Download Thiobenzoate Photochemistry

Photoinduced Electron Transfer Reactions of Thiobenzoates
Introduction and Background
Irradiation of thiobenzoates can produce alkenes by way of a Norrish Type II process.
Barton and others have studied the synthetic applications of the photochemical -hydrogen
abstraction of thiobenzoates.1-6 The elimination of thiobenzoic acid to form an alkene is
stereo- and regioselective for the axial allylic hydrogen in the cholesterol derivative 1 (Figure
1).1 Likewise, photolysis of 2-ethoxyethyl thiobenzoate (2) gives ethyl vinyl ether in 46% yield
in less than one hour.4
Figure 1. Alkene Formation from Thiobenzoates
Me C H
8 17
Me
Me
Me
(D)H
C8H17
h
O
Ph C
D(H)
D(H)
1
S
O
+
OEt
O
OEt
h
Ph C
SH
C
2
S
While the elimination occurs with compounds possessing a -bond or heteroatom at the carbon, it fails with saturated hydrocarbons. The success of the reaction depends on the
strength of the -C-H bond. For example, the quantum yields for the disappearance of
Ph(C=S)OCH2CH2R are 0.46, 0.12 and 0.01 when R is p-MeOPh, EtO and Me, respectively.
The corresponding bond -C-H bond energies are 81, 91 and 96 kcal/mol. Thus, an -alkoxy
group only weakly activates the reaction.6
We recently reported the photochemistry of thiobenzoates 3 and 4 (Figure 2).7 Although
these compounds possess both -alkoxy and -hydroxy groups, they do not undergo
photochemical elimination. Instead, they suffer a complicated transformation that begins with
a photoinduced electron-transfer (PET) reaction.
Figure 2. Glucose Thiobenzaotes giving PET Reactions
Ph
O
O
HO
O
Ph
O
O
O
O OCH3
Ph
C
S
3
O
Ph
HO OCH3
C
S
4
Irradiation of 3 and 4 in CH2Cl2 in the presence of NEt3 results in nearly quantitative product
formation within 15 min. Each thiobenzoate gives two main products that are assigned as 5a,b
and 6a,b, respectively (Figure 3).
Figure 3. Products from the PET Reactions of 3 and 4
Ph
O
Ph
O
O
O
O
O
OCH3
O
O
O
R2
R1
S
R2 R1
5a, R1 = H, R2 = Ph
5b, R1 = Ph, R2 = H
OCH3
O
S
6a, R1 = Ph, R2 = H
6b, R1 = H, R2 = Ph
We proposed that these products arise by a mechanism that involves a photoinduced
electron-transfer step (Scheme 1).8-11 Irradiation of the thiobenzoate rapidly produces the
excited n* triplet state through intersystem crossing. The quantum yield for triplet state
formation is nearly unity. The excited triplet-state abstracts an electron from NEt3. The
resulting radical anion reacts with the solvent by nucleophilic substitution to generate a
chloromethyl sulfide benzyl radical. The radical abstracts a hydrogen atom from the
triethylamine radical cation. In the presence of NEt3, the chain cyclizes at the adjacent
hydroxyl group.
Several experiments with deuterium-labeled solvents and reagents helped elucidate the
mechanism. Irradiation in CD2Cl2 gives complete loss of the signals assigned to the
diastereotopic CH2 groups. Irradiation using NEt3-d15 leads to partial loss of the thiobenzylic
resonances (65% and 66% loss for 3 and 4, resp.). Finally, with 3- and 4-D (exchanged using
MeOD) in CH2Cl2 using NEt3, irradiation also leads to partial loss of the thiobenzylic
resonances (27% and 31% loss for 3 and 4, resp.).
Scheme 1. PET Mechanism for the Formation of 5 and 6
S
Ph
1) h
S
2) isc Ph
O
S
H
Ph
3*
NEt3
NEt3 +
O
NEt2
+
H
Ph
O
O
H3CCH
NEt3
H
C
Cl
S
CH3 HC H
Ph
Cl
CH2 OH
S
Ph
Cl
NEt2
CH2Cl
O
H
S
CH2Cl
O
O
S
H
Ph
+ HNEt3 Cl
O
The electron-transfer mechanism is supported by several observations. Most revealing is
that the reaction occurs only with added NEt 3. Cyclic voltammetry studies on 3 and 4 give
reduction potentials of -1.56 and -1.57 V in CH3CN, respectively, vs. Ag/Ag+. According to the
2
Weller equation (Equation 1) and using a value of 2.75 eV for E T0-0 the proposed electrontransfer reaction is exothermic by 0.24 eV.
G  E(D  / D)  E( A / A)  E(0  0)  E(coul )  E(solv )
(1)
The failure of photoexcited 3 and 4 to undergo -H abstraction is likely due to constraints
imposed by the fused ring systems. The added activation energy counteracts the weakly
activating effects of the -alkoxy or -hydroxy groups.
The photochemistry of the thiocarbonyl group has been documented in a number of
reviews.12-14 Besides the Norrish Type II process, photoexcited thiocarbonyls can dimerize,
oxidize, undergo cycloadditions and suffer cleavage reactions. The excited thiocarbonyl group
has rarely been implicated in electron transfer reactions. In fact, one review goes so far as to
claim that "the excited thiones in both the S2 1(, *) and the lowest 3(n,*) states seem to react
with donor or acceptor molecules with a small contribution from charge transter".15 Recent
studies of pyridinethione, 6H-purine-6-thione, and N-hydroxyacridine-9-thione have shown that
their excited triplet states are reduced by tetrmethylbenzidine, but no subsequent reactions
were reported.16-18
This proposal asks for support to investigate the mechanism, scope and synthetic
applications of electron-transfer reactions of thiobenzoates. The justification for the study is
that this type of reactivity of thiocarbonyl compounds is new, unusual, and may lead to useful
synthetic methods. The PET reactions are unusual in that they differ dramatically from oxygen
analogues. Excited carbonyls suffer photoreduction with amines. The ketyl anions react by
protonating the negatively charged oxygen. With 3 and 4 the negatively charged sulfur reacts
by nucleophilic substitution and not by protonation. Transformations involving sulfur are
important for the synthesis pencillins, thiosugars and peptides.19,20
Proposed Research
1. Mechanistic Investigations
(a) Test Substrate and Delineation of the H-abstraction/PET Borderline
The glucose thiobenzoates are acid-sensitive, and are poor substrates for mechanistic
investigations. We propose to use several alternative substrates to validate the electrontransfer mechanism (Figure 4). These test molecules we will also probe the borderline
between PET and the -H abstraction processes.
Figure 4. Thiobenzoate Substrates
OH
OH
O
S
Ar
7
S
Ar = Ph, p-NO2Ph, p-CH3OPh
O
OH
O
Ar
8
9
S
Ar
3
Compound 7 is derived from trans-1,2-cyclohexanediol. Of the three, it most nearly
resembles the glucose compounds. If the PET transfer pathway is not competitive here, then
we will have established that the cyclohexane ring is flexible enough for the H-abstraction
pathway and that it is the benzylidene ring that shuts down the H-abstraction pathway in the
glucose compounds. The second substrate (8) comes from cis-1,2-cyclohexanediol. The
equatorial -H should disfavor the H-abstraction route, but it may also affect the cyclization with
methylene chloride. The last substrate (9) is a derivative of a vicinal trans-diol. The bicyclic
ring structure should be inflexible enough to prevent -H abstraction. One of these substrates
will be chosen to study the PET process in detail. The substrate will be irradiated in methylene
chloride in the presence of amine or other electron donor. The extent of the reaction will be
monitored by loss of the characteristic yellow absorption of the thiobenzoate chromophore.
(b) Dependence on Donor Oxidation Potential
We have established that triethylamine is necessary for the formation of the unusual
products. A reaction that proceeds by an electron-transfer pathway will be dependent on the
oxidation potentials of the donor (vide supra). Electron-transfer will occur rapidly for reagents
with a sufficiently low oxidation potential, but not with those with higher potentials. We will
observe the dependence of the reaction progress with the following amine donors (Table 1).
Each amine has a higher oxidation potential than triethylamine. 21,22 PET should be
endothermic for those amines with an oxidation potential greater than 1.0 V. A number of nonamine donors will also be tested. In the choice of alternative donors, it is important to make
sure that the donor's triplet energy is higher that that of the thiobenzoate. Otherwise, the donor
will quench the excited triplet thiobenzoate by energy transfer, not electron transfer. The
naphthalene compound is borderline in this requirement, but the triplet-state of the
trimethoxybenzene is out of the range of the thiobenzoate.
Table 1. Amine and Other Donors Designed to Corroborate the PET Mechanism
O
N
amine
N
N
N
N
N
N
.
E(D+ /D)
vs. SCE
0.76
0.81
0.83
OCH3
0.96
0.99
OCH3
OCH3
other
donors
+.
E(D /D)
vs. SCE
1.20
OCH3
1.10
OCH3
1.12
4
(c) Dependence on Thiobenzoate Reduction Potential
The electron-transfer reaction depends on both the oxidation potential of the donor and the
excited state reduction potential of the thiobenzoate. The presence of the benzene moiety
allows for the modulation of the reduction potential by the introduction of substituents. We will
examine two extreme cases: a nitro substituent and a methoxy substituent. The same series
of amines will be used as the electron donors. If the PET hypothesis is correct, then the cutoff
will occur at a lower oxidation potential with the nitrophenyl Ar group and at a higher oxidation
potential with the methoxyphenyl Ar group. If a H-abstraction/PET borderline is established in
(a), then the nitrophenyl and methoxyphenyl substrates will be examined to see if the
substituents can affect the mechanistic pathway. Barton has shown that these substituents do
not significantly affect the H-abstraction pathway in O-phenethyl thiobenzoates, although the
reaction is slightly slower with the nitrophenyl group.3
(d) Solvent Effects
The formation of an ion pair can be controlled to some extent by solvent polarity. It is
unusual to have ion pairs form in methylene chloride since this solvent is not very polar ( =
9.1). The photochemical H-abstraction reactions cited above were often conducted in
cyclohexane ( = 2.0) where the formation of ion pairs would not be energetically feasible. We
will conduct irradiations in solvents of different polarity using donor-substrate pairs that are on
the H-abstraction/PET borderline. The PET pathway should be made more favorable as more
polar solvents are selected. Suitable solvents would be acetone ( = 20.7) and acetonitrile ( =
37.5)
(e) H-atom Source
Triethylamine is the major source of the benzylic hydrogen atom. We will explore what
happens when that source is removed and/or modified. The alternate amine donors are
shown in Table 2.
Table 2. Alternate amine donors.
C-H
N-H
Y
Y
Y
CH3
Y
N
N
H
Y
N
Y
N
Ph
N
N
N
N
H
N
N
Y
C-H
H3C
CH2
N H
H3C
CH2
H3C
N
H3C
H3C
N
H3C
Ph
Ph
Ph
Ph
5
The secondary amines (diethyl, dimethyl and diphenyl) allow for the possibility of proton
transfer from the amino group in analogy with the reaction of singlet stilbenes. 23 Proton
transfer could result in the photoreduction of the thiocarbonyl group. Triphenylamine is a
tertiary amine without -C-H bonds. It will reveal what happens in the reaction when the
radical is denied a facile a H-atom source. Radical coupling may be ultimate result. Finally,
trimethylamine offers an -C-H, but will produce an iminium cation that may react with a
thiocarbonyl radical anion, but only if H-atom abstraction preceeds nucleophilic attack on
methylene chloride.
(f) CIDNP Studies
The formation of a geminate triplet radical ion pair by photoinduced electron-transfer from
the triplet excited state of the thiobenzoate should give rise to net CIDNP effects. 24 There are
several steps that must occur to account for the product formation, but the order in which they
occur is an educated guess. CIDNP studies are ideally suited to answer these mechanistic
questions. The thiobenzoate radical ion should have a large g-factor that the amino radical
cation due to the sulfur. Significant hyperfine coupling should be present with the aryl protons
(- hfc) and the protons next to the nitrogen in the donor amine (+ hfc). The first question that
CIDNP can address is the reaction efficiency. If polarized amine and thiobenzoate are
observed, it means that the initial triplet radical ion pair is slow to react, at least slow enough to
allow for intersystem crossing to the singlet radical ion pair, which then suffers reverse
electron-transfer to regenerate the reactants (Scheme 2).
Scheme 2. Potential Radical Pairs giving rise to CIDNP Effects.
3
Ar
O
proton
transfer
H3C
SH
C
R
3
NEt2
Ar
H
1
S
O
R
NEt3
back eNEt3 transfer
S
Ar
O
R
Ar
O
C
R
Ar
H
H atom
transfer
H2C
C
R +
Ar
O
H
H
S
NEt2
O
S
S
NEt2
Ar
O
R + NEt3
R +
O
Ar
R
Ar
O
NEt2
CH2Cl2
H2
C O
S
CH2Cl
CH2Cl
R
C
H
H
CH2Cl2
H atom
transfer
SH
R + NEt3
H3C
S
H3C
SH
O
H atom
transfer
cage escape
1
S
Ar
S
H-atom
source
Ar
O
H
R
Ar
O
H
The CIDNP effects can provide insight into a number of other possible steps. For example,
the final product can arise from either the singlet (recombination) manifold or the triplet
(escape) manifold. The signs of the observed effects will indicate which pathway is operating.
If the product derives from the triplet manifold, CIDNP effects will provide some indication of
how fast the thiocarbonyl radical anion attacks methylene chloride. If nucleophilic attack is
slow, then substitution will occur after escape from the initial radical ion pair as shown in
6
Scheme 2. The methylene protons should not show any polarization. On the other hand, a
fast reaction with methylene chloride could lead to substitution of the initial radical ion pair with
a radical/radical cation pair. In this case the methylene protons should show some
polarization. Finally, in analogy with carbonyl photochemistry, the radical ion pair could react
by proton transfer to give a thiobenzyl/--aminoethyl radical pair. This pathway would be
indicated by the polarization of the methyl groups in triethylamine. 25,26 We did not determine
the fate of the triethylamine in our previous study. CIDNP effects should reveal into which
compound it transforms.
2. Reaction Scope and Synthetic Applications
(a) Variation of the Electrophile
Methylene chloride is a convenient solvent and reactant. It offers two leaving groups, but in
our mechanism only one of these reacts with the photogenerated thiocarbonyl radical anion.
We will conduct the PET reactions in inert solvents that contain other possible electrophiles
(Figure 5).
Figure 5. PET Reactions with other Electrophiles
OH
OH
h
7
CH3CH2
+
Cl
O
NEt3
CH3CN
O
S
S
Ar
OH
Ar
O
10
+
O
Ar
h
S
CHCl3
Ar
NEt3
O S
O
OH
OH
7
Cl
+
O
h
CH2 CH2
NEt3
CH3 CN
O
S
Cl
Ar
S
O
Ar
For example, chloroethane as the electrophile only allows for one nucleophilic
displacement with 7. The cyclization that occurs with 3 and 4 is not possible here. The
product itself isn't particularly interesting, but it will show that the second displacement is not a
critical part of the PET mechanism. The reaction of photoexcited 10 with chloroform is
arguably a stretch. The three chlorine atoms will retard the nucleophilic substitution by an S N2
process, and the double cyclization will be difficult. The reaction of photoexcited 7 with 1,2dichloroethane will measure of the facility of the second cyclization. In the reaction with 3 and
4, a chloromethyl sulfide reacts with a hydroxyl group to form a seven-membered ring. The
sulfur likely assists the chloride leaving, making way for attack by the neutral oxygen. In the
case here, a 2-chloroethyl sulfide will form. The sulfur can still participate in the cyclization by
7
displacing the chloride to form an episulfonium ring. This reaction would result in the formation
of an eight-membered ring.
(b) Intramolecular Nucleophilic Substitution
In our previous work the thiobenzoate radical anion reacts with methylene chloride to form
a chloromethyl group. In subsequent steps a remote hydroxyl group reacts with this group to
form a seven-membered ring. We will explore whether the reduced thiobenzoate group can
displace intramolecular leaving groups.
These displacements are similar to classic
neighboring group participation reaction; however, the bridged intermediate is trapped in this
case. The series of substrates and potential products is shown in Figure 6.
Figure 6. Thiobenzoate Neighboring Group Effects
Br
Br
11
Ph
h
S
O C
S
NEt3
O
Ph
13
O
h
S
H
NEt3
O
Ph
H
S
Ph
Br
h
Br
Ph
12
O C
S
h
S
NEt3
O
Ph
H
S
NEt3
14
S
O
H
O
Ph
Ph
Compound 11 is the simplest substrate bearing an internal bromine leaving group. Since
the strength of the (Br)C-H bond in CH3Br is several kcal higher than the (CH3O)C-H in
CH3OCH3, the possibility of H-abstraction is remote.27 The CH3CBr2-H bond strength has
been experimentally determined as 94.9 kcal/mol. 28 We will use information from the
mechanistic studies to maximize the PET process by choosing the optimal aryl group and
solvent. The product is a benzylidene thioacetal, a 1,3-oxathiolane. The benzylidene group
can be removed by acid hydrolysis to liberate the vicinal mono-thioglycol.29 The H-abstraction
mechanism is not possible in 12. The product is also a benzylidene thioacetal, but instead a
1,3-oxathiane. Deprotection generates a 1,3-mercaptoalcohol. The next two substrates are
ring analogues of 11 and 12. Compound 13 should give a cis-thioglycol. In compound 14 the
reaction requires the thiobenzoate group to be axial and the leaving group equatorial. The
latter requirement may the cyclization problematic.
If these substrates prove viable, they may also be used in the mechanistic studies.
(c) Ramburg-Bäcklund Reaction: Formation of Aryl Vinyl Ethers
The chloromethyl sulfide intermediate formed as a result of the PET reaction seems a
convenient precursor for the Rämburg-Backlund reaction of -halo sulfones.30-32 If the
second hydroxyl group of our thiobenzoates is removed, as in 15, then it may be possible to
isolate chloromethyl sulfide intermediate (Figure 7). Oxidation to the sulfone followed by
treatment with base should produce the aryl vinyl ether.
8
Figure 7. Ramburg-Bäcklund Reaction
h
CH2Cl2
+
O
NEt3
O
15
S
S
Ar
Ar
CH2Cl
MCPBA
NaOCH3
CH3OH
O
H2C
O
Ar
O2S
Ar
CH2Cl
(d) Internal Amine Donor: C-H Substitution
Placing the amine donor as part of the thiobenzoate may promote an alternative reaction
course (Scheme 3). The enforced proximity of the benzylic radical to the -amino C-H may
promote H-atom abstraction. The thiolate ion should react readily with the iminium ion
resulting in net substitution of the H with S.
Scheme 3. C-H Substitution with an Internal Amine Donor
S
Ar
h
O
N
H
S
CH3CN
O
N
Ar
16
S
N
Ar
H
O
S
N
Ar
H
O
(e) Internal Radical Traps
The radical portion of the thiocarbonyl radical anion could react with an accessible -bond.
A 1,5 exo-trig cyclization would be optimal. Three substrates satisfying this description are
shown in Figure 8.
9
Figure 8. Thiobenzoates with Internal Radical Traps
S
h
O
Ar
O
S
Ar
NEt3
CH3 CH2Cl
CH3CN
17
S
O
O
h
Ar
Ar
S
CH3CN
18
NMe 2
NMe 2
Ar
Ph
O
Br
S
h
S
NEt3
CH3 CN
O
Ph
Ar
19
Compounds 17 and 18 have norbornane skeletons to avoid the Norrish II process which is
efficient with allylic C-H bonds. Compound 17 offers a simple alkene trap, whereas in
compound 18 the trap is also the electron donor. The overall process with 18 is an internal
[2+2] cycloaddition. In compound 19 the internal nucleophilic displacement of the bromine is
followed by an 1,6 exo-trig cyclization.
10
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12
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