Download Applications of trivalent and pentavalent tantalum in organic synthesis

Indian Journal of Chemistry
Vol. 43B, April 2004, pp. 813-838
Advances in Contemporary Research
Applications of trivalent and pentavalent tantalum in organic synthesis
Srivari Chandrasekhar*",Tokala ~amachandar~
& Tokala Shyamsunder"
"Indian Institute of Chemical Technology, Hyderabad 500 007, India
?emple University, Philadelphia, Pennsylvania 19122, USA
E-mail: [email protected]
Received 1I July 2003; accepted (revised) 7 January 2004
IPC: 1 n t . ~ 1C. ~01 G 35/00
Of the Group B elements, the group V is relatively
unexplored especially with reference to Niobium (Nb)
and Tantalum (Ta). Interestingly, these two elements
have very similar chemical properties but not as similar as Zirconiun~(Zr) and Hafnium (Hf).
Oxidation states and stereochemistries of Nb and
Ta are shown below (Table I).
Nb and Ta have very little cationic behaviour,
however they can complex well in all the oxidation
states viz., 11, 111, IV and V. In the case of I1 and 111
oxidation states, metal-metal bonds are rather routinely formed. With reference to abundance, Nb is 1012 times more abundant in the earth crust than Ta.
The main source being columbite-tantalite series of
minerals having general chemical composition
(Fe/Mn) ( N b 1 T a ) ~ 0with
~ varying ratios. Both metals
are bright, high melting (Nb 2468 "C, Ta 2996 "C) and
very resistant to acids.
The oxides of these metals are vast', inert and insoluble except in concentrated HF. Accordingly the
chemical applications especially in higher oxidation
states are rather restricted. However, the penta halides
especially chlorides are yellow to purple red solids
and easily prepared by direct reaction of metals with
excess of halogens.2 The halides are soluble in various
organic solvents including diethylether and carbon
tetrachloride. The entire penta halides combine with
- with amines, NbC15 ofhalide ions to form M X ~ and
ten undergoes reduction. The penta halides due to
their Lewis acidity are used as catalysts in cyclotrimerizing or lineraly polymerizing acetylenes and in
Friedal-Crafts and related reactions to little e ~ t e n t It. ~
is the own experience of the authors that Ta (111 and
V) are more user friendly in the laboratory, whereas
Nb (I11 and V) are more difficult to handle and also
NbC15 was air oxidising after a single use of the bottle
despite precautions. Surprisingly. even though the Ta
and Nb coordinate complexes are known for a long
time, its synthetic applications in organic chemistry
has a history of only 10-12 years.
Low valent tantalum complexes react with inactivated acetylenic triple bonds and being sterically congested, the only C-C bond formation effected was
cyclotrimerisation. However, some contribution in CC bond formation and also Ta in its V oxidation state
was effective as Lewis acid catalyst. Keeping aside
the elaborate studies carried out on organo-metalllic
complexes of Ta and Nb, the present review will describe the synthetic reactions using tantalum chloride
as a reagentlcatalyst in both oxidation states V and 111.
The literature available on Ta in organic synthesis can
be clasified into two major areas. (i) Low valent tantalum (LVT) in combination with acetylenes acting as
dianion equivalent (ii) TaCls as a Lewis acid for catalysed reactions.
(I) Low valent tantalum (LVT) in combination
with acetylenes acting as dianion equivalents
(a) Synthesis of substituted naphthol derivatives
Tantalum-alkyne complex can be produced in situ
from acetylenes and combination of TaCls and Zn
((reduction of Ta (V) to Ta (111) J . A careful examination of these complexes proved that the two carbons
of acetylene act as vicinal d~anionand can add onto
electrophiles.
Also, the study between Nb-alkyne complexJ ' and
Ta-alkyne complex7 l 4 showed that Ta-alkynes complexes are formed more smoothly and reaction\ with
o-phthalaldehyde afforded 1-naphthol derivative
when terminal acetylene was used (Scheme I)."
In case of internal acetylenes, 2.3-disubstituted
naphthols were obtained in good yields (Table 11).
INDIAN J. CHEM., SEC B, APRIL 2004
814
Table IOxidation
state
~ b - ' Ta"
,
~ b - '7ra-l
,
Oxidation states and stereochemistries of niobium and tantalum
Coordination Geometry
number
5
Tbp
6
Octahedral
Examples
n~omp~ex
7
Distored capped octahedron
(non-rigid)
6
6
Octahedral
Trigonal prism
Octahedral
Complex
Dodecahedra1
Octahedral
Distorted pentagonal
bipyramidal
Capped octahedron
7
8
6
7
7
8
4
5
NbO
TaC12(d~npe)2
LiNb02
N~>CI:-, M2Cl6(SMed;
TaC13(CO)(PMe2Yh)3.EtOH
Ks[Nb(CN)xI
(NbClA), TaC14py2,~ ~ 1 6 ' KWJF~
Non rigid in solution
Square antiprism
Dodecahedral
6
Tetrahedral
T ~ P
Distorted tetragonal pyramid
Octahedral
6
Trigonal prism
8
Distorted pentagonal bipyrarnidal NbO(S2CNEt2),
Pentagonal bipyrarnidal, fluxional S=Ta(S2CNEt2)?,Ta(NMe?) (S2CNMe2)3,
(S2CNR212, TaMei
Bicapped trigonal prism
INb(trop)q 1'
Sqaure antiprism
NaiTaFx
Dodecahedra1
Ta(S2CNMe2)4+
n Complex
(n' - C S H S ) T ~ H ~
9
CHO
MCI,
~b(p-dike),,M2CldPme3)4,
Nb(SCN), (dipy12
K4Nb(CN)82H20
(q-C5H5)2NbMe2
ScNbO,
MCl,(vapour), ?'aMe5. Nb(NR2)5
Nb(NMe2)5
NaMO? (perovskite), NbC15.0PC13,
TaC15.S(CH3)2,TaF;, NbOCl;,
M2Cllo. MCIL
[M(S~C~HI.)~~-
-
---
OH
2,6-lutidine
DME, PhH
25 OC
OH
Scheme I
Mechanistically, it is anticipated that low valent Ta
(Nb) produces a three membered ring metal-alkyne
complex. Insertion of formyl group into metal carbon
bond of the complex followed by second insertion,
elimination of metaloxy group and naphthol is liberated after an aqueous work up.
(b) Synthesis of allyl alcohols
A variety of Ta-alkyne complexes 1 and 2 have
been prepared (without isolation) and were reacted
with various aldehydes in a one-to-one fashion t o
yield the corresponding allyl alcohols with exclusive
E-geometry (Scheme 11). Interestingly, in the absence
of carbonyl compound and quenching the complex 1
with NaOD-D20 furnished dideuterated cis-olefin 3 in
good yields.'6
The results of the reaction reveals that electronic
effects play a role in the regiochemistry of rlie
815
ADVANCES IN CONTEMPORARY RESEARCH
Table II-Regioseleclive
Run
R'
C5Hl
R?
M
Time
t (hr)
TaCI5, Zn
C5Hl 1
+
DME, PhH
25 OC, 30 min
synthesis of 2,3-disubstituted-l-naphthols
Aldehyde
Equiv.
Product ratio
C5Hvc5H1
>=(
NaOD / D20
C5Hi 1
C5Hi I
+
Ta
25 OC, 1 h
3
D
INDIAN J. CHEM.,SEC B. APRIL 2004
R~+R~
THF
TaC15, Zn+
DME, PhH
25Oc, t hr
R'
R~R~C=O
R2
b
Pyridine
25 OC, 15min
OH
A
B
Scheme II(b)
A. NaOH/H20
~1-7~2
TaCI5, Zn
TH F
DME, PhH
25 OC, t hr
Pyridine
R3R4c=0
25 OC, t hr
25OC, 1 hr
HO
A
R4
B
Scheme I11
Table 111Run
RI
Synthesis of allylic alcohols from acetylenes and aldehydes
R~
R3
product. Preference is generally that the acetylenic
carbon having smaller substitution adds onto C=O
(Table 111).
(c) Synthesis of (Z)-alkenyl sulfides and (E)-3hydroxy-1-propenyl methyl sulfides
It was observed that tantalum-alkyne complexes
form more readily from alkynyl sulfides than the dialkynes. There is a side reaction of a-chlorination,
which could be prevented by the addition of pyridine
in the reaction medium (Scheme III)."
As usual quenching of complex in absence of
electrophile (R-CO-R') resulted in (Z)-alkenyl
sulfides, whereas the electrophile presence made the
new C-C bond formation
to the thio group
preferentially yielding (E)-3-hydroxy- 1-propenyl
methyl sulfides. The bulkiness and electronic nature
R4
Time
t (hr)
Yield
A/B
("/.)
The bulkiness and electronic nature of the substituents
of alkynes influences the regiochemistry of the coupling reaction (Table IV).I8
The results have special advantage over zirconocene-(methy1thio)-1-alkyne complex in that zirconocene complex yields as 1:l mixture of regioisorners,
whereas the Ta-complex produced one stereo isomeric allylic alcohol predominantly.
(d) High oxidation state transition metal carboxylates as acylating agents
The abilities of metallocene ~sobutyratelo ' ~ c t '14
acylating agent was demonstrated by Koln~ct ~ r l .
wherein the metallocene complex was prepared b)
treatment of TaCls with isobutqric 'ic~d to 14olate 1'
817
ADVANCES IN CONTEMPORARY RESEARCH
Table IV -Reactions
Run
of alkynyl sulfides and sulfones with carbonyl co~npoundsby means of a TaCI,-Zn system
R'
ZR~
R~
R~
t/h r
Yield
A/B
(a)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
11-C10H21
11-c10H21
n-CloH21
11-C10H21
n-CloH21
c-ChH I I
c - C ~ HI I
Ph
Ph
n-CloH21
n-CloH21
n-CloH21
n-CloH21
c-C6H~I
Ph(CH212
Ph(CH212
c - C ~ HI I
-(CHz)s-(cHz)8Ph(CH2)z
-(CH2)5Ph(CH2)2
-(CHz)5Ph(CH2)2
Ph(CH212
c-C~H
II
-(CH2)5Ph(CHz)z
SMe
SPh
SMe
SMe
SPh
SMe
SMe
SMe
SMe
S02Me
S02Ph
S02Me
S02Me
S02Me
0.2
0.5
0.2
0.2
0.5
0.2
0.2
0.5
0.5
2.5
21
2.5
2.5
2.5
H
H
H
H
H
H
H
H
H
TaCI,
73
85
74
77
75
68
70
64
54
54
43
46
62
59
+
15 min
5
Scheme IV
-<
Figure 1
coordinated complex (Figure I)'', which on exposure
to benyzl amine yielded the N-benzyl isobutyramide 6
in less than 15 minutes." The same experiment with
other metallocenes such as Ti and Zr yielded the corresponding 6 albeit at a very slow rate compared to
Ta (Scheme IV). This may be due to high positive
charge on the metal.
(e) Synthesis of (E)-a, P-unsaturated amides from
reaction of tantalum-alkyne complexes with isocyanates
Reaction of metal-alkyne complexes 7, 8 and 9 is
well studied with few metals."-'"E)-a,
P-unsaturated
amides can be stereoselectively synthesized25926
by the
reaction between low valent Ta-alkyne complex and
isocyanates. In a typical experiment, the reaction was
performed between 6-dodecyne, tantalum (111) and
PhNCO to generate (E)-N-phenyl-2-pentyl-2-octenamide 1 0 in 80% yield [Scheme V(a)]. The lijgh
yields are obtained subject to filtration of Ta-alkyne
complex from reaction mixture and later isocyanate
was added. Quenching the reaction with DIO/NaOD
yielded deutero amides, which are hitherto unacce5i1ble or difficult to make.
Again, as in the case of addition to carbonyi group.
the substitution pattern of acetylene plays a majol- [-ole
in the regio chemistry. Only in the case of methylthio
substituted alkyne, the regioisomer A was produced
exclusively because of the electronic nature of the
substituent (Table V).
(f) Chlorination of alcohols using TaClj
Selective and ,ilmost complete convcriioii o f
cyclohexanol 11 to chloro cyclohexane 12 (96V) was
achieved whereas other metal halides naniel) WC16.
did not give
a good
conversion and NbCi? 2aL.c. only
70% conversion after prolonged reaction hours.
Thus, this is one of the rare cl-block elements ~ v h i c l ~
is comparable with halides of p-bloch eieinznts
(S02C12. PClr) in conversion of 2" alcohol a, the respective chloride^.'^-'^ Thus TaClS proved :o he :i
1
1
1
INDIAN J. CHEM., SEC B, APRIL 2004
--
TaCI5, Zn
R+
l R2
DME, PhH
25 OC, 1 h
PhN=C=O
NaOD / D 2 0
2 5 0 ~
NaOH / H 2 0
*
RqN,
(H)D
0
10
Scheme V(a)
R-1
TaC15, i n
R2
R~N=C=O
NaOH 1 H 2 0 R1<
DME, PhH
25 OC, 1 h
25 OC, t hr
+
*
w
250C, 1 hr
0
NHR~
6
A
Scheme V(b)
Table V -Reactions between alkynes and isocynates by means of TaCIS and Zn
Run
R1
Timelhr
n-CsH1 I
n-CsH I I
n-CsH1 I
n-CsH I I
n-CsH1 I
c - C ~ HI I
c-ChH I I
Ph
Fh
Me3%
Me3Si
MeS
MeS
3
3
Yield
(%)
1
2
3
4
5
6
7
8
9
10
I1
12
13
better halogenating
(Scheme VI).~'
agent
20
20
20
3
3
3
3
3
3
compared
to
NbCIS
(g) Carbocations of alkyl benzenes with TaC15-CH2CI2
TaCIS-CH2CI2performed C-C and C-E-I bond cleavage in 1,3,5-triisopropyl benzene 13 to yield stable
indane cation 14 as shown in ( Scheme VIJ ).
The transformation generally demands superor very acidic metal halides viz., TiCl,, ZrCI,
and ~ f c l ~ . " . "It is rather interesting to note that even
though 'Ta' in oxidation state V is almost nonmetallic and not a strong acidic metal halide it is able
acids3h-39
80
72
79
63
38
69
62
51
0.2
74
60
33
58
0.2
90
to perform such a transformation albeit in moderate
yields ( 5 0 % ) . ~ ~
(h) Allylic arnine synthesis via tantalum-alkyne
complexes
Nucleophilic
addition of organo ~netallic
compounds to carbon-nitrogen double bonds
constitute an important method for the synthexis of
amines."-" Tantalum-alkyne complex 15 adds onto
N,N-dimethylhydrazones 16 at elevated temperatures
(80°C) to yield allylic hydrazine in rather low yields.
Attempt to add activation such as Me3AI. Me;Ga
helped to improve the yields. The regiocher:listsy of
the allylic hydrazine depends on the b~:lkiness of
ADVANCES IN CONTEMPORARY RESEARCH
Table VI -Reactions between tantalum-alkyne colnplex and hydrazones in the presence of MelAl
Run
1
2
3
4
5
6
7
8
R~
R~
RI
II-C~H~~
l1-C5Hl1
II-C~H~
I I - C ~ IH ~
c-C6HII
Ph
Me3Si
MeS
Ph(CH212
C
-
~I
~
~
I
(E)-PrCH=CH
(cyclohexanone)
Ph(CH2I2
Ph(CH2)2
Ph(CH2)?
Ph(CH212
tl/hr
t2/hr
Yield
(%I
2
2
2
2
3
4
3
0.2
16
16
16
16
16
16
16
26
80
71
32
5
64
76
57
69
A/B
Unreacted
alkene (%)
Reco~,
Hydrazone
(%I
substitutions on the acetylenes as expected. Electronic
effects are also observed in regiochemistry \\lien
Me& and -SMe substitutions (Run 7 and 8.
Table VI) are placed on acetylene. This reaction ~vas
restricted to aldehyde hydrazones (Scheme VIII)."'
(96%)
12
NbCI,
Interestingly, insertion of isocyanide into tantalumalkyne complex was achieved to synthesize 2.3,4trisubstituted N-amino pyrrox 17 albeit in only 17%
yield (Scheme IX).
(70%)
11
It may be inferred that external Lewis acid addition
is essential to promote the addition of tantalum-alkq ne
complex to imines.
(50%)
12
Scheme VI
(i) Selective reduction of acetylene to olefin with Zstereoselectivity using LVT
14
13
Scheme VII
R'
=
2
Tzicl5,.Z
DME, PhH
55 O C , t' 1 h
Selective reduction of acetylenes to olefins \s.ith
precise cis geornetry is an important task." Catalytic
hydrogenation gives cis-olefin (some time reaction
does not stop at olefin stage and proceeds t o
'yl1
15
TliF +
Me3AI
,NMe2
N
1 1 6
R3
H *
55 OC, t2 / h
NaOH I H,O *
25 O C , 1 h
INDIAN J. CHEM., SEC B, APRIL 2004
C5HI
C 5 H ~1
DME, PhH
25 OC, 30 min
NaOH 1 H 2 0
17
Scheme IX
R1 Z I
R~
TaCI5, Zn
NaOH I H 2 0
*
R'
R2
HHH
DME, PhH
25 OC, t
Scheme X
saturation) whereas metal/NH3 reduction generally
yields the E-olefin. Low valent tantalum produced
from TaC15-Zn which is much faster than analogues
NbC15-Zn for complexation with acetylene after basic
hydrolysis with H20/(NaOH) furnished the protonated
olefin in good yields with more than 99% Z selectivity
(Scheme x).~'
It is observed that in the case of NbC15, HMPA as
additive is desired which is carcinogenic and not
available on many catalogues. Thus 'Ta' proved to be
a superior metal over 'Nb' in this transformation. Entries 10 and 11 (Table VII) are inert to a possible cyclization despite possessing an additional olefinic system thus proving of 'Ta' is different from low valent
'Zr' wherein an intramolecular cyclisation with concomitant reduction of triple bond is observed
(Table V I I ) . ~ ~ . ~ ~
Cj) Synthesis of substituted furans from tantalumalkyne complexes
Although insertion of isocyanide into Ta-alkyne
~ process has not
was recognized as ealry as 1 9 7 4 , ~the
been utilized in organic synthesis. Insertion of isocyanide into the tantalum-carbon bond was successfully
achieved for the synthesis of highly substituted furans
18 (Scheme X I and Table VIII).
Thus a variety of 2,3,4-trisubstituted furans were
prepared by the treatment of various tantalum-alkyne
complexes with the aldehydes followed by addition of
an isocyanide in DME-PhH-THF (1: 1:1). The general
approach is shown in below.57
(k) Preparation of (E)-allylic amines by reaction between tantalum-alkyne complex with metalloimines
There are two typical approaches for the preparation of amines from acetylenes and imine derivatives.
Table VII -Reductions of alkynes to (Z)-alkcncs by means a TaC1,-Zn system
Entry
Yield (%)
39
52
68
85
69
85
80
82
79
81
82
80
ZIE
ADVANCES IN CONTEMPORARY RESEARCH
Scheme XI
Table VIII-Synthesis
of 2,3,4-trisubstituted furans
Run
t l hr
1
0.5
8
2
0.5
66
3
0.5
28
4
2
55
5
5
40
6
2
57
7
3.5
42
8
3.5
54
In one approach as reported by ~ u c h w a l and
d ~ Liv~~~
inghouse60, the reaction between ziroconocene-imine
complex and acetylene would lead to allylic amines.
Alternatively, Ta-alkyne complexes can add onto N,
N-dimethyl hydrazones of aldehydes in the presence
of Me3A1 to yield (E)-allylic hydrazines, which in
principle can be reduced to amines.
Since both the approaches are restricted to imines
of aldehydes, the methods cannot be used for primary
hydrazinelamine synthesis having 3" carbon due to
~~'
the steric factor^.^' It is observed that l i t h i ~ i m i n e ' as
C=N component62 which is brought close to the Taalkyne complex by ligand exchange.63264Alkaline
work-up as usual to cleave tantalum complex furnished trisubstituted allyl primary amines. In a typical
experiment addition of nonane nitrile 19 to an ether
solution of MeLi (in situ to generate lithioimine), followed by treatment with low valent tantalum-(6dodecyne) complex 20 gave allylic amine 21, which
was acetylated for characterisation (Scheme X I I ) . ~ ~
(1) Low valent T a mediated Reformatsky reaction
R~formatskyreaction is a powerful protocol for the
synthesis of P-hydroxy esters. Generally this reaction
is performed at elevated temperatures using Zn
However, Sm (11) iodide has been used as one
electron reducing agent in this reaction which is per-
Yield (70)
~ formed at low temperature. TaC15-Znwas found to be
an efficient combination for the Rqformatsky reaction
at 0 "C (Table 1x1~'.Typically to a suspension of low
valent tantalum prepared from TaC15 (3 mmol) and Zn
(4.5 mg atom) in THF (15 mL,) at room temperature
was added a-bromopropionate 23(1 mmol) and carbony1 compound 24 (1 mmol) at 0°C under argon atmosphere. After 2 h of stirring at 0°C and usual workup produced P-hydroxy ester 25 (Scheme XI11 and
Table IX)
(m) Insertion of carbon-carbon double bonds into
tantalum-alkyne complexes
Not only tantalum-alkyile con~plexesadd onto C=O
and C=N but can also be inserted into sirnplz alk e n e ~ This
. ~ ~ has a great potential, as this protocol
enables one to create a new C-C bond between internal acetylenes and terminal oiefin~
In a typical experiment Ion valent tantalum-hdodecyne complex with lithium-3-buten-1-oiate 26 in
DME-benzene-THF (1: 1: 1) at 3-5 "C for 3 hr gave t w ~
alkylated alkanols 27 and 28 in 38:2 ratio as regio isomers (Scheme XIV). Prior coordination of the tantdlum-alkyne complex with the nyciroxyl group of the
homo allyl alcohol facilitates both i~lsertionsas well a:,
regio chemistry (Table X). Anchimeric assistance of.
phenols and amines is also a noteworthy feature.""?"
I
INDIAN J. CHEM., SEC B, APRIL 2004
822
I
I
I
I
I
I
I
I
I
+
n-C8Hl,CN
19
Tact5, Zn
+
C5H1 1+C5H1
1
MeM
[c5Hxc5H1]
1
THF
1
=
DME, PhH
25 OC, 30 min
~1~1
n-C8H7~
additive
I
I
I
I
I
Ac20 1 Et3N
NaOH / H20
25 OC, 1 h
*
CH2CI2
A: M = Li
C 5 H ~ ~C 5 H ~ 1
%Me
AcHN
addi.tive = none
n-C8H17
50 OC
25 OC
6: M = Mg
4h
67%
20h 43%
additive = none
50 OC
6h 65%
C: M = AIMe2 additive = none
50 OC
6h
75%
Scheme XI1
Table IX -The
Entry
reactions of ethyl a-bromopropionate with ketones and aldehydes in the presence of the LVT reagent
Carbonyl
compound
tl hr
Solvent
Cyclohexanone
Cyclohexanone
Cyclohexanone
Cyclohexanone
Cyclohexanone
Cyclohexanone
Cyclohexanone
Cyclohexanone
Cyclohexanone
Cyclohexanone
Cyclohexanone
Cyclopentanone
Acetophenone
Benzophenone
Octan-2-one
Cyclohexenone
D-Camphor
Benzaldehyde
Cyclohexylaldehyde
Octanal
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
12
2.5
2.0
2.0
2.0
0.5
3.0
I .O
2.0
THF
Benzene
Et,O
CH2C12
MeCN
DMF
DMF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
1.5
T / "C
Yield (56)of
b-hydroxy-ester\
67
Manyproductc
0
44
Manyproducts
78
79
Manyproductc
40
59
81(17: 13)
62
751:3:2)
16(3: 1)
19
75(tl~reo:c~r-!~i/1ro=4:3)
57,(tl1reo:er~.rli1'0=6:5)
6O(tlzreo:or~rllro=4:3)
+
ADVANCES IN CONTEMPORARY RESEARCH
23
25
24
Scheme XI11
C5H1
C5H1 1
TaCI,, Zn
m
26 O L i
DME, PhH
25 OC, 2 h
-
C Y ~ H I I
NaOH / H 2 0*
-
+
C5H7C5Hll
OH
Scheme XIV(a)
[w
BuLi
R=n-C,Hll
R]
*-
NaOH
Hzo
A
Scheme XIV(b)
Parallel to the above observation, Takai et al., observed that ally1 alcohols and amines could be inserted into low valent tantalum-alkyne complexes.77
N,N-dimethylallylamine was found to react with the
tantalum complex at 55 "C and 1, 4-diene 29, a formal
cis-hydroallylation product of 6-dodecyne 30, was
obtained (Scheme XV).
Mechanistically, insertion of an olefinic bond of allyl alcohol into the tantalum-carbon bond of the com-
plex 31 form two oxatantalab~cycloheptene intermdiates 32 and 33. Deoxygenative ring openlng of
oxatantacyclo butane ring of interl-nediate 32 follo\teci
by hydrolysis providci the product 34134a.
(n) Selective cyclotrin~erisationof IA\.'T-alk\.lle
Low valcnt tantalum-alh~ne (inte~nal)complex I c act with terminal dlynes 35 in THF to y v e t r ~ i a b u b i r ~ tuted b e n ~ e n ed e n v a t i ~e5 36 (Scheme XVI) "
INDIAN J. CHEM., SEC B, APRIL 2004
C5H1
C5H1 1
TaCI5, Zn
ex
NaOH i
b
DME, PhH
a:
fiNMe2
NMe2
m
N
H
55
20
54%
Not detected
25
4
86%
Not detected
Scheme XV
When acetylenic nitrile 37 was used, pyridine ring
38 was formed in 73% yield (Scheme XVII).
Functionalised acetylenes viz., TMS-acetylene and
methylthio substituted alkynes also behaved well
(Table XI).
(11) TaC15 as a Lewis acid for catalysed reactions
Differential behaviour of TaClj and NbCl5 in
Sakurai Reaction
An unusual reaction was observed, when NbC15
was used instead of TiClj in the Sakurai ~eaction"
INDIAN J. CHEM., SEC B, APRIL 2004
C5Hl
C5Hl 1
TaC15, Zn
THF
35
-
r
DME, PhH
Pyridine
50 OC
NaOH / H 2 0
36,82%
Scheme XVI
C5H1
C5H1I
-
TaCI5, Zn
THF
DME, PhH
Pyridine
*
-
NaOH 1
~
~
0
0
38,73%
Scheme XVII(a)
RI-R~
-TaCI5, Zn
THF
DME, PhH Pyridine
R3-(cH2)n-
NaOH
50°C, t hr
"20
R-,
Scheme XVII(b)
Mechanism
4
- Me3SiCI
+741
Scheme XVIII
between aldehyde 39 and ally1 trimethyl silane 40
(Scheme XVIII).~' It was visualised that niobium
alkoxide species is a leaving group and cyclopropanation is forced via homoallyl group participation. As
the intermediacy of long lived carbon~um run5 I \
unlikely, it is assumed that the cations are trapped b j
a chloride ion so that the cyclopropyl methyl chlonde
41 is formed to yield the unexpected product.
ADVANCES IN CONTEMPORARY RESEARCH
R-
CHO
+
e S i M e 3
R = Ph, naphthyl, cyclohexyl
Scheme XIX
exo
R' = Me, R2 = H
Scheme XX
Interestingly, when TaC15 was used in the reaction,
in presence of AczO, a one-step conversion followed
by acetylation is observed (Scheme XIX).~'
(b) TaCl5 as a catalyst in Diels-Alder reaction
In fact the first application of TaC15 as Lewis acid
was di~monstratedin a Diels-Alder reaction. A comparison between NbC15 and TaC15 for Lewis acid catalysed Diels-Alder reaction was attempted and found
that NbC15 catalysed the reaction better between
cyclopentadiene 42 and m e t h a c r o ~ e i n . ~ ~
However, replacing the chloride ligands with chiral
ligands (preferably bidentate) was attempted and
found that, even though enantioselective Diels-Alder
reaction could not be achieved, endoselective DielsAlder to an extent of 90% (Scheme XX). However,
the ee's were in the range of 10-40%. Thus, the niobium tantalum complexes with bidentate ligands have
a good potential (Table XI1 and XIII).
(c) Synthesis of imides from anhydrides catalysed
by TaC15
In depth studies on the Lewis acid catalysed reactions of TaCl5 revealed that TaC15 adsorbed on silicage1 acts as a better catalyst compared with the TaC15
alone. This is in agreement with the observations
made by Howarth et al., who also noticed that when
the chloride ligands were replaced with other oxoligands, the efficiency of catalyst was enhanced. In a
typical experiment, equimolar phthalic anhydride 41
and benzyl amine were adsorbed on silicagel, admixed with 10 mol% of TaC15-silicagel and exposed
to microwave irradiation for 5 min, furnished N-
TaCI, - SiO,
MW, R-NH2
cfiR
41
42
R = N-benzyl amine
Scheme XXI
--
Table XI1 - Results for the Diels-Alder reaction bet\vee~l
cyclopentadiene and crotonaldehyde or melhacrolein in tlic
presence of niobiuin complexes of some bidentaate ligands
Bidentate
ligand
1 L-Phenylalanine
2. L-Alanine
3. L-Leucine
4. L-Isoleucine
5. L-Tryptophan
6. L-Valine
7 . Diethyl L-tartrate
8. Diisopropyl Ltartrate
Dienophile
Crotonaldehyde
Methacrolei~~
18%; 85: 15
37%: 95: 5
10%: 9-3: 7
20%: 94: 6
10%: 84: 16
5 % : 95: 5
9%; 93: 7
16%: 94: 6
43-%: 75: 75
44%: 7: 93
77%: 13: 87
40%; 8: 92
57%. 32: 7X
l i ' % : 13: X7
6l(;,t,6:94:25(;
52Ik: .?:97: 40c;
-
benyzl phthalimide 42 in 92% isolated yield (Scheme
XXI). When optically active R-(+)-@-methyl benzy l
amine was used, the product was obtained u,ithout
any racemisation (Table XIV).'"
The same reaction could also be accomplished o n
solid support such as Merrifield resin (Scheme XXII
and Table X V ) . ' ~
Solid supported amino acid 43 wherein thc 1-chin
part was connected through ester linkage. ~ i l i e n
INDIAN J. CHEM., SEC B, APRIL 2004
828
Table XI11 -Results for the Diels-Alder reaction between cyclopentddiene and crotonaldehyde or methacrolein in the presence of tantalum complexes of some bidentaate ligands.
Bidentate ligand
1
2.
3.
4.
5.
Dienophile Crotonaldehyde
Diethyl-L-tartrate
Diiosopropyl-L-tartrate
42%; 90: 10
24%; 95:5
(+)-2,3-0-Isopropylidene-L-threitol
14%; 99: 1
(-)-2,3-0-Isopropyliden 1,1,4,4-tetraphenyl-L-threitol
72%; 86: 14
(-)-2,3-0-Isopropyliden 1,1,4,4-tetra(2-naphthy1)-L-threitol 42%; 99: 1
Table XIV -Synthesis
Entry
Anhydride
Amine
Methacrolein
78%; 6:94;7%
55%; 6:94
49%; 16:84
50%; 9:91
75%; 10:90
of N-alkyl and N-aryl imides
Amide
Time
(min)
Yield
($6)
829
ADVANCES IN CONTEMPORARY RESEARCH
Table XIV -Synthesis of N-alkyl and N-aryl imides-Xontd
Entry
Anhydride
Amine
Ti me
(min)
Amide
Yield
(%I
1. TaCI5 - Si02
MW, 5 rnin
2. TFA in CH2C12
43
Scheme XXII
treated with phthalic anhydride 41 in presence of
TaCls-silicagel, after reaction and cleavage of bound
resin yielded the phthalimido acids 44 in good yields.
R-
TaCI, - Si02
EtSH
(d) Use of TaCIs-silicagel as Lewis acid for protection of organic functional groups
SEt
OH
OTHP
TaCI, - S102
R+
Highly oxophylic TaC15 and TaCls-silicagel has
also been utilized as a catalyst in protective group
introduction namely -0THP formation and thioacetalisation of aldehydes (Scheme X X I I I ) . ~ Series
~
of alcohols both lo, 2" and 3" alcohols converted to
tetrahydropyranyl ethers in good yields. The yields
were higher when Ta-silicagel was used instead of
CHO
R2
DHP
+
R+R'
R
R = A I ~ ~R',
I , R~ = H or A I ~ ~ I
Scheme XXlII
TaCIS alone even at higher concentration of TaClj
(Table XVI).
'
.
t*:~
INDIAN J. CHEM.. S E C B, APRIL 2001
830
Table XV-Synthesis
Entry
Polymer-bound ,ilnlnc
of N-alkyl imides o n solid support in solid htate
Anhyd~~de
Table XVI-Thioacetalisatiori
Substrate
Entry
ph-
Ph
4.
5
TBDMSOC
-HO
-
and tetruhycil-opyr-;i~~yi~~~~or~
lieaction
condition
CHO
CHO
5
X
B
1
I
si '\ 311111 1
")
T a D ~ , ~ , l s o / \ / - ~ ~ ~ s '
--
c
111 1
ADVANCES IN CONTEMPORARY RESEARCH
Table XVI-Thioacetalisation
Entry
Substrate
and tetrahydropyranylatiodorltd
Reaction
condition
-
C
0
Product
(time)
%
7
\
\
P
11.
12.
B
~
n
O
83 1
~
O
H
n 0THP
C
C
P
~
0,
OTHP
-7
~
O
( 1 0 min)
71
P
( 5 min)
88
0
0
0--\
\
T
H
Bnon
( 6 ~nin)
86
A.TaCI5-silicagel, ethanethiol, CH2C12;
B.TaC1,-silicagel, propanedithiol, CH2CI2;
C.TaC1,-silicagel, dihydropyran, CH2CI2.
TaCI,
+
b
R2
solvent, r.t
Scheme XXIV
Thioketalisation of aldehydes in presence of ketones is an added advantage of this catalyst. The aldehydes in absence of propanedithiol underwent selective cyclo trimerisation (Scheme XXIV).
Which helps in separating aldehydes from ketones
if they have close boiling points. However, a side reaction observed was self-aldolisation. Changing the
solvent could alter the ratios of self-aldolisation vs
trimerisation. Different ratios of products were obtained in DME, ether, CH2C12and in the absence of
solvent (Table XVII, Table V).
(e) TaC15-silicagel catalysed Prim reaction
Pri~lsreaction, wherein a new C-C bond formation
is achieved with simultaneous functionalisation of 1.3
carbons as diol is generally performed with mineral
acids and clay besides few Lewis a ~ i d s . ~ " ~ ~ u ~
Si02 proved to be a powerful catalytic system for
P r i m reaction between olefin and paraformaldehyde
(Scheme XXV, Table X V I I I ) . ~ ~
INDIAN J. CHEM., SEC B, APRIL 2004
832
Table XVII-Tri~nerisationand/oraldolisation
Entry
5.
Aldehyde
Solvent
Reactiontim(hr)
la
Ether
4
la
CH2C12
4
la
DME
4
Neat
2
f i CHO
Trimer
Aldolproduct(%)
(%I
2a
2c(O)
6.
7.
8.
2a
2a
Ether
CH2CI2
4
5
2b( 100)
2b(40)
2b(50)
2a
DME
6
2c(50)
'f"
9.
Ph
A
CHO
Neat
2
3a
Ph
Table XVIII-Prins
Entry
Substrants
Ph
reaction catalysed by TaCI5-Si02
Product
Reaction time & Yield (%)
Microwave Conventional
irradiation
heating
Overall
yield(%)
5-33
ADVANCES IN CONTEMPORARY RESEARCH
Table XVIII-Prins
Entry
reaction catalysed by TaC15-SiO&otltcl
Subslrants
Product
Reaction lime & Yield (9)
Microwave Conventional
irradiation
heating
4 rnin ( 8 5 )
12 hr (78,
5 min (80)
13 hs (70)
C1
02N
MW / 3-5 min / TaC15-Si02(or)
R
H
Dioxane, A, 10-13 h / TaC15-Si02
R' = H or CH3, R2 = H or CH3 or C6H5
Scheine XXV
k2
INDIAN J. CHEM., SEC B, APRIL 2004
834
(f) Multicomponent coupling catalyzed by TaC15SiOz
a-Amino phosphonates were synthesized in one
pot by coupling three fragments namely carbonyl
compouhd, amine and diethyl phosphite in a one-toone fashion catalysed by 10 mol% TaC1,-SiO,
(Scheme XXVI). In a typical experiment, ptolualdehyde, aniline and diethyl phosphite and stored
in CH2C12 in presence of 16 mol% TaCls-Si02 and
after 22 h, clean formation of a-amino phosphonate
was observed (Entry 1, Table XIX). The reaction is
performed at room temperature.89
This is unlike lanthanide triflates, which are also
known to catalyse the addition of phosphonate to imines that require reflux temperatures.90
(g) Kinetic resolution of 2' alcohols via acetylation
Acetylation of alcohols was also reported by treat-
6,
"4'2
R'.
I
+
+
OEt
ment of alcohols with Ac20 in presence of TaC15-SiO?
(Scheme XXVII) .
Keeping the size of Ta in view, it was anticipated
that complexation with chiral ligands should give a
bulky chiral catalyst which enables efficient kinetic
resolution. Two ligands were prepared using TaCli
and TADDOL (A)la,a-diphenyl prolinol (B) and
used as catalysts in acetylation of 2" alcohols by allowing the reaction to only 50% alcohol will be enantiomerically enriched (Scheme XXVIII)." Unexpectedly, however at best only 40% ee of chiral alcohol
was recovered (Table XX).
(h) Cleavage of epoxides
Symmetrical epoxides are opened with aryl amines
in presence of TaC15-Si02 (Scheme XXIX). Interestingly, the protocol is restricted to aryl amines as external nucleophiles whereas aliphatic amines were
innert." This reaction has a potential to desymmetrise
meso epoxides to chiral aminols (Table XXI).
TaCI5 - Si02 (10 mol%) -
HO-P; OEt
9
-
CH2CI2, 18-24 h, r.t
NH
R = Alkyl, R' = H, Alkyl
R~ = H, NO2, OMe, OH, F
Scheme XXVI
OH
TaCI, - SiO, (10 mol%)
OAc
Scheme XXVII
OH
TaCI, - Chiral ligand
OH
b
Ac20, CH2CI2
n = 1,2, 4
Scheme XXIX
'AR,
R
Scheme XXVIII
OAc
+
R~.-4R2
835
ADVANCES IN CONTEMPORARY RESEARCH
Table XIX-Tantalum
Entry
1.
(V) chloride silica gel catalysed one-pot synthesis of a-amino phosphonates
from aldehydes, ketones and amines
Aldehydelketone
H
3
c
oc H o
Amine
Time(hr)
Yield(%)
INDIAN J. CHEM., SEC B, APRIL 2004
836
TableXX-Enantioselective
Entry
Substrate
Ratio of TaCI5 and
ligand in mole %
acylation of 2"-alcohols
Rccoveredalcohol
Convcrsion
(96)
c.
c
(% i
Table XXI-TaCI5-Silica gcl catalyscd epoxide opening with
aromatic amines
Entry
Aminoalcohols
Yield
(%)
Conclusion
In conclusion, this review attempted to cover
various aspects of organic chemistry of Ta and its
complexes for various transformations. Eventhough in
asymmetric transformations not vary bigh selectivities
are obtained, modifications in ligands and experimental protocols will allow great improvements. Also
the new C-C bond forming reactions and synthesis of
substituted heterocycles which are hitherto n o t
possible by other metals Catalysis would be a great
advantage.
Acknowledgement
TS thanks CSIR New
assistance.
Delhi,
r
r'innncini
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