Application of capillary electrophoresis to stereochemical conversions. A case study: the photoracemization/hydrolysis of Cr(diimine)(3)(3+) species

Capillary electrophoresis (CE) is shown to be a powerful probe of the stere ochemical changes accompanying photoinitiated racemization and ligand subst itution of optically active Cr(diimine)(3)(3+) complexes (diimine = 2,2'-bi pyridine or 1,10-phenanthroline) in aqueous solution. With dibenzoyl-L-tart rate as a chiral additive in the CE capillary buffer, the electropherograms of photolyzed samples of Lambda -Cr(diimine)(3)(3+) species display excell ent separations of the Lambda and Delta optical isomers of both the parent Cr(diimine),3+ complex and the hydrolysis products. Therefore, we have util ized CE analysis as a direct probe of the extent of racemization of the par ent Lambda -Cr(diimine)(3)(3+) complexes, while simultaneously determining the optical purity of the hydrolysis product. For pH 2 irradiations, racemi zation of both Cr(diimine)(3)(3+) complexes proceeds without concomitant ci s-Cr(diimine)(2)(H2O)(2)(3+) formation. The corresponding data at pH 7.4 re veal that racemization and hydrolysis are concurrent processes, with format ion of the hydrolysis product proceeding with complete loss of optical conf iguration. For photolyses at pH 11.6, extensive base hydrolysis is apparent (again with no retention of absolute configuration), but is accompanied by the absence of direct Lambda -Cr(diimine)(3)(3+) racemization. These resul ts provide support for racemization and hydrolysis proceeding via a common intermediate. (C) 2000 Elsevier Science B.V. All rights reserved.