Triethylborane-initiated room temperature radical addition of hypophosphites to olefins: Synthesis of monosubstituted phosphinic acids and esters

A novel and practical approach to monosubstituted phosphinic acid (alkylpho sphonous acid) derivatives from hypophosphite salts or esters is described. Phosphorus-centered radical formation is initiated with Et3B/O-2, and the reaction is conveniently conducted at room temperature in an open flask. In contrast to previously reported conditions for, the radical reaction of hy pophosphorous acid and sodium hypophosphite (peroxide initiators, acid cata lysis, heat), the method proceeds under neutral conditions and therefore to lerates a wide range of functional groups. Previously inaccessible phosphin ic acids can be prepared in a single step from cheap starting materials. Ex cellent selectivity is observed for monoaddition, and symmetrical dialkyl p hosphinates do not form in significant amounts. Monosubstituted phosphinic acids are usually obtained in better than 90% purity by a simple extractive workup; however, isolated yields are diminished if the substituent is pola r. Because radicals derived from hypophosphites are electrophilic, the reac tion is limited to the use of electron-rich olefins. The reaction condition s can also be employed in the room temperature radical reduction of alkyl h alides and provide an exceptionally mild and environmentally friendly alter native to the use of tributyltin hydride. The remarkable mild nature of the reaction conditions allows for the radical reaction of sensitive alkyl hyp ophosphites to occur, in which case, a catalytic amount of Et3B suffices to deliver alkyl phosphinate esters in reasonable yield.