Intermediates in the decomposition of aliphatic diazo-compounds. Part 14. Chemically induced dynamic nuclear polarisation during benzylic insertion of diphenylmethylene into diastereoisomeric esters of α-fluorophenylacetic acid. Evidence for a dual mechanism of C–H insertion

Abstract

Diazodiphenylmethane and a number of its substituted analogues have been decomposed thermally in esters of α-fluorophenylacetic acid as solvent, and the ¹H and ¹⁹F CIDNP effects recorded. The pattern of the principal polarisation in the case of the methyl ester is consistent with benzylic C–H insertion by triplet diphenylmethylene. A minor ¹⁹F polarisation showing no corresponding depolarised product is attributed to a metastable product formed by bonding of PhĊFCO₂Me at the para-position of the diarylmethyl radical. CIDNP has been observed during benzylic insertion into diastereoisomeric esters formed by esterification of racemic and resolved α-fluorophenylacetic acid with chiral alcohols (1 methylheptanol, bornan-2-endo-ol, p-menthan-3-ol). In the case of single diastereoisomers, the polarised ¹⁹F signals of the insertion product indicate that geminate radical pair recombination occurs with complete loss of stereochemistry at the reaction centre. However, depolarised n.m.r. spectra indicate a 55 :45 preference for retention of configuration. The species responsible for insertion with retention but without the generation of polarisation is considered to be singlet diphenylmethylene.