The Role Played by Phosphorus Hexacoordination in Driving the Stereochemical Outcome of a Phosphination Reaction

Asymmetric alkyl phospholanes have been diastereoselectively synthesized by addition of an unsymmetrical bis-Grignard reagent and of amono-Grignard reagent to benzothiadiphosphole (1) and isolated as sulfides. The relative cis/trans ratio of the products depends on the steric hindrance of themono-Grignard used.An accurate analysis of NMR and stereoselective data revealed the fundamental role played by hexacoordinated phosphorus intermediates in driving the stereochemical outcome of the reaction. The particular bicyclic and folded structure of reagent 1 strongly stabilizes hypercoordinated phosphorus species involved in the reaction and favors their formation. Pentacoordinate and metastable hexacoordinate phosphorus species have been detected and their evolution studied through 31P NMR spectroscopy. The diastereoselective outcome of the reaction between reagent 1 and the couple bis-Grignard reagent 5/mono-Grignard reagent has been explained through a pathway involving hexacoordination of the phosphorus as a key step.