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

Journal of the American Chemical Society

Volumn 121, Issue 15, 1999, Pages 3785-3786

  DeCosta, Dayal P ^a   Bennett, Amy K ^a   Pincock, James A ^a  

^a DALHOUSIE UNIVERSITY   (Canada)

Author keywords

[No Author keywords available]

Indexed keywords

ESTER;

ELECTRON TRANSPORT; LETTER; MOLECULAR INTERACTION; OLIGOMERIZATION; PHOTOCHEMISTRY; PHOTOLYSIS; PROTON TRANSPORT; REACTION ANALYSIS;

EID: 0033594534     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja9840347     Document Type: Letter

References (37)

1. (a) Wagner, P. J. In CRC Handbook of Organic Photochemistry and Photobiology; Horspool, W. M., Song, P.-S., Eds; CRC Press: New York, 1995; pp 449-470 and references therein.
2. (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.
3. 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).
4. Barltrop, J. A.; Coyle, J. D. J. Chem. Soc. (B) 1971, 251-255.
5. Gano, J. E. Chem. Commun. 1971, 1491-1492.
6. Pincock, J. A. Acc. Chem. Res. 1997, 30, 43-49.
7. Jaeger, D. A. J. Am. Chem. Soc. 1976, 98, 6401-6402.
8. Jaeger, D. A.; Bernhardt, E. A. Tetrahedron. Lett. 1983, 24, 4521.
9. Cristol, S. J.; Mahfuza, B. A.; Sankar, I. V. J. Am. Chem. Soc. 1989, 111, 8207-8211 and references therein.
10. Morrison, H.; Miller, A.; Bigot, B. J. Am. Chem. Soc. 1983, 105, 2398-2408.
11. Bhalerao, V. K.; Nanjundiah, B. S.; Sonawane, H. R.; Nair, P. M. Tetrahedron 1986, 42, 1487-1496.
12. Abdel-Wahab, A. A.; Ismail, M. T.; Mohamed, O. S.; Khalaf, A. A. Gazz. Chim. Ital. 1985, 115, 591-594.
13. 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.
14. The corresponding acetates of the 2-phenylethanols were photochemically inert.
15. Minor amounts of ethylbenzenes, 3-aryl-1-propanols, and 1-aryl-2,2-dimethoxyethanes were also formed.
16. The structures were determined either by isolation of products from the reaction mixtures or by independent synthesis. All compounds gave satisfactory spectral analysis.
17. The styrenes are also formed in acetonitrile, but they rapidly disappear to form nonvolatile products, presumably oligomers.
18. McEwen, J.; Yates, K. J. Phys. Org. Chem. 1991, 4, 193-206.
19. 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.
20. Neunteufel, R. A.; Arnold, D. A. J. Am. Chem, Soc. 1973, 95, 4080-4081.
21. 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.
22. 22
23. Kawakami, J.; Iwamura, M. J. Phys. Org. Chem. 1994, 7, 31-42.
24. 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.
25. Irreversible, in volts versus SCE: -1.65, -1.66, and -1.66 for 1a, 1b, and 1c, respectively.
26. The long-wavelength absorption is mainly the 4-cyanobenzoate chromophore, particularly for 1a.
27. 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.
28. Nicholas, A. M. P.; Arnold, D. R. Can. J. Chem. 1982, 60, 2165.
29. a as a function of reduction potential.
30. 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.
31. Estimated from Benson's rules.
32. Calculated from the heat of combustion.
33. 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 diastereomers of 2,3-bis(4-methoxyphenyl)butane.
34. Reference 1b, p 337.
35. 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.
36. Cram, D. J. J. Am. Chem. Soc. 1952, 74, 2129-2137.
37. At 84% conversion, the ratio of disappearance of 12 to formation of the diastereomer was about 50:1.