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 [ZnR₂] (R = Me, Et, CH₂SiMe₃) in a 1:2 molar ratio afforded the dinuclear chiral zinc alkyls [Zn(R)(κ²-NNµ-O)Zn(R)₂] (1–4) [κ²-NNµ-O = bpzbe, R = Me (1), Et (2), CH₂SiMe₃ (3); bpzte, R = Et (4)]. Subsequent alcoholysis or thioalcoholysis reaction with ArEH (1 equiv.; E = O, S; Ar = 2,6-C₆H₃Me₂) 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), CH₂SiMe₃ (7); bpzte, E = O, R = Et (8); bpzbe, E = S, R = Me (9), Et (10), CH₂SiMe₃ (11); bpzte, E = S, R = Et (12)]. The alcoholysis reaction of the previously reported monoalkyls [Zn(Me)(κ³-NNO)] or [Zn(R)(κ-NNµ-O)]₂ with ArOH (1 equiv., Ar = 2,6-C₆H₃Me₂) afforded the chiral aryl oxides [Zn(OAr)(κ²-NNµ-O)]₂ (13–14) [κ²-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).

A new series of chiral dinuclear trisalkyl- and alkyl–aryl oxide-containing zinc heteroscorpionates complexes, namely, [Zn(R)(κ²-NNµ-O)Zn(R)₂] and [(ZnR)₂(κN:κN-µ-O)(µ-OAr)], respectively, has been synthesized. Their dynamic behavior has been studied. Both families act as single-component living initiators for the ring-opening polymerization of ϵ-CL and L-/rac-LA.

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