Identification of a four-center intermediate in a Grignard addition reaction to a P–S bond

The reaction between tert-butylmagnesium chloride (or tert-pentylmagnesium chloride) and the particular phosphorus–sulfur bond of a benzothiadiphospholic system showed, for the first time, evidence of formation of intermediates with a four-center structure. The possibility, for the phosphorus atom, to have very stable hypervalent coordinations makes it possible to observe its hypervalent states during the course of a reaction. The benzothiadiphosphole, with its bicyclic folded structure, further stabilizes the hypervalent coordinations thus making the intermediates sufficiently stable to be detected during the course of the reaction by 31P NMR spectroscopy, which revealed the nature and the stability of the species involved in this reaction, carried out also using other Grignard reagents.