Synthesis and Dynamic Behavior of Chiral NNO‐Scorpionate Zinc Initiators for the Ring‐Opening Polymerization of Cyclic Esters

Abstract

The reaction of chiral alcohol–scorpionate compounds bpzbeH [bpzbeH = 1,1‐bis(3,5‐dimethylpyrazol‐1‐yl)‐3,3‐dimethyl‐2‐butanol] or bpzteH [bpzteH = 2,2‐bis(3,5‐dimethylpyrazol‐1‐yl)‐1‐ para ‐tolylethanol] with [ZnR 2 ] (R = Me, Et, CH 2 SiMe 3 ) in a 1:2 molar ratio afforded the dinuclear chiral zinc alkyls [Zn(R)(κ 2 ‐NNµ‐O)Zn(R) 2 ] ( 1 – 4 ) [κ 2 ‐NNµ‐O = bpzbe, R = Me ( 1 ), Et ( 2 ), CH 2 SiMe 3 ( 3 ); bpzte, R = Et ( 4 )]. Subsequent alcoholysis or thioalcoholysis reaction with ArEH (1 equiv.; E = O, S; Ar = 2,6‐C 6 H 3 Me 2 ) yielded the chiral dinuclear mixed alkyl–alryl oxides/thioaryl oxides [(ZnR) 2 (κN:κN‐µ‐O)(µ‐EAr)] ( 5 – 12 ) [κN:κN‐µ‐O = bpzbe, E = O, R = Me ( 5 ), Et ( 6 ), CH 2 SiMe 3 ( 7 ); bpzte, E = O, R = Et ( 8 ); bpzbe, E = S, R = Me ( 9 ), Et ( 10 ), CH 2 SiMe 3 ( 11 ); bpzte, E = S, R = Et ( 12 )]. The alcoholysis reaction of the previously reported monoalkyls [Zn(Me)(κ 3 ‐NNO)] or [Zn(R)(κ‐NNµ‐O)] 2 with ArOH (1 equiv., Ar = 2,6‐C 6 H 3 Me 2 ) afforded the chiral aryl oxides [Zn(OAr)(κ 2 ‐NNµ‐O)] 2 ( 13 – 14 ) [κ 2 ‐NNµ‐O = bpzbe ( 13 ); bpzte ( 14 )]. The X‐ray crystal structures of 3 , 5 , 6 , and 14 confirmed a dinuclear structure in all cases with the alkoxide of the heteroscorpionates in a µ‐bridging mode between the Zn II centers. Variable‐temperature NMR spectroscopic studies were carried out to study their dynamic behavior in solution. Complexes 1 – 9 and 12 can act as single‐component initiators for the ROP of ε‐CL and l ‐/ rac ‐LA, and afforded materials with low molecular weights and narrow monomodal molecular weight distributions. MALDI‐TOF mass spectra confirmed that for 5 – 8 the initiation occurred through nucleophilic attack by the alkoxide group, rather than the alkyl group, on the lactide monomer. Furthermore, inspection of the kinetic parameters showed propagations with a pseudo‐first‐order dependence on monomer and catalyst concentrations. Microstructural analysis of poly( rac ‐lactide) revealed that the mixed alkyl/aryl oxide‐substituted initiators exert a moderate level of heteroselectivity ( P s = 0.66).