Mechanism of hydrogen atom transfer in the photolytic rearrangement of N-bromophenylalaninamide derivatives

Abstract

Photolysis of N-bromo-N-tert-butyl-N^α-phthaloylphenylalaninamide gave a 1∶1 mixture of the diastereomers of 3-bromo-N-tert-butyl-N^α-phthaloylphenylalaninamide. Reactions carried out with various concentrations of the N-bromoamide, and in the presence of 4-tert-butyltoluene, showed that the ratio of the product bromophenylalanine derivatives to 4-tert-butylbenzyl bromide varied as a function of the concentration of the N-bromoamide, indicating that the bromophenylalanine derivatives are formed through an intermolecular process and not by intramolecular 1,4-hydrogen atom transfer of the corresponding amidyl radical. Results of reactions of N-bromo-N-tert-butyl-N^α-phthaloyl-p-methylphenylalaninamide are also consistent with this interpretation. Reactions of stereoselectively β-deuteriated derivatives of N-bromo-N-tert-butyl-N^α-phthaloylphenylalaninamide established that the pro-S benzylic hydrogen of the phenylalaninamide is selectively abstracted, by a factor of ca. 3.8, in this intermolecular process.