The Norrish Type II photofragmentation of esters induced by intramolecular electron transfer [4]

Full text of DOI:10.1021/ja9840347

J. Am. Chem. Soc. 1999, 121, 3785-3786
3785
Scheme 1. Products (bottom) and Proposed Mechanism (top)
for Photolysis of 4-Cyanobenzoate Esters, 1, in Methanol
The Norrish Type II Photofragmentation of Esters
Induced by Intramolecular Electron Transfer
Dayal P. DeCosta,† Amy K. Bennett, and James A. Pincock*
Department of Chemistry, Dalhousie UniVersity
Halifax, NoVa Scotia, Canada B3H 4J3
ReceiVed NoVember 23, 1998
The classic Norrish Type II photofragmentation reaction of aro-
matic ketones played a significant role in the development of both
mechanistic and theoretical organic photochemistry; several
reviews are available.1 The established reaction mechanism pro-
ceeds by intramolecular hydrogen atom transfer to the carbonyl
oxygen from the γ carbon to form a 1,4-biradical that then
fragments to the observed products,2 eq 1, X ) CH2. When the
lowest energy excited state is of the n,π* triplet configuration,
the efficiency for biradical formation is essentially 100%.
In contrast, for aromatic esters, eq 1, X ) O, the reaction pro-
ceeds with very low efficiency. For instance, irradiation of a num-
ber of alkyl benzoates in acetonitrile does result in Norrish Type
II fragmentation from both the π,π* singlet and triplet state, but
the quantum yields average about 0.01 for the singlets and 0.001
for the triplets with the variation depending on the substituents
on the aromatic ring.3 Moreover, this inefficiency is not a result
of preferential disproportionation of the intermediate 1,4-biradical
back to starting material because a pair of diastereomers photo-
equilibrate in acetonitrile or pentane with a quantum yield (Φ )
0.008) that is even lower than that for fragmentation (Φ ) 0.01).4
This difference in reactivity between ketones and esters is ascribed
to the fact that the esters have lowest energy π,π* states so that
their carbonyl oxygen atoms are expected to be much less reactive
toward hydrogen atom abstraction. We now report that the Norrish
Type II fragmentation of esters can occur by intramolecular
proton transfer if intramolecular electron transfer to form a
radical ion pair occurs as the primary photochemical event.
As an extension of earlier work on arylmethyl ester (ArCH2-
O2CR) photochemistry,5 we have begun to examine arylethyl
compounds with leaving groups (ArCH2CH2-LG) in the hope of
obtaining a better understanding of â-cleavage reactions, a few
of which have been observed previously.6-11 To our surprise, the
esters, 1, reacted more efficiently than expected12,13 on irradiation
at 254 nm in methanol to give as the major14 volatile (GC)
products15 those shown in Scheme 1 along with 4-cyanobenzoic
acid. The yields, Table 1, indicate that at low conversions the
styrenes, 2a-c, are the major photoproduct and that the mass
balance is good. However, at higher conversions, the styrenes,
which are more strongly absorbing than 1, rapidly disappear by
secondary photochemistry and the mass balance drops, cor-
respondingly.16 These observations suggest (particularly for 1b)
that the styrenes and 4-cyanobenzoic acid are the major and
perhaps only primary photoproducts. This conclusion is reinforced
by irradiation, to complete conversion, of 2,2-diphenylethyl
4-cyanobenzoate, which gave 1,1-diphenylethylene in 80% yield,
along with 1,1-diphenylethane (6%) and the Markovnikov (8%)
and anti-Markovnikov (5%) ethers.
The secondary photolysis observed for the styrenes is of three
types: (1) Markovnikov addition of methanol to give 3, presum-
ably by the photochemical acid-catalyzed addition to alkenes17
with 4-cyanobenzoic acid, the other primary photoproduct, serving
as the acid catalyst;18 (2) anti-Markovnikov addition of methanol
to give 4 and 5, presumably by the PET mechanism19 through
the radical cation of the styrene, with either 4-cyanobenzoic acid
or the starting ester, 1, serving as the electron acceptor;20 and (3)
photooligomerization to nonvolatile products.
As summarized in Scheme 1, the primary photoproducts for
these reactions are analogous to those expected from the Norrish
† Present address: University of Colombo, Sri Lanka.
(1) (a) Wagner, P. J. In CRC Handbook of Organic Photochemistrty and
Photobiology; Horspool, W. M., Song, P.-S., Eds; CRC Press: New York,
1995; pp 449-470 and references therein. (b) Wagner, P. J.; Park, B.-S. In
Organic Photochemistry; Padwa, A., Ed.; Marcal Dekker: New York, 1991;
Vol. 11, pp 227-366 and references therein.
(12) The quantum yield for disappearance of 1a was 0.05, between 5 and
50 times higher than the previously reported values for the Norrish Type II
reaction of esters.
(13) The corresponding acetates of the 2-phenylethanols were photochemi-
cally inert.
(14) Minor amounts of ethylbenzenes, 3-aryl-1-propanols, and 1-aryl-2,2-
dimethoxyethanes were also formed.
(15) The structures were determined either by isolation of products from
the reaction mixtures or by independent synthesis. All compounds gave
satisfactory spectral analysis.
(2) The enol is usually isolated as the ketone tautomer but can often be
observed spectroscopically. The fragmentation products are often accompanied
by cyclobutanols (the Yang reaction).
(3) Barltrop, J. A.; Coyle, J. D. J. Chem. Soc. (B) 1971, 251-255.
(4) Gano, J. E. Chem. Commun. 1971, 1491-1492.
(16) The styrenes are also formed in acetonitrile, but they rapidly disappear
to form nonvolatile products, presumably oligomers.
(17) McEwen, J.; Yates, K. J. Phys. Org. Chem. 1991, 4, 193-206.
(18) The relative reactivity of styrenes by this pathway is 3-methoxy:4-
methoxy:hydrogen ) 72:26:0.8 and therefore 3 is formed most efficiently
from 1b.
(5) Pincock, J. A. Acc. Chem. Res. 1997, 30, 43-49.
(6) Jaeger, D. A. J. Am. Chem. Soc. 1976, 98, 6401-6402.
(7) Jaeger, D. A.; Bernhardt, E. A. Tetrahedron. Lett. 1983, 24, 4521.
(8) Cristol, S. J.; Mahfuza, B. A.; Sankar, I. V. J. Am. Chem. Soc. 1989,
111, 8207-8211 and references therein.
(9) Morrison, H.; Miller, A.; Bigot, B. J. Am. Chem. Soc. 1983, 105, 2398-
2408.
(19) Neunteufel, R. A.; Arnold, D. A. J. Am. Chem. Soc. 1973, 95, 4080-
4081.
(10) Bhalerao, V. K.; Nanjundiah, B. S.; Sonawane, H. R.; Nair, P. M.
Tetrahedron 1986, 42, 1487-1496.
(11) Abdel-Wahab, A. A.; Ismail, M. T.; Mohamed, O. S.; Khalaf, A. A.
Gazz. Chim. Ital. 1985, 115, 591-594.
(20) The radical cations of styrenes react by this pathway with a relative
reactivity of 4-methoxy:H ) 0.03:180; the 3-methoxy compound is expected
to react at a rate similar to that of the unsubstituted one. Therefore, the yield
of 4 and 5 from 1c is low.
10.1021/ja9840347 CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/03/1999

3786 J. Am. Chem. Soc., Vol. 121, No. 15, 1999
Communications to the Editor
Table 1. Yields of Products from the Photolysis of Esters 1 in
Methanol
Also relevant, in contrast to 1, are the photocleavage reactions
of several 2-phenethyl derivatives which have been studied pre-
viously. A good example is reported by Jaeger6 for the photolysis
of the mesylate, 6a, in aqueous methanol, eq 2. Moreover, pho-
product yields (%)a
compd
% conv
2
3
4
5b
total
1a: X ) H
11
24
85
7
27
66
4
20
75
52
26
2
16
24
22
3
33
48
1
3
5
7
7
9
2
6
5
7
5
6
75
57
33
91
88
92
62
29
24
1b: X ) 3-OCH3
1c: X ) 4-OCH3
84
34
18
44
7
2
15
21
10
14
13
0
tolysis of the deuterated isomer, 6b, demonstrated that although
the starting material had only partially isomerized to 6c (ratio of
6b:6c ) 6.1:1), the products 7 and 8 had the -CD2- group es-
sentially equally distributed between the two possible sites. These
results were rationalized by proposing competing photoinduced
cleavage to give 10 (major) and 11 (minor), or by 10 being a
transition state between equilibrating open 2-arylethyl cations.
An extension of this work by Cristol and co-workers8 using anal-
ogous diastereomeric mesylates demonstrated that diastereomeric
scrambling had occurred in the formation of the products. The
contrast in reactivity between the 4-cyanobenzoates, 1, and the
mesylate presumably results from two facts: the poorer leaving
group in 1 and the higher basicity of the 4-cyanobenzoate radical
anion.
a For instance, for 1a, at 11% disappearance of starting material,
the products 2-5 account for 75% of that 11%. b Mixture of two
diastereomers in about equal amounts.
Type II photofragmentation of ketones.1 Our proposed mechanism
begins with excitation to S1 followed by intramolecular electron
transfer to give a radical ion pair with the 4-cyanobenzoate group
being the electron acceptor. As expected for substrates that
undergo rapid intramolecular electron transfer in S1, compounds
1a-c do not fluoresce efficiently in methanol.21,22 The oxidation23
and reduction24 potential for esters 1a-c along with the singlet
excitation energy of 95 kcal/mol25 allow an estimate of the free
energy of electron transfer.26 These values are exergonic; -2,
-17, and -19 kcal/mol for 1a, 1b, and 1c, respectively. The
formation of the radical cation will increase the acidity of the
benzylic hydrogen (pKa ca. -1227) and radical anion formation
will increase the basicity of the carbonyl oxygen (pKa of the
conjugate acid ca. 828). The thermodynamic driving force for
proton transfer is therefore 20 pKa units, apparently enough to
overcome the kinetic barrier for proton transfer from carbon.
Exergonic back electron transfer is likely the dominant pathway
in competition with proton transfer. The resulting 1,4-biradical
then fragments to the two primary photoproducts.29 Heats of
formation for 1a (-52 kcal/mol30), styrene (+23 kcal/mol31), and
benzoic acid (-56 kcal/mol31) allow an estimate for the overall
enthalpy of the reaction at +19 kcal/mol. Consequently, the reac-
tion itself is endothermic but, after the initial excitation to S1,
the mechanism proceeds through a cascade of exothermic steps.32
A mechanism analogous to the one above has been proposed
for a number of other carbonyl compounds but, to our knowledge,
not for esters. For instance, both aryl alkyl ketones33 and N-alkyl
phthalimides34 with oxidizable substituents, such as amino,
hydroxy, and aryl groups on the alkyl chain, also react by an
electron transfer followed by proton transfer pathway.
Three other experiments have contributed to the development
of the proposed electron transfer mechanism for 1. (1) Irradiation
of 1d gave products deuterated only on the carbons expected for
the mechanism in Scheme 1. No scrambling was observed as
required if intermediates such as 10 or equilibrating open cations
were involved. (2) Compounds 1c and 1e reacted with relative
efficiencies of 1.3:1. This small isotope effect is reasonable if
the exothermic proton transfer in the intramolecular charge transfer
complex is at least partially rate determining. (3) Photolysis in
methanol of 12, prepared from the known racemic (2S,3R):
(2R,3S)-3-phenyl-2-butanol,35 indicated inefficient formation of
its diastereomer.36 Therefore, fragmentation of the biradical in
methanol is much faster than disproportionation back to starting
material. The same is true for ketones in protic solvents.
In conclusion, we have discovered that a classic photochemical
reaction, the Norrish Type II fragmentation, can occur for suitably
substituted esters. Future experiments will be directed at exploring
the scope and efficiency of this process.
(21) For instance, in methanol, the relative fluorescence emmission intensity
at 283 nm for toluene (ΦF ) 0.13):1a is 175:1. In addition, 1a has a weak,
long-wavelength, exciplex emission band. We are in the process of obtaining
absorbance and both steady-state and time-resolved fluorescence spectra on
these compounds as a function of solvent polarity. Electron transfer in
analogous pyrenyl esters has been studied in some detail by fluorescence
measurements but no reaction of the esters was reported.22
(22) Kawakami, J.; Iwamura, M. J. Phys. Org. Chem. 1994, 7, 31-42.
(23) Reversible, in volts versus SCE: 1.72 and 1.62 for 1b and 1c,
respectively. A value of 2.4 V for 1a is estimated from the value given for
toluene: Eberson, L. In Electron-Transfer Reactions in Organic Chemistry;
Springer-Verlag: Berlin, 1987; p 44.
Acknowledgment. We thank NSERC of Canada for financial support
and A. L. Pincock for technical help. D.P.D. thanks the University of
Columbo, Sri Lanka, for sabbatical leave and a travel grant.
JA9840347
(24) Irreversible, in volts versus SCE: -1.65, -1.66, and -1.66 for 1a,
1b, and 1c, respectively.
(25) The long-wavelength absorption is mainly the 4-cyanobenzoate
chromophore, particularly for 1a.
(32) Preliminary results for the corresponding intermolecular reaction
support this proposed mechanism. For instance, irradiation in methanol of
4-methoxyphenylethane and methyl 4-cyanobenzoate resulted in even more
rapid disappearance of the former than for 1c. The products, derived from
the intermediate intermolecular radical pair, include 4-methoxystyrene, 2c (and
its Markovnikov methanol addition product), and three radical coupling
products, methyl 4-(4-methoxyphenyl-1-ethyl)benzoate and the two diaster-
eomers of 2,3-bis(4-methoxyphenyl)butane.
(26) The Coulombic term, which will make the values more negative, was
not included because its magnitude will depend on the conformation adopted
for the radical ion pair. See ref 22.
(27) Nicholas, A. M. P.; Arnold, D. R. Can. J. Chem. 1982, 60, 2165.
(28) Hayon, E.; Simic, M. Acc. Chem. Res. 1974, 7, 114-121. This estimate
of 8 for the pKa is made on the basis of the value of 5.5 reported for the
conjugate acid of the radical anion of methyl benzoate and a plot of the change
in pKa as a function of reduction potential.
(29) GC/MS analysis revealed two very weak peaks (∼1% yield) isomeric
with the starting ester. These are possibly products resulting from cyclization
of the 1,4-biradical.
(30) Estimated from Benson’s rules.
(31) Calculated from the heat of combustion.
(33) Reference 1b, p 337.
(34) Griesbeck, A. G.; Henz, A.; Hirt, J.; Platschek, V.; Engel, T.; Loffler,
D.; Schneider, F. W. Tetrahedron, 1994, 50, 701-714. We thank a referee
for informing us of this work.
(35) Cram, D. J. J. Am. Chem. Soc. 1952, 74, 2129-2137.
(36) At 84% conversion, the ratio of disappearance of 12 to formation of
the diastereomer was about 50:1.