Unusually Stable Chiral Ethyl Zinc Complexes: Reactivity and Polymerization of Lactide

Chiral diaminophenoxy proligands, H(NNR'OR), where R = t-Bu, H and R' = Me, H, have been developed and their respective zinc ethyl complexes, (NNR'OR)ZnEt, 3a (R = t-Bu, R' = Me), 3b (R = H, R' = Me), 3c (R = t-Bu, R' = H), have been synthesized. The reactivity of 3a with alcohols was explored in detail and compared to a compound reported by Hillmyer, Tolman et al., LZnEt (L = 2,4-di-tert-butyl-6-{[(2'-dimethylaminoethyl)methylamino]-methyl}phenolate). Unlike LZnEt, 3a was inert toward ethanol (as well as methanol, isopropanol, and water). It reacted with phenol and with hydrochloric acid to form (NNMeOtBu)ZnOPh, 4a, and (NNMeOtBu)ZnCl, 5a, respectively. Racemic and enantiopure forms of 4a, (+/-)-4a and (R,R)-4a, were synthesized. The phenoxide complex catalyzed the ring opening polymerization of lactide to atactic poly(lactic acid).