Photochemical generation and structure of vinyl radicals

Article abstract of DOI:10.1002/ejoc.200700882

Photolysis of a series of E or Z stereoisomeric α-R-substituted bromostyrenes (R = CH₃, F or CN) in methanol yields E and/ or Z stereoisomeric styrenes, which stem from the corresponding vinyl radicals. The results show that the α-Me vinyl radical is a rapidly equilibrating, bent structure, while the α-F vinyl radical is a stable bent species, in agreement with earlier thermal results. The α-CN vinyl radical is assigned as a rapidly inverting bent and not a linear species from the stereochemical results as a function of temperature. Stereochemical data for the α-C(H)=O system in diethyl ether indicate a stable bent vinyl radical as product forming species. The conclusions are supported by quantum chemical computations. Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/ejoc.200700882

FULL PAPER
DOI: 10.1002/ejoc.200700882
Photochemical Generation and Structure of Vinyl Radicals
Theodorus P. M. Goumans,[a][‡] Kaj van Alem,[a] and Gerrit Lodder*[a]
Keywords: Photochemistry / Vinyl radicals / Structure elucidation / Stereoselectivity / Ab initio calculations
Photolysis of a series of E or Z stereoisomeric α-R-substituted
bromostyrenes (R = CH3, F or CN) in methanol yields E and/
or Z stereoisomeric styrenes, which stem from the corre-
sponding vinyl radicals. The results show that the α-Me vinyl
radical is a rapidly equilibrating, bent structure, while the α-
F vinyl radical is a stable bent species, in agreement with
earlier thermal results. The α-CN vinyl radical is assigned as
a rapidly inverting bent and not a linear species from the
stereochemical results as a function of temperature. Stereo-
chemical data for the α-C(H)=O system in diethyl ether indi-
cate a stable bent vinyl radical as product forming species.
The conclusions are supported by quantum chemical compu-
tations.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
vinyl radicals from stereoisomeric E or Z α-substituted vi-
nylic precursors or by addition of a radical to a carbon–
carbon triple bond, and comparison of the product compo-
sitions after chemical trapping. Both approaches, however,
suffer from drawbacks, which make it difficult to distin-
guish experimentally between a configurationally un-
stable,[5] rapidly inverting σ-type radical (Scheme 1, B), ef-
fectively linear, and a π-type, linear, vinyl radical.
Vinyl radicals with an α-hydrogen, -alkyl, -alkoxy, or
-halogen substituent have since long been known to have a
bent structure.[4] The α-silyl group induces a π-type, linear,
vinyl radical structure.[6] The structures of vinyl radicals
bearing unsaturated α-substituents that allow delocalization
of the single electron are a topic of controversy. The cyano,
carboxyl, phenyl and alkenyl group, for example, are all
capable of π-delocalization and were thought to impose lin-
earity on the radical center to effect maximum resonance
stabilization. This assumption appeared to be corroborated
by ESR data.[7] However, the linearity of α-carboxyl-,[8–10]
α-phenyl-,[9,10] and α-cyano-substituted[10,11] vinyl radicals
has been questioned.
Introduction
Since the pioneering study of Stork and Baine[1] in 1982,
vinyl radical reactions have developed into a versatile tool
for synthetic organic chemists.[2] The structure of these radi-
cals is a key factor in controlling the outcome of their reac-
tions providing stereoisomeric products.[3] Vinyl radicals
have either a bent or a linear structure, and are usually des-
ignated as σ- and π-type radicals, respectively (Scheme 1,
A). These appellations refer to the presence or absence of
any s-character in the singly occupied MO.
Investigations of the structures of vinyl radicals by quan-
tum chemical calculations also led to controversies and did
not settle arguments. Guerra found that the α-ethynyl, α-
cyano, and α-carboxyl substituent favor a bent vinyl radical
structure, with a low inversion barrier, using the post-HF
method UMP4/TZP.[12] The corresponding linear radicals
were calculated to be transition states. In contrast, Galli,
Mencarelli and co-workers calculated α-ethenyl, α-phenyl,
α-cyano, and α-aldehyde-substituted vinyl radicals to have
their minimum energy for a linear, π-type radical, with the
single electron in conjugation with the α-π-electrons.[13] The
latter authors used the DFT BLYP functional for their cal-
culations, with a 6-31G(d,p) basis set. Similarly, the α-cya-
novinyl radical is computed to have a bent structure using
Scheme 1.
The configuration of vinyl radicals depends on the nature
of the α-substituent R, and mainly two approaches have
been used to deduce their structures.[4] The most straight-
forward method is evaluation of ESR hyperfine splitting
constants. The other method is the generation of isomeric
[a] Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden
University,
P. O. Box 9502, 2300 RA Leiden, The Netherlands
E-mail: lodder@chem.leidenuniv.nl
[‡] Present address: Department of Chemistry, University College
London,
20 Gordon Street, London, WC1H 0AJ, United Kingdom
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T. P. M. Goumans, K. van Alem, G. Lodder
FULL PAPER
HF and post-HF methods, but a linear structure, using
DFT methods.[14]
In this paper we report how, and to which extent, the
photochemistry of vinyl halides can contribute to answering
the question about the structure of α-substituted π- and σ-
isomeric vinyl radicals. Upon irradiation, vinyl halides give
two types of bond cleavage.[15] Heterolytic cleavage of the
carbon–halogen bond yields vinyl cations, while homolytic
cleavage leads to vinyl radicals. The cation to radical ratio
depends on the nature of the α-substituent R as well as on
the polarity of the solvent. The effects of a series of six α-
substituents are considered: α-CH3, α-F, α-CN, α-C(H)=O,
α-CϵCH, and α-CH=CH2. The compounds under experi-
mental investigation are the E and Z isomeric bromosty-
renes 1–6. If these molecules undergo homolysis, they yield
α-substituted β-phenyl vinyl radicals. When these radicals
are bent, the stereoisomeric vinyl halides a and b initially
generate two stereoisomeric, σ-type, radicals. If the re-
sulting vinyl radical is linear, a and b produce the same π-
type radical intermediate.
Scheme 2.
The vinyl cation-derived products include dehydrohalo-
genation and nucleophilic substitution products, sometimes
after 1,2-rearrangement across the C=C double bond. Also
hydride and halide shifts occur, typical of carbocation
chemistry.[19]
Under oxygen-deficient conditions, the products c and d
constitute the total of vinyl radical-derived products formed
upon irradiation of the vinyl bromides under study.[20] The
rates of cis-H and trans-H product formation have to be
and are determined at low conversion of starting material
to ensure that E/Z isomerization of substrates a and b or
products c and d does not influence the yields of radical-
derived products. The ratio of cis-H to trans-H product for-
mation is kd/kc, or equivalently Φd/Φc, in terms of quantum
yields. In the remainder of this paper, when referring to in-
version (retention), the product obtained after inversion (re-
tention) at the radical center of the intermediate is meant.
This implies that kd/kc equals the ratio of inverted to re-
tained product for Z-vinyl halides a, while the reciprocal
value kc/kd corresponds to the same ratio for the E-isomers
b.
Because of the discrepancy between the computational
results obtained using post-HF[12,14] and DFT[13,14] meth-
ods for the structures of vinyl radicals with unsaturated α-
substituents, the structures of the series H2C=C·–R with R
= CϵN, C(H)=O, CϵCH and CH=CH2 are subjected to a
theoretical study and computed using the high-level
QCISD(T)/6-311+G(d,p)//QCISD/6-311+G(d,p) approach.
Results and Discussion
α-CH3- and α-F-Substituted Vinyl Radicals
Photochemistry
Irradiations of the α-methylstyrene bromides 1, and the
The photoreactions of the styryl bromides 1–6 typically α-fluorostyrene bromides 2 in methanol yield vinyl radical-
yield E/Z isomerization, homolysis and heterolysis prod- derived products. Homolysis is a minor reaction pathway
ucts, as depicted in Scheme 2. The points of focus in this for 1,[17] but is effectively the only bond rupture process
study are the reductive dehalogenation products c and d, as observed for 2.[21] In Table 1 the quantum yields of forma-
they stem from an intermediate vinyl radical. This has been tion of the two reductively dehalogenated isomers c and d
shown by the independent thermal generation of such spe- and their ratio observed in the irradiation of the E- and Z-
cies by reaction of vinyl halides with tri-n-butyltin hydride isomers of 1 and 2 are summarized.
and azobis(isobutyronitrile) as an initiator.[16–18] The inter-
Photohomolysis of the carbon halogen bond in the Z-
mediate radical reacts with the solvent by transfer of a isomer 1a and the E-isomer 1b gives nearly the same prod-
hydrogen atom. For the Z-bromides 1a–6a, the trans-H uct composition (Table 1, entries 1 and 2). These results are
products 1c–6c are the result of vinyl radical trapping with similar to those of thermal experiments,[22,23] and are in
retention, while the formation of the cis-H products 1d–6d agreement with a rapid E/Z equilibration of the bent β-
has occurred with inversion at the radical center. The oppo- phenyl-methylvinyl radicals E-1· and Z-1· (Scheme 3), which
site is true for the E-isomers b.
is faster than scavenging by the solvent. The slight deviation
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Photochemical Generation and Structure of Vinyl Radicals
Table 1. Quantum yields of formation (102Φ) of the reductive de-
halogenation products c and d and their ratios upon irradiation
(λexc = 248 nm) of 1a, 1b, 2a, and 2b in methanol.
The reductive dehalogenation of the E and Z isomeric α-
fluoro-substituted compounds 2a and 2b occurs with com-
plete retention of configuration (Table 1, entries 3 and 4).
Despite the possible involvement of initial radical pairs, not
even a trace of inversion product is observed, which means
that the radical itself does not invert. The α-fluoro vinyl
radicals 2· (Scheme 3) thus are configurationally stable, bent
species. This conclusion is in accordance with theoretical
predictions[13,26] and with the results found for thermally
generated α-fluorovinyl radicals.[13,27]
Φc (ϫ102)
Φd (ϫ102)
Φd/Φc
1a
1b
2a
2b
9.7
9.7
2.7
0
4.1
3.4
0
0.42
0.35
ϱ
3.8
0
in stereoconvergence corresponds to a preference for inver-
sion. This observation implies that non-complete equilib-
rium-establishment prior to scavenging can be excluded, be-
cause in that case the product compositions should reflect a
bias for retention of configuration. Radical pair formation,
however, can explain incomplete stereoconvergence. If the
rate of radical pair diffusion is slow or comparable to that
of scavenging by solvent, inversion of configuration will be
favoured over retention. This is due to steric shielding of
the incoming scavenger by the halogen atom. The prepon-
derance of 1c over 1d is due to two factors, (a) the hydrogen
atom transfer from the solvent methanol to 1· is a highly
exothermic process,[24] and (b) the rates of hydrogen atom
transfer from the small H-donor methanol to the two iso-
meric radicals are about equal. This implies[25] that the
product ratio is determined by the equilibrium constant K
for inversion between the trans- and the cis-vinyl radical E-
1· and Z-1·, which will be less than unity, leading to a ratio
of Ͼ1:1 of 1c vs. 1d.
α-CN-Substituted Vinyl Radicals
Upon irradiation in methanol, the α-bromocinnamo-
nitrile stereoisomers 3a and 3b show nearly stereoconver-
gence in their formation of 3c and 3d, with a slight bias for
inversion as was the case for 1a and 1b. (See data in
Table 2). The similar 3d/3c ratios observed starting from 3a
or 3b are compatible with a linear 3· as well as with rapidly
equilibrating bent 3· s. To determine the structure of the α-
cyanovinyl radical 3·, 3b was irradiated in methanol at five
different temperatures over a 40 °C temperature range.
Table 2. Relative ratios of reductive dehalogenation (kd/kc = kcis-H
trans-H) of compounds 3a and 3b upon irradiation (λexc = 254 nm)
in methanol at various temperatures.
/
k
Substrate T [K] 265
270
283
297
306
3a
3b
1.30
1.15
1.50
1.35
1.83
1.58
1.22
1.52
Analysis of cis- to trans-H product ratios as a function
of temperature can distinguish between rapidly inverting
bent and linear vinyl radicals (Scheme 4).
From this Scheme, which was first proposed by Sing-
er[4a,22] and elaborated by Metzger and Blumenstein,[10] the
following kinetic expressions for the ratio of cis- to trans-H
formation can be deduced. For the bent radical (the left
part of the Scheme) Equation (1) is valid, when starting
with 3b from which the Z-radical is photogenerated.
(1)
In a borderline case, valid here, inversion is fast relative
Scheme 3.
to scavenging (Curtin–Hammett conditions), i.e. the inver-
Scheme 4.
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437

T. P. M. Goumans, K. van Alem, G. Lodder
FULL PAPER
sion barrier is low. Equation (1) is reduced to Equation (2) bent. A more qualitative consideration of such results leads
because kcis[scav]ϽϽkinv
.
to the same conclusion: when the thermodynamically fav-
oured product is formed (relatively) less at elevated
temperatures, a pre-equilibrium must be operative. When
∆∆H‡
is larger than ∆∆H0(Z-rad)–(E-rad), how-
(cis-H)–(trans-H)
(2)
ever, a positive slope will be found. In that case it is imposs-
ible to discriminate between the bent and the linear
species. If a reactive, small radical scavenger is used,
K
Z-rad/E-rad is the equilibrium constant for the bent radicals,
∆∆H‡
is reduced to a minimum[30] and is ex-
kЈinv/kinv, and is expected to be less than unity.
(cis-H)–(trans-H)
pected to be less than ∆∆H0(Z-rad)–(E-rad), which relieves the
indistinguishability of the two situations.
For a linear radical, Scheme 4 yields [Equation (3)].
The cis-H to trans-H product ratios from 3b were calcu-
lated from the measured rates of formation of the two iso-
meric products at the five different temperatures.[29] Also,
the 3a isomer was irradiated at three different temperatures
to corroborate the results found for 3b. In Table 2 the kd/kc
ratios (which are equal to kcis-H/ktrans-H) for compounds 3a
and 3b at the various temperatures used are reported. When
log (kcis-H/ktrans-H) is plotted against 1/T, for both 3a and 3b
a straight line with a negative slope is found (Figure 1), with
a correlation coefficient exceeding 0.990.
(3)
The scavenging reaction constants, k and kЈ, are – a priori –
not the same for an interconverting bent radical and a lin-
ear radical, because of the different angle of approach of
the scavenger.[28]
∆∆H‡
is negative because cis-scavenging is
(cis-H)-(trans-H)
less hindered than trans-scavenging. Therefore, if the loga-
rithm of the kcis-H/ktrans-H ratio[29] is plotted against 1/T, a
linear radical will give a straight line with a positive slope.
This follows from Equation (4), applying Eyring TS theory
and expressing relative rates in terms of enthalpy and en-
tropy differences.
(4)
Figure 1. Plot of the log of the relative rates of formation of cis- to
trans-olefin in the irradiations of 3a and 3b vs. 1/T.
For a bent radical a different equation is obtained [(i.e.
Equation (5)], due to the presence of the equilibrium con-
stant KZ-rad/E-rad in Equation (2).
If 3a and 3b were to yield the same product-forming in-
termediate upon homolysis, the lines in Figure 1 would be
coincident. This is clearly not the case: the cis-H/trans-H
ratio is higher for 3a than that for 3b at a given temperature,
but the lines are nearly parallel. Initial radical pair forma-
tion, which hinders attack at the initially formed radical,
favouring inversion products slightly, can account for the
non-coincidence. The same argument applied to the incom-
plete stereoconvergence in the case of α-methyl analogues
1a and 1b.
Figure 1 clearly shows that both lines have a negative slope,
which amounts to [∆∆H‡(cis-H)-(trans-H) + ∆∆H0(Z-rad)–(E-rad)]/R,
as can be seen from Equation 5. For 3a and 3b the values of the
(5)
∆∆H0
is positive, because the E-radical is slopes are the same, and equal –1.3 kcal/mol. The difference
(Z-rad)–(E-rad)
more stable than the Z-radical, and its value opposes the in enthalpy of formation of the cis and the trans-radical thus
negative ∆∆H‡(cis-H)-(trans-H)-value. The difference is ex- exceeds the value of ∆∆H‡
in methanol, by
(cis-H)–(trans-H)
pected to be larger, the bulkier the scavenger. If ∆∆H0
1.3 kcal/mol. The overall negative slope establishes unambigu-
rad
predominates, a negative slope will be found in the plot, ously that the intermediate β-phenyl-α-cyanovinyl radical 3· is
and the radical is unambiguously established to be a rapidly inverting bent species (Scheme 5).
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Photochemical Generation and Structure of Vinyl Radicals
isomer is not very stable and tends to reisomerize to 4a
thermally. Upon irradiation of 4b in diethyl ether, eventu-
ally a trace of the reductive dehalogenation product 4c is
observed. The other radical product, 4d, however, is not
formed. It is questionable whether 4c is produced from the
vinyl bromide isomer 4b or 4a, as the latter was already
present in considerable amounts (20%) by the time re-
ductive dehalogenation became detectable. It may be that
the only photoproduct of 4b is the Z-isomer 4a and that
the latter undergoes photohomolysis, as described above,
exclusively with retention to 4c.
Scheme 5.
Metzger and Blumenstein have shown that the related β-
cyclohexyl-α-cyanovinyl radical is a rapidly inverting bent
species at high temperatures.[10] They stated that at room
temperature, the radical might be linear because it may have
a lower enthalpy than the bent vinyl radical, which is out-
weighed at higher temperatures by the higher entropy con-
tent of the bent species. Although such an argument is valid
for the parent α-cyanovinyl radical H2C=C·–CN, which is
symmetrical when linear, this is not the case for the mole-
cules under consideration because they have no sym-
metry.[31] Recently, however, flash photolysis of acrylonitrile
has shown that the parent cyanovinyl radical is bent at
room temperature.[32] We have now shown that also 3· is
nonlinear at room temperature.
Irradiations of the acetylene-substituted 5a and ethenyl-
substituted 6a in methanol yielded no reductive dehaloge-
nation products. Instead, merely cation-derived products
and E/Z-isomerization products are formed.[34,18]
Quantum Chemical Calculations
The parent vinyl radicals 3, 4, 5 and 6 with the unsatu-
rated α-substituents depicted in Scheme 6 were subjected to
a theoretical study. The structures were first optimized at
the MP2/6-311+G(d,p) level followed by a frequency calcu-
lation to assure the nature of the stationary point. The
structures were subsequently optimized at the QCISD-level,
using the 6-311+G(d,p) basis set and single point energies
were calculated at the QCISD(T)-level with the same basis
set.[35] The optimized MP2 and QCISD structures are very
similar, but preliminary B3LYP calculations showed a
strong tendency to straighten the vinyl radical towards the
linear π-type (vide infra). The angles of bending at the radi-
cal center (θ), the relative energies and the natures of the
stationary points are summarized in Table 3. The calcula-
tions predict that for the substituents with a double bond,
C(H)=O and CH=CH2, the π-type structures π4 and π6 are
energetically favored over the σ-type radicals. In order to
effect allylic delocalization of the single electron, the π-sub-
stituent has to rotate out of the plane, and becomes perpen-
dicular to the vinyl π-system. The imaginary frequency of
the transition structure σ6 rotates the α-CH=CH2 out of
this plane, and leads to π6.
In an α-cyanovinyl radical, the cyano group is capable of
delocalizing the single electron. The resonance contributor,
however, is not very stabilizing, because the single electron
is situated on an sp-hybridized electronegative nitrogen.
Apparently, resonance stabilization, which is at maxi-
mum for a π-type (sp-hybridized) vinyl radical, is counter-
acted by a destabilizing effect, which imposes a bent struc-
ture. Qualitatively, the non-linearity of the α-cyanovinyl
radical is explained best by the model postulated by Dewar
and Bingham:[33] α-substituents with low-lying π-electrons
tend to destabilize a π-type radical and therefore to decrease
the C=C–R angle. Apparently, the subtle opposing effects
nearly cancel each other in 3·, rendering the energy surface
for bending shallow and the inversion barrier low.
α-C(H)=O, α-CϵCH, and α-CH=CH2 Vinyl Radicals
The method to produce stereoisomeric vinyl radicals un-
der mild conditions by photolysis of vinyl halides did not
work well for compounds 4–6. Irradiation of the aldehyde-
substituted 4a in methanol gives a complex mixture of pho-
toproducts (see ref.[16]), with the reductive dehalogenation
products 4c/d as minor components. This makes it difficult
to obtain kinetic data. When 4a is irradiated in diethyl ether
only reductive dehalogenation occurs, next to E/Z isomer-
ization. Remarkably, only the retention product 4c is
formed. This selectivity may be the result of scavenging a
configurationally stable σ-type vinyl radical with retention
or stereoselective scavenging of a linear intermediate lead-
ing to the trans-product.
To establish which case prevails, the E-isomer 4b was
synthesized by triplet-sensitized irradiation of 4a. The E- Scheme 6.
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T. P. M. Goumans, K. van Alem, G. Lodder
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Table 3. Results of quantum chemical calculations of the parent The bent radical with the cyano trans to the phenyl ring, at
vinyl radicals 3, 4, 5 and 6 at the QCISD(T)/6-311+G(d,p)//
an angle of 135.5° was found to be the global minimum
(Figure 2). The cis isomer, with an angle of 139.0°, is
0.85 kcal/mol higher in energy.
QCISD/6-311+G(d,p) level.
Substituent Stationary point Angle θ Energy [kcal/mol][a]
σ3 CϵN
minimum
transition state 180
minimum
minimum
minimum
transition state 180
transition state 136.6
144.0
0
1.82
0
–2.88
0
1.23
0
π3 CϵN
σ4 C(H)=O
π4 C(H)=O
σ5 CϵCH
π5 CϵCH
σ6 CH=CH2
π6 CH=CH2
130.8
180
149.0
minimum
180
–7.83
[a] relative energy, with the σ-type radical set to 0 each time.
Figure 2. MP2/6-311+G(d,p)-optimized structure of the trans-β-
phenyl-α-cyanovinyl radical 3·.
Upon photohomolysis of the vinyl bromide bond in
either 4a or 4b, the α-C(H)=O-substituted vinyl radical will
initially have the local minimum structure σ4. Since we only
find radical product 4c and not 4d as a photoproduct of 4a,
it could be that scavenging of the vinyl radical in structure
σ4 is more rapid than its isomerization to the linear struc-
ture π4. At the MP2/6-311+G(d,p) level the barrier for this
isomerization in vacuo is 1.43 kcal/mol. Solvent reorganiza-
tion effects will significantly increase the entropic barrier.
For the vinyl radicals bearing an α-CϵN and an α-
CϵCH group the bent structures σ3 and σ5 are favored
over the linear ones, although the energy differences are
small. The linear structures are in fact the transition struc-
tures for inversion, as their one imaginary frequency corre-
sponds to the θ-angle bending coordinate. The activation
barrier for inversion in the gas phase is equal to the energy
difference between the linear and the bent structures, and
is thus quite small. This flat potential energy surface along
the vinyl bending coordinate implies that in solution these
radicals will rapidly invert, with an activation barrier that
is largely determined by the barrier associated with solvent
reorganization. The triple bond substituents can participate
in the electron delocalization while maintaining overlap
with the vinyl moiety. This means that the structures of the
bent and the linear species are alike. This contrasts with the
vinyl radicals with the double bond substituents, in which
the two π-systems are perpendicular in the linear species,
but coplanar in the bent structure.
Conclusions
Photolysis of E/Z isomeric vinyl bromides is found to be
a convenient method to produce stereoisomeric vinyl radi-
cals, under mild reaction conditions. The results of thermal
experiments for the β-phenyl-α-methyl vinyl radical, which
indicate a rapidly equilibrating bent species, are perfectly
reproduced, if initial radical pair formation is taken into
account. The α-fluorovinyl radical is proven to be a stable
σ-radical.
Photolysis of E and Z α-cyanovinyl bromides, in combi-
nation with kinetic monitoring, has demonstrated that the
β-phenyl-α-cyanovinyl radical is not linear but a rapidly in-
verting bent species. The experimental observation is sup-
ported by quantum chemical computations.
The method applied here is rather easy to generalize, but
for some substituents adjustments in reaction conditions
are necessary to get an appreciable yield of vinyl radicals.
Care must be taken in studying the π- vs. σ-vinyl radical
structure question by theoretical calculations, as the out-
come is rather sensitive to the methodology.
Experimental Section
Irradiations: Products studies were carried out at λexc = 248 nm in
a cuvette set up, with a Hanovia 977 B0010 high pressure Xenon
lamp and an Oriel Grating monochromator model 77250. Triplet-
Our results for the α-cyanovinyl radical 3 are in line with
other post-HF results[12,14] and disagree with earlier DFT sensitized irradiations were carried out in acetone at λexc = 313 nm
calculations[13,14] which predict the linear structure to be
using a Hanau TQ-81 high-pressure mercury arc and a filter solu-
tion consisting of 0.025  potassium biphthalate, 1.0  NiSO4, and
more stable. We found that with the B3LYP/6-311+G(d,p)
0.25  CoSO4. The isothermal irradiations at various temperatures
approach radicals 3 and 6 are both bent, but with very
were performed at λexc = 254 nm with a Hanau TNN-15/32 low
small deviations from linearity (167° and 178°) and negligi-
pressure mercury lamp. The lamp is separated from the solution,
bly small activation barriers (ϽϽ0.1 kcal/mol). Guerra has
by immersing it in a tube with double quartz walls, separated by
vacuum, filled with water. The tube is submerged in the cylindrical
pyrex reaction vessel. The temperature of the reaction vessel is kept
found that the stability of π-delocalized radicals is overesti-
mated compared to that of σ-localized ones by the currently
used density functionals.[36] This overestimation is probably
constant by submerging it in a dewar flask filled with water at a
the reason why Galli, Mencarelli, and co-workers[13] find
set temperature. All photochemical experiments were carried out
the linear structure of 3 to be the more stable one using under argon atmosphere.
BLYP/6-31G(d,p).
Methanol (Rathburn Chemicals) and ether (p.a. quality, Baker)
were used as supplied after checking for UV impurities. All sub-
The β-phenyl-α-cyano radical 3·, which we experimen-
tally determined to be bent and rapidly inverting, was also
strates used in the photochemical experiments were 98+% purity
subjected to a theoretical study, using MP2/6-311+G(d,p). (GC), except for 4b (90%), which reisomerized to 4a.
440
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Photochemical Generation and Structure of Vinyl Radicals
The solutions were about 10–15 m in the pyrex reaction vessel
and about 2–5 m in the cuvette set-up (optically dense). An ap-
proximately equimolar amount of n-decane was used as an internal
standard. The irradiations were carried out in triplicate. The quan-
tum yields were calculated from the kinetic data obtained in the
irradiations by using least-squares treatment and the intensity of
light at the wavelength used. The values of the quantum yields re-
ported are average values; the error (i.e., deviation from the mean)
is typically 5%. The intensity of the light employed was determined
by using a chemical actinometer: Actino Chromo 1R (248/334)
from Photon Technology International. Each reaction constant kc,
kd and quantum yield Φc, Φd was determined by using the least-
squares method on at least seven samples taken at different times.
All samples were taken within the first 4% of conversion of the
starting materials. Typical correlation coefficients are in the range
0.94–0.99.
(100), 200 (90), 121 (70), 101 (55); high resolution MS: m/z =
199.9623 (calcd. 199.9637).
The products (E)-1-fluoro-2-phenylethene (2c) and (Z)-1-fluoro-2-
phenylethene (2d) were synthesized according to published pro-
cedures.[39]
3
3
2c: 1H NMR: δ = 6.38 (dd, 1 H, JHF = 19.4 Hz, JHH = 11.4 Hz),
3
3
7.15 (dd, 1 H, JHF = 83.3 Hz, JHH = 11.4 Hz), 7.35 ppm (m, 5
H). 19F NMR: δ = –53.9 ppm (dd, 1 H, 3JHF(α) = 83.3 Hz, 3JHF(β)
= 19.4 Hz). MS: m/z (rel. int.) = 122 (100), 121 (45), 101 (40), 96
(50); high resolution MS: m/z = 122.0553 (calcd. 122.0523).
1
3
3
2d: H NMR: δ = 5.60 (dd, 1 H, JHF = 44.9 Hz, JHH = 5.4 Hz),
6.65 (dd, 1 H, 3JHF = 81.5 Hz, 3JHH = 5.4 Hz), 7.35 ppm (m, 5 H).
19F NMR: δ = –46.2 ppm [dd, 1 H, JHF(α) = 81.5 Hz, JHF(β) =
3
3
44.9 Hz]. MS: m/z (rel. int.) = 122 (100), 121 (45), 101 (35), 96 (45).
(Z)- (3a) and (E)-1-Bromo-1-cyano-2-phenylethene (3b): To a chilled
(0 °C) solution of 5.0 g of cinnamonitrile in 100 mL tetrachloro-
methane, 12 g of bromine was added. The solution was stirred for
1 d at room temperature, and the excess bromine and solvent re-
moved. The oily residue was dissolved in 40 mL of dry toluene, and
3.6 g triethylamine was added, after which the products crys-
tallized. The solution was filtered and the precipitate washed with
toluene. After evaporation of the solvent, the crude product (yield
88%) was purified into the separate isomers by silica column
chromatography using a mixture of diethyl ether and petroleum
ether (1:1) as an eluent.
Analyses: Products were identified using gas chromatography and
GC-MS of samples, coinjecting the alleged products with the prod-
uct mixture. GC analyses were carried out with a Packard model
433 gas chromatograph, fitted with a CP-SIL 19 CB column (30 m,
Ø = 0.32 mm), and hydrogen as a carrier gas. Preparative GC was
performed using a Varian 920 Aerograph, fitted with a column
(6 m, Ø = 8 mm) filled with 20% SE-30 on Chromosorb WAW,
using hydrogen as the carrier gas. For the kinetic analyses, an auto-
matic GC was used, from the HP 6890 Series, equipped with a CP-
Sil-5B column (25 m, Ø = 0.25 mm).
1H-, 19F and noise decoupled 13C-spectra were recorded on a Jeol
JNM FX-200, a Bruker WM 300 or Bruker MSL 400, using CDCl3
as a solvent and TMS (1H), CF3COOH (19F) or CDCl3 (13C) as a
reference. UV spectra were recorded on a Varian DMS 200 spectro-
photometer, with methanol as a solvent. The low resolution mass
spectra were recorded on a GC-MS set-up, consisting of a Hewlett–
Packard model 438A gas chromatograph, fitted with a CP-SIL 5
CB column (25 m, Ø = 0.25) using helium as a carrier gas, coupled
with a Finnigan Mat ITD 700 mass spectrometer. The high resolu-
tion mass spectra were recorded on a Finnigan Mat 900 mass spec-
trometer.
1
3a: H NMR: δ = 7.46 (m, 3 H), 7.75 ppm (m, 3 H). 13C NMR: δ
= 86.4, 128.3, 129.4, 130.8, 132.0, 145.0 ppm. UV: λmax = 283 nm
(ε = 1.7ϫ104), λmax = 213 nm (ε = 6.2ϫ103), λmax = 204 nm (ε =
8.1ϫ103). MS: m/z (rel. int.) = 209 (46), 207 (48), 128 (100), 101
(45), 77 (45).
1
3b: H NMR: δ = 7.44 (m, 3 H), 7.54 (s, 1 H), 7.67 ppm (m, 2 H).
UV: λmax = 290 nm (ε = 1.4ϫ104), λmax = 222 nm (ε = 7.3ϫ103),
λmax = 203 nm (ε = 6.0ϫ103). MS: m/z (rel. int.) = 209 (42), 207
(44), 128 (100), 101 (38), 77 (37).
Product trans-cinnamonitrile (3c) is commercially available and con-
tains some cis-cinnamonitrile (3d). This mixture was used as such
for product identification.
Reactants and Products: (Z)- and (E)-2-bromo-1-phenylpropene (1a,
1b) were synthesized in a 40:60 ratio from α-methylcinnamic acid
and NBS using a modified Hunsdiecker reaction.[37] The E- and Z-
isomers were separated using column chromatography (silica/petro-
leum ether, boiling range 40–60 °C) in Ͼ98% purity. Their spectro-
(Z)-2-Bromo-3-phenylpropenal (4a) is commercially available (α-
bromocinnamaldehyde) and is isomerized to give (E)-2-bromo-3-
phenylpropenal (4b) by triplet-sensitation using λexc = 313 nm and
acetone as a solvent. The E-isomer is separated and purified by
silica column chromatography using petroleum ether as an eluent.
scopic characterizations are reported in ref.[17]
.
Photoproduct (E)-1-phenylpropene (1c) is commercially available
and (Z)-1-phenylpropene (1d) was prepared from 1c by irradiation
in acetone using light of λexc = 313 nm.[17]
1
4b: H NMR: δ = 7.40 (m, 6 H), 9.55 ppm (s, 1 H). MS: m/z (rel.
int.) = 212 (31), 210 (33), 211 (56), 209 (56),131 (24), 103 (83) [100]
102, 77 (96).
(Z)-1-Bromo-1-fluoro-2-phenylethene (2a) and (E)-1-Bromo-1-
fluoro-2-phenylethene (2b): To a solution of 13 g of triphenylphos-
phane in 125 mL freshly distilled, dry dichloromethane, 6.8 g tri-
bromofluoromethane was added. Subsequently, in 15 min 2.6 g of
benzaldehyde was added, and the solution was stirred for 7 h under
nitrogen atmosphere. The reaction mixture was purified by silica
column chromatography using hexane as the eluent to yield a 5:4
E/Z-mixture in a 0.4 g (15%) yield. The isomers were separated to
an isomeric purity over 98% by preparative GC.
The photoproduct (E)-3-phenyl-2-propenal (4c) is commercially
available (trans-cinnamaldehyde) and (Z)-3-phenyl-2-propanal (4d)
was prepared from 4c by irradiation in acetone using light of λexc
= 313 nm.
1
4d: H NMR: δ = 6.19 (q, 1 H), 7.5 (m, 6 H), 9.96 ppm (d, 1 H).
MS: m/z (rel. int.) = 132 (38), 131 (100), 104 (42), 103 (60), 78 (67),
77 (47).
(Z)-3-Bromo-4-phenylbut-3-en-1-yn (5a) was synthesized using a
modification of a reported procedure.[40] To a solution of 13.1 g
[Ph3PCH2Br]Br in 130 mL THF cooled to –78 °C, under nitrogen
atmosphere, 3.4 g (1 equiv.) tBuOK in 70 mL THF was added. The
solution was stirred for 30 min, after which 6.4 g of α-bromocinna-
maldehyde in 50 mL of THF was added dropwise. The solution
was kept at –78 °C, and stirred for another 30 min. Subsequently,
3
2a:[38] 1H NMR: δ = 6.70 (d, 1 H, JHF = 15.2 Hz), 7.27 ppm (m,
5 H). UV: λexc = 245 nm (ε = 1.1ϫ104). MS: m/z (rel. int.) = 202
(100), 200 (90), 121 (70), 101 (55); high resolution MS: m/z =
199.9625 (calcd. 199.9637).
3
2b:[38] 1H NMR: δ = 5.78 (d, 1 H, JHF = 32.6 Hz), 7.28 ppm (m,
5 H). UV: λmax = 252 nm (ε = 2.9ϫ104). MS: m/z (rel. int.) = 202
Eur. J. Org. Chem. 2008, 435–443
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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441

T. P. M. Goumans, K. van Alem, G. Lodder
FULL PAPER
Press, Boca Raton, 1995; c) T. Kitamura, chapter 85 in CRC
Handbook of Organic Photochemistry and Photobiology (Eds.:
W. M. Horspool, P.-S. Song), CRC Press, Boca Raton, 1995;
d) G. Lodder, J. Cornelisse, chapter 16 in The Chemistry of
Functional Groups: Supplement D2 (Eds.: S. Patai, Z. Rappo-
port), Wiley, Chichester, 1995.
a) E. S. Krijnen, G. Lodder, Tetrahedron Lett. 1993, 34, 729–
732; b) E. S. Krijnen, K. Van Alem, D. H. J. Van der Vlugt, H.
Zuilhof, G. Lodder, Manuscript in preparation.
K. Van Alem, G. Belder, G. Lodder, H. Zuilhof, J. Org. Chem.
2005, 70, 179–190.
the second portion of 3.4 g tBuOK in 70 mL of THF was added,
and the solution was warmed up to room temperature and stirred
overnight. The solution was filtered, and the precipitate washed
with petroleum ether. After evaporation of the solvent, the brown
solid was washed with 4 portions of 150 mL of petroleum ether.
The brown, oily residue after solvent removal was purified by silica
column chromatography with a mixture of ether in petroleum ether
[16]
1
as eluent (yield 18%). H NMR: δ = 3.24 (s, 1 H), 7.32 (m, 4 H),
7.64 (m, 2 H), 7.98 ppm (s, 1 H). 13C NMR: δ = 79.1, 83.1, 99.0,
128.3, 129.1, 129.2, 134.4, 138.1, 192.9 ppm. UV: λmax = 276 nm (ε
= 3.4ϫ104), λmax = 204 nm (ε = 2.2ϫ104). MS: m/z (rel. int.) =
208 (33), 206 (35), 127 (100), 77 (24).
[17]
[18]
[19]
E. S. Krijnen, H. Zuilhof, G. Lodder, J. Org. Chem. 1994, 59,
8139–8150.
The photochemistry of compounds 1 (R = CH3), 3 and 4 [R
= CN and C(H)=O], and 6 (R = CH=CH2) is also reported
in ref.[17,16,18], where the points of focus are the cation-derived
products.
In the presence of oxygen, next to c and d, other vinyl radical-
derived products are produced. See: J. M. Verbeek, M. Stapper,
E. S. Krijnen, J.-D. Van Loon, G. Lodder, S. Steenken, J. Phys.
Chem. 1994, 98, 9526–9536.
A trace amount of Ph–CϵC–F is produced via β-proton loss
from the vinylic cation.
a) L. A. Singer, N. P. Kong, Tetrahedron Lett. 1966, 7, 2089–
2094; b) L. A. Singer, N. P. Kong, J. Am. Chem. Soc. 1966, 88,
5213–5219; c) L. A. Singer, J. Chen, Tetrahedron Lett. 1969, 10,
4849–4854.
(Z)-2-Bromo-1-phenyl-1,3-butadiene (6a): To a cooled solution
(–40 °C) of 9 g [Ph3PCH3]I in THF, 14 mL of 1.6  BuLi in hex-
anes was added, and the solution was stirred for 50 min. After
dropwise addition of 4.5 g α-bromocinnamaldehyde in 30 mL THF,
the solution was warmed to room temperature and stirred for an
hour. The solution was filtered, and the precipitate washed with
petroleum ether. After evaporation of the solvent, the brown solid
was washed with 4 portions of 100 mL of petroleum ether. The
brown, oily residue after solvent removal was purified in a 38%
yield by silica column chromatography with a mixture of dichloro-
methane in petroleum ether as an eluent. The spectroscopic charac-
terization of 6a is reported in ref.[18]
[20]
[21]
[22]
[23]
[24]
a) G. T. Whitesides, C. P. Casey, J. K. Krieger, J. Am. Chem.
Soc. 1971, 93, 1379–1389; b) X.-Y. Jiao, W. G. Bentrude, J. Org.
Chem. 2003, 68, 3303–3306; c) A. B. Chopa, V. B. Dorn, M. A.
Badajoz, M. T. Lockhart, J. Org. Chem. 2004, 69, 3801–3805.
The C–H bond dissociation energies of ethene and methanol
are 108 and 92.6 kcal/mol, respectively, according to: K. W.
Egger, A. T. Cocks, Helv. Chim. Acta 1973, 56, 1516–1536.
J. A. Kampmeier, G. Chen, J. Am. Chem. Soc. 1965, 87, 2608–
2613.
A. C. Simmonett, S. E. Wheeler, H. F. Schaefer, J. Phys. Chem.
A 2004, 108, 1608–1615.
J. R. McCarthy, E. W. Huber, T.-B. Le, F. M. Laskovics, D. P.
Matthews, Tetrahedron 1996, 52, 45–58.
[1] G. Stork, N. H. Baine, J. Am. Chem. Soc. 1982, 104, 2321–
2323.
[2] a) S. Z. Zard, Radical Reactions in Organic Synthesis, Oxford
University Press, Oxford, 2003; b) P. Renaud, M. P. Sibi, Radi-
cals in Organic Synthesis, Wiley-VCH, Weinheim, 2001.
[3] a) D. P. Curran, N. A. Porter, B. Giese, Stereochemistry of Rad-
ical Reactions, VCH, Weinheim, 1995; b) B. Giese, Angew.
Chem. Int. Ed. Engl. 1989, 28, 969–980.
[4] a) L. A. Singer, in Selective Organic Transformations (Ed.: B. S.
Thyagarajan), Wiley-Interscience, New York, 1972, vol. 2, 239–
268; b) A. L. J. Beckwith, K. U. Ingold, in Rearrangements in
Ground and Excited States (Ed.: P. De Mayo), Academic Press,
New York, 1980, vol. 1, 280–310; c) O. Simumara, Top. Ste-
reochem. 1969, 4, 1–37; d) C. Galli, Z. Rappoport, Acc. Chem.
Res. 2003, 36, 580–587.
[5] P. R. Jenkins, M. C. R. Symons, S. E. Booth, C. J. Swain, Tetra-
hedron Lett. 1992, 33, 3543–3546.
[6] C. J. Rhodes, E. Roduner, J. Chem. Soc. Perkin Trans. 2 1990,
1729–1733 and references cited therein.
[7] a) L. Bonazzola, S. Feinstein, R. Marx, Mol. Phys. 1971, 22,
689–695; b) G. W. Neilson, M. C. R. Symons, J. Chem. Soc.
Perkin Trans. 2 1973, 1405–1410.
[8] H.-G. Korth, J. Lustztyk, K. U. Ingold, J. Chem. Soc. Perkin
Trans. 2 1990, 1997–2007.
[25]
[26]
[27]
[28]
[29]
R. M. Kopchik, J. A. Kampmeier, J. Am. Chem. Soc. 1968, 90,
6733–6741.
a) Singer
[4a,22]
and Metzger and Blumenstein[10] plot final con-
centration ratios instead of reaction rate ratios. Care must be
taken, however, to avoid free radical induced cis/trans isomer-
ization or, in the case of photochemical approaches, photo-
chemical isomerization; b) R. C. Neuman, G. D. Holmes, J.
Org. Chem. 1968, 33, 4317–4322.
The difference in activation enthalpy for an exothermic radical
scavenging will be mainly composed of the difference in steric
repulsion for cis and trans attack, which follows from the Bell–
Evans–Polanyi principle.
The vibrational contribution to the molecular partition func-
tion, and thus to the entropy is usually the largest. The normal
modes of a molecule, in turn, are determined by symmetry; cf.,
for instance, P. W. Atkins, Physical Chemistry, Oxford Univer-
sity Press, Oxford, 1995, 5th edition, chapters 16, 19 and 20.
L. Letendre, H.-L. Dai, J. Phys. Chem. A 2002, 106, 12035–
12040.
a) R. C. Bingham, M. J. S. Dewar, J. Am. Chem. Soc. 1973, 95,
7180–7182; b) R. C. Bingham, M. J. S. Dewar, J. Am. Chem.
Soc. 1973, 95, 7182–7183.
Irradiation of 5a yields Ph–CϵC–CϵC–H and Ph–
C(H)=C=C=C(H)–Br (tentative structure), produced from the
vinylic cation by β-proton loss and 1,3-bromide shift, respec-
tively.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tom-
asi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
[30]
[31]
[9] O. Ito, O. Ryoichi, M. Matsuda, J. Am. Chem. Soc. 1982, 104,
3934–3937.
[10] J. O. Metzger, M. Blumenstein, Chem. Ber. 1993, 126, 2493–
2499.
[11] M. Ohno, K. Ishizaki, S. Eguchi, J. Org. Chem. 1988, 53, 1285–
1288.
[12] M. Guerra, Res. Chem. Intermed. 1996, 22, 369–379.
[13] C. Galli, A. Guarnieri, H. F. Koch, P. Mencarelli, Z. Rappo-
port, J. Org. Chem. 1997, 62, 4072–4077. The vinyl radicals
with α-substituents H, Cl and F were also calculated at the
higher level B3LYP/6-311G(2d,2p).
[14] a) P. M. Mayer, C. J. Parkinson, D. M. Smith, L. Radom, J.
Chem. Phys. 1998, 108, 604–615; b) C. J. Parkinson, P. M.
Mayer, L. Radom, Theor. Chem. Acc. 1999, 102, 92–96.
[15] a) G. Lodder, chapter 8 in Dicoordinated Carbocations (Eds.:
Z. Rappoport, P. J. Stang), Wiley, West Sussex, 1997; b) P. J.
Kropp, chapter 84 in CRC Handbook of Organic Photochemis-
try and Photobiology (Eds.: W. M. Horspool, P.-S. Song), CRC
[32]
[33]
[34]
[35]
442
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Eur. J. Org. Chem. 2008, 435–443

Photochemical Generation and Structure of Vinyl Radicals
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratch-
ian, J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J.
W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Sal-
vador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D.
Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Ba-
boul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Lia-
shenko, P. Piskorz, L. Komaromi, R. L. Martin, D. J. Fox, T.
Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M.
Challa-Combe, P. M. W. Gill, B. Johnson, W. Chen, M. W.
Wong, C. Gonzalez, J. A. Pople, Gaussian 03, Revision B.3,
Gaussian, Inc., Pittsburgh, PA, 2003.
[36] M. Guerra, Theor. Chem. Acc. 2000, 104, 455–460.
[37] S. Chowdhurry, S. Roy, J. Org. Chem. 1997, 62, 199–200.
[38] For previous characterization, see ref.[13] and J. Kvícˇala, R.
Hrabal, J. Czernek, I. Bartosˇová, O. Paleta, A. Pelter, J. Fluor-
ine Chem. 2002, 113, 211–218.
[39] D. G. Cox, N. Gurushamy, D. J. Burton, J. Am. Chem. Soc.
1985, 107, 2811–2812.
[40] M. Matsumoto, K. Kuroda, Tetrahedron Lett. 1980, 21, 4021–
4024.
Received: September 17, 2007
Published Online: December 18, 2007
Eur. J. Org. Chem. 2008, 435–443
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