Further examples of photocycloadditions of 1,3-diketonatoboron difluorides to olefins: a description of the reaction pattern and mechanism

Abstract

The photocycloaddition of dibenzoylmethanatoboron difluoride (DBMBF2) with conjugated nitriles, ketones, and esters occurs more slowly than that with simple olefins and with fair regioselectivity. For these electron-deficient olefins, the cyclic analogues react more slowly than the acyclic ones and the cyano analogues react more slowly than all others; also a β-methyl substituent reverses the regioselectivity in adduct formation. Benzoylacetonatoboron difluoride (BABF2) photocycloadds to simple cyclic olefins from the excited state isomer in which the double bond is conjugated with the phenyl group, and occurs more slowly than that of the corresponding reaction of DBMBF2. The efficiency and regiochemistry of these cycloadditions have been explained with respect to the exciplex as the immediate intermediate. Keywords: photocycloaddition, dibenzoylmethanatoboron difluoride, benzoylacetonatoboron difluoride, exciplex.