Sterically constrained tricyclic phosphine: redox behaviour, reductive and oxidative cleavage of P-C bonds, generation of a dilithium phosphaindole as a promising synthon in phosphine chemistry.

The redox behaviour of sterically constrained tricyclic phosphine was investigated by spectroelectrochemistry. The data suggested a highly negative reduction potential with the reversible formation of a dianionic species. Accordingly, reacted with two equivalents of Li/naphthalene by reductive cleavage of a P-C bond of one of the PC heterocycles. The resulting dilithium compound represents a phosphaindole derivative with annulated aromatic C and PC rings. It is an interesting starting material for the synthesis of new heterocyclic molecules, as was shown by treatment with MeSiCl and PhPCl. The structures of the products ( and ) formally reflect ring expansion by insertion of silylen or phosphinidene fragments into a P-C bond of . Treatment of with HO did not result in the usually observed transfer of a single O atom to phosphorus, but oxidative cleavage of a strained PC ring afforded a bicyclic phosphinic acid, RPOH.