J . Am. Chem. SOC.1988, 110, 8660-8664
8660
Photodimerization of Singlet trans- and cis-Anethole. Concerted or Stepwise? Frederick D. Lewis* and Masanobu Kojimat Contribution from the Department of Chemistry, Northwestern University, Evanston, Illinois 60208. Received June 9, 1988
Abstract: The singlet states of both tram- and cis-anethole are moderately long-lived (8.5 and 6.1 ns, respectively) and undergo photodimerization as well as cis,trans isomerization. Both the singlet lifetimes and fluorescence rate constants are larger than those for related phenylalkenes. Dimerization of singlet trans-anethole yields exclusively a syn head-to-head dimer with a rate constant of 3.2 X lo8 M-l s-l. Reaction of singlet tram-anethole with ground-state cis-anethole also yields a syn head-to-head dimer with a lower rate constant. Dimerization of singlet cis-anethole yields an anti head-to-head dimer with a rate constant of 0.8 X lo8 M-l s-l. Whereas the two former reactions occur with retention of anethole stereochemistry, dimerization of cis-anethole occurs with retention at one double bond and inversion at the other. Dimerization of cis-anethole is proposed to occur via sequential formation of a singlet exciplex with optimized *-orbital overlap and a singlet 1,Cbiradical intermediate.
Introduction
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The [2, 2,] cycloaddition reaction of a singlet olefin and ground-state olefin is a prototypical symmetry-allowed process.' There are numerous examples of stereospecific [2 21 photocycloaddition reactions, in accord with the simplest interpretation of covalent bond formation as a concerted process.2 For example, cycloaddition of singlet trans-stilbene with cis- or trans-2-butene3 and with dimethyl maleate or fumarate4 occurs with complete retention of olefin stereochemistry. A more stringent test of photocycloaddition stereospecificity would be posed by the reaction of singlet and ground-state cis olefins. With the exception of the photodimerization of ~ i s - 2 - b u t e n e ,attempts ~~ to observe such processes have been frustrated by competing cis trans photoisomerization and by the lower inherent reactivity of cis vs trans olefins as the singlet or ground-state component in photocycloaddition reaction^.^^^ We report here the results of our inves21 photodimerization reactions of singlet tigation of the [2 trans-anethole ( 1-(p-methoxyphenyl)propene, t-A) and cis-anethole (c-A). Whereas the reaction of singlet t-A with either ground-state t-A or c-A occurs with retention of stereochemistry, the dimerization of c-A occurs with complete inversion of stereochemistry a t one of the two double bonds.
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Results and Discussion Photophysics and Photoisomerization. Absorption and fluorescence data for t-A and c-A in hexane solution are reported in Table I. The absorption spectra of the trans and cis isomers exhibit maxima a t 258 and 254 nm and less intense long-wavelength shoulders at 300 and 290 nm (estimated from first derivative spectra). By analogy with the absorption spectra of the 1-phenylpropenes, the maxima are assigned to an allowed HOMO LUMO transition to a delocalized 'A, state and the shoulders to a forbidden benzene-localized transition to a 'B2 state.6 The higher energy and intensity of the A band for the cis vs trans isomer is attributed to nonplanarity of the cis isomer which results from nonbonded repulsion of the methyl and aryl groups.' Both t-A and c-A exhibit structureless fluorescence with maxima at 326 and 318 nm, respectively. Long-wavelength maxima in the fluorescence excitation spectra occur a t 297 nm for t-A and 288 nm for c-A. Fluorescence is presumed to originate from the low-energy 'B2 state, as is the case for the I-phenylpropenes6 Fluorescence lifetimes and quantum yields (Table 11) were obtained with dilute solutions ( M) in order to avoid self-absorption of fluorescence, which might result from overlapping absorption and emission spectra and shortening of the singlet lifetime resulting from self-quenching.* The singlet lifetimes of the anetholes are longer than that of ~ the singlet lifetime and ?runs-1-phenylbutene (4.3 n ~ ) . Both fluorescence quantum yield are larger for t-A vs c-A. However,
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'Visiting scholar on leave from Shinshu University, Matsumoto, Japan.
0002-7863/88/ 15 l0-8660$01.50/0
Table I. Absorption and Fluorescence Spectral Data for the Anetholes absorption fluorescenceo isomer 4" c A,, Af trans 258 19750 297 326 300 (sh) 1780 cis 254 18380 288 318 290 (sh) 1390 a Data for IO4 M hexane solution. Table 11. Kinetics and Isomerization Quantum Yields for Singlet and Triplet Anetholes" isomer 7.. ns Or 10-'kr O;(S)b 9;(TY trans 8.5 f 0.1 0.57 f 0.03 6.7 0.12 f 0.01 0.23 4.9 0.06f0.01 0.16 cis 6 . 1 f 0 . 1 0.30f0.02 Fluorescence data for M deoxygenated hexane solutions. *Quantum yield extrapolated to zero concentration for direct irradiation (313 nm) in deoxygenated acetonitrile solution. cValues for triplet-sensitized irradiation (365 nm) of the anetholes (0.05 M) with Michler's ketone (3 X lo-) M) in degassed acetonitrile solution. Table 111. Quantum Yields for Photoisomerization of the Anetholes"
ai concn, M 0.02
trans 0.119 f 0.005
cis 0.06 f 0.01
0.03
0.06 0.112 0.06 0.104 0.06 0.08 0.102 0.06 0.10 0.100 0.07 'Data for 3 13-nm irradiation of deoxygenated acetonitrile solutions. 0.04 0.06
the fluorescence rate constants, calculated from the measured singlet lifetimes and fluorescence yields (kr = @rr;'), are similar for the two isomers. The similarity of kf for the trans and cis isomers is consistent with the occurrence of fluorescence from a (1) Woodward, R. B.; Hoffman, R. The Conservation of Orbital Symmetry; Academic: New York, 1970. (2) (a) McCullough, J. J. Chem. Reu. 1987,87, 81 1. (b) Yamazaki, H.; CvetanoviC, R. J.; Irwin, R. S . J. Am. Chem. Soc. 1976, 98, 2198. (3) Chapman, D. L.; Lura, R. D.; Owens, R. M.; Plank, E. D.; Shin, S. C.; Arnold, D. R.; Gillis, L. B. Can. J. Chem. 1972, 50, 1984. (4) Green, B. S.; Rejt6, M.; Johnson, D. E.; Hoyle, C. E.; Ho, T.-I.; McCoy, F.; Simpson, J. T.;Lewis, F. D. J. Am. Chem. Soc. 1979, 101, 3325. (5) Caldwell, R. A.; Ghali, N. 1.; Chien, C.-K.; DeMarco, D.; Smith, L. J. Am. Chem. SOC.1978, 109, 2857. ( 6 ) (a) Rcckley, M. G.; Salisbury, K. J. Chem. SOC.,Perkin Trans. 2 1973, 1582. (b) Ghiggino, K. P.; Hara, K.; Salisbury, K.; Phillips, D. J. Chem. Soc., Faraday Trans. 2 1978, 20, 607. ( 7 ) Fueno, T.; Yamaguchi, K.; Naka, Y. Bull. Chem. SOC.Jpn. 1972, 45, 3294. (8) Condirston, D. A.; Laposa, J. D. Chem. Phys. Lett. 1979, 63, 313. (9) Bonneau, R. J . Am. Chem. SOC.1982, 104, 2921.
0 1988 American Chemical Society
J . Am. Chem. SOC.,Vol. 110, No. 26, 1988 8661
Photodimerization of Singlet trans- and cis- Anethole benzene-localized IB2 state.6 The values of k f (ca. 5 X lo7 s-I) are somewhat larger than the value for styrene (1.6 X lo7 s-l) A similar increase in kf is observed and other phenylalkene~.~J~ for anisole vs benzene (3.5 X lo7 s-I vs 2.4 X IO6 s-') and is attributed to the interaction of the unpaired electrons on oxygen with the benzene T electrons." Since the kf values for t-A and c-A are similar, the shorter lifetime of the cis isomer must result from more rapid nonradiative decay. Nonradiative decay of singlet styrene derivatives can occur via twisting of the double bond, internal conversion, and intersystem c r o ~ s i n g .Quantum ~ yields for isomerization (aPi) of t-A and c-A, eq 1, were measured for 0.02-0.10 M acetonitrile so-
lutions with 313-nm irradiation (Table 111). The measured value of ai for t-A decreases with increasing concentration. A value of ai = 0.12 f 0.01 is obtained by extrapolating the low-concentration data to zero concentration. The concentration dependence of ai,like the previously reported concentration dependence of r, for styrene, is presumably a consequence of excimer formation (vide infra).8 Values of ai for c-A proved more difficult to measure due to the stronger absorption of the trans vs cis isomer (e3l3 = 310 vs 30) and its larger isomerization quantum yield. Correction of the measured values for light absorption by t-A provides values of ai = 0.06 f 0.01 which are not noticeably dependent upon concentration. The photostationary state obtained upon irradiation of M t-A or c-A is 92 f 2% c-A, in reasonable agreement with the value of 95% calculated from the measured quantum yields and absorbance at 3 13 nm. Dimerization does not compete effectively with isomerization in M solutions. The lower quantum yield for isomerization of c-A vs t-A indicates that more rapid twisting about the double bond is not responsible for the shorter singlet lifetime of the cis isomer. Since the focus of this investigation was the singlet-state reactions of the anetholes, their triplet-sensitized reactions received only cursory attention. In agreement with the nonquantitative observations of Nozaki et aI.,'* we observed that triplet sensitization using Michler's ketone and 365-nm irradiation results in more efficient photoisomerization than direct irradiation and that prolonged triplet-sensitized irradiation fails to produce even trace amounts of dimers. The photostationary state obtained with Michler's ketone is 68 f 2% c-A. Quantum yields for tripletsensitized isomerization were determined for acetonitrile solutions containing Michler's ketone (3 X M) and t-A or c-A (0.05 M). The measured quantum yields for triplet-sensitized isomerization (Table 11) are substantially larger than those obtained upon direct irradiation. Since it has not been established that energy transfer is completely efficient under our reaction conditions, the measured values represent lower bounds for the actual triplet isomerization quantum yields. Larger quantum yields, but a similar photostationary state, were reported by Caldwell et al.I3 for the benzophenone-sensitized photoisomerization of fi-methylstyrene. While intersystem crossing followed by triplet isomerization could account for the isomerization observed upon direct irradiation, more extensive sensitization and quenching experiments would be necessary to test this possibility. The role of intersystem crossing in phenylalkenes remains a topic of controversy in the photochemical l i t e r a t ~ r e . ~Bonneaug ~ ~ ~ ' ~ reports a value of 3 x IO'S-' for the rate constant of intersystem crossing for several ( I O ) Zimmerman, H. E.; Kamm, K. S.; Werthemann, D. P. J . Am. Chem. SOC.1974, 96, 7821. ( I 1 ) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic: New York, 1971; p 71. (12) Nozaki, H.; Otani, I.; Noyori, R.; Kawanisi, M . Tetrahedron 1968, 24, 2192. (13) Caldwell, R. A,; Sovocool, G. W.; Peresie, R. J. J . Am. Chem. S o t . 1973, 95, 1496.
Table IV. Quantum Yields for Photodimerization of the Anetholes" concn, M (trans) a2(cis) 9, (cis) 0.015 0.01 0.02 0.037 0.0054 0.0040 0.03 0.0067 0.0046 0.04 0.06 0.08 0.10
0.010 0.016 0.021
0.064 0.085 0.099
0.0069 0.0088
0.010 0.013
0.027
Data for 3 13-nmirradiation of deoxygenated acetonitrile solutions. Table V. Quantum Yields and Kinetics for Photodimerization of the Anetholes' @Ab k7,M-' 10-8k., M-I s-I isomer trans 0.56 f 0.05 2.9 f 0.3 3.2 f 0.3 0.49 f 0.2 0.8 f 0.4 cis 0.56 f 0.1 1? Data for 3 13-nm irradiation of deoxygenated acetonitrile solutions. bLimiting quantum yield at infinite concentration. Scheme I t-A
hv
ll-Ai
1-A
kcp
lc1-A)2~
k,
knr
t -A
C-A
dimer
2 t-A
phenylalkenes. A similar value (5 X lo7 s-I) would be necessary in order for intersystem crossing to be the major nonradiative decay pathway for t-A. Photodimerization Products and Kinetics. As previously reported by Nozaki et al.,12 irradiation of 0.25 M t-A in cyclohexane solution results in the formation of the syn head-to-head dimer 1 as well as c-A (eq 2). Even at conversions >50%, the recovered (2) 1
An
anethole is largely (ca. 70%) t-A and the only dimer detected is 1. Similar results were obtained in cyclohexane and acetonitrile solution. Quantum yields for dimerization (Table IV) were determined for acetonitrile solutions of 0.01-0.10 M t-A with 313-nm irradiation. Conversions of t-A were
