Chemically induced dynamic polarisation of 19F nuclei in the dimerisation of α-fluorobenzyl radicals

Abstract

Generation of α-fluorobenzyl radicals during thermal decomposition of diazodiphenylmethanes or benzoyl peroxide in benzyl fluoride as solvent or in the decomposition of t-butyl α-fluorophenylperacetate gives rise inter alia to meso and (±)-difluorobibenzyl. However, CIDNP observed in ¹⁹F-spectra does not appear to match the pattern of signals of these products. The inference, in a preliminary report, that the dimerisation takes place in two stages, a head-to-tail co-ordination giving a semibenzene followed by rearrangement, is shown to be incorrect. Examination of the effect on the pattern of ¹⁹F-polarisation of introducing deuterium into the phenyl ring and into the side-chain of α-fluorobenzyl radicals indicates that the predominant mode of dimerisation must be head-to-head giving directly meso- and (±)-1,2-difluoro-1,2-diphenylethanes. This conclusion is confirmed by computer simulation of the polarised spectrum.