DFT calculations in solution systems: solvation energy, dispersion energy and entropy

DFT calculations of reaction mechanisms in solution have always been a hot topic, especially for transition-metal-catalyzed reactions. The calculation of solvation energy is performed using either the polarizable continuum model (PCM) or the universal solvation model SMD. The PCM calculation is very sensitive to the choice of atomic radii to form a cavity, where the self-consistent isodensity PCM (SCI-PCM) has been recognized as the best choice and our IDSCRF radii can provide a similar cavity. Moving from a gas-phase case to a solution case, dispersion energy and entropy should be carefully treated. The solvent-solute dispersion is also important in solution systems, and it should be calculated together with the solute dispersion. Only half of the solvent-solute dispersion energy from the PCM calculation belongs to the solute molecules to maintain a thermal equilibrium between a solute molecule and its cavity, similar to the treatment of electrostatic energy. Relative solute dispersion energy should also be shared equally with the newly formed cavity. The entropy change from a gas phase to a liquid phase is quite large, but the modern quantum chemistry programs can only calculate the gas-phase translational entropy based on the idea-gas equation. In this review, we will provide an operable method to calculate the solution translational entropy, which has been coded in our THERMO program.

Theoretical Insights into Enantioselective [3 + 2] Cycloaddition between Cinnamaldehyde and Cyclic N-Sulfonyl Trifluoromethylated Ketimine Catalyzed by N-Heterocyclic Carbene

The density functional theory (DFT) method was used to investigate the mechanism and the origin of stereoselectivity of N-heterocyclic carbene (NHC)-catalyzed [3 + 2] cycloaddition between enals and cyclic imine N-sulfonyl trifluoromethyl ketimines at the M06-2X/SMD/6-311+G(d,p)//M06-2X/SMD/6-31G (d,p) level. The results show that the favorable reaction path consists of five steps: nucleophilic attack, proton transfer, the formation of the C-C bond, the tautomerism of the enol intermediate, the formation of the five-membered ring, and the regeneration of the catalyst. For the process of proton transfer, the base-assisted reaction can reduce the activation free energy and make the reaction easier to occur compared with the direct proton transfer process. The formation of the C-C bond is the crucial step of stereoselectivity, in which two chiral centers and four configurations of intermediates (RR/RS/SR/SS) were generated. The free energy barriers obtained and the noncovalent interaction analysis confirm that the dominant configuration is SS, becoming the final trans-type product observed in experiment. Furthermore, through the analyses of the conceptual DFT and natural atomic charges, it is revealed that NHC acts as a double catalyst, which can not only increase the nucleophilicity of reactants by Lewis base but also activate the C-H bond and promote the proton transfer process. The understanding of the mechanism obtained in this study should be helpful to the other organic catalytic reactions with high stereoselectivity.

Phosphine-Catalyzed Synthesis of 3-Allyl-4-pyrones by the Tandem Reaction of Diynones and Allylic Alcohols

A simple and effective tandem reaction of diynones and allylic alcohols was developed to afford functionalized 3-allyl-4-pyrones in moderate to excellent yields. This protocol underwent a Michael addition─Claisen rearrangement─O-cyclization process, which exhibited broad substrate tolerance, high regioselectivity, and atom economy under a metal-free condition. Moreover, functional transformation of the products was also further studied.

Mechanism and regio- and stereoselectivity in an NHC-catalyzed Mannich/lactamization domino reaction

Density functional theory (DFT) calculations (M06-2X) have been employed to disclose the mechanisms and regio- and stereo-selectivities of the N-heterocyclic carbene (NHC)-catalyzed reaction of 2-benzothiazolimines and α-chloroaldehydes. The preferred mechanism is initiated by the nucleophilic attack of NHC on α-chloroaldehyde (first step), followed by 1,2-proton transfer which was assisted by the Brønsted acid DABCO·H+ to generate the Breslow intermediate (second step). The cleavage of the C-Cl bond (third step) and deprotonation (fourth step) form the enolate intermediate. This further reacts with 2-benzothiazolimine which leads to the formation of a new C-C bond (fifth step). Subsequent cyclization takes place via the formation of a new C-N bond (sixth step). Catalyst regeneration completes the whole catalytic cycle and affords the final product (seventh step). The DFT results indicate that the fifth step determines the stereochemistry of the reaction and leads to benzothiazolopyrimidinone with the SS configuration, which agrees well with experimental observations. Intramolecular cyclization is found to be the regioselectivity-determining step, for which the [4+2] annulation pathway is more preferred than that via [2+2] annulation, which again agrees well with experimental observations. Based on the mechanism proposed, the origins of regio- and stereoselectivities have also been investigated by performing distortion/interaction, natural bond orbital (NBO) and non-covalent interaction (NCI) analyses. The mechanistic insights gained in this work should be helpful in the rational design of potential catalysts for analogous reactions.

Mechanistic study on the NHC-catalyzed [3+4] annulation of enals and thiazolones

Reaction mechanisms and origins of regio- and stereo-selectivities of NHC catalyzed [3+4] annulation of enals and thiazolones.