1519 by a Vidar 6210 digital integrator when possible and by Disc or Triangulation.
Acknowledgment. We wish to thank the National Science Foundation and the Alfred P. Sloan Foundation for financial support of this work. References and Notes (1) Taken in part from the Ph.D. theses of C. N. Shih and A. Yeshurun, lndiana University, January 1972 and January 1973, respectively. Supported in part by the National Science Foundation Grant No. GP 27621 and the Alfred P. Sloan Foundation. (2) Fellow of the Alfred P. Sloan Foundation, 1971-1973. (3) (a) J. J. Gajewski and C. N. Shih, J. Amer. Chem. SOC., 92, 4457 (1970): (b) ibid.. 89, 4532 (1967); (c) ibid.. 91, 5900 (1969); (d) [bid..94, 1675 (1972): (e) W. von E. Doering and W. R. Dolbier, ibid., 89, 4534 (1967). (4) R. Srinivasan, J. Amer. Chem. SOC.,84, 4141 (1962). (5) R. Srinivasan. J. Amer. Chem. SOC.,85, 4045 (1963). (6) R. Srinivasan, J. Amer. Chem. Soc., 90, 4498 (1968). (7) S.Boue and R. Srinivasan, J. Amer. Chem. Soc., 92, 3226 (1970). (8) K. lnuzuka and R. S.Becker, Bull. Chem. SOC.Jap., 44, 3323 (1971). (9) D. H. Aue and R. N. Reynolds, J. Amer. Chem. SOC., 95,2027 (1973). (10) W. G. Dauben, R. L. Cargill, R. M. Coates, and J. Saltiel, J. Amer. Chem. SOC.,88, 2742 (1966). (11) W. G. Dauben, et a/.. Pure Appl. Chem., 33, 197 (1973). (12) W. J. Nebe and G. J. Fonken, J. Amer. Chem. SOC.,91, 1249 (1969). (13) W. G. Dauben and R. S. Ritscher. J. Amer. Chem. Soc., 92, 2925 (1970). (14) W.G.'Dauben, C. D. Poulter, and C. Sutzer, J. Amer. Chem. SOC.,92, 7408 (1970).
(15) W. G. Dauben and C. D. Poulter, Tetrahedron Lett.. 3201 (1967). and references therein. (16) E. F. Kiefer and C. H. Tanna, J. Amer. Chem. Soc., 91, 4478 (1469). (17) (a) J. SaRil, L. Mens, and M. Wrighton, J. Amer. Chem. Soc.,93, 550 (1971): see also (b) J. Saltiel, eta/., Org. Photochem., 3, 79 (1973). (18) R. Srinivasan and S. Boue, J. A w r . Chem. Soc., 93, 550 (1971). (19) I. Haller and R. Srinivasan, J. Chem. Phys.,40, 1992 (1964). (20) R. Srinivasan. J. Chem. Phys., 38, 1039 (1963). (21) R. Srinivasan, Advan. Phofochem., 4, 113 (1966). (22) W. G. Dauben in "Reactivity of the Photoexcited Molecule" (Proceedinos of the 13th Solvay Conference on Chemistry), Interscience, New York, N.Y., 1967, p 17i. (23) N. J. Turro, "Molecular Photochemistry," W. A. Benjamin. New York. N.Y., 1965. (24) J. Saltiel, L. Metts, and M. Wrighton, J. Amer. Chem. Soc., 91, 5084 ( 1969). (25) J. Saltiel, L. Metts, A. Sykes, and M. Wrighton, 93, 5302 (1971). (26) P. Dowd, J. Amer. Chem. Soc., 92, 1066 (1970). (27) G. S. Hammond, Advan. fhotochem., 7, 1 (1969). (28) W. Th. P. M. van der Lugt and J. Oosterhoff, J. Amer. Chem. Soc., 91, 6042 (1969). (29) (a) J. 1. Brauman, W. E. Farneth, and M. 8. D'Amore, J. Amer. Chem. Soc., 95, 5043 (1973); (b) G. P. Andrews, M. Davalt. and J. E. Baldwin, ibid., 95, 5044 (1973). (30) M. C. Flowers and H. M. Frey, J. Amer. Chem. SOC.,94, 8636 (1972). (31) W. T. Borden. L. Sharpe, and I. L. Reich, Chem. Commun., 461 (1970). (32) The fact that DMCB does not close to a cyclobutene makes it a unique cisoid diene. Aue8 has suggested that ring strain plays an important role here. It should be noted that this interesting olefin has been prepared by K. B. Wiberg, G. J. Burgmaier, and P. Warner, J. Amer. Chem. Soc., 93, 246 (1971). (33) For a discussion of this and other propelbnes, see W. D. Stohrer and R. Hoffman, J. Amer. Chem. Soc., 94, 779 (1972). (34) S. W. Benson, "The Foundations of Chemical Kinetics." McGraw-Hill, New York, N.Y., 1960, Chapters 10 and 11.
Photochemical a Cleavage of Benzoin Derivatives. Polar Transition States for Free-Radical Formation' F. D. Lewis,*2s R. T. Lauterbach,zbH.-G. Heine,*2c W. Hartmann? and H. Rudolphzc Contributionfrom the Department of Chemistry, Northwestern University, Evanston, Illinois 60201, and the Zentralbereich Forschung, WissenschaftlichesHauptlaboratorium der Bayer AG, 415 Krefeld-Uerdingen, Germany. Received July 5, 1974
Abstract: The photochemical a cleavage reactions of benzoin, benzoin ethers, benzoin acetate, and several related phenyl ketones have been investigated. CY cleavage is the only primary process observed for benzoin and the benzoin alkyl ethers in benzene solution. /3 cleavage is a minor competing reaction for benzoin phenyl ether. Substituents at the a carbon have little effect on the efficiency (quantum yield) of cleavage; however, they have a pronounced effect on the rate constant for cleavage. Benzoin alkyl ethers are about lo5 times more reactive toward CY cleavage than deoxybenzoin. Substituents capable of stabilizing an adjacent positive charge are far more effective in accelerating CY cleavage than are substituents capable of stabilizing free radical centers. It is concluded that the transition state for CY cleavage has considerable ionic character and does not resemble the free radical products.
B e n z ~ i n , ~benzoin -~ ether^,^-^ and benzoin estersI0 undergo photochemical a cleavage to form a benzoyl-substituted benzyl radical pair (eq 1). The photochemical a cleavage of benzoin acetate is a relatively slow triplet-state reactiont0 as is the a cleavage of deoxybenzoin ( k , = 1.6 X IO6 sec-l).'l In contrast, the a cleavage of benzoin ethers
,R
Ph
cannot be quenched by standard triplet q ~ e n c h e r s ' - ~and must be either an exceptionally rapid triplet process ( k , > l o t o sec-I) or a singlet process which competes efficiently with intersystem crossing.'* y-Hydrogen abstraction (eq 2 ) , which is the predominant photochemical reaction for a-alkoxyacetophenones (k, > lo9 sec-I),l3 does not compete with the a-cleavage reaction of benzoin Benzoin and benzoin ethers are widely used as initiators for photopolymerization.I4 Therefore elucidation of structure-reactivity relationships for the a cleavage of benzoin and its derivatives has practical as well as mechanistic consequences.
Results and Discussion Benzoin Ethers. Irradiation of the benzoin ethers 1-4 in benzene solution results in the formation of benzaldehyde, benzil, and an equimolar mixture of diastereomeric pinacol ethers. Isolated yields of the pinacol ethers are 60-70%. All Lewis et al.
/
Polar Transition States for Free-Radical Formation
1520 of the products can be accounted for by photochemical cy cleavage followed by free-radical reactions of the benzoylalkoxybenzyl radical pair (Scheme I). There is no evidence Scheme I
Table 1. Quantum Yield and Kinetic Data for oi Cleavage of PhCOCRHPh
1
0.44
3
0.44 0.47b 0.39
5
R1 5 CH?
R' H
3, zC,H; C2H5 H 27
4' 5, P h
0.1 Ze
io3
CH,
I 1 RCN=NCRb I /
0
I
CH,
CH3
I I
CH3
CH,
CH3
I
I
RCN=NCR I
CH,
--+
A
+
2RC
N,
(12)
I
CH3
tochemical a cleavage and perester thermolysis are similar. The acetoxy group is weakly activating, in contrast to its deactivating effect on azoalkane thermolysis. Alkoxy and phenoxy substituents also have a much larger rate-enhancing effect for a cleavage and perester thermolysis than for azoalkane decomposition. Alkoxy substituents are known to stabilize adjacent carbonium ion centers. For example, chloromethyl ethyl ether undergoes solvolysis in aqueous dioxane lo9 times more rapidly than n-butyl chloride.34Alkoxy groups have a much smaller effect on free-radical stabil i t i e ~ The . ~ ~effect ~ of an a-cyano group on a cleavage and azoalkane thermolysis also is in accord with ionic and freeradical transition states, respectively, for these reactions.35 The effect of an a-phenyl substituent on a cleavage may be underestimated by comparing a-phenyl and a-methyldeoxybenzoin." However, a comparison of the rate constants for a cleavage of a-tert-butoxyacetophenone ( 6 ) and deoxybenzoin" reveals that an a phenyl is less effective than an a alkoxy1 in stabilizing the transition state for a cleavage. Once again, this result is the same as that for perester thermolysis. The analogous effects of substituents on photochemical a cleavage and perester thermolysis can be used to provide an estimate of the rate constant for benzoin ether triplet a cleavage. The linear free-energy relationship for the two reactions is shown in Figure 1. Extrapolation of the available data using k , = 2.1 X lo7 sec-' for a-methyldeoxybenzoin" provides a value of k , 1O'O sec-' for benzoin methyl ether. The effects of substituents on photochemical y-hydrogen abstraction of y-substituted butyrophenones (eq 13) have been investigated by Wagner and K e m ~ p a i n e n(Table ~ ~ 11). Their results are similar to those previously observed by
March 19, 1975
-
1523
4
log ktherrn
(re].)
,
1
tones1' and c y c l ~ a l k a n o n e sdemonstrate ~~~ that rate constants do not depend upon the stability of the radical pair. tert-Alkyl ketones undergo a cleavage a t least an order of magnitude faster than the corresponding benzyl ketones. Evidence for a transition state with a moderate amount of ionic character was provided by the effect of nonconjugated aromatic substituents on the rate constants for deoxybenzoin a cleavage.Ib The rate constants give an excellent fit to the Hammett equation with the use of u+, indicative of the development of partial positive charge at the a carbon (eq 15). Further evidence for the importance of polar effects on
OCH,
i
hu
PhCOCH,Ar
0
6-
[PhCO.
-
6* a
*
CH,Ar]
--+
PhCO
-2
+/ CN
2
1
0
3
log kl ( r e l . )
Figure 1. Linear free-energy relationship for photochemical a cleavage and perester thermolysis (Table 11).
+
CH,Ar (15)
photochemical a cleavage is provided by the present study. Substituents capable of stabilizing an adjacent positive charge are the most effective in accelerating a cleavage. Finally, the effects of substituents upon photochemical a cleavage are readily understood using ground-state concepts. Photochemical a cleavage and perester thermolysis (eq 1 1)32 display remarkably similar structure-reactivity relationships.'b,ll Since a cleavage of a 3n,7r* state to form a triplet diradical intermediate is an adiabatic process,39 a transition-state model for the reaction seems justified.
Experimental Section Walling and coworkers3' for intermolecular hydrogen abstraction by tert-butoxy radical (eq 14). The effects of a-
t-BuO.
+
I I
HCR
-----t
t-BuOH
I + .CR
(14)
I
-
oxygen substituents upon photochemical a cleavage and yhydrogen abstraction are similar (OCH3 > OH OPh >> OAc). This trend reflects the availability of the nonbonded electrons on o ~ y g e n .The ~ ~two ? ~ reactions ~ differ in that acetoxy is rate enhancing for a cleavage and rate retarding for y-hydrogen abstraction. I n addition, the magnitude of the substituent effects for a cleavage or perester thermolysis is much greater than for hydrogen abstraction. W e conclude that there is considerably more ionic character in the transition state for photochemical a cleavage than for yhydrogen abstraction. Benzoin and its derivatives are commercially important as initiators for photopolymerizations. l 4 Free-radical polymerization is initiated by the radicals formed upon a cleavage. The effects of a substituents upon reactivity of benzoin derivatives in the light-induced curing of unsaturated polyester resinsI4 parallel a-cleavage reactivities ( O C H 3 > OPh > OH >> OAc). In degassed benzene solution, all of the benzoin derivatives form free radicals with comparable efficiency and might be expected to initiate polymerization equally well. However, under the conditions used for photopolymerization quenching by the high concentrations of vinyl monomer becomes important. Thus the benzoin derivatives with the shortest triplet lifetimes are the most efficient initiators.
Conclusions Prior to our investigation of the a-cleavage reactions of aryl alkyl ketones,Ib.l1 it was assumed that photochemical reactivity was determined by the stability of the radical-pair produced.38 The effects of a-methyl and a-phenyl substituents on the rate constants for a cleavage of alkyl phenyl ke-
Ketones. Benzoin alkyl ethers 1-3 were prepared by the method of Fischer40 and purified by crystallization from hexane-ether: 1, mp 49-51' (lit.40 mp 49-50'); 2, mp 62-63' (lit.40 mp 62'); 3, mp 78-79' (lit.41 m p 72-75'), a-Methylbenzoin methyl ether (4), amethylbenzoin (lo), and a-n-butylbenzoin (11) were prepared and purified as previously described.42 Benzoin phenyl ether (5) was synthesized from desyl chloride and and recrystallized from ethanol, mp 87' (lit.43 mp 85'). a-tert-Butoxyacetophenone ( 6 ) was synthesized from a-diazoacetophenone and tert-butyl al~ o h o and l ~ purified ~ by chromatography on silica gel and distillation, bp 80-85' (2 mm). Benzoin acetate was prepared by the method of Corson and Saliani,45a mp 82' (lit.45a mp 81.5-82.5'). a-Hydroxymethylbenzoin (12) was synthesized by aldol condensation of benzoin (8) with formaldehyde and recrystallized from carbon tetrachloride, mp 84-85' (lit!5b mp 85-86'), a-Cyanodeoxybenzoin (13) was synthesized by the method of Chase and Walker46 and recrystallized from ethanol, mp 98-99O (lit.46 mp 949.5'). - a-Dimethylaminodeoxybenzoin (14) was prepared from desyl chloride and dimethylamine and isolated as the hydrochloride, mp 230' (lit.47mp 232-234'). The free amine 14 was liberated with aqueous sodium hydroxide and extracted into benzene prior to spectroscopic and photochemical studies. Absorption and emission spectral data for ketones 1-14 are given in Table III.48 No room temperature emission was observed for the benzoin alkyl ethers.x Photolysis of Benzoin Methyl Ether (1). A solution of 3.4 g of ketone in 300 ml of benzene was irradiated for 6 hr under nitrogen in a water-cooled Pyrex apparatus with a Philips 125-W high-pressure mercury lamp. Solvent and benzaldehyde were removed by distillation, and the residue was separated by chromatography on silica gel (benzene) to yield 0.15 g of benzil (10%) and 1.3 g of a 1 : 1 mixture of pinacol ethers l a and l b (72%). Rechromatography of the pinacol ethers gave the meso pinacol ether la, mp 140-141 ' mp 139-140'), and the d,/ pinacol ether lb, mp 92-93O (lit.49 90-91°), both of which were identical with authentic samp l e ~ . ~ ~ Photolysis of Benzoin Ethyl Ether (2). Irradiation of 3.6 g of ketone in 300 ml of benzene for 6 hr followed by work-up similar to that described for 1 afforded 0.2 g of benzil (13%), 0.65 g of 2a (32%), mp 62-63', and 0.6 g of 2b (30%). mp 69-70', Meso pinacol ether 2a was identical with an authentic sample prepared by alkylation of m e s o - h y d r ~ b e n z o i nN . ~M ~ R (CDC13) for 2a: 6 0.94 (t, 6 H, J = 7 Hz), 2.83-3.65 (m, 4 H), 4.30 (s, 2 H), 7.09-7.58 (m, I O H). Anal. Calcd for C18H2202: C , 79.96; H , 8.20; 0, 11.84. Found: C, 79.80; H, 8.30; 0, 12.20. d,/ pinacol ether 2b was identical with an authentic sample prepared by alkylation of d,/-hydro-
Lewis et al.
/
Polar Transition States for Free-Radical Formation
1524 Table Ill. Spectral Data for Ketones 1-14
measurements, and Stern-Volmer kinetics have been previously described.' I 7,
nma E E T , kcallmolb msecC 345 (2.20) 71.5 345 (230) 71.5 345 (235) 71.5 4 344 (185) 71.5 5 342 (275) 72.5 3.0 6 340 (sh) 7 329 (242) 8 340 (sh) (120) 73.4 9.1 9 345 (sh) (167) 10 344 (167) 72.4 2d 11 340 (sh) (110) 72.2 ld 12 340 (150) 71.1 2.8 14 336 (131) UWavelength of n,n* absorption maximum in cyclohexane at room temperature. 'Estimated from the wavelength of the highest energy-emission maximum at 77 K in ether-ethanol. CLifetime of 77 K emission in methylcyclohexane for 5 and in ether-isopentane for 8-1 1. dApproximate values from low-intensity emission. Ketone ,,,,A 1 2 3
b e n z ~ i n . ~N'M R (CDC13) for 2b: 6 1.13 (t, 6 H, J = 7 Hz), 3.43 (q' H' = Hz)' 4'42 (" H)' 6'92-7'33 (m' l o Found for C18H2202: C, 80.05; H, 8.09; 0, 11.90. Photolysis of Benzoin Isopropyl Ether (3). Irradiation of 3.81 g of 3 in 300 ml of benzene for 5 hr gave after work-up 0.1 g of benzil o'8 g Of pinacol ether 3a (36%)3 mp and 0'7 g of d,l pinacol ether 3b (32%), mp 37-38O. Pinacol ether 3a was characterized by its N M R spectrum and elemental analysis. N M R (CDC13) for 3a: 6 0.73 (d, 6 H , J = 6 Hz), 0.90 (d, 6 H, J = 6 Hz)* 3.30 (sept9 H$ = Hz)9 4.25 7.08-7.60 = 10 Hz). Anal. Calcd for C2oH2602: c , 80.49; H , 8.78; 0, 10.72. Found: C, 80.50; H, 8.97; 0, 10.75. d.1 pinacol ether 3b: N M R (CDCI,), 6 1.02 (d, 6 H , J = 6 Hz), 1.08 (d, 6 H , J = 6 Hz), 3.55 (sept, 2 H, J = 6 Hz), 4.43 (s, 2 H), 7.10 (s, 10 H). Anal. Found for C2OH2602: C , 80.55; H, 8.78; 0, 10.85. Photolysis of a-Methylbenzoin Methyl Ether (4). Irradiation of 3.6 of 4 i n 300 ml of benzene for 6,5 hr followed by repeated chromatography on silica gel (hexane) afforded 2.5 g of a 1:l mixture of 4a and 4b (62%). Fractional crystallization from acetone yielded pure 4a, mp 170- 17 1 O (lit.s2 17 1- 173O), identified by comparison with an authentic sample. N M R (CDCI3) for 4a: 6 1.42 (s, 6 H), 3.01 (S, 6 H), 7.25 (5. 10 HI. The isomeric PjnacOl ether 4b was obtained after tedious fractional crystallization from ether-hexane, mp 114-1]60, N M R ( c D C I ~ )for 4b: 6 1.65 (s, 6 H), 3.10 (', 6 6.70-7.40 (m, l o H). Found for C18H2202: c , 80.00; H, 8.28; 0, 11.97. Photolysis of Benzoin Phenyl Ether (5). Irradiation of 3.4 g of 5 in 300 ml of benzene for 6 hr resulted in the formation of benzaldehyde, benzil, phenol, and a mixture of pinacol ethers 5a and 5b. Chromatography on silica gel (hexane-benzene) afforded 1.4 g of 5a and 5b (65%) and 0.1 1 g of benzil (9%). Fractional crystalliof 5a, mp 172-1730. zation from acetone yielded a pure N M R (CDC13) for Sa: 6 5.41 (S, 2 HI. 6.70-7.45 (m, 20 HI. Anal. Calcd for C26H2202: C , 85.21; H , 6.05; 0, 8.73. Found: C, 84.95; H, 5.95; 0, 8.53. Pure 5b could not be obtained by fractional crystallization; however, slow chromatography using a high excess of silica gel afforded a Pure Sample of 5b, mp 150-152°. N M R (CDC13) for 5b: 6 5.48 (s, 2 H ) , 7.15 (s, 10 H ) , 6.70-7.35 (m, 10 H). Anal. Found for C26H2202: C, 84.99; H, 6.25; 0, 8.86. Photolysis of Benzoin (8). Irradiation of 3.18 g of 8 in 300 rnl of benzene for 6 hr resulted in only 28% conversion. VPC showed benzaldehyde and benzoin benzoate were the only volatile products formed. Column chromatography on silica gel afforded 2.1 g of unreacted 8 (66%) and 0.21 g of benzoin benzoate (13% based on benzoyl radical, eq 9). Photolysis of a-Alkylbenzoins 10-12. Irradiation of 3.4 g of 10 in 300 ml of benzene followed by distillation of solvent, benzaldehyde, and acetophenone and chromatography on silica gel afforded 0.23 g of benzoin benzoate (15%) and 0.15 g of benzoin (9%). No 1-phenylethanol or acetophenone pinacol could be detected by VPC, TLC, or N M R analysis. Analogous results were obtained for 11 and 12. General Procedures for purification of materials, quantum yield (7%)3
82-8303
(rn7
('3
Journal of the American Chemical Society
/
97.6
/
Acknowledgment. The authors a t Northwestern thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, and PPG Industries for support of this research. W e gratefully acknowledge the phosphorescence spectra provided by Professor H.-H. Perkampus, University of DGsseldorf, and Professor H.-D. Scharf, University of Aachen. References and Notes ( 1 ) (a) Part Vi\: Photocherliical a Cleavage of Ketones in Solution. (b) Part Vi: F. D. Lewis. C. H. Hoyle. J. G. Magyar, H.-G. Heine. and W. Hartmann, J. Org. Chem., in press.
(2) (a) Camille and Henry Dreyfus Teacher-Scholar, 1973-1978; (b) PPG Industries Fellow, 1971-1972, National Science Foundation Trainee, 1972-1974; (c) Bayer AG. (3) G. Kornis and P. deMayo, Can. J, Chem., 42, 2822 (1964). (4) (a) M, Cocivera and A, M, Trozzolo, Am, them, sot,, 92, 1772 (1970);(b) G. L. Closs and D. R. Paulson, ibid.. 92, 7229 (1970). (5) A. Ledwith. P. J. Russell, and L. H. Sutcliffe. J. Chem. SOC., Perkin Trans. 2, 1925 (1972). (6) T. Dominh, hd. Chim. Belge., 36, 1060 (1971). (7) H.-G. Heine, Tetrahedron Lett., 4755 (1972). (8) S. P. Pappas and A. Chattopadhyay, J. Am. Chem. SOC., 95, 6484 (1973). (9) (a) M. R. Sandner and C. L. Osborn, Tetrahedron Lett,, 415 (1974); (b) S. Adam, Dissertation, University of Karlsruhe, 1973. (IO) (a) J. C. Sheehan and R. M. Wilson, J. Am. Chem. Soc.. 86, 5277 (1964); (b) J. C. Sheehan, R. M. Wilson, and A. W. Oxford, ibid.. 93, 7222 (1971). J,
( 1 1 ) H,-G, Heine, W, Hartmann, D, R, Kory, J, G,Magyar, C,E, Hoyle, J, K, McVey, and F. D. Lewis, J. Org. Chem., 39, 691 (1974). (12) The rate constant for intersystem crossing from the lowest vibronic level of benzophenone is 2 X 10" sec-'. P. M. Rentzepis and G. E. Busch, Mol. Photochem.. 4, 353 (1972). (13) F. D. Lewis and N. J. Turro, J. Am. Chem. SOC., 92, 31 1 (1970). (14) H.-G. Heine, H.-J. Rosenkranz, and H. Rudolph, Angew. Chem., ht. Ed. Engl., 11, 974 (1972). (15) F. D. Lewis and J. G. Magyar, J. Am. Chem. SOC., 95, 5973 (1973). (16) The quantum yield for pinacol ether formation has been reported to depend upon excitation wavelength.* in view of the rapid rate constants for a cleavage. free radical reactions subsequent to a cleavage must be responsible for the reported wavelength dependence. (17) P cleavage is the major primary process for desyl halidestoa and SUIfides.18 The &cleavage process is under further investigation in our laboratories. (18) (a) A. Schonberg, A. K. Fateen, and S.M. A. R. Omran, J. Am. Chem. SOC.,78, 1224 (1956); (b) J. R. Collier and J. Hill, Chem. Commun., 640 (1969);(c) Y. Saburi. K. Minami, and T. Yoshimoto, Nippon Kagaku Zasshi, 88, 557 (1967). (19) The quantum yield for benzaldehyde formation in the absence of thiol is -0.02, thus explaining previous failure to observe its f ~ r m a t i o n . ~ (20) E. J. Baum, L. D. Hess, J. R. Wyatt, and J. N. pi%. Jr., J. Am. Chem. SOC.,91, 2461 (1969). (21) Benzoin could be formed via type II photoelimination from 9. However, efforts to detect 1-butene, the other product expected from photoeiimination, were unsuccessful.
(24)
~rJ;w~~~;d:'&y~ ~ & & ~ ~ ~ , ( ~communication^ ~ ~ ~ ; a J.
S. Bradshaw, R. D. Knudsen, and W. W. Parish,
l
J. Chem. SOC.,
Chem. Commun., 1321 (1972). (25) D. L. Bunbury and T. T. Chuang, Can. J. Chem., 47,2045 (1969). (26) F. F. Rust, F. H. Seubold, and W. E. Vaughan, J. Am. Chem. Sot., 70, 3258 (1948). (27) (a) R . G. Zepp and P. J. Wagner, J. ~ m Chem. . Soc., 92, 7466 (1970); (b) N. J. Turro and T. J. Lee, ibid.. 92, 7467 (1970); (c) M. E. Long, B. Bergman, and E. C. Lim, Mol. Photochem., 2, 341 (1970); (d) A. Padwa and A. Au, J. Am. Chem. Soc., 96, 1633 (1974); (e)A. Padwa and G. A. Lee, ibid., 96, 1634 (1974). (28) G. Rio and D. Masure, BUI. SOC. Chim. Fr., 3232 (1971). (29) P. J. Wagner, A. E. Kemppainen, and T. Jellinek, J. Am. Chem. SOC., 94, 7512 (1972). (30) The maximum errors in k, resulting from this approximation (
