Nucleophilic Substitution at Tetracoordinate Phosphorus. Stereochemical Course and Mechanisms of Nucleophilic Displacement Reactions at Phosphorus in Diastereomeric - and -2-Halogeno-4-methyl-1,3,2-dioxaphosphorinan-2-thiones: Experimental and DFT Studies.

Geometrical - and - isomers of 2-chloro-, 2-bromo- and 2-fluoro-4-methyl-1,3,2-dioxaphosphorinan-2-thiones were obtained in a diastereoselective way by (a) sulfurization of corresponding cyclic P-halogenides, (b) reaction of cyclic phosphorothioic acids with phosphorus pentachloride and (c) halogen-halogen exchange at P-halogenide. Their conformation and configuration at the C-ring carbon and phosphorus stereocentres were studied by NMR (H, P) methods, X-ray analysis and density functional (DFT) calculations. The stereochemistry of displacement reactions (alkaline hydrolysis, methanolysis, aminolysis) at phosphorus and its mechanism were shown to depend on the nature of halogen. Cyclic - and -isomers of chlorides and bromides react with nucleophiles (HO, CHO, MeNH) with inversion of configuration at phosphorus. DFT calculations provided evidence that alkaline hydrolysis of cyclic thiophosphoryl chlorides proceeds according to the S2-P mechanism with a single transition state according to the potential energy surface (PES) observed. The alkaline hydrolysis reaction of - and -fluorides afforded the same mixture of the corresponding cyclic thiophosphoric acids with the thermodynamically more stable major product. Similar DFT calculations revealed that substitution at phosphorus in fluorides proceeds stepwise according to the A-E mechanism with formation of a pentacoordinate intermediate since a PES with two transition states was observed.