Photochemistry of allyloxybenzophenones: a pseudo-Paternò–Büchi rearrangement accompanied by hydrogen transfer induced 1,5-cyclization

Abstract

The solution photochemistry of the ortho allyloxy-substituted benzophenone 1 has been investigated in detail. Product analysis revealed formation of a diastereomeric mixture of dihydrobenzofuran derivatives 3 by cyclization via a short-lived intermediate 1,5-biradical and an unusual acetal 4 by a pseudo-Paternò-Büchi rearrangement. The latter reaction pathway was supported by means of laser flash photolysis, where a long-lived intermediate with a maximum absorption band at 380 nm was observed. Besides, theoretical calculations (TD-DFT) of this UV-transient resulted in a band with maximum intensity at 390 nm showing a good correlation between experimental results and theoretical calculations. For comparison, the meta-substituent substrate 2 was also investigated showing preferred triplet–triplet energy transfer.