Selectivity in Homolytic Substitution at a Tertiary Phosphine Center: Mechanistic Insights into Dealkanation Reaction

Here, we present a comprehensive mechanistic investigation of a transformation that reverses the usual reactivity of an established radical P-C bond-forming process. Peri-substituted compounds bearing tertiary and secondary phosphine groups undergo dealkanation to afford an alkane and a diphosphine. To examine the influence of the chain-carrying radical, we synthesized precursors with different alkyl substituents at the tertiary phosphine center and correlated the rates and selectivities of their reactions with computational results. The reactivity is governed by the leaving-group ability of the carbon-centered radical expelled in the SH2 step, which is rate-limiting in most cases. Competitive elimination of propane and n-butane occurs when both pathways are accessible, whereas only propane is eliminated when propane and methane formation compete. Substrates bearing a tert-butyl group eliminate isobutane even at low temperature. Low-intensity UV irradiation, including ambient daylight, promotes the reaction at room temperature at rates comparable to those observed under thermal conditions at 140 °C. Calculations indicate a weakly bound, phosphoranyl-radical-like SH2 transition state and reveal that peri-induced preorganization and altered bond strengths enable this unusual reactivity.