Alkali Metal Alkoxide Clusters: Convenient Catalysts for the Synthesis of Methylphosphonates

Alkali metal alkoxides are good catalysts (1-8 mol %) for promoting the interchange reaction between carbonyl and phosphorus esters. This reactivity leads to convenient methodologies for the synthesis of symmetric and unsymmetric alkyl-substituted methylphosphonates from dimethyl methylphosphonate (DMMP). Reactions rates are high with initial turnover frequencies (N(t)) in excess of 1 x 10(6) h(-)(1) observed and with KO(t)Bu > NaO(t)Bu > LiO(t)Bu. The reactions were sensitive to steric effects in the product methylphosphonates with reaction rates paralleling the size of the transferring alkoxide (n-alkyl > isoalkyl >> tert-alkyl). For the test reaction DMMP + isopropyl acetate, substitution kinetics were consistent with a scenario wherein each methoxide is replaced sequentially, and the substitution rate for the second displacement is substantially slower than the first. Kinetic studies on the first substitution process were indicative of a concentration dependent rate law; a scenario most easily accounted for by a coupled transesterification wherein alkoxide reversibly and independently adds to phosphonate and ester.