Factors affecting the electronic states of amidyls: evidence for Π—Σ mixing in simple amidyls

Abstract

N-Methyl- and N-acetyl-biphenyl-2-carboxamidyls, generated from the corresponding N-bromides in photochemically initiated alkyl radical or bromine atom chain reactions, cyclise to give Ar₁-5 and Ar₂-6 products, indicating the accessibility of a low-energy excited Σ-state for these species. N-Methoxy-biphenyl-2-carboxamidyl radical, from photolysis of the N-chloro derivative, yields no products from Ar₁-5 or Ar₂-6 cyclisation; this indicates a thermodynamically stable Π-ground state and a high-energy and thermally inaccessible excited Σ-state. The N-methylbiphenyl-2-sulphonamidyl radical from photolysis of the N-iodo or N-bromo derivative affords only the Ar₂-6 product; this represents chemical evidence for a large Π—Σ energy separation. MNDO calculations predict the Π-ground states of N-alkoxy-amidyls and -amiyls to be more nucleophilic than those of amidyls, imidyls, and sulphonamidyls, in keeping with the observed reactivities. The reactivities and regioselectivities observed, together with e.s.r. data and MNDO results, support mixing between the Π-ground and Σ-excited states of amidyls, whereas alkoxyamidyls, cyclic amidyls, and sulphonamidyls have pure Π-ground states. Mixing in amidyls is facilitated by their acyclic nature and by a small energy difference between their Π-ground and Σ-excited states.