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 has been investigated in detail. Product analysis revealed formation of a diastereomeric mixture of dihydrobenzofuran derivatives by cyclization via a short-lived intermediate 1,5-biradical and an unusual acetal 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 was also investigated showing preferred triplet-triplet energy transfer.

Photochemical reactions of aromatic compounds and the concept of the photon as a traceless reagent

“…The final product 142 is obtained in a bicyclization reaction. Similar reactions were observed with allyl ethers of ortho-hydroxybenzophenone 113 and 1-hydroxyanthraquinone. 114 Electron transfer mediated photochemical reactions of aromatic compounds Photochemical electron transfer mediated electrophilic substitution is involved in the Witkop cyclization.…”

Strategies for the Synthesis of 2,3-Dihydrobenzofurans

“…Benzophenone 84, bearing a prenyl ether subsituent was shown to cyclise under irradiation to give DHB 85 in 40% yield as a mixture of diastereoisomers (Scheme 18). 70 However, the major product of this reaction was acetal 86.…”

The arene–alkene photocycloaddition

“…Griesbeck et al reported the formation of benzoxepines from the benzophenone analogue upon irradiation at slightly lower wavelengths [102]. The formation of the compound was observed in 50% yield, along with a diastereoisomeric mixture of dihydrobenzofurans in 40% yield (Scheme 38).…”

“…Griesbeck et al investigated the mechanism for this photocycloaddition, as he suggested that the mechanism proposed by Jones is unlikely, because the regiochemistry of proton catalyzed addition of alcohols to enols or enol ethers has the opposite regiochemistry to that observed in the product [102]. Furthermore, an electron transfer intermediate was ruled out, as the reaction is not thermodynamically feasible according to the Weller equation.…”