Thiophosphonates of 1,1-binaphthol as chiral equivalents of H2S. Preparation of 2-mercaptonorbornanes and 2-mercaptonorbornenes

Isomerically pure exo- and endo-2-mercaptonorbornanes and 2-mercaptonorbornenes were prepared by radical addition of the binaphthol-derived thiophosphonates 1a and 1b to norbornene and norbornadiene, separation of the exo and endo isomers by flash-chromatography and reduction with lithium aluminium hydride. The reaction of 1b with norbornadiene performed with enantiopure 1,1′-binaphthol, produced a 1:1 mixture of the exo-diastereoisomers, one of which could be obtained in pure form by fractional crystallization. The latter, upon reduction with lithium aluminium hydride, afforded enantiomerically pure exo-2-mercaptonorbornene showing that the binaphthylthiophosphonates can be considered as safe, chiral synthetic equivalents of hydrogen sulfide.