6998 Table VII. Solvolysis of p-Nitrophenyl Palmitate Catalyzed by Imidazole and/or Phenol-Containing Compounds in 30 (v/v) 1-Propanol-Water at 26", Ionic Strength 0.02
Catalyst HPNI MPNI p-Cresol Imidazole
-koa$, 7.08
l./mol min, at pH7.90 8.98
7.20
9.61
10.86
2.15
2.74
2.08
9.97 12.56 14.58 1.10 2.43
The results in Table VI1 indicate that imidazole and p-cresol are less efficient in catalyzing the solvolysis of a long-chain substrate than a short-chain substrate (Table VI). The decrease in catalyst efficiency can be attributed to steric hindrance caused by the long aliphatic chain located at the acyl portion of the substrate. The imidazole-catalyzed hydrolysis of esters of p-nitrophenol have been reported to be subject to steric hindrancea26 The steric hindrance should be even larger
if the long-chain compounds HPNI and MPNI are used as the catalyst. In spite of more severe steric crowding, the MPNIor HPNI-catalyzed solvolysis of p-nitrophenyl palmitate (PNPP) was more efficient (relative to imidazole) than the solvolysis of p-nitrophenyl acetate (PNPA) as previously expected. Polyoxyethylene (23) lauryl alcohol was reportedz7 not to form micelles in aqueous solutions which contain 2 5 x (by volume) or more of an additive, such as ethanol or 1,4-dioxane. The small observed rate enhancement during the solvolysis of PNPP catalyzed by long-chain compounds can, therefore, be attributed to a hydrophobic attraction between the substrate and the catalyst. Acknowledgment. The authors are grateful for financial support by the National Institutes of Health under Grant No. GM-15256. (26) T. F. Fife, J . Amer. Chem. Soc., 87, 4597 (1965). (27) P. Becher, J . Colloid Sci., 20,728 (1965).
Electron Transfer in the Type I1 Photoelimination of a-Aminoacetophenones Albert Padwa,* William Eisenhardt, Robert Gruber, and Deran Pashayan
Contribution from the Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214. Received February 23, 1971 Abstract: The type I1 photoelimination reaction of a series of a-aminoacetophenones has been studied. Quantum yields and rate constants for product formation were determined in several solvents. These ketones were found to undergo photoeliminationwith extraordinarily high rates but with modest quantum efficiency. Photoelimination of a-amino ketones with low-lying T-T* triplet states also occurs with enhanced reaction rates. Attachment of an electron-withdrawing benzoyl group to the nitrogen atom decreases the rate of photoelimination. The results can best be rationalized by an electron-transfer route for which the rate constant may exceed diffusion control.
S
tudies of the photochemistry of ketones that possess a hydrogen-bearing y carbon have shown that two major reaction pathways are available.z-21 The (1) Alfred P. Sloan Foundation Research Fellow, 1968-1970. (2) This work was presented in part at the 161st National Meeting of the American Chemical Society, Los Angeles, Calif., March 1971. (3) P. J. Wagner and G. S. Hammond, Aduan. Photochem., 5, 21 (1968); J . Amer. Chem. Soc., 88, 1245 (1966). (4) P. J. Wagner, ibid., 89, 5898 (1967). (5) P. J. Wagner and A. E. Kamppainen, ibid., 90, 5896, 5898, (1968). (6) N. C. Yang, A. Marduchowitz, and D. H. Yang, ibid., 85, 1017 (1963). (7) N. C. Yang and S. P. Elliott, ibid., 91, 7550 (1969); N. C. Yang, S. P. Elliott, and B . Kim, ibid., 91, 7551 (1969). (8) J. N. Pitts, Jr., D. R. Burley, J. C. Mani, and A. D. Broadbent, ibid., 90, 5900 (1968). (9) E. J. Baum, J. K. S. Wan, and J. N. Pitts, Jr. ibid., 88, 2652 (1966). (IO) J. I
