The reaction mechanism for the Zn(salphen)/NBu4X (X = Br, I) mediated cycloaddition of CO2 to a series of epoxides, affording five-membered cyclic carbonate products has been investigated in detail by using DFT methods. The ring-opening step of the process was examined and the preference for opening at the methylene (Cβ) or methine carbon (Cα) was established. Furthermore, calculations were performed to clarify the reasons for the lethargic behavior of internal epoxides in the presence of the binary catalyst. Also, the CO2 insertion and the ring-closing steps have been explored for six differently substituted epoxides and proved to be significantly more challenging compared with the ring-opening step. The computational findings should allow the design and application of more efficient catalysts for organic carbonate formation.

Al(III) complexes of amino-tris(phenolate) ligand scaffolds have been prepared to attain highly Lewis acidic catalysts. Combination of the aforementioned systems with ammonium halides provides highly active catalysts for the synthesis of organic carbonates through addition of carbon dioxide to oxiranes with initial turnover frequencies among the highest reported to date within the context of cyclic carbonate formation. Density functional theory (DFT) studies combined with kinetic data provides a rational for the relative high activity found for these Al(III) complexes, and the data are consistent with a monometallic mechanism. The activity and versatility of these Al(III) complexes has also been evaluated against some state-of-the-art catalysts and the combined results compare favorably in terms of catalyst construction, stability, activity, and applicability.

Amino-triphenolate derived Al(III) complexes combined with suitable nucleophiles have been investigated as binary catalysts for the coupling of limonene oxide and carbon dioxide to afford alternating polycarbonates. These catalysts are able to produce stereo-regular, perfectly alternating trans-polymers from cis-limonene oxide, whereas the pure trans isomer and cis/trans mixture give rise to lower degrees of stereo-regularity. The best Al(III) catalyst shows the potential to mediate the conversion of both stereo-isomers of limonene oxide with high conversion levels of up to 71% under neat conditions indicating a high robustness and atom-efficiency of this catalytic process. Computational studies have revealed unique features of the binary catalyst system among which is the preferred nucleophilic attack on the quaternary carbon centre in the limonene oxide substrate.

A detailed study on the mechanism for the alternating copolymerization of cyclohexene oxide (CHO) and CO mediated by an [Al{amino-tri(phenolate)}]/NBu I binary catalyst system was performed by using DFT-based methods. Four potential mechanisms (one monometallic and three bimetallic) were considered for the first propagation cycle of the CHO/CO copolymerization. The obtained Gibbs free energies provided a rationale for the relative high activity of a non-covalent dimeric structure formed in situ and thus for the feasibility of a bimetallic mechanism to obtain polycarbonates quantitatively. Gibbs free energies also indicated that the alternating copolymerization was favored over the cyclic carbonate formation.

Entropic corrections are found to be crucial for evaluating the catalytic performance in solution for the reaction of CO2 with epoxides.