by Breder and co-workers.
the mechanism, a cross experiment by using PhSeBr and tolylSeBr as the catalysts have been
The
authors discovered
thatco-workers.
ArSeBr could
the reaction
and
diselenides
were generated
after
conducted
by Breder and
The catalyze
authors discovered
that
ArSeBr
could catalyze
the reaction
77 Se NMR of the diselenide, two new signals were
theand
fulldiselenides
conversion
of
substrate.
By
analyzing
were generated after the full conversion of substrate. By analyzing 77Se NMR of the
detected
and
putatively
belonged
to the
mixedand
diselenide
tolSeSePh.
compound
was further
diselenide, two new signals
were
detected
putatively
belongedThis
to the
mixed diselenide
77 Se, 77 Se COSY experiments. According 77
77
confirmed
by
2D
to
these
results,
the
Se-Se
bond
was
possible
tolSeSePh. This compound was further confirmed by 2D Se, Se COSY experiments. According
to
to suffer
cleavage/recombination
during
the catalytic
reaction
and oxyselenenylation–deselenenylation
these results,
the Se-Se bond was
possible
to suffer
cleavage/recombination
during the catalytic
mechanism
was
reasonable for this cyclization [59].
reaction and
oxyselenenylation–deselenenylation
mechanism was reasonable for this cyclization [59].
R1
R2
R4
N
N
14 Ts
N
Ts
79%
N
Ts
O
Ph
O
87%
N
Ts
Ph
NC
76%
Me
Ph
N
R2
N
Pr
N
Ts
N
Ts
Ph
Me
Ph
N
Ts
81%
R1
15
n
Ph
97%
H
Cat.
PhSeSePh
R4
NFSI (1.05 equiv)
N
Ts
16, 99%
N
Ts
H
Scheme7.7.Organoselenium-catalyzed
Organoselenium-catalyzed synthesis
Scheme
synthesisofofindoles.
indoles.
3. ESC with Fluoropyridinium Salts as the Oxidants
3. ESC with Fluoropyridinium Salts as the Oxidants
Stereospecific Syn-Dichlorination of Alkenes
3.1.3.1.
Stereospecific
Syn-Dichlorination of Alkenes
NFSI as the oxidant was efficiently utilized in ESC process. Versatile allylic or vinylic functionality
NFSI as the oxidant was efficiently utilized in ESC process. Versatile allylic or vinylic functionality
compounds were synthesized by the oxidative system. Compared to NFSI, another type of electrophilic
compounds were synthesized by the oxidative system. Compared to NFSI, another type of electrophilic
N-F reagents, fluoropyridium salts possess higher redox potential (Scheme 1) [42] and contain a
N-F reagents, fluoropyridium salts possess higher redox potential (Scheme 1) [42] and contain a weak
weak nucleophile pyridine. Owing to the unique properties of these reagents, it might be promising
nucleophile
pyridine. Owing to the unique properties of these reagents, it might be promising to
to develop novel transformations with them and organoselenium compounds.
develop
novel
transformations
them and
organoselenium
compounds.with simple olefins was
In 2015, the
first example with
of catalytic,
stereoselective
syn-dichlorination
In
2015,
the
first
example
of
catalytic,
stereoselective
syn-dichlorination
with simple
disclosed by Denmark et al., and a novel PhSeSePh/PyF-BF4 system was employed
in olefins
this
was
disclosed
by
Denmark
et
al.,
and
a
novel
PhSeSePh/PyF-BF
system
was
employed
in
this
4
transformation (Scheme 8) [64]. Owing to the inevitable anti-addition issue in traditional direct
transformation
(Scheme
8)
[64].
Owing
to
the
inevitable
anti-addition
issue
in
traditional
direct
dichlorination of olefins via chloriranium ion, synthetic chemists were always plagued by the synthesis
dichlorination
of olefins
via chloriranium
ion,
syntheticofchemists
always
plagued by[65–69],
the synthesis
of syn-dichlorides.
Inspired
by the known
feasibility
PhSeCl3 were
to afford
syn-dichloride
the
of authors
syn-dichlorides.
bycatalytic
the known
feasibility of PhSeCl
afford
syn-dichloride
proposedInspired
a route of
syn-dichlorination
through
ESC
process.
The critical[65–69],
point tothe
3 to
authors
proposed
a route of catalytic
ESC process.
The critical
accomplish
this transformation
was tosyn-dichlorination
identify an oxidantthrough
which satisfied
some criteria:
(1) it point
could to
oxidize Sethis
(II) transformation
into Se (IV); (2) itwas
could
not reactan
with
substrates
directly;
(3) some
it could
not oxidize
Cl−
accomplish
to identify
oxidant
which
satisfied
criteria:
(1) it could
−
into Cl
background
it could
release competitive
nucleophiles
compared
oxidize
Se2 to
(II)avoid
into Se
(IV); (2) it reaction;
could not(4)
react
withnot
substrates
directly; (3) it
could not oxidize
Cl to
into
−; and (5) it could promote the SN2 reaction of selenenylated intermediate rather than elimination [64].
Cl2Clto
avoid background reaction; (4) it could not release competitive nucleophiles compared to Cl− ;
these
assumptions
mind, theofauthors
speculated
that N-F reagents
might
be a suitable
and (5)With
it could
promote
the SNin
2 reaction
selenenylated
intermediate
rather than
elimination
[64].
oxidant.
It
was
found
that
when
simple
alkenes
(17)
reacted
with
PyF-BF
4 in the presence of
With these assumptions in mind, the authors speculated that N-F reagents might be a suitable
TMSCl
and
PhSeSePh,
syn-dichlorides
(18)
were afforded
stereoselectivitely
in most cases.
BnEt3NCl,
oxidant.
It was
found
that
when simple
alkenes (17)
reacted
with PyF-BF
4 in the presence of BnEt3 NCl,
Additional 2,6-lutidine N-oxide (19) was able to accelerate the reaction. This transformation was
TMSCl and PhSeSePh, syn-dichlorides (18) were afforded stereoselectivitely in most cases. Additional
compatible with a variety of functional groups including free hydroxyl group, tert-butyl-diphenylsilyl
2,6-lutidine N-oxide (19) was able to accelerate the reaction. This transformation was compatible
(TBDPS), acetal and so on. Control experiment revealed that ca. 35% anti-dichlorides were generated
with a variety of functional groups including free hydroxyl group, tert-butyl-diphenylsilyl
(TBDPS),
without PhSeSePh. It indicated that PyF-BF4 was capable of oxidizing Cl− to Cl2 but the background
acetal and so on. Control experiment revealed that ca. 35% anti-dichlorides were generated without
reaction could be suppressed in the catalytic reaction. A possible mechanism of this catalytic
PhSeSePh. It indicated that PyF-BF4 was capable of oxidizing Cl− to Cl2 but the background reaction
syn-dichlorination is depicted as follows (Scheme 9). In the initial reaction, PhSeSePh is oxidized to
could
be suppressed
in the catalytic reaction. A possible mechanism of this catalytic syn-dichlorination
PhSeCl
3 species in the presence of PyF-BF4 and TMSCl. The PhSeCl3 might react with the alkene to
Molecules 2017, 22, 835
6 of 13
Molecules 2017, 22, 835
6 of 13
Molecules 2017,
22, 835 (Scheme 9). In the initial reaction, PhSeSePh is oxidized to PhSeCl species
6 of
is depicted
as follows
in13the
3
−, anti-stereospecific
presence
PyF-BF4 and
react
with
the alkene to generate
seleniranium
3 might
generateofseleniranium
ionTMSCl.
20. AfterThe
thePhSeCl
nucleophilic
attack
of Cl
chloroselenenylated
generate seleniranium ion 20. After the nucleophilic
attack of Cl−, anti-stereospecific chloroselenenylated
−
−
ionintermediate
20. After the21nucleophilic
attackSNof
Cl , anti-stereospecific
chloroselenenylated
intermediate
21 is
is formed. Then,
2 reaction
of Se (IV) moiety
by Cl releases the desired
product
intermediate 21 is formed. Then, SN2 reaction of Se
(IV) moiety by Cl− releases the desired product
formed.
SN[64].
2 reaction of Se (IV) moiety by Cl− releases the desired product 18 and PhSeCl [64].
18 andThen,
PhSeCl
18 and PhSeCl [64].
Scheme 8. Organoselenium-catalyzed syn-dichlorination of alkenes.
Scheme
syn-dichlorinationofofalkenes.
alkenes.
Scheme8.8.Organoselenium-catalyzed
Organoselenium-catalyzed syn-dichlorination
Scheme 9. Proposed mechanism of syn-dichlorination of alkenes.
Scheme 9. Proposed mechanism of syn-dichlorination of alkenes.
Scheme 9. Proposed mechanism of syn-dichlorination of alkenes.
3.2. Synthesis of Oxygen- and Nitrogen-Containing Heterocycles via Oxidative Cyclization
3.2. Synthesis of Oxygen- and Nitrogen-Containing Heterocycles via Oxidative Cyclization
3.2. Synthesis of Oxygen- and Nitrogen-Containing Heterocycles via Oxidative Cyclization
In 2016, Zhao and co-workers disclosed an organoselenium catalyzed oxidative cyclization to
In 2016, Zhao and co-workers disclosed an organoselenium catalyzed oxidative cyclization to
synthesize
and
nitrogen-containing
[70]. Thecatalyzed
authors conceived
olefinic to
In 2016, oxygenZhao and
co-workers
disclosed heterocycles
an organoselenium
oxidativethat
cyclization
synthesize
oxygenand
nitrogen-containing
heterocycles
[70]. The authors conceived
that
olefinic
alcohols (22)
were and
ablenitrogen-containing
to undergo exo-trig cyclization
in [70].
the presence
of PyF-OTf
and that
PhSeSePh
synthesize
oxygenheterocycles
The
authors
conceived
olefinic
alcohols (22) were able to undergo exo-trig cyclization in the presence of PyF-OTf and PhSeSePh
(Scheme
10).were
This able
protocol
has beenexo-trig
successfully
appliedininto
the
formation
of different
α-alkenyl
alcohols
(22)
to
undergo
cyclization
the
presence
of
PyF-OTf
and
PhSeSePh
(Scheme 10). This protocol has been successfully applied into the formation of different α-alkenyl
tetrahydrofurans
or -pyrans
(23)
withsuccessfully
excellent regioand stereoselectivities.
It is
mentioning
(Scheme
10). This protocol
has
been
applied
into the formation
ofworth
different
α-alkenyl
tetrahydrofurans
or -pyrans
(23)
with excellent regioand stereoselectivities.
It is
worth
mentioning
that,
when
NFSI
or
Selectfluor
was
employed
as
the
oxidant,
the
reaction
gave
the
product
in lower
tetrahydrofurans
or
-pyrans
(23)
with
excellent
regioand
stereoselectivities.
It
is
worth
mentioning
that, when NFSI or Selectfluor was employed as the oxidant, the reaction gave the product in lower
yield
with
unidentified
or fluorinated
byproducts.
This result
an the
appropriate
redox
that,
when
NFSI
or Selectfluor
was employed
as the oxidant,
theemphasized
reaction gave
product in
lower
yield
with
unidentified
or fluorinated
byproducts.
This result
emphasized
an appropriate
redox
potential
of
oxidant
was
important
for
the
transformation.
Furthermore,
the
desired
products
were
yield
with unidentified
orimportant
fluorinated
This result
emphasized
an appropriate
redox
potential
of oxidant was
for byproducts.
the transformation.
Furthermore,
the desired
products were
able to rearrange into seven-membered heterocycles (24) under the acidic conditions. In order to
able to of
rearrange
(24) under
the acidic
conditions.
In order were
to
potential
oxidant into
was seven-membered
important for theheterocycles
transformation.
Furthermore,
the
desired products
avoid the formation of byproducts, NaF was employed as the base to neutralize HF putatively
avoid
the
formation
of
byproducts,
NaF
was
employed
as
the
base
to
neutralize
HF
putatively
able to rearrange into seven-membered heterocycles (24) under the acidic conditions. In order to avoid
generated in the reaction. It should be mentioned that olefinic amides also underwent the
reaction. NaF
It should
be mentioned
amides
underwent
the
thegenerated
formationinofthe
byproducts,
was employed
as the that
baseolefinic
to neutralize
HFalso
putatively
generated
cyclization under the similar conditions. Both five- and seven-membered heterocycles were
the similar
conditions.
fiveand also
seven-membered
heterocycles
in cyclization
the reaction.under
It should
be mentioned
that Both
olefinic
amides
underwent the
cyclizationwere
under
obtained in good yields by means of slight modification of bases. Some experiments have been
obtained
in good yields
byfivemeans
slight modification
of bases. Some
been
theconducted
similar conditions.
Both
andofseven-membered
heterocycles
wereexperiments
obtained in have
good
yields
to elucidate the mechanism. It was found that a new signal was detected in 77
NMR
77Se
conducted
to
elucidate
the
mechanism.
It
was
found
that
a
new
signal
was
detected
in
Se
NMR
bywhen
means
of
slight
modification
of
bases.
Some
experiments
have
been
conducted
to
elucidate
PhSeSePh reacted with equimolar PyF-OTf after 4 h. When the mixture of PhSeSePh andthe
when PhSeSePh
reacted
witha equimolar
PyF-OTf
after 4 h.
When
thewhen
mixture
of PhSeSePh
77 Se
mechanism.
It was
found that
new
signal
was detected
NMR
PhSeSePh
reactedand
with
PyF-OTf was
employed
to treat
with
olefinic
alcohol, ainselenenylated
product
was reasonably
PyF-OTf was employed to treat with olefinic alcohol, a selenenylated product was reasonably
equimolar
h. When
the mixture
of PhSeSePh
and
PyF-OTf was
employed
to desired
treat with
affordedPyF-OTf
in 61% after
NMR4 yield.
Subsequent
oxidation
of this
intermediate
generated
the
afforded in 61% NMR yield. Subsequent oxidation of this intermediate generated the desired
Molecules 2017, 22, 835
7 of 13
Molecules 2017, 22, 835
7 of 13
olefinic
alcohol, a selenenylated product was reasonably afforded in 61% NMR yield. Subsequent
oxidation
of this intermediate generated the desired product smoothly. Based on these results, the
product smoothly. Based on these results, the authors considered that PhSeSePh was initially oxidized
authors
considered
PhSeSePh
initially
oxidizedand
to PhSeX
(X =cyclization
F or OTf)could
in theundergo
presence of
to PhSeX
(X = Fthat
or OTf)
in the was
presence
of PyF-OTf
the entire
PyF-OTf
and
the
entire
cyclization
could
undergo
selenenylation–deselenenylation
process
[46,70].
selenenylation–deselenenylation process [46,70].
R4
R3
H
Nu
R1
2
1,2 R
NuH = OH, NHTs
22
Me
Me
Cat.
PhSeSePh
PyF-OTf, NaF
R4
R3
Ph
67%
R1
2
R
1,2
NTs
or
2
R
23
O
O
Nu
Me
Ts
N
O
O
Ph
Me
OBn
88%
Me
70% (E/Z = 5:1)
(4-Cl)C6H4
R1
R3 = R4 = H
24
Ph
96% (E/Z = 8:1)
Me
NTs
Cl
81% (1 equiv NaF)
82% (0.5 equiv NaF)
Scheme 10. Organoselenium-catalyzed synthesis of oxygen- and nitrogen-containing heterocycles.
Scheme 10. Organoselenium-catalyzed synthesis of oxygen- and nitrogen-containing heterocycles.
3.3. Regioselective Pyridination of 1,3-Dienes
3.3. Regioselective Pyridination of 1,3-Dienes
Selective functionalization of alkenes is a challenge in organic synthesis, especially for the
Selective
functionalization
of alkenes
is a challenge
regioselective
C-H functionalization
of 1,3-dienes
due to in
theorganic
nature synthesis,
of the highespecially
reactivity for
of the
conjugated
dienes.
Many efforts have
been devoted
thisnature
field. of
However,
functionalization
regioselective
C-H
functionalization
of 1,3-dienes
due totothe
the highC-1
reactivity
of conjugated
products
usually
by means
of cross-coupling
reactions
[71–77]. However, there
werewere
dienes.
Manywere
efforts
haveaccessed
been devoted
to this
field. However,
C-1 functionalization
products
rare
documented
transformations
with
respect
to
regioselective
C-H
functionalization
at
the
other
usually accessed by means of cross-coupling reactions [71–77]. However, there were rare documented
position of with
1,3-dienes.
the excellent
selectivity in selenenylation–deselenenylation
transformations
respectConsidering
to regioselective
C-H functionalization
at the other position of 1,3-dienes.
process, electrophilic selenium catalysis could be an ideal strategy to overcome this issue.
Considering the excellent selectivity in selenenylation–deselenenylation process, electrophilic selenium
Recently, Zhao and co-workers reported a regioselective C-H pyridination of olefins through
catalysis could be an ideal strategy to overcome this issue.
BnSeSeBn/fluoropyridinium salts system [78]. In this transformation, fluoropyridinium salt served
Recently,
co-workers
reported
a regioselective
C-Hscope
pyridination
of olefins was
through
as pyridineZhao
sourceand
[79] and
terminal oxidant
(Scheme
11). The substrate
of this transformation
BnSeSeBn/fluoropyridinium
[78].
transformation,
fluoropyridinium
salt served as
broad. Different 1,3-dienessalts
weresystem
suitable
to In
thethis
pyridination.
Surprisingly,
only C-2 pyridinated
pyridine
source
[79]
andformed
terminal
(Scheme
11). The
substrate
scope
this transformation
products
(27)
were
byoxidant
this method.
Styrene
derivatives
were
alsoofcompatible
with the was
pyridination
28). Moreover,
exogenous
pyridine
sources could
selectively
broad.
Different (see
1,3-dienes
were suitable
to and
the endogenous
pyridination.
Surprisingly,
onlybe
C-2
pyridinated
installed
on
1,3-dienes
using
co-oxidant
TMPyF-BF
4
and
Selectfluor
instead
of
PyF-BF
4
(see
25
and
It the
products (27) were formed by this method. Styrene derivatives were also compatible26).
with
should
be
point
out
that
when
alkyl
terminal
alkenes
were
treated
with
fluoropyridinium
salt
pyridination (see 28). Moreover, exogenous and endogenous pyridine sources could be selectively
under the standard conditions, a mixture of terminal pyridinated products was formed. This result
installed
on 1,3-dienes using co-oxidant TMPyF-BF4 and Selectfluor instead of PyF-BF4 (see 25 and
emphasized the impact of conjugated aryl or vinyl group was crucial to achieve the remarkable
26). It should be point out that when alkyl terminal alkenes were treated with fluoropyridinium salt
selectivity. Further modification of pyridinium salts, such as Diels–Alder reaction, nucleophilic
underaddition
the standard
conditions, a mixture of terminal pyridinated products was formed. This result
and aromatization, afforded valuable synthetic intermediates and demonstrated the
emphasized
impact of
conjugated
aryl or
vinyl group was crucial to achieve the remarkable
potential the
applicability
of this
transformation
[78].
selectivity.
Further
modification
of
pyridinium
salts, such
as Diels–Alder
reaction,
nucleophilic
In order to make a profile of the selective pyridination
[80–82],
some mechanistic
studies
have
addition
and
aromatization,
afforded
valuable
synthetic
intermediates
and
demonstrated
the potential
been conducted. It was found that the selenenylated intermediate 32 along with N-benzylpyridinium
salt transformation
35 was formed when
applicability
of this
[78].1,3-diene 29 reacted with an equimolar mixture of BnSeSeBn
andorder
PyF-OTf.
Further
oxidation
of selective
32 afforded
the desired [80–82],
product some
34 andmechanistic
byproduct 35.
These have
In
to make
a profile
of the
pyridination
studies
proved
that found
the selenenylation–deselenenylation
process was
rational
to N-benzyl-pyridinium
the pyridination.
been results
conducted.
It was
that the selenenylated intermediate
32 along
with
Therefore, the authors proposed that diselenide was initially oxidized into the real catalyst BnSeX.
salt 35
was formed when 1,3-diene 29 reacted with an equimolar mixture of BnSeSeBn and PyF-OTf.
Then addition of 1,3-diene 29 forms intermediate 30 or 31. Subsequently, pyridine selectively
Further oxidation of 32 afforded the desired product 34 and byproduct 35. These results proved that
attacks to C-2 position owing to the more stability of intermediate 30. After oxidation of
the selenenylation–deselenenylation
process was rational to the pyridination. Therefore, the authors
selenenylated intermediate 32 and the following elimination, pyridinium salt 34 and RSeX species
proposed
that
diselenide
was
initially
oxidized into the real catalyst BnSeX. Then addition of 1,3-diene
are produced (Scheme 12) [78].
29 forms intermediate 30 or 31. Subsequently, pyridine selectively attacks to C-2 position owing to the
more stability of intermediate 30. After oxidation of selenenylated intermediate 32 and the following
elimination, pyridinium salt 34 and RSeX species are produced (Scheme 12) [78].
Molecules 2017, 22, 835
Molecules 2017,
2017, 22,
22, 835
835
Molecules
R55
R
R55
R
N
N
Ph
Ph
8 of 13
of 13
13
88 of
R33
R
N
BF44 N
BF
8
R8
R
26
26
N
N
OTf
OTf
R66
R
N
N
OTf
OTf
Me
Me
76%
76%
Me
Me
N
N
OTf
OTf
Me
Me
28
R77 28
R
Me
Me
N
N
Ph
Ph
94%
94%
OTf
OTf
R11
R
N
N
R22 27
27
R
BnSeSeBn (5
(5 mol%)
mol%)
BnSeSeBn
1,3-diene
TMPyF-BF44 (1.2
(1.2 equiv)
equiv) 1,3-diene
BnSeSeBn (20
(20 mol%)
mol%)
TMPyF-BF
BnSeSeBn
or
or
PyF-OTf (1.2
(1.2 equiv)
equiv)
olefin
Selectfluor (0.5
(0.5 equiv)
equiv) olefin
PyF-OTf
Selectfluor
MeCN/THF, rtrt
MeCN, rtrt
MeCN/THF,
MeCN,
25
25
Me
Me
R44
R
N
N
BF44
BF
53%
53%
BF44
BF
N
BF44N
BF
Ph
Ph
64%
64%
Scheme 11.
11. Organoselenium-catalyzed
regioselective pyridination
pyridination
of
1,3-dienes.
Organoselenium-catalyzed regioselective
pyridination of
of 1,3-dienes.
1,3-dienes.
Scheme
Scheme 12.
12. Proposed
Proposed
mechanism of
of regioselective
regioselective pyridination
pyridination of
of 1,3-dienes.
1,3-dienes.
Scheme
Scheme
Proposed mechanism
mechanism
4. Asymmetric Conversion
Conversion
4.
4. Asymmetric
Asymmetric Conversion
Asymmetric conversion plays
plays an important
important role in
in modern synthesis
synthesis owing to
to the realization
realization of the
the
Asymmetric
Asymmetric conversion
conversion plays an
an important role
role in modern
modern synthesis owing
owing to the
the realization of
of the
different properties
properties of
of enantiomer
enantiomer in
in biology.
biology. Over
Over the
the past
past years,
years, several
several ingenious
ingenious organoselenium
organoselenium
different
different properties of enantiomer in biology. Over the past years, several ingenious organoselenium
catalyzed enantioselective reactions
reactions have been
been developed, but
but most successful
successful cases focused
focused on the
the
catalyzed
catalyzed enantioselective
enantioselective reactionshave
have beendeveloped,
developed, butmost
most successfulcases
cases focused on
on the
utilization of selenium
selenium in Lewis-base
Lewis-base catalysis [83–92].
[83–92]. To date,
date, efficient asymmetric
asymmetric conversion is
is
utilization
utilization of
of selenium in
in Lewis-base catalysis
catalysis [83–92]. To
To date, efficient
efficient asymmetric conversion
conversion is
still
rare
in
electrophilic
selenium
catalysis
although
considerable
efforts
have
been
devoted
in
this
still
electrophilic selenium
selenium catalysis
still rare
rare in
in electrophilic
catalysis although
although considerable
considerable efforts
efforts have
have been
been devoted
devoted in
in this
this
field [93–97].
[93–97].
field
field [93–97].
In 2016, Maruoka
Maruoka and co-workers
co-workers reported an
an enantioselective
enantioselective synthesis
synthesis of γ-lactone
γ-lactone through
In
In 2016,
2016, Maruoka and
and co-workers reported
reported an enantioselective
synthesis of
of γ-lactone through
through
electrophilic
selenium
catalysis
(Scheme
13)
[98].
The
desired
products
were
obtained
with excellent
excellent
electrophilic
selenium
catalysis
(Scheme
13)
[98].
The
desired
products
were
obtained
with
electrophilic selenium catalysis (Scheme 13) [98]. The desired products were obtained with
enantioselectivities because
because of
of the
the employment
employment of
of aa chiral
chiral selenide
selenide catalyst
catalyst with
with the
the rigid
rigid indane
indane
enantioselectivities
excellent enantioselectivities
because
of the employment
of a chiral selenide
catalyst
with the
rigid
scaffold (36).
(36). In
In electrophilic
electrophilic selenium
selenium catalysis,
catalysis, diselenides
diselenides were
were generally
generally utilized
utilized as
as the
the pre-catalysts.
pre-catalysts.
scaffold
indane scaffold (36). In electrophilic selenium catalysis, diselenides were generally utilized as
When the
the authors
authors tried
tried to
to synthesize
synthesize indane-based
indane-based diselenide,
diselenide, an
an inseparable
inseparable mixture
mixture was formed.
formed. To
To
When
the pre-catalysts.
When the
authors tried to synthesize
indane-based
diselenide, was
an inseparable
solve this problem,
problem, they judiciously
judiciously synthesized
synthesized chiral
chiral indane-based
indane-based selenide
selenide 36
36 bearing
bearing
solve
mixturethis
was formed. they
To solve this problem,
they judiciously
synthesized chiral indane-based
p-methoxy-benzyl
group
instead
of
the
corresponding
diselenide.
It
could
also
generate
electrophilic
p-methoxy-benzyl
group
instead of the group
corresponding
diselenide.
It could also
generate Itelectrophilic
selenide 36 bearing
p-methoxy-benzyl
instead of
the corresponding
diselenide.
could also
selenium catalyst
catalyst via
via an
an oxidative
oxidative process
process (Scheme
(Scheme 14).
14). To
To test
test the
the catalyst,
catalyst, β,γ-unsaturated
β,γ-unsaturated
selenium
generate electrophilic selenium catalyst via an oxidative process (Scheme 14). To test the catalyst,
carboxylic acid
acid (37)
(37) was
was selected
selected as
as the
the model
model substrate
substrate to
to undergo oxidative
oxidative cyclization.
carboxylic
β,γ-unsaturated
carboxylic
acid (37) was
selected as
the model undergo
substrate to undergocyclization.
oxidative
Surprisingly,
γ-lactones
(38)
were
formed
smoothly
with
high
stereoselectivities
in the
the presence
presence of
of
Surprisingly,
γ-lactones
(38)
were
formed
smoothly
with
high
stereoselectivities
in
cyclization. Surprisingly, γ-lactones (38) were formed smoothly with high stereoselectivities in the
pre-catalyst 36
36 and
and NFSI.
NFSI. The
The authors
authors mentioned
mentioned that
that the
the gem-dimethyl
gem-dimethyl group
group on
on catalyst
catalyst was
was
pre-catalyst
critical to
to the
the acquisition
acquisition of
of high
high enantioselectivity.
enantioselectivity. By
By this
this protocol,
protocol, different
different kinds
kinds of
of alkyl
alkyl
critical
Molecules 2017, 22, 835
9 of 13
presence
pre-catalyst
36 and NFSI. The authors mentioned that the gem-dimethyl group on catalyst
Moleculesof
2017,
22, 835
9 of 13
was critical
to
the
acquisition
of
high
enantioselectivity.
By
this
protocol,
different
kinds
of
alkyl
Molecules 2017, 22, 835
9 of 13
β,γ-unsaturated
carboxylic
acids
were
converted
into
γ-lactones
with
excellent
enantioselectivities
β,γ-unsaturated carboxylic acids were converted into γ-lactones with excellent enantioselectivities
β,γ-unsaturated
carboxylic
acids
were converted
into γ-lactones
with excellentwith
enantioselectivities
ranging
from
93%
97%.Aryl
Aryl
substrates
also underwent
underwent
the
erosion
of of
ranging
from
93%
toto
97%.
substrates
also
the lactonization
lactonization
withslightly
slightly
erosion
ranging
from
93%
to
97%.
Aryl
substrates
also
underwent
the
lactonization
with
slightly
erosion
of
enantioseletivities.
should
pointedout
outthat
thatthe
theutilization
utilizationofofpersulfate
persulfateor
orhypervalent
hypervalentiodide
iodide as
enantioseletivities.
It It
should
bebe
pointed
enantioseletivities.
Itthe
should
be reactivity
pointed out[98].
that the
utilization
of persulfate
orunique
hypervalent
iodide
as
the
oxidant
led
to
poor
This
result
indicated
the
feature
of
N-F in
the oxidant
led
to the poor reactivity
[98]. This result indicated the unique feature of N-F reagents
as the
oxidant led toselenium
the poorcatalysis.
reactivity [98]. This result indicated the unique feature of N-F
reagents
in
electrophilic
electrophilic
selenium
catalysis.
reagents
in electrophilic
selenium catalysis.
R
R
COOH
COOH CaCO
3 3(3
(3equiv)
equiv) or
or
CaCO
TMSOTf
TMSOTf(1.2
(1.2 equiv)
equiv)
37 37
rtrtoror10
10°C
°C
R
R
O
OO
PMBSe
MeO
MeO
OO
O O
O O
OO
93%, 95% ee
93%, 95% ee
OO
NC
NC
O
O
O
O
O
O
O
91%, 84% ee
>99%, 94% ee
Me Me
Me Me
MeO
MeO
O
O
>99%, 94% ee
OTBS
OTBS
36 36
38
38
NC
NC
Me Me
O
PMBSe
Cat. 36 (10 mol%)
Cat. 36 (10 mol%)
NFSI
NFSI(1.1
(1.1equiv)
equiv)
O
>99%, 75% ee
91%, 84% ee
>99%, 75% ee
Scheme 13. Organoselenium-catalyzed enantioselective synthesis of γ-lactones.
Scheme
13.13.Organoselenium-catalyzed
synthesisofofγ-lactones.
γ-lactones.
Scheme
Organoselenium-catalyzed enantioselective
enantioselective synthesis
Ar*Se
R'
XY (oxidant)
XY (oxidant)
Ar*Se
R'6H4 (PMB)
R'
= 4-MeOC
Ar*
Se
R'
Ar* X Y
Se R'
X Y
Ar*Se X
Y
R'
Ar*Se X
R' =14.
4-MeOC
H4 (PMB)
Y catalyst
R'
Scheme
New 6route
to generate electrophilic selenium
with selenide.
Scheme
Newroute
routetotogenerate
generate electrophilic
electrophilic selenium
Scheme
14.14.New
seleniumcatalyst
catalystwith
withselenide.
selenide.
5. Conclusions
The application of N-F reagents in ESC process provides a powerful tool to functionalize the
5. Conclusions
5. Conclusions
carbon–carbon double bond. Several elegant transformations have been developed by the catalytic
The
application
of N-F reagents
in ESC process
provides
a powerful
to functionalize
system
diselenide/N-F
The
featuresprovides
of these reactions
are tool
easy
mild thethe
The
application
of N-Freagents.
reagents
incommon
ESC process
a powerful
tooltotohandle,
functionalize
carbon–carbon
Several
transformations
have been
developed
by thewith
catalytic
conditions, double
excellentbond.
selectivity
and elegant
good functional
group tolerance.
However,
ESC process
carbon–carbon double bond. Several elegant transformations have been developed by the catalytic
system
reagents.
The
common
features ofconversion,
these reactions
easy to handle,
N-Fdiselenide/N-F
reagents is still in
its infancy,
especially
in asymmetric
which are
is a challenging
issue. mild
system The
diselenide/N-F
reagents.
The common
features of
these reactions
are easy to handle,
mild
current
catalytic
system is and
limited
in the
modification
of alkenes.
It hasHowever,
not been reported
for its with
conditions,
excellent
selectivity
good
functional
group
tolerance.
ESC process
conditions,
excellent
selectivity
and
good
functional
group
tolerance.
However,
ESC
process
with
application
other
unsaturated
substrates such
as allenes orconversion,
alkynes. Furthermore,
details ofissue.
N-F reagents
is in
still
in its
infancy, especially
in asymmetric
which is asome
challenging
N-F
reagents
iscatalytic
still
its
infancy,
especially
asymmetricofconversion,
which
is a challenging
this system
arein
unclear
andisneed
to bein
further
investigated.
The
current
system
limited
theinmodification
alkenes. It has
not been
reported forissue.
its
Theapplication
current catalytic
system
is
limited
in
the
modification
of
alkenes.
It
has
not
been
reported
in other unsaturated substrates such as allenes or alkynes. Furthermore, some
detailsfor
ofits
Acknowledgments: We thank Sun Yat-Sen University, the “One Thousand Youth Talents” Program of China
application
inare
other
unsaturated
such
as allenes or alkynes. Furthermore, some details of
this system
unclear
needsubstrates
toofbe
further
investigated.
and the Natural
Scienceand
Foundation
Guangdong
Province (Grant No. 2014A030312018) for financial support.
this system are unclear and need to be further investigated.
Author Contributions: R.G. collected the references and wrote the draft of the manuscript; L.L. critically
Acknowledgments: We thank Sun Yat-Sen University, the “One Thousand Youth Talents” Program of China
analyzed and reviewed the manuscript; and X.Z. revised the manuscript.
Acknowledgments:
We thank
Sun Yat-Sen
University,
the “One
Thousand
Youth Talents”for
Program
of support.
China and
and the Natural Science
Foundation
of Guangdong
Province
(Grant
No. 2014A030312018)
financial
the Natural
Science
Foundation
of Guangdong
Province
(Grant No. 2014A030312018) for financial support.
Conflicts
of Interest:
The authors
declare no conflict
of interest.
Author Contributions: R.G. collected the references and wrote the draft of the manuscript; L.L. critically
Author Contributions: R.G. collected the references and wrote the draft of the manuscript; L.L. critically analyzed
and
the manuscript;
and X.Z.
the manuscript.
References
andanalyzed
reviewed
thereviewed
manuscript;
and X.Z. revised
therevised
manuscript.
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Patai,
S.; Rappoport,
Z. The
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ofconflict
Organic
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of
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Conflicts
The
authors
declare
no
of
interest.
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2.
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