Mechanism of Phosphorus−Carbon Bond Cleavage by Lithium in Tertiary Phosphines. An Optimized Synthesis of 1,2-Bis(phenylphosphino)ethane

Abstract

Conditions influencing the extent of P−C_aryl vs P−C_alkyl bond cleavage in the reaction of Ph₂P(CH₂)₂PPh₂ with lithium in THF have been investigated. The results complement and elucidate earlier work; they indicate that the mechanism of P−C bond cleavage in tertiary phosphines of this type involves a thermodynamic equilibrium between P−C_aryl and P−C_alkyl cleaved radicals and anions, followed by reaction and stabilization of these as lithium salts. The addition of water to the reaction mixture causes a reestablishment of the cleavage equilibrium prior to the formation of the secondary phosphines. A mechanism involving competitive release of leaving groups as the thermodynamically most stable anion or radical has been proposed. The preparation of (R*, R*)-(±)/(R*, S*)-PhP(H)(CH₂)₂P(H)Ph by this route has been optimized.