- Design of radical clocks to probe the reactivity of the intermediated in arylmethyl ester photochemistry
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The photochemistry in methanol of the esters 1-6 was examined. These reactions normally proceed through radical pairs that result from homolytic cleavage of the carbon-oxygen bond in the excited singlet state. Each of the esters was designed to probe the intervention and reactivity of the substituted arylmethyl radical by incorporating a potential radical clock at the carbon of the reactive bond. For esters 1-5, the products isolated indicated that the radical clock was not reactive enough to compete with the very rapid alternate processes of the radical pair, namely, electron transfer to form the corresponding ion pair and decarboxylation of the phenylacyloxy radical (k = 4.6×109 s-1). Ester 6, which incorporates the extremely rapid fluorenylcyclopropylcarbinyl clock, showed very unusual reactivity. On thermal solvolysis in methanol, 6 rearranged quantitatively to the ester 20. No methyl ethers were detected. In contrast, photolysis of 6 in benzene resulted in an alternate rearrangement to the cyclobutyl ester, 22, resulting from the aryl version of the cyclopropyl-π-methane photochemical rearrangement. No ester cleavage occurred on excitation. A rationale for the latter conversion was based on stereoelectronic arguments provided by a crystal structure of 6.
- Nevill,Pincock
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p. 232 - 247
(2007/10/03)
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- Internal return in the photochemistry of ring-substituted 1-(1-naphthyl)ethyl esters of phenylacetic acid
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The photochemistry in methanol of the esters 12a-d been studied in order to determine the importance of internal return of both ion pair and radical pair intermediates.The efficiency of internal return, determined by monitoring 18O exchange between the alcohol and carbonyl oxygens, was shown to be substituent dependent, varying from approximately 10percent for the 4,7-dimethoxy substrate to nearly 50percent for the 4-cyano case.The corresponding ground state solvolysis reactions gave about 10percent internal return and, within experimental error, were substituent independent.Internal return was also examined by racemization of the chiral center in 12a and 12d.In summary, these combined results reveal that internal return probably occurs mainly through a contact (not solvent-separated) radical pair.More important, internal return has little effect on previously calculated electron transfer rate constants for converting the radical pair to the ion pair.Therefore, the previously reported Marcus' correlations are valid.Key words: photochemistry of benzylic esters, internal return, photosolvolysis, electron transfer, radical pairs.
- Kim, J. M.,Pincock, J. A.
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p. 885 - 895
(2007/10/03)
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