Modeling the Mechanism of CO2/Cyclohexene
Oxide Copolymerization Catalyzed by Chiral Zinc β‑Diiminates:
Factors Affecting Reactivity and Isotacticity
Posted on 2020-07-27 - 18:14
Copolymerization of CO2 and cyclohexene oxide (CHO)
upcycles CO2 into the value-added, chemically recyclable,
thermoplastic poly(cyclohexene carbonate) (PCHC). Using density functional
theory, the Zn-catalyzed copolymerization mechanism has been characterized
with a particular focus on the effects of chiral β-diiminate
(BDI) ligands as they influence the reactivity and enantioselectivity
in the epoxide ring-opening step, where the latter is required for
isotacticity. Theory indicates that both mono- and binuclear forms
of the catalyst are involved along the reaction path, with the turnover-limiting
step being ring-opening of the epoxide mediated by a binuclear catalyst.
Subsequent CO2 insertion is predicted to be kinetically
facile and preferentially mediated by a mononuclear catalyst. The
predicted preference for epoxide opening to give R,R-stereocenters in the copolymer when N-(4-(((1S,2S)-2-(benzyloxy)cyclohexyl)amino)-5,5,5-trifluoropent-3-en-2-ylidene)-2,6-dimethylaniline
is used as the BDI ligand agrees with the experiment and is attributed
to differential ligand distortions associated with key non-bonded
interactions in the competing transition-state structures. Further
analysis predicts that 2,6-dichloro and dibromo substitutions of the
BDI ligand N-aryl group(s) should result in increased
rates and enantioselectivities for copolymerization.
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Shao, Huiling; Reddi, Yernaidu; Cramer, Christopher J. (2020). Modeling the Mechanism of CO2/Cyclohexene
Oxide Copolymerization Catalyzed by Chiral Zinc β‑Diiminates:
Factors Affecting Reactivity and Isotacticity. ACS Publications. Collection. https://doi.org/10.1021/acscatal.0c02299
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AUTHORS (3)
HS
Huiling Shao
YR
Yernaidu Reddi
CC
Christopher J. Cramer
KEYWORDS
BDI ligandcyclohexene oxideIsotacticity CopolymerizationSubsequent CO 2 insertionFactors Affecting Reactivitychiral β- diiminateupcycles CO 2dibromo substitutionsCO 2Zn-catalyzed copolymerization mechanismligand distortionsepoxide openingcatalystturnover-limiting stepepoxide ring-opening stepnon-bonded interactionstransition-state structuresPCHCCHOreaction pathBDI ligand N