Racemization and deracemization of poly(quinoxaline-2,3-diyl)s

Racemization of optically active oligo-and poly(quinoxaline-2,3-diyl) derivatives with a single screw-sense, prepared by the polymerization of 3,6-diaryl-1,2-diisocyanobenzenes catalyzed by methylpalladium(II) initiator, was investigated. Pentameric and hexameric oligomers with a living palladium terminus were reluctant to undergo racemization in solution at room temperature. However, the (oligoquinoxalinyl)bis(phosphine)palladium(II) complexes underwent a phosphine exchange reaction in the presence of an excess of phosphine ligand with partial racemization at room temperature. Oligomers without palladium terminus, prepared by a treatment of the corresponding palladium derivatives with methylmagnesium bromide, underwent racemization more rapidly, the rate of which depended upon the bulkiness of the substituents at the 5,8-positions of the quinoxaline rings and oligomerization degree, i.e., the number of the quinoxaline units in the oligomer chain. Starting from the racemic polymers with a living palladium terminus, substantial deracemization of the helical poly(quinoxaline-2,3-diyl) was induced by means of optically active 2,3-O-isopropylidenedioxy-1,4-bis(diphenylphosphino)butane ligand, which coordinated to the palladium terminus.