Ylidic radical reactivity towards ethylene and acetylene

A detailed computational study at the Gaussian-3//B3LYP/6-31G(d) level is performed on the PH3CH + C2H4/C2H2 reactions to unravel the chemical reactivity of the ylidic radical PH3CH towards π-bonded species. Both reactions are associated with the addition-elimination mechanism, eventually leading to PH3 + cCH2CHCH2, PH2CHCH2 + CH3 and PHCH2 + C2H5 for the PH3CH + C2H4 reaction, and most favorably to PHCHCHCH2 + H for the PH3CH + C2H2 reaction. The four-membered ring species (cPH3CHCH2CH2 and cPH3CHCHCH, respectively) are important entrance intermediates for both reactions, whereas they are of negligible importance for normal unsaturated radical reactions with alkenes and alkynes. For both the PH3CH + C2H4/C2H2 reactions, the λ5-P bonding within PH3CH is effectively transformed into the λ3-P bonding within the products, also in contrast to normal reactions of unsaturated radicals, where the π-bonding within the radical is well kept throughout the reactions. The distinct feature of the ylidic reactions can be explained by the fact that the ylidic radical PH3CH has a P-C π-bonding LUMO, whereas normal unsaturated radicals have σ-bonding LUMO localized on one atom. Finally, the solvent effects of benzene, methanol and water on the title reactions are considered. The present study should be useful for understanding the ylidic radical chemistry.