The photochemical (4 + 2)‐cycloaddition of ortho‐quinones to triarylketenimines

Abstract

AbstractThe photochemical reaction of ortho‐quinones with triarylketenimines 4 yields quantitatively the dihydrodioxin derivatives 5 via a (4 + 2)‐cycloaddition. Kinetic studies on the photochemical reaction of phenanthrenequinone (PQ) with N‐(p‐tolyl)dipheriylketenimine show that the reaction proceeds irreversibly, probably via an exciplex of triplet PQ with the CC double bond and an allylic biradical: k2 = (1.18 ± 0.03)·109 1·mol−1·s−1 at 25°C in benzene (Scheme 2). Competition experiments between triplet PQ and N‐phenyl‐substituted triphenylketenimines (Ph2C = C = N − C6H4 − X) gave a good correlation between the relative rate constants and the normal Hammett σ‐constants ρ = −0.31, which indicates the absence of conjugation between the substituent X and the reaction centre at the C  C double bond and confirms the electrophilic character of triplet phenantrenequinone. From the Taft's DSP equation it follows that the blending constant λ = ρI/ρR = 1.02.