Theoretical Investigation On The Reaction Mechanism Of Green Chemistry Catalysts Assisted By Water Or Peptide

Green chemistry is a leading topic in modern chemistry. However, in order to achieve the goal of chemical sustainable waste minimization, the use of catalyst has been identified as crucial. The investigation of catalysts, especially the green catalyst has attracted the interest of a great number of research groups. As one of the essential research tools, computation chemistry has an advantage of obtaining structures and energies of catalysts and energy minima for reactants, catalytic intermediates and products, and can be used for computing the structures of the hypothetical activated complexes associated with transition states, which can never be observed directly by experiment. In this way, one can model the reaction pathways and the corresponding reaction kinetics in detail, gaining insight into the various elementary steps in the catalytic cycle and predict the most reasonable mechanisms for further practical investigation and design. In this work, we focused on the computational investigation on some typical green catalysts. The main results are summarized as follows.Water is the most preferred green catalyst because of its availability, ease of use, and safe disposal into the environment. The mechanism of the aromatic Claisen rearrangement of 1-（4-chloronaphthyl） 1,1-dimethylallyl ether under neat condition and“on water”were investigated at the B3LYP/6-311++G（d,p） level. The on water condition was modeled by a combination of the“oil”droplet/water interface and neat condition inside the oil droplet. The MD simulation was used to obtain the most reliable interaction position between reactant and solvent water, which was further used as a starting material for the water-catalyst mechanism to model the surface reaction. We found that the chairlike [3,3]-intramolecular shift became the rate-limiting step for the water-catalyst mechanism with lower energy barrier in both gas-phase and QM/MM simulated bulk water, compared with those under neat condition. The experimental findings were further explained by binding energy, NPA, and donor-acceptor NBO calculations.We also investigated the mechanism of the water acceleration of the“on water”nucleophilic opening of epoxide using the B3LYP/6-311++G（d,p） method. The water-free and water-catalyst mechanisms were used as models for neat and surface reactions, respectively. Two pathways of water-free mechanism are competitive. A four-water cluster was considered as a key surface fragment in the water-catalyst mechanism. The most favorable pathway in the water-catalyst mechanism has only one step, with the epoxide opening of the rear-side attack and the proton-transfer process taking place in a concerted fashion. This suggests that the rate acceleration of“on water”reactions depends mainly on the surface reaction where the larger“oil”droplet/water interface corresponds to a faster reaction. The binding energy and electron density results revealed that the effect of water-cluster catalysis is more favorable in the transition state than in the other states. The NPA analysis revealed that the water cluster accelerated the reaction not only by assisting the proton transfer, but also by strengthening both the entering and the leaving groups through a charge-transfer process induced by different strengths between the two proton-transfer processes. The enhancing of the entering- and leaving-group effects were qualitatively supported by the evaluation of the nucleophilicity index and the stabilization energy, respectively. Practically, pulverization of substrates before reaction and vigorous stirring during reaction might be necessary requirements for the“on water”reaction.The mechanism of the deamination of C and 5mC under both water and bisulfite conditions was investigated by theoretical methods. Our calculation results suggest that the deamination of C in bisulfite can occur at room temperature because of an“earlier”transition state on the reaction coordinate, which is induced by the hydrogen bond between the adducted–SO₃^- group and water molecule. The comparatively slower deamination of 5mC in bisulfite is mainly ascribed to the stable intermediate of its competing rate-limiting step, because of an additional hydrogen bond between methyl group and water molecule.We investigated the mechanism of peptide-catalyzed asymmetric Simmons–Smith cyclopropanation for unfunctionalized olefins at the B3LYP/6-31G（d） level. The calculated results of model reactions showed that the coordinated Lewis acidic zinc halide ZnX2 （X=Cl, I） and/or halo-methylzinc halide XZnCH2X in the catalyst play an important role in the enantioselectivity. The catalyst not only forms the ring with the substance in the reaction centre, but also establishes two steric repulsions that can lead to an explanation for the high enantioselectivity. Hence, these results highlight some important insights for the prerequisites of an effective catalyst and a proper substrate towards high enantioselectivity for this kind of reaction.Organometallic catalyst is of great contemporary interest and has broad potential in catalyzing C-H activation/functionalization. The mechanism of water-assisted Pd（Ⅳ）-catalyzed benzylic C-H amination was investigated both experimentally and theoretically. Our calculation at the B3LYP/6-311++G（d,p） （with ECP for Pd atom） level. Our results reveal that right amount of water affects the reaction dramatically. The favorable pathway proceed three steps: the first step is the oxidation addition of N-fluorobenzenesulfonimide （NFSI） to the complex formed by N-tolylacetarmide and Pd（OAc）₂ to a generate Pd（Ⅳ）-F catalyst, then two protons in N-tolylacetarmide were abstracted by this catalyst and formed quinoid intermediate, which can be added by the N-（phenylsulfonyl）benzenesulfonamide （NHSI） and form the final product. In the catalyst, the Pd（Ⅳ）-F bond can also be cleaved with the assistance of water and form Pd（Ⅳ）-OH bond in the effectual catalyst, which has similar activation as Pd（Ⅳ）-F catalyst. Moreover, we discovered the spiro-cyclopalladation intermediate for the first time. The NBO calculation reveals that spiro-Pd-C bond is mainly contributed by d orbital and/or s orbital of Pd and p orbital of C.We also investigated the mechanism of water-assisted palladium catalyzed allylic C-H amination of unactivated alkenes using the B3LYP/6-31G（d,p） （with ECP for Pd atom） method. Our results suggest that the reaction undergo the Pd（Ⅱ）/Pd（Ⅳ） catalytic cycle and water evidently plays an important role in this reaction. The initial Pd（0） was oxidated addicted by NFSI and formed the effectual Pd（Ⅱ） catalyst. This suggests that with the assistance of water both Pd（Ⅱ）-F and Pd（Ⅳ）-OH catalyst activates the allylic C-H bond. This is because more NFSI was oxidated to the intermediate and C-N elimination process was carried out for form the final product.