TOPOCHEMICAL INDUCTION TO AN ALTERNATING ZIGZAG LINEAR AND SYNDIOTACTIC CHAIN STRUCTURE IN THE COURSE OF A [2+2]PHOTOREACTION OF ALKYL ALPHA-CYANO-4-[2-(2-PYRIDYL)ETHENYL]CINNAMATE CRYSTALS

Methyl and ethyl alpha-cyano-4-[2-(2-pyridyl)ethenyl]cinnamate (1a and 1b) crystals are highly photoreactive and are converted by a [2+2] photocycloaddition reaction into dimers (2a and 2b) in high yields or upon further photoirradiation into a tricyclic dimer (3a) or oligomers. The photochemical conversion of 1 into 2 was reasonably intepreted by a crystallographic analysis of 1, based on topochemical considerations. The formation of 3a by an intramolecular cycloaddition of monocyclic dimer 2a at an extremely low temperature was attributed to an insufficient thermal energy for 2a to be rearranged into its stable form. On the other hand, the [2+2] photodimerization of 1b did not occur at random at the reactive site in the crystal, but was topochemically controlled, resulting in an alternating arrangement of both enantiomers in the solid state. Upon irradiation, crystal 2a afforded a polymer via an intermediate tetramer (4a), whereas from crystal 2b a tetramer (4b) and an octamer were obtained. The tetramers (4a and 4b) were isolated and shown to contain three cyclobutane rings of beta-hetero, alpha-homo, and beta-hetero-type structures in this order. The two beta-type cyclobutanes in a molecule of these tetramers are opposite in configuration to each other. The structures of 4, predictable from the crystal structures of 2, were not predictable from the crystal structures of the corresponding monomers. The high molecular weight polymer (Mn = 12 000), obtained from crystal 2a, has an unusual repeating octamer unit, in which three types of cyclobutane structures are incorporated in a sequence of [alpha-beta-epsilon-beta-alpha-beta--1epsilon-1] (''double syndiotactic'') where beta and beta--1 are opposite in configuration to each other. In contrast, the octamer derived from crystal 2b has a repeating unit in which two types of cyclobutane structures are arranged in a sequence of [alpha-beta-alpha-beta--1] (''syndiotactic'').