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.
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