Green chemistry strategies using crystal-to-crystal photoreactions: stereoselective synthesis and decarbonylation of trans-alpha,alpha'-dialkenoylcyclohexanones.

This paper describes steps to develop green chemistry strategies to prepare compounds with adjacent quaternary centers by stereospecific photodecarbonylation of crystalline ketones bearing radical-stabilizing alkenyl substituents in their alpha-positions. Crystals of trans-2,6-dimethyl-2,6-di(benzyloxycarbonyl-trans-ethenyl)-cyclohexanone, trans-2, prepared by double Michael addition of benzyl propiolate to 2,6-dimethyl-cyclohexanone iso-butylimine (1), were investigated along with those of the dicarboxylic acid (trans-3) and a dibenzylammonium salt derivative (trans-4) to establish the best substrates for the solid-state reaction. While reactions of the ester and the acid lose CO to give radical combination products with high selectivity at low conversions, the crystalline salt gave a similar product in >97% yield. Explorative reactions carried out under sunlight and in semipreparative (1.5 g) scale highlight the potential of reactions in crystals as a viable strategy for the development of green chemistry.