Photochemical Hydrogen Abstraction from Cymenes

Article abstract of DOI:10.1021/jo00270a025

The three isomeric cymenes are examined in photochemical reactions involving hydrogen transfer with 1,4-naphthalenedicarbonitrile (NDN), α,α,α-trifluoroacetophenone (TFA), and benzophenone (BP).Selectivity for attack at primary vs tertiary position (P/T) and quantum yield are determined.The reactions are rationalized as involving loose perpendicular complexes yielding a low P/T ratio in the products (with BP n? triplet), a face-to-face charge transfer complex yielding a high P/T (with TFA ??^* triplet and also NDN singlet, but only as far as the benzylnaphthalenes 1 and 2 are concerned), and a tighter charge transfer complex yielding again a low P/T in the formation of dibenzomethanocyclooctenes 3, the main products from NDN.The energetic limit of such a directing effect of the conformation of the intermediate complex onto photochemical reactivity is evidenced by the different results obtained with o-cymene.