Unexpected Reaction of Dialkyl α-Hydroxy-benzylphosphonates with Dialkyl Phosphites and a Few Related Reactions

The condensation of dialkyl α-hydroxy-benzylphosphonates with dialkyl phosphites and that of α-hydroxybenzyl-diphenylphosphine oxide with diphenylphosphine oxide unexpectedly gave the corresponding phosphorylated α-hydroxy derivatives. This new reaction proved to be general. The formation of the two products may be similar and involves the attack of the hydroxy group of the α-hydroxyphosphonate or α-hydroxyphosphine oxide on the phosphorus atom of the trivalent tautomer form (Y₂POH) of the Y₂P(O)H reagent (Y= MeO, EtO, or Ph) going with the elimination of an alcohol and water molecule, respectively. The mechanism was supported by DFT computations at the M062X/6-31G (d,p) level of theory, including suitable proton transfer networks. The condensations discussed are typical autocatalytic reactions promoted by the alcohol or water molecules formed. The initial promoters are the traces of water inevitably present in the mixture. In the reaction of α-hydroxyphosphonates with dialkyl phosphites, the −P(O)(OR)H derivative is the primary product that is partially hydrolyzed to the −P(O)(OH)H species by the traces of water under the conditions of the reaction. Arbuzov reaction of diethyl α-bromobenzylphosphonate with ethyl diphenyphosphinite afforded the target-like phenylmethylene-phosphine oxide─phosphonate derivative.