Silicon-phosphorus analogies : Participation of external nucleophiles to activated processes of racemization and hydrolysis of chlorophosphono derivatives

Abstract

Kinetic and stereochemical studies show nucleophilic assistance by dimethylformamide (DMF), dimethylacetainide (DMA), hexamethylphosphotriamide (HMPT) and N-methylimidazole (NMI) in racemization and solvolysis of menthylchloro(phenyl)phosphonate, 1a, and O-ethylchloro(phenyl)thiophosphonate, 2. Similar orders of nucleophilic reactivity (Nu = NMI⪢HMPT>DMF>DMA), and identical rate-laws (v rac=k [M-Cl] [Nu] 2 and v H 2O = k' [M-Cl] [H 2O] [Nu]) are consistent with a common mechanism, governed by entropy (−60 u.e< ΔS ≠<−40u.e). Analogies between reaction mechanisms at silicon and phosphorus are clearly evidenced. A two-step process, involving rate-determining attack on a pentacoordinate complex is discussed.