A study of solvent effect on photochemically induced reations between pyridinedicarbonitriles and alkenes: an easy approach to the synthesis of cyclopenta [b or c]pyridines

Photochemically induced reactions of pyridinedicarbonitriles and alkenes show an interesting dependence on solvent polarity. In non-polar solvents ipso-substitution of the cyano groups in positions alpha or gamma to the heterocyclic nitrogen occurs to a larger extent, while in polar solvents the reaction provides a path to the formation of a new ring between the carbon atom of one of the cyano groups and a ring position, forming a cyclopenta[b or c]pyridine derivative (pyrindine). Studies on the multiplicity of the excited state controlling the reaction show that the singlet state is involved in the ipso-substitution, while the triplet state controls the formation of the pyrindine. An explanation for the solvent effect is given in terms of shift of the excited states with the solvent used. Theoretical calculations justify the position of the cyclisation, although no correlation was found for the regioisomers ratio. This reaction represents an effective entry to the biologically interesting pyrindine systems.