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-st ate photodimerization. UV irradiation of crystals of (4RS,1'RS)-methyl 1-phenyl-2-piperidinoethyl 6-dimethyl-4-(2-thienyl)pyridine-3,5-dicar boxylate (1), however, affords a single product hydro-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 olefi ns can undergo solid-state photodimerization. Although the molecular s ystem and the molecular arrangement in the crystal of dimethyl methyl- 4-(3-nitrophenyl)pyridine-3,5-dicarboxylate (3) are quite similar to t hose 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 zo ne, buffers the steric hindrance from which the reacting molecules suf fer when they approach each other. The buffer zone is formed by the di sordered piperidine rings in 1, but there is no extra space in the cry stal structure of 3. The present study has shown that the buffer zone in the crystal structure must be one of the prerequisite controlling f actors for solid-state photodimerization.