A buffer zone in the crystal structure that governs the solid-state photodimerization of bulky olefins with the 1,4-dihydropyridine skeleton

Due to steric hindrance, bulky olefins cannot readily undergo solid-state photodimerization. UV irradiation of crystals of (4RS,1'RS)-methyl 1-phenyl-2-piperidinoethyl 1,4-dihydro-2,6-dimethyl-4-(2-thienyl)pyridine-3,5-dicarboxylate (1), however, affords a single product (4RS,8SR)-4a,8a-dimethoxycarbonyl-2,4b,6,8b-tetramethyl-3-[(1RS)-1-phenyl-2-piperidinoethoxycarbonyl]-7-[(1SR)-1-phenyl-2-piperidinoethoxycarbonyl]-4,8-di(2-thienyl)-1,4,4a,4b,5,8,8a,8b-octahydro-trans-cyclobuta-[1,2-b:3,4-b']dipyridine(2), in quantitative yield. X-Ray analyses of 1 and 2 showed that 2 is a photodimer of 1 and proved that bulky olefins can undergo solid-state photodimerization. Although the molecular system and the molecular arrangement in the crystal of dimethyl I,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)pyridine-3,5-dicarboxylate (3) are quite similar to those of 1, crystals of 3 cannot undergo solid-state photodimerization, Detailed inspection of the crystal structure of 1 revealed that there is a certain space between reacting molecules in the crystal to allow initiation of photodimerization, The space, designated as a buffer zone, buffers the steric hindrance from which the reacting molecules suffer when they approach each other. The buffer zone is formed by the disordered piperidine rings in 1, but there is no extra space in the crystal structure of 3. The present study has shown that the buffer zone in the crystal structure must be one of the prerequisite controlling factors for solid-state photodimerization.