Synthesis and Conformational Behavior of 2-Phosphonio- and 2-Phosphinyl-1,3-dithianes. Operation of the Generalized Anomeric Effect in the S-C-P(+) System

Article abstract of DOI:10.1021/jo00121a043

A stereoselective preparation of various 2-phosphinyl- and 2-phosphonio-1,3-dithianes by desulfurization of the appropriate 2-thiophosphoryl-1,3-dithianes was described.The structure of the title compounds was studied by means of 1H, 13C, and 31P NMR methods.Configurational assignments were also based on chemical correlation and X-ray structure determination.Both the NMR studies of conformationally labile models and equilibration of diastereomeric compounds showed an increased preference of the phosphinyl and phosphonium groups for the axial orientation.Magnitude of the anomeric effect found varies in the range from ca. 6 kJ/mol in phosphines to more than 10 kJ/mol in phosphonium salts.The anomeric effect could stem from the n_s-?^*_C-P hyperconjugative interaction.If phenyl groups are connected with phosphorus, overlap repulsion involving lone electron pairs of the endocyclic sulfur atoms and ?-electrons of phenyl rings should also be taken into account.The reverse anomeric effect was not observed.No manifestation of the exo anomeric effect in 2-phosphinyl-1,3-dithianes was found.