Chirality induction in cyclocopolymerization. 12. Conformational effect ofmonomer on asymmetric cyclocopolymerization of 2,3-bis-O-(4-vinylbenzoyl)-L-tartrates with styrene

Methyl, ethyl, and isopropyl 2,3-bis-O-(4-vinylbenzoyl)-L-tartrates (1a, 1b , and 1c, respectively) were cyclocopolymerized with styrene, and the resul ting copolymers 2a-c were hydrolyzed, followed by methyl esterification usi ng diazomethane to yield the poly [(methyl 4-vinylbenzoate)-co-styrene]s (3 a-c). Although 1a-c had the chiral twist of the 4-vinylbenzoyl groups, 3a-c showed extremely low optical activities with specific rotations of -2.9 de grees for 3a, -1.1 degrees for 3b, and -0.8 degrees for 3c. As is distinct from the general cases, this is the first exceptional case, in which the te mplate-free copolymers 3a-c exhibited the specific rotations of the same si gn as those of monomers 1a-c. On the other hand, the cyclocopolymerization of (2S,3S)-1,4-dimethoxy-2,3-butanediyl bis(4-vinylbenzoate) (that is, the decarbonyl analog of 1a) (1d) with styrene produced copolymer 3d, which exh ibited optical activity with a specific rotation of -21.9 degrees. The spec ific rotation had the opposite sign to that for monomer 1d. Thus, the carbo nyl groups in the L-tartrate template clearly reduced the chirality inducti on. The influence of the carbonyl groups on the chirality induction was dis cussed by analyzing the conformation of these monomers based on the exciton chirality method.