Iterative tandem catalysis of secondary diols and diesters to chiral polyesters

The well-known dynamic kinetic resoln. of secondary alcs. and esters was extended to secondary diols and diesters to afford chiral polyesters. This process is an example of iterative tandem catalysis (ITC), a polymn. method where the concurrent action of two fundamentally different catalysts is required to achieve chain growth. In order to procure chiral polyesters of high enantiomeric excess value (ee) and good mol. wt., the catalysts employed need to be complementary and compatible during the polymn. reaction. We here show that Shvo's catalyst and Novozym 435 fulfil these requirements. The optimal polymn. conditions of 1,1'-(1,3-phenylene) diethanol (1,3-diol) and diisopropyl adipate required 2 mol% Shvo's catalyst and 12 mg Novozym 435 per mmol alc. group in the presence of 0.5 M 2,4-dimethyl-3-pentanol as the hydrogen donor. With these conditions, chiral polyesters were obtained with peak mol. wts. up to 15 kDa, an ee value up to 99% and with 1-3% ketone end groups. Also with the structural isomer, 1,4-diol, a chiral polyester was obtained, albeit with lower mol. wt. (8.3 kDa) and slightly lower ee (94%). Aliph. secondary diols also resulted in enantio-enriched polymers but at most an ee of 46% was obtained with mol. wts. in the range of 3.3-3.7 kDa. This low ee originates from the intrinsic low enantioselectivity of Novozym 435 for this type of secondary aliph. diols. The results presented here show that ITC can be applied to procure chiral polyesters with good mol. wt. and high ee from optically inactive AA-BB type monomers. [on SciFinder (R)]