| Abstract
| - A novel and practical approach to monosubstituted phosphinic acid (alkylphosphonous acid)derivatives from hypophosphite salts or esters is described. Phosphorus-centered radical formationis initiated with Et3B/O2, and the reaction is conveniently conducted at room temperature in anopen flask. In contrast to previously reported conditions for the radical reaction of hypophosphorousacid and sodium hypophosphite (peroxide initiators, acid catalysis, heat), the method proceeds underneutral conditions and therefore tolerates a wide range of functional groups. Previously inaccessiblephosphinic acids can be prepared in a single step from cheap starting materials. Excellent selectivityis observed for monoaddition, and symmetrical dialkyl phosphinates do not form in significantamounts. Monosubstituted phosphinic acids are usually obtained in better than 90% purity by asimple extractive workup; however, isolated yields are diminished if the substituent is polar. Becauseradicals derived from hypophosphites are electrophilic, the reaction is limited to the use of electron-rich olefins. The reaction conditions can also be employed in the room temperature radical reductionof alkyl halides and provide an exceptionally mild and environmentally friendly alternative to theuse of tributyltin hydride. The remarkable mild nature of the reaction conditions allows for theradical reaction of sensitive alkyl hypophosphites to occur, in which case, a catalytic amount ofEt3B suffices to deliver alkyl phosphinate esters in reasonable yield.
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