Theoretical Calculations on the Mechanism of the T3P<sup>®</sup>-Promoted Esterification and Amidation of Phosphinic Acids

The mechanism for the T3P®-promoted derivatizations of five-membered cyclic phosphinic acids was studied by theoretical calculations using the DFT method at the B3LYP/6- 31g(d,p)\\6-311g(2d,2p) level in order to elucidate the role of T3P® and interpret the experimental results. The esterification and amidation of 1-hydroxy-3-methyl-3-phospholene 1-oxide, along with the esterification of 1-hydroxy-3,4-dimethylphospholane oxide with special regard to the diastereoselectivity of the latter reaction were investigated. While the direct derivatizations with butanol and butylamine were found to be slightly exothermic and endothermic, respectively, in the presence of the T3P® reagent, the energetics of the derivatizations became by -59.2 kJ mol-1 and -31.6 kJ mol-1 exothermic, respectively. In the latter case, the reactions take place via the adduct of the cyclic phosphinic acid and the T3P® reactant. The unexpected diastereoselectivity observed in the T3P®-assisted esterification of the 1-hydroxy- 3,4-dimethylphospholane oxide was explained by a steric effect. Keywords: Phosphinic acids, esterification, amidation, T3P® reagent, mechanism, energetics.