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 3a was investigated by spectroelectrochemistry. The data suggested a highly negative reduction potential with the reversible formation of a dianionic species. Accordingly, 3a reacted with two equivalents of Li/naphthalene by reductive cleavage of a P–C bond of one of the PC(4) heterocycles. The resulting dilithium compound 5 represents a phosphaindole derivative with annulated aromatic C(6) and PC(4) rings. It is an interesting starting material for the synthesis of new heterocyclic molecules, as was shown by treatment with Me(2)SiCl(2) and PhPCl(2). The structures of the products (6 and 7) formally reflect ring expansion by insertion of silylen or phosphinidene fragments into a P–C bond of 3a. Treatment of 3a with H(2)O(2) did not result in the usually observed transfer of a single O atom to phosphorus, but oxidative cleavage of a strained PC(4) ring afforded a bicyclic phosphinic acid, R(2)PO(2)H.