Homolytic organometallic reactions. Part IX. An electron spin resonance study of the mechanism of β-scission of tetra-alkoxyphosphoranyl radicals. Formation of trialkoxy(methyl)phosphoranyl radicals

Abstract

The rate of β-scission of the tetra-alkoxyphosphoranyl radicals (RO)₄P·, where the alkyl groups may be the same or different, is determined by configurational, steric, electronic, and solvent effects. The rate of C–O cleavage increases along the series R^p < R^s < R^t < PhCH₂. In the distorted trigonal bipyramidal radical (RO)₄P·, alkyl groups in apical or equatorial positions are not lost at the same rate, probably those in the equatorial position under-going more rapid β-scission. As a result of this configurational selectivity, bulkyl alkyl groups are retained preferentially when (RO)₄P· decomposes. The rate of scission of (EtO)₃POBu^t increases somewhat with the solvent polarity, but there appears to be little steric acceleration when (RO)₄P· contains bulky alkyl groups. Methyl radicals react with trialkyl phosphites to give the radicals (RO)₃[graphic omitted]Me at a rate which is governed primarily by steric effects. PhCH₂O[graphic omitted](OEt)₂Me undergoes rapid β-scission to give a benzyl radical.