Novel photo-induced deracemization of [Co(acac)(3)] (acac=acetylacetonate) with a chiral ruthenium(II) complex, Delta-[Ru(menbpy)(3)](2+) (menbpy=4,4 '-bis{(1R,2S,5R)-(-)-menthoxycarbonyl}-2,2 '-bipyridine). Reaction mechanism and significant effects

摘要

Deracemization of a racemic mixture of Delta- and Lambda-[Co(acac)(3)] (acac=acetylacetonate) took place with a chiral ruthenium(II) complex, Delta-[Ru(menbpy)(3)](2+) (menbpy=4,4'-bis{(1R,2S,5R)-(-)-menthoxycarbonyl}-2,2'-bipyridine), under visible light (420 < lambda < 470 nm) irradiation in the presence of either triethylamine or NaOH. The enantiomeric excess (e.e.) of Lambda-[Co(acac)(3)] was 28% in acetonitrile-water (7:3 v/v; ionic strength (I)=0.1 mol dm(-3) (KCl)) without acetylacetone and 38% with acetylacetone (50 mmol dm(-3)) added to the solution. Quenching experiments on Delta-*[Ru(menbpy)(3)](2+) led to the conclusion that this deracemization occurs through combination of the preferential photo-reduction of Delta-[Co(acac)(3)] by Delta-*[Ru(menbpy)(3)](2+) to afford [Co(acac)(2)] and the thermal oxidation of [Co(acac)(2)] by Delta-[Ru(menbpy)(3)](3+) to afford preferentially Lambda-[Co(acac)(3)]. The selectivity of deracemization remarkably depends on the solvent; the e.e. value is 51% in acetone-water (7:3 v/v), 38% in ethanol-water (7:3 v/v), and 37% in DMF-water (7:3 v/v). The e.e. value significantly decreases to 33% from 51% in acetone-water (7:3 v/v) when the KCl concentration is increased to 0.3 mol dm(-3) from 0.1 mol dm(-3). When either KF or CH3CO2K is added to acetone-water (7:3 v/v) instead of KCl the deracemization proceeds much more rapidly without decrease of the e.e. value (approximate to 50%). These counter anion effects are interpreted in terms that the reduction potential of [Co(acac)(3)] becomes more negative by addition of either F- or CH3CO2-. [References: 26]