Intermediates in the decomposition of aliphatic diazo-compounds. Part XII. Chemically induced dynamic polarisation of fluorine-19 and proton nuclei in the insertion of diarylmethylenes into benzyl fluoride

Abstract

The patterns of polarisation of ¹⁹F and ¹H n.m.r. signals of 2,2-diaryl-1-phenylethyl fluoride formed during thermal decomposition of diaryldiazomethanes in solvent benzyl fluoride are described. Using diphenyldiazomethane, the ¹⁹F quartet shows net (A)+ multiplet (A/E) polarisation and this changes to E + A/E as the g-factor of the intermediate diarylmethyl radical is increased beyond 2.003 0 using substituted diphenyldiazomethanes. With monosubstituted diphenyldiazomethanes, two diastereoisomeric products are formed in equal amounts, and these show identical polarisation. Over almost the whole range of substituents, little net polarisation is detectable in the n.m.r. signals of the protons geminal and vicinal to the fluorine despite wide variations in Δg; both ¹H quartets show strong multiplet polarisation, A/E for H-1 and E/A for H-2. By the use of phenyl t-butyl nitrone which traps intermediate free radicals it is shown that polarised insertion products arise from both recombination of geminate radical pairs and from diffusive encounter of diarylmethyl and fluorobenzyl radicals. The results, particularly the differential effect of substituent on the polarisation of ¹⁹F and ¹H nuclei, are interpretable in terms of the radical-pair theory of CIDNP, but it is shown by simulation studies that nuclear spin relaxation can play a significant role in certain instances in modifying the relative intensities of polarised signals.