Organometallic Rhenium Complex Catalysts Theoretical Study Of The Epoxy Compounds

In this paper, density functional theory (DFT) calculations at the B3YLP level are carried out to study and analyze the following three problems on the basis of experiments. Our major intention is to investigate the molecular sturetures, bonding, and mechanisms on the organometallic reactions mentioned below.(1) In 2009, Abu-Omar and coworkers reported MeReO3-catalyzed deoxygenation of epoxides and diols and proposed a mechanism starting with addition of H2 to MeReO3 (abbreviated as MTO). Our calculations showed that addition of H2 to MTO via the [2+3] mechanism is significantly endothermic and the barrier is inaccessibly high. In this work, we proposed an alternative mechanism with the aid of density functional theory (DFT) calculations. In the alternative mechanism, MTO and epoxide react first to give a five-membered-ring rhenium diolate intermediate (2). To the intermediate, addition of H2 via a [2+3] mechanism gives an oxo-hydroxy species (4). A proton transfer in 4 results in formation of a rhenium diolate intermediate (6) having a water ligand. Subsequent extrusion of olefin from the rhenium diolate intermediate (6) completes the reaction and regenerates the catalyst MTO.Formation of diol at the beginning of the reaction observed experimentally is related to the hydrolysis of epoxide with the co-product water through the rhenium diolate intermediate (2) via two possible paths. Our calculation results confirmed that the rhenium diolate intermediate (2)+ H2O and the hydrolysis products (MTO+diol) are in fast equilibrium, resulting in the eventual conversion of diol to olefin. Meanwhile, we obtained the mechanism of the reactions about the diol as the reactant. MTO and diol react first to give a five-membered-ring rhenium diolate intermediate (2). To the intermediate, addition of H2 via a [2+3] mechanism gives an oxo-hydroxy species (4). A proton transfer in 4 results in formation of a rhenium diolate intermediate (6) having a water ligand. Subsequent extrusion of olefin from the rhenium diolate intermediate (6) completes the reaction and regenerates the catalyst MTO. This explained Abu-Omar's experimental phenomenon successfully.(2) In 2005, Jiang and coworkers reported the first example of the Re(CO)5Br or CpRe(CO)3-catalyzed coupling of CO2 with epoxides to give cyclic carbonates at 383.15 K under solvent-free conditions and the yield up to 97%. Re(CO)5Br was shown to be an efficient, simple catalyst in this coupling reaction. They also postulated a possible mechanism for the reaction based on experimental observations, which is closely related to that of the Ni-catalyzed reactions. Most recently, Guo and coworkers carried out a DFT study on the mechanism of the coupling reaction between chloromethyloxirane and carbon dioxide catalyzed by Re(CO)5Br. Two possible mechanisms I and II were proposed. Mechanism I starts with epoxide oxidative addition, while mechanism starts with the CO2 activation by Re(CO)4Br. But we found that the overall barriers calculated for the two mechanisms are inaccessibly high, which promotes us to explore an alternative mechanism for the reaction on the basis of our DFT calculations. In this paper, we proposed an alternative mechanism in which the overall activation barrier height is much lower and the intermediates involved are found to be significantly more stable. In the alternative mechanism (V-b), initially, decarbonylation of Re(CO)5Br affords the active 16e intermediate, Re(CO)4Br. Oxidative addition of the C-0 bond of epoxides to Re-Br bond of Re(CO)4Br gives the metallaoxetane intermediate a'. Insertion of CO2 into the Re-O bond gives the metallaoxetane intermediate c', and finally reductive elimination of C-0 bond from f produces the cyclic carbonate and regenerates the active form of the catalyst Re(CO)4Br. In summary, three main stages are involved:epoxide oxidative addition, CO2 insertion and reductive elimination of product. It is noteworthy that the Br atom played an important role in the catalyticcycle reaction.In this paper, we also investigated the mechanism of the reactions where the CpRe(CO)3 was employed as catalyst. We proposed three mechanismsⅠ',Ⅱ' andⅢ'. The mechanismⅢ' is found to be reasonable. Initially, decarbonylation of CpRe(CO)3 affords the active 16e intermediate, CpRe(CO)2. Oxidative addition of the C-0 bond of epoxides to CpRe(CO)2 gives the metallaoxetane intermediate i. Insertion of CO2 into the Re-O bond gives the metallaoxetane intermediate n', and then isomerization of n'results in the metallaoxetane intermediate n. Finally, reductive elimination of C-0 bond from n produces the cyclic carbonate and regenerates the active form of the catalyst CpRe(CO)2.(3) On the basis of Templeton's experiment (West, N. M.; et al. Organometallics 2008,27, 5252), the mechanisms of the main and the side reactions between (Cl-nacnac)Pt(H) (Cl-nacnac: bis(N-aryl)-β-diiminate) and a terminal alkyne were investigated by density functional theory. Our study shows that the 1,2-insertion of t-BuC=CH into the Pt—H bond generates the main products and that C—C bond formation is the rate-determining step. The 2,1-insertion of t-BuC≡CH into the Pt—H bond generates the byproducts and alkyne insertion is the rate-determining step. Based on the mechanisms of the main and side reactions the presence of the main product and the by-product could be explained. We found that the main product is thermodynamically controlled while the side product is kinetically controlled.