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

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_H2O = k' M-Cl] H₂O] 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.