The photochemistry of ring-substituted cinnamyl acetates

Article abstract of DOI:10.1139/V06-140

The photochemistry of the (E)-cinnamyl acetates ((E)-1-aryl-3-propenyl acetates, 8a-8e) with substituents H, 4-CH₃O, 3-CH₃O, 4-CF₃, and 3-CF₃, respectively, was examined in both cyclohexane and methanol solvents. Alkene isomerization (E to Z) occurred more efficiently than other reactions and evidence is presented that this process occurs from the excited triplet state. In a slower process, 1,3-migration of the acetoxy group led to the rearranged 3-aryl-3-propenyl acetate isomers (9a-9e) as the major pathway, particularly in cylohexane. In methanol, the isomeric ethers 3-aryl-3-methoxypropene (14) and 1-aryl-3-methoxypropene (15) were formed by reaction of methanol with the photochemically generated cation. The combined yield of 14 and 15 (95% and 5%, respectively) was quantitative for the 4-methoxyphenyl compound (8b). Independent irradiations of the isomers 9a-9c demonstrated that the ethers 14 and 15 were primary photoproducts from 8 and not secondary photoproducts from 9. Fluorescence quantum yields and excited singlet state lifetimes indicated that the reactions, other than the E to Z isomerization, are from the excited singlet state.

Full text of DOI:10.1139/V06-140

1146
The photochemistry of ring-substituted cinnamyl
acetates
S.A. Fleming, L. Renault, E.C. Grundy, and J.A. Pincock
Abstract: The photochemistry of the (E)-cinnamyl acetates ((E)-1-aryl-3-propenyl acetates, 8a–8e) with substituents H,
4-CH3O, 3-CH3O, 4-CF3, and 3-CF3, respectively, was examined in both cyclohexane and methanol solvents. Alkene
isomerization (E to Z) occurred more efficiently than other reactions and evidence is presented that this process occurs
from the excited triplet state. In a slower process, 1,3-migration of the acetoxy group led to the rearranged 3-aryl-3-
propenyl acetate isomers (9a–9e) as the major pathway, particularly in cylohexane. In methanol, the isomeric ethers 3-
aryl-3-methoxypropene (14) and 1-aryl-3-methoxypropene (15) were formed by reaction of methanol with the photo-
chemically generated cation. The combined yield of 14 and 15 (95% and 5%, respectively) was quantitative for the 4-
methoxyphenyl compound (8b). Independent irradiations of the isomers 9a–9c demonstrated that the ethers 14 and 15
were primary photoproducts from 8 and not secondary photoproducts from 9. Fluorescence quantum yields and excited
singlet state lifetimes indicated that the reactions, other than the E to Z isomerization, are from the excited singlet
state.
Key words: cinnamyl acetates, photochemistry, E/Z isomerization, photocleavage, fluorescence.
Résumé : Opérant tant dans le cyclohexane que dans le méthanol comme solvants, on a étudié la photochimie des acé-
tates de (E)-cinnamyle (acétates de (E)-1-aryl-3-propényle, 8a–8e) portant des substituants H, 4-OCH3, 3-OCH3, 4-CF3,
et 3-CF3. L’isomérisation de l’alcène (E à Z) est la réaction qui se produit de la façon la plus efficace par rapport aux
autres et on présente des données qui montrent que cette réaction se produit à partir de l’état triplet excité. Dans un
processus plus lent, particulièrement dans le cyclohexane, la voie réactionnelle principalement provoque un réarrange-
ment avec migration-1,3 du groupe acétoxy qui conduit aux acétates de 3-aryl-3-propényle isomères (9a–9e). Dans le
méthanol, les éthers isomères 3-aryl-3-méthoxypropène (14) et 1-aryl-3-méthoxypropène (15) se forment par réaction
du méthanol avec le cation généré photochimiquement. Le rendement combiné des composés 14 et 15 (95 % et 5 %)
est quantitatif pour le composé 4-méthoxyphényle (8b). Des irradiations indépendantes des isomères 9a–9c ont démon-
tré que les éthers 14 et 15 correspondent aux photoproduits primaires des composés 8 et non pas des photoproduits se-
condaires des composés 9. Les rendements quantiques de fluorescence et les temps de vie de l’état excité singulet
indiquent que les réactions, à l’exception de l’isomérisation E à Z, se font à partir de l’état singulet excité.
Mots clés : acétates de cinnamyle, photochimie, isomérisation E/Z, photoclivage, fluorescence.
[Traduit par la Rédaction] Fleming et al. 1154
Introduction
The initial report of the “meta effect” for these reactions
by Zimmerman and coworkers (4, 5) presented the observa-
tion that 3-methoxybenzyl acetate (1b) gives a higher chemi-
cal (29%–35%) and quantum yield (0.13) of formation of
the ion-derived alcohol product than 4-methoxybenzyl ace-
tate (1a) (“minor” amount and 0.016, respectively) on
photolysis in aqueous dioxane (eq. [1]). This observation is
contrary to expectations on the basis of ground state stability
of the corresponding cations, the 4-methoxybenzyl cation
being considerably more stable than the 3-methoxy substi-
tuted one.
For some time, we have been interested in the effect that
substitutents on aromatic rings have on photochemical pro-
cesses. In particular, we have examined the photocleavage
reactions of arylmethyl systems with leaving groups
(ArCH2-LG), the competition between radical and ion-
derived products in polar nucleophilic solvents like alcohols,
Scheme 1, and the pathway for their formation (1–3).
Received 14 July 2006. Accepted 7 September 2006.
Published on the NRC Research Press Web site at
http://canjchem.nrc.ca on 2 November 2006.
S.A. Fleming. Department of Chemistry and Biochemistry,
C100 BNSN, Brigham Young University, Provo, UT 84602,
USA.
[1]
L. Renault, E.C. Grundy, and J.A. Pincock.1 Department of
Chemistry, Dalhousie University, Halifax, NS B3H 4J3,
Canada.
As outlined in Scheme 1, two quite different mechanistic
proposals have been advanced to explain this observation.
1Corresponding author (e-mail: james.pincock@dal.ca).
Can. J. Chem. 84: 1146–1154 (2006)
doi:10.1139/V06-140
© 2006 NRC Canada

Fleming et al.
1147
Scheme 1.
Path 1, as initially proposed and reinforced by more recent
computational results by Zimmerman (6, 7) assumes that the
ion pair is formed directly from the first excited singlet state,
S1, of the substrate and that the 3-methoxybenzyl cation is
formed more efficiently than the 4-methoxy substituted one.
Path 2, as developed by us, assumes that the ion pair is
formed by redox electron transfer from an initially formed
radical pair; direct formation of the ion pair from S1 is, at
most, a minor process. The yield of the ion pair is then de-
termined by ket, the rate constant for electron transfer that
converts the radical pair to the ion pair and therefore by the
driving force as determined by the reduction potential of the
acyloxy radical and the oxidation potential of the arylmethyl
radical. Results for substituted benzyl and 1- naphthylmethyl
acetates 1, 2 have been particularly important in developing
this mechanism (1).
More recently, Rao et al. (8) reported on the photo-
chemistry2 of (E)-1-naphthylallyl acetates (3) for X = H, 4–
CH3 and 4–OCH3 (eq. [2]) in hexane, methanol, and
acetonitrile. In hexane, E to Z isomerization was the major
process observed although low yields of the rearranged ace-
tate 5 were reported for 3b and 3c. In acetonitrile, the effi-
ciencies of conversion were similar but the major product
was now the rearranged acetate 5 for all three cases. In
methanol, the major photosolvolysis product for all three
was the methyl ether 6 with yields of 30% from 3a, 36%
from 3b, and 80% from 3c. The ether 7, resulting from di-
rect substitution, was not observed. Remarkably, the 4-
methoxy substrate 3c reacted considerably more efficiently
than either of the other two in methanol (conversion after
30 min of 40%, 43%, and 95% for 3a, 3b, and 3c, respec-
tively). No products obviously derived from radical interme-
diates were reported for any of the three substrates, even in
cyclohexane as solvent, although the rearranged acetates 5
could be so formed.
[2]
We have observed (9) a similar high yield (74% of the
methyl ether) of ion-derived product for the photolysis in
methanol of the 4-methoxy-1-naphthyl substrate 2a, com-
pared with the corresponding 3-methoxy derivative 2b
(31%).
We now report on the photochemistry in both cyclohexane
and methanol of the cinnamyl acetates 8a–8e (eq. [3]).
2 For 30 min irradiations of 10–3 mol/L solutions of substrate in a Rayonet reactor with 300 nm lamps.
© 2006 NRC Canada

1148
Can. J. Chem. Vol. 84, 2006
Table 1. Product yields for the photolysis of esters 8a–8e for 1 h at 25 °C.
Compound
X
Solventa
8 (E + Z)
9
13 (E + Z)
14
15 (E + Z)
8a
8b
H
C
C
24 + 46
—
30
96
—
4
4-CH3O
8c
8d
8
3-CH3O
4-CF3
C
C
C
30 + 43
7 + 16
16
50
56
11
20 + 7
3
3-CF3
11 + 28
8a
8b
H
M
M
—
—
57
—
—
—
43
95
—
5
4-CH3O
8c
8d
8
3-CH3O
4-CF3
M
M
M
—
24
38
58
—
52
6
23
3
5 + 14
7 + 15
21 + 13
5 + 5
3-CF3
9
2
Note: Product yields are in % and normalized to 100% (see text). A dash indicates that the yield was less than 1%.
aC = cyclohexane, M = methanol.
These compounds were chosen to examine both the “meta
effect” for a system with extended conjugation relative to
the prototypical benzyl acetates 1 and the connection
between the arylmethyl compounds 1 and 2 and those
with more extended allylic conjugation, 3 and 8. The CF3-
substituted cases were included to examine the contrast in
reactivity with substrates having electron-withdrawing sub-
stituents.
ported in Table 1 after 1 hour of irradiation. Dark control
experiments in methanol resulted in no alkene isomerization
or solvolysis for any of the compounds 8. For photolysis of
all substrates, the mass balance for volatile products was low
(e.g., typical of most cases only 55% after 85% conversion
of 8a in methanol). The nonvolatile products are assumed to
be oligomers formed from the arylallyl radical and ionic in-
termediates. This suggestion is supported by the observation
of broad structureless signals in the aromatic and aliphatic
1
[3]
region of the H NMR spectra of crude reaction mixtures.
Therefore, the values of product yields in Table 1 have been
normalized to 100%. On the basis of product yields, the
most reactive compound in both cyclohexane and methanol
is the 4-methoxy one, similar to the previous observations
(8) for the 1-naphthyl derivatives 3 (eq. [2]).
A main photoproduct for each of the substrates was the
isomeric 3-aryl-3-propenyl acetate (9). Although they could
be isolated by chromatography from the photolysis mixtures,
9a–9c were also prepared by the reaction of vinylmagnesium
bromide with the appropriate substituted benzaldehyde fol-
lowed by quenching with acetyl chloride.
Photophysical properties of 8a–8e and the siloxanes
10b–10e in cyclohexane and methanol
The photophysical properties of the esters 8 in both cyclo-
hexane and methanol are reported in Table 2 along with
those for the corresponding siloxanes 10, which serve as
useful models because they have the same chromophore but
are photochemically unreactive to C–O bond cleavage. The
siloxanes 10b–10e were available in our laboratory because
they had been prepared for a related project on the photo-
chemistry of dicinnamyl siloxanes. Another good model for
the chromophore of the esters are 1-arylpropenes, which
have been studied previously; literature values for the spec-
tra of 1-phenylpropene 11a are also included in Table 1.
The UV absorption spectra of compounds 8, 10, and 11
are as expected on the basis of the reported values for 1-
arylpropenes (11, 12) with a longer wavelength lower inten-
sity band with vibrational structure for the S0 to S1 transition
and a higher intensity broad structureless band for the S0 to
S2 transition at shorter wavelength. As an example, for
cinnamyl acetate 8a in methanol the bands observed are at
291 nm (ε = 780 (mol/L)–1 cm–1) and 282 nm (ε = 1190
(mol/L)–1 cm–1) for the S1 transition and 252 nm (ε = 16 100
Results and discussion
Synthesis and photolysis experiments
The synthesis of the (E)-1-aryl-3-propenyl acetates 8a–8e
was performed as described in the literature (10). The corre-
sponding commercially available cinnamic acids were con-
verted to their ethyl esters, reduced with DIBAL, and the
resulting alcohols were esterified by reacting with acetyl
chloride. The acetates (100 mg in 100 mL of solvent) were
irradiated at 25 °C in a Rayonet apparatus equipped with
254 nm lamps. The reaction progress was monitored by
1
GCMS and product ratios (by GC and H NMR) are re-
© 2006 NRC Canada

Fleming et al.
1149
Table 2. Photophysical properties of the compounds 8–11 at 25 °C.
λ
max, ε/103
kf
Solventa
(nm, (mol/L)–1 cm–1)
τ (ns)
(107 s–1)b
Φf
Compound
X
7.2
5.6
11.8
7.2
3.0
2.9
3.0
0.36
12.7
8a
H
H
H
H
C
253, 15.5
252, 16.1
250c
256, 0.23
263, 22.2
0.22
0.16
0.35
0.026
0.23
8a
M
H
M
C
11ac
9a
8b
4-CH3O
1.8
10b
8b
10b
9b
8c
4-CH3O
4-CH3O
4-CH3O
4-CH3O
3-CH3O
3-CH3O
3-CH3O
3-CH3O
3-CH3O
4-CF3
C
261, 22.5
262, 22.1
260, 24.4
281, 1.58
253, 14.0
253, 15.2
252, 14.4
252, 14.0
271, 1.66
255, 19.0
254, 18.8
253, 20.0
254, 20.9
249, 16.6
254, 18.4
249, 17.9
249, 18.3
0.59
0.029
0.34
0.098
0.50
0.44
0.091
0.64
0.008
0.24
0.25
0.22
0.16
0.24
0.21
0.24
0.27
6.1
9.7
d
M
M
M
C
—
3.2
5.25
6.5
5.8
0.89
6.8
1.1
4.0
3.8
4.0
3.5
4.8
4.2
5.4
5.0
10.6
1.9
7.7
7.6
10.2
9.3
0.72
6.0
6.6
5.5
4.6
5.0
5.0
4.4
5.4
10c
8c
10c
9c
C
M
M
M
C
8d
10d
8d
10d
8
4-CF3
C
4-CF3
M
M
C
4-CF3
3-CF3
10
3-CF3
C
8
3-CF3
M
M
10
3-CF3
aC = cyclohexane, M = methanol, H = hexane
bkf = Φ / τ.
f
cFrom ref. 11.
d<0.5 ns.
(mol/L)–1 cm–1) for the S2 transition. The irradiation experi-
ments were done at 254 nm into the S2 transition and only
those bands are reported in Table 1. This band is shifted to
longer wavelength (263 nm) and higher molar absorptivity
(ε = 22 200 (mol/L)–1 cm–1) by the conjugating 4-methoxy
substituent 8b but affected only slightly by the other sub-
stituents. The change in solvent from cyclohexane to metha-
nol also has little effect. A comparison of the absorption
bands and the molar absorptivities for the esters 8 and the si-
loxanes 10 indicates that, as expected, the chromophores are
essentially the same for the two.
ing the similarity of the excited states of the esters and the
model siloxanes. This similarity is also apparent for the
trifluoromethyl substituted compounds 8d and 8e, but now
the Φ and τs values suggest that excited state C–O bond
f
cleavage is a relatively inefficient process in both cyclohex-
ane and methanol.
[4]
[5]
Φ = kf / kt = kfτs
f
1 / τs = kt = kf + kisc + kic + kr
Photolysis of esters 8 in cyclohexane
In contrast, the quantum yields of fluorescence (Φ) for
Although the main goal of this study was to examine the
ion-derived products formed by photosolvolysis of the
cinnamyl esters, the results in cyclohexane provide impor-
tant information on other mechanisms. Several observations
and trends are worth noting. First, from percent conversion
vs. time plots, E to Z isomerization occurs more rapidly than
other photochemical processes to give mixtures enriched in
the Z isomer. This result is apparent from the photolysis
yield results shown in Table 1 for those substrates that are
not completely consumed after one hour of irradiation. Fur-
ther product formation then occurs from this equilibrated
mixture, which is rich in the Z isomer. Because the second-
ary photochemistry must compete with the more rapid
isomerization process, the photoreactions are relatively inef-
ficient.
f
the methoxy substituted esters 8b and 8c are considerably
lower than for the corresponding siloxanes 10b and 10c, par-
ticularly in methanol. This quantum yield is defined by
eq. [4], where kf and kt are the rate constants for fluores-
cence and the total rate of decay for the excited singlet state,
respectively. The rate constant kt (the reciprocal of τS) is de-
fined by eq. [5], where kic, kisc, and kr are the rate constants
for internal conversion, intersystem crossing, and reaction,
respectively. The decreases in Φ are a consequence of the
f
fact that the excited singlet states of the esters have a path-
way, C–O bond cleavage, for deactivation that competes
with fluorescence but is unavailable for the silanes. In agree-
ment with this concept, the excited singlet state lifetimes (τs)
are also considerably shorter for the esters. The parallel de-
crease in the quantum yield of fluorescence and singlet state
lifetime means that kf remains nearly constant, again indicat-
On the basis of results by Lewis and Zuo (11) for (E)- and
(Z)-1-phenylpropene, which have an analogous chromophore
© 2006 NRC Canada

1150
Can. J. Chem. Vol. 84, 2006
Scheme 2.
to the unsubstituted cinnamyl acetate 8a, the E isomer
should have a high barrier (~9 kcal/mol, 1 cal = 4.184 J) in
the excited singlet state for rotation about the C=C double
bond. At room temperature, the dominant mode of the ex-
cited singlet state decay should be intersystem crossing and
isomerization would then occur by a barrierless process in
the triplet manifold. In contrast, the Z isomer should have a
lower barrier (~4 kcal/mol) for excited singlet state rotation
and this process would be the dominant mode for its decay
at room temperature.
Attempts to confirm that the triplet state of 8a did un-
dergo E to Z isomerization were unsuccessful because of
rapid reaction of the sensitizers, either benzophenone or
xanthone with the solvent cyclohexane. However, a previous
report (13) on the triplet sensitization of the photochemistry
of cinnamyl acetate 8a in acetonitrile indicated that only
isomerization occurred to give a ratio of 35% E and 65% Z.
A very small amount (2.5%) of the rearranged isomer 9a
was also reported but it was ascribed to direct absorption of
the incident light by 8a rather than by the sensitizer.
Second, the predominate product for the irradiations in
cyclohexane is the 3-aryl-3-propenyl acetate 9 that results
from a 1,3-shift of the acetoxy group (eq. [3]). This observa-
tion is similar to that reported recently (8) for the formation
of the products 5 on photolysis in hexane of the 1-(1-
naphthyl)-3-propenyl acetate derivatives 3 (eq. [2]). How-
ever, in contrast to the high yields observed in the current
study (94% for 8b; X = 4-CH3O, for example, the compound
where this process is the most efficient) the yields reported
for the naphthyl derivatives are very low (3% for 3c; X = 4-
CH3O). Because these products are being formed in cyclo-
hexane where ionic intermediates are unlikely to be formed,
the mechanism for their formation must be by a radical or
concerted pathway similar to a photo-Claisen rearrangement.
The other major product of the photolyses in cyclohexane
is 13, another deconjugated isomer of the starting material
but formed by a 1,3-hydogen shift and as a mixture of E and
Z isomers. The yield is highest (27%) for the 4-trifluoro-
methyl substrate 8d. Again, this process does not likely in-
volve ion pairs.
mation of the rearranged isomers 9 and 13 in cyclohexane as
solvent occurs by a concerted process without discrete inter-
mediates.
Photolysis of esters 8 in methanol
Photolysis of each of the substrates 8 in methanol resulted
in the formation of the ion-derived products 13 and 14 along
with, in most cases, those obtained in cyclohexane irradia-
tions. However, for the 4-methoxy compound 8b the ethers
13 and 14 are the only products observed with a high selec-
tivity for unconjugated isomer 14. The evidence that these
products are derived from the excited singlet state is pro-
vided by a sensitized irradiation in methanol for 8a.
Xanthone, as opposed to benzophenone, was chosen as the
sensitizer on the basis of the fact that its triplet state is not
efficiently quenched by methanol (14). Irradiation using a
Pyrex filter, conditions that led to no reaction in the absence
of xanthone, resulted in efficient E to Z isomerization. No
other products were detected indicating that the excited trip-
let state of 8a does not react by C–O bond cleavage in con-
trast to direct irradiations, which proceed through the excited
singlet state. As discussed previously, this proposal is sup-
ported by the decreases in Φ and τf for 8a (Table 1) relative
f
to the (E)-1-phenylpropene 11a.
None of the compounds irradiated in methanol, as in cyc-
lohexane, gave observable quantities of radical coupling
products. Therefore, the observed photochemistry is consis-
tent with three possible mechanisms: (i) heterolytic cleavage
of the C–O bond from S1 to form the ion pair directly,
(ii) homolytic cleavage from S1 followed by electron transfer
that is faster than decarboxylation of the acetyloxy radical,
and (iii) bimolecular reaction of methanol with S1 to form
14 (SN2′) or 15 (SN2). The second would require that elec-
tron transfer be more than an order of magnitude faster than
109 s–1, the rate of decarboxylation of the acetyloxy radical
(15) and therefore faster than previously determined rates for
benzyl systems (1). The third also seems unlikely because,
to our knowledge, convincing evidence for this possibility in
photosolvolysis reactions has not been reported. The first
possibility, direct heterolytic cleavage from S1, is therefore
most probable. This suggestion is supported by the greater
efficiency of reaction for the methoxy substituted com-
pounds, which should favour cation formation relative to
those with trifluoromethyl substituents.
An important point to make here is that no products re-
sulting from radical coupling were obtained. In the photo-
chemistry of both benzyl and 1-naphthylmethyl acetates,
products (ethylbenzene and ethylnaphthalene derivatives) re-
sulting from the loss of carbon dioxide from the acetyloxy
radical followed by coupling of the methyl radical with the
arylmethyl radical are always observed (1). The lack of anal-
ogous products for the photolysis of 8 suggests that the for-
By reaction of the solvent at the aryl-substituted end C1
of the allylic cation, Scheme 2, the photosolvolysis products
primarily give the unconjugated ether 14. The same observa-
tion was made for the 1-naphthylmethyl derivatives 3
© 2006 NRC Canada

Fleming et al.
1151
(eq. [2]) with 6 formed in preference to 7. Moreover, irradia-
tion of the conjugated ether 15a in methanol did not result in
formation of the unconjugated isomer 14a. If the two iso-
mers were equilibrating photochemically then 14a, with the
weaker chromophore, would have been the major one in the
mixture. Finally, because the conjugated isomers 15 will be
more stable than the unconjugated ones 14, any explanation
for the preference for 14 must be a kinetic rather than a ther-
modynamic one.
This selective regiochemistry could simply be the result of
the distribution of the positive charge in the cationic inter-
mediate 16. However, calculated positive charges (Mulliken,
with attached hydrogens summed into the carbons) by
Gaussian 03 (B3LYP, 6-31G(d), optimized geometry) (16)
for 16 gave values (X, charge at C1, charge at C3) that were
significantly higher at C3 than C1: 16a (H, 0.091, 0.200),
16b (4-methoxy, 0.055, 0.155), 16c (3-methoxy, 0.081,
0.192), 16d (4-trifluoromethyl, 0.098, 0.214), 16e (3-
trifluoromethyl, 0.099, 0.215). This result leads to a predic-
tion opposite to that observed. However, examination of the
LUMO, a pure π orbital dominated by contributions from the
AO’s at C1 and C3 so that it resembles an allyl system, was
more satisfactory. The values for 16 were (X, contribution
from 2px + 3px at C1, contribution from 2px + 3px at C3, dif-
ference): 16a (H, 0.710, –0.614, 0.096), 16b (4-methoxy,
(eq. [6]). As discussed above, these latter products were not
observed as photoproducts in methanol from compounds 8.
This implies that the excited states of 8 and 9 behave quite
differently, the latter being comparable to normal benzylic
ester photochemistry.
For irradiation of the 3- or 4-trifluoromethyl substituted
compounds 8d and 8e in methanol, photosolvolysis is not
the major pathway. In both cases the major photoproducts
are the rearranged acetates 9 and 13, the former being pre-
ferred. This result is consistent with a less efficient hetero-
lytic cleavage because the electron-withdrawing groups do
not assist with the bond breaking process. As discussed pre-
viously, this conclusion is supported by their small changes
in fluorescence quantum yield and excited singlet state life-
time (Table 1) relative to the siloxanes 10d and 10e. The re-
versed polarity of the excited states as a consequence of the
electron-withdrawing groups may also explain the prefer-
ence for 1,3-hydrogen and 1,3-acetoxy migrations to give
compounds 9 and 13, respectively.
Conclusions
The photochemistry of the (E)-cinnamyl acetates is simi-
lar to that of benzylic esters in that processes involving C–O
bond cleavage occur, including photosolvolysis in methanol.
These processes from the excited singlet state are in compe-
tition with the more efficient intersystem crossing followed
by E to Z isomerization from the triplet state. In contrast to
benzylic systems, where 3-methoxy derivatives react consid-
erably more efficiently than the 4-methoxy substituted ones
in polar protic solvents, both methoxy compounds give high
yields of photosolvolysis for the cinnamyl derivative. In fact,
the 4-methoxy compound gives essentially 100% yield of
ion-derived ethers in methanol.
0.694, –0.579, 0.115), 16c (3-methoxy, 0.0.723,
–0.617,
0.105), 16d (4-trifluoromethyl, 0.718, –0.616, 0.102), 16e
(3-trifloromethyl, 0.725, –0.620, 0.105). Therefore, the
LUMO is dominated by the contribution from C1 for all
substituents, the largest difference being for the 4-methoxy
compound where the selectivity is also high for reaction at
C1 to give 14 (95%) over C3 to give 15 (5%) (Table 1). A
similar observation that LUMO coefficients are better than
positive charge distribution at predicting the regioselectivity
of cation reactivity with methanol has been reported previ-
ously for photosolvolysis reactions (17). The asymmetry of
the allyl fragment in the cations 16 is also apparent from the
calculated bond lengths. The C1–C2 bond is significantly
In both cyclohexane and methanol, no evidence for
radical-derived products was obtained. The rearranged ace-
tates appear to be derived from concerted 1,3-hydrogen or
acetoxy migrations.
longer (1.420
0.002) than that for C2–C3 (1.358
0.0023), this difference being essentially independent of
substituent.
Experimental
The possibility that the methyl ethers 14 and 15 were
formed as secondary photoproducts from the initially formed
rearranged esters 9 was also independently tested by study-
ing the photochemistry of 9. As indicated in Table 2 and as
expected, the esters 9a–9c absorbed at similar wavelengths
to 8a–8c, but the molar absorptivity of compounds 9 was
more than one order of magnitude lower. Therefore, compet-
itive photochemistry of 9 is unlikely because it cannot be ab-
sorbing a significant percentage of the photons until very
high conversion. Moreover, the rearranged acetate 9b was
not even observed in the photolysis of 8b in methanol. The
other interesting observation from the photochemistry of
compounds 9 is that the products were exclusively the
unconjugated ethers 14 and the radical coupling products 17
Materials and instruments
All NMR spectra were obtained in CDCl3 using a Bruker
250 MHz instrument unless otherwise noted. High resolution
mass spectrometry (HRMS) was performed on a CEC 21-
110B sector instrument using electron impact ionization.
Solvent extractions were dried with magnesium sulfate. To-
luene was distilled from CaH. Ethyl acetate and hexane were
flash distilled prior to chromatography. All other solvents
were used without further purification. Solvents used for
photochemical experiments were obtained commercially as
HPLC grade. Column chromatography was performed using
60–250 mesh silica gel under flash conditions. Chiral chro-
matography was performed on a Varian Star #1 instrument
using a Chiracel AD-RH column eluted with methanol–water.
Irradiations
A solution of ~100 mg of the appropriate compound in
100 mL of methanol or cyclohexane was purged with nitro-
gen and then irradiated in a Rayonet photochemical reactor
using 16 lamps (75 W, 254 nm). The temperature was con-
[6]
© 2006 NRC Canada

1152
Can. J. Chem. Vol. 84, 2006
trolled ( 1 °C) by circulating water in an immersion tube.
Reaction progress was monitored by GC followed by NMR
analysis after solvent evaporation. For sensitized reactions in
methanol, ~250 mg of 8a and xanthone (3.6 × 10–4 mol/L,
A = 2.3 at 340 nm) in 280 mL of solvent was irradiated in
the well of a 450 W Hanovia lamp using a Pyrex filter. In
the absence of the sensitizer, no reaction occurred under
these conditions.
206.0943; found: 206.0939. The literature reference for the
synthesis provides no spectral data (22).
3-Methoxycinnamyl acetate 8c
1H NMR δ: 7.25 (m, 1 H), 6.95 (d, 1H), 6.92 (bs, 1H),
6.83 (dd, 1H), 6.63 (d, 1H), 6.27 (dt, 1H), 4.73 (dd, 2H),
3.81 (s, 3H), 2.10 (s, 3H). 13C NMR (62.5 MHz) δ: 171.0,
160.1, 137.8, 134.3, 129.8, 123.8, 119.5, 114.0, 112.1, 65.2,
55.4, 21.2. The literature reference for the synthesis provides
no spectral data (10).
GC method
GCMS analyses were done a PerkinElmer Autosystem us-
ing a Supelco 30 mm × 0.25 mm MDN-5S 5% phenyl
methylsiloxane column, film thickness 0.50 µm, temperature
programmed as follows: 60 °C for 1min, 20 °C/min to
240 °C, 240 °C for 10 min.
4-Trifluoromethylcinnamyl acetate 8d
1H NMR δ: 7.57 (d, 2H, J = 7 Hz), 7.47 (d, 2H, J = 8 Hz),
6.67 (d, 1H, J = 16 Hz), 6.37 (dt, 1H, J = 16 and 6 Hz), 4.75
(d, 2H, J = 6 Hz), 2.11 (s, 3 H). 13C NMR (62.5 MHz) δ:
170.8, 139.9 (q, J = 1 Hz), 132.4, 129.9 (q, J = 32 Hz),
126.9, 126.2, 125.7 (q, J = 3.6 Hz), 124.3 (q, J = 270 Hz),
64.7, 21.0. HRMS calcd. for C12H11F3O2: 244.0711; found:
244.0714 (23).
Absorbance measurements
Absorbance spectra were recorded at 1 nm resolution us-
ing a Cary 100 UV–vis spectrometer thermostated at 25 °C.
3-Trifluoromethylcinnamyl acetate 8e
1H NMR δ: 7.7–7.4 (m, 4H), 6.67 (d, 1H, J = 16 Hz), 6.36
(dt, 1H, J = 16 and 6 Hz), 4.35 (d, 2H, J = 6 Hz), 2.11 (s, 3
H). 13C NMR (62.5 MHz) δ: 170.9, 137.1, 132.5, 131.2 (q,
J = 32.3), 129.8 (q, J = 1.3), 129.2, 125.5, 124.7 (q, J = 3.7),
124.2 (q, J = 271), 123.4 (q, J = 4.0), 64.7, 21.0. HRMS
calcd. for C12H11F3O2: 244.0711; found: 244.0713.
Fluorescence measurements
All samples were degassed by three freeze-pump-thaw cy-
cles and then thermostated at 25 °C. Fluorescence intensity
measurements were done using a PTI-210M fluorescence
spectrometer with dual Model 101 monochromators, a 75W
Xenon lamp, and a Model 814 photomultiplier detector. Cor-
rected spectra were obtained.
Singlet lifetimes were measured by monitoring fluores-
cence decay using a PRA time-correlated single photon
counting apparatus with a hydrogen flash lamp with pulse
width of about 1.8 ns. The cell compartment was
thermostated at 25.0 °C with circulating water from a con-
stant temperature bath.
General synthetic procedure for substituted 3-aryl-3-
propenyl acetates 9a–9c
The aldehyde (9.5 mmol) in 100 mL of anhydrous ether
was cooled in an ice bath and 8.5 mL (8.5 mmol) of vinyl
magnesium bromide was added. The reaction mixture was
stirred for 3 h followed by the addition of acetyl chloride
(2.4 mL, 38 mmol) followed by further stirring for 20 h. The
reaction mixture was then poured into aq NaOH (5%) and
extracted with ether. The extract was washed with water and
saturated brine and then dried with magnesium sulfate. The
crude ester was then purified by column chromatography
with yield approximately 75%. Spectral data agreed with
those reported previously: X = H (24), X = 4-methoxy (25),
X = 3-methoxy (26).
General synthetic procedure for substituted (E)-
cinnamyl acetates 8
(E)-Cinnamyl acetate 8a is commercially available. For
the other acetates, the commercially available substituted
(E)-cinnamic acid was converted to its respective ester by
refluxing in 100% ethanol in the presence of a catalytic
amount of p-toluenesulfonic acid for 24–36 h. The isolated
ester was then treated with 3 equiv. of diisobutyl aluminum
hydride (DIBAL) in toluene in an ice bath. The reaction was
allowed to stir for 2 h. The reaction was quenched with aq
HCl (5%) and extracted with ether. The organic layer was
washed with aq HCl (5%), water, brine, and then dried. The
resulting alcohol was purified by recrystallization or chro-
matography to give the known X-substituted cinnamyl alco-
hols: X = 4-methoxy (18), X = 3-methoxy (19), X = 4-
trifluoromethyl (20), X = 3-trifluoromethyl (21). Treatment
of the alcohol with excess acetyl chloride in benzene then
purification by chromatography yielded the substituted (E)-
cinnamyl acetate.
Characterization of photoproducts from irradiation of
the cinnamyl esters 8
The following compounds were isolated from photolysis
mixtures by separation using column chromatography. Spec-
tra were compared with those from the literature. Other
photoproducts were not obtained in pure form but were iden-
1
tified by GCMS and analysis of H NMR of mixtures.
3-(4-Trifluoromethylphenyl)-3-propenyl acetate 9d (26)
1H NMR δ:7.62 (d, 2H, J = 8.5 Hz), 7.46 (d, 2H, J =
8.5 Hz), 6.28 (bd, 1H, J = 6 Hz), 5.98 (ddd, 1H, J = 17, 10.3
and 6 Hz), 5.30 (m, 2H, =CH2), 2.13 (s, 3H). GCMS m/z
(rel. intensity): 244 (1), 202 (11), 165 (49), 164 (19), 145
(29, 133 (31), 116 (24), 115 (100), 75 (14), 69 (25), 63 (13).
4-Methoxycinnamyl acetate 8b
1H NMR δ: 7.32 (d, 2H, J = 8.5 Hz), 6.85 (d, 2H, J =
8.7 Hz), 6.60 (d, 1H, J = 16 Hz), 6.15 (dt, 1H, J = 16 and
6.2 Hz), 4.69 (d, 2H, J = 6.5 Hz), 3.80 (s, 3H), 2.01 (s, 3H).
13C NMR (62.5 MHz) δ: 171.1, 159.8, 134.2, 129.1, 128.0,
121.0, 114.2, 65.5, 55.4, 21.2. HRMS calcd. for C12H14O3:
3-(3-Trifluoromethylphenyl)-3-propenyl acetate 9e
1H NMR δ: 7.7–7.4 (m, 4 H), 6.35 (bd, 1 H, J = 7 Hz),
5.94 (ddd, 1 H, J = 17, 10 and 7 Hz), 5.30 (m, 2H, =CH2),
2.12 (s, 3 H). GCMS m/z (rel. intensity): 244 (3), 202 (33),
© 2006 NRC Canada

Fleming et al.
1153
185 (12), 184 (20), 165 (38), 164 (19), 145 (22), 133 (24),
116 (202), 115 (100), 69 (12).
Acknowledgements
We thank Natural Sciences and Engineering Research
Council (NSERC) of Canada for financial support. SAF also
thanks the Department of Chemistry and Biochemistry,
Brigham Young University, for the leave necessary to pursue
this research.
3-Methoxy-3-phenylpropene 14a (27)
1H NMR δ: 7.5–7.2 (m, 5H), 5.93 (ddd, 1H, J = 17, 10.4
and 6.8 Hz), 5.23 (m, 2H, =CH2), 4.61 (bd, H, J = 6.8 Hz),
3.43 (s, 3H). GCMS m/z (rel. intensity): 148 (1), 116 (30),
115 (86), 91 (53), 89 (15), 77 (68), 63 (38), 62 (20), 51
(100).
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(Z)-3-Methoxy-1-(3-methoxyphenyl)propene 15c
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