Engineering Reactions in Crystalline Solids: Prediction of Intramolecular Carbene Rearrangements

The unimolecular reactivity and stereoselectivity of four arylalkyl carbenes generated by a steady state irradiation of crystalline 1,2-diaryldiazopropanes were correctly predicted by taking advantage of known reaction trajectories applied to structures obtained by simple molecular mechanics calculations. It is suggested that the conformation of the diazo compound in the crystal determines the conformation of the carbene intermediate which is predisposed for a conformationally stereospecific 1,2-H shift. The results of irradiations of 1,2-diaryldiazoalkanes in crystalline media were compared with those obtained in solution and in amorphous solids. It was demonstrated that rigidity alone is not sufficient for carbene reactions to proceed with high stereoselectivity.