DFT Rationalization of Gold(I)-Catalyzed Couplings between Alkynyl Thioether and Nitrenoid Derivatives: Mechanism, Selectivity Patterns, and Effects of Substituents

Density Functional Theory Rationalization of the Mechanism, Selectivity, and Role of Substituents in Au(I)-Catalyzed Synthesis of Pyrazolines and Dihydropyridines

A comprehensive theoretical investigation into the gold-catalyzed synthesis of polysubstituted pyrazolines and dihydropyridines from imines and methyl phenylpropiolate was conducted in this study. Three imines with distinct substituents were selected as model reactants. The computational outcomes reveal that four-membered intermediates generated from aza-enyne metathesis significantly affect the reaction selectivity. For nitrogen-centered NHCO2Me substituents (series A), an outward ring opening occurs during the metathesis of the aza-alkyne. This leads to the formation of Z-butadiene intermediates and ultimately to pyrazoline products. Conversely, with an aromatic substituent at the nitrogen site (series B and C), an inward ring opening takes place. This results in E-butadiene intermediates and the synthesis of dihydropyridine derivatives. The dihydropyridine product's configuration is determined by the aromatic ring's substituent. Electron-donating groups tend to directly form 1,4-dihydropyridine through a 6π electrocyclization (series B). In contrast, strong electron-withdrawing substituents initially undergo azayne metathesis, followed by 6π electrocyclization to produce 1,2-dihydropyridine products (series C). Furthermore, the distinctive selectivities were investigated in depth using global reactivity index and distortion/interaction methods. This research may contribute to the design of more effective and selective protocols to access pyrazolines, dihydropyridines, and related compounds.

DFT rationalization of the mechanism and selectivity in a gold-catalyzed oxidative cyclization of diynones with alcohols

The mechanism, regioselectivity, and chemoselectivity in a gold-catalyzed oxidative cyclization of diynones with alcohols to give furan-3-carboxylate derivatives were explored by density functional theory (DFT). The obtained results revealed that the first step of the global reaction involves a nucleophilic attack of a pyridine-N-oxide derivative on the catalyst-ligated diynone, forming a vinyl intermediate that can isomerize to an α,α'-dioxo gold carbene upon the cleavage of the N-O bond. In the second step, a nucleophilic addition is also completed via pyridine-N-oxide instead of an alcohol proposed in the experiment. In the following steps, the selective nucleophilic addition of alcohol, 1,2-alkynyl migration, five-membered cyclization, and protodeauration lead to the furan-based products with the regeneration of the gold catalyst. The unique features of regio- and chemoselectivity were investigated in detail by the global reactivity index (GRI) and distortion/interaction analyses. Apart from fully rationalizing the experimental data, the DFT results provide an important contribution to understanding, optimizing, and further developing the related types of organic transformations.

Regiodivergent Synthesis of 4- and 5-Sulfenyl Oxazoles from Alkynyl Thioethers

The regiodivergent synthesis of 4- and 5-sulfenyl oxazoles from 1,4,2-dioxazoles and alkynyl thioethers has been achieved. Gold-catalysed conditions are used to favour the formation of 5-sulfenyl oxazoles via β-selective attack of the nitrenoid relative to the sulfenyl group. In contrast, 4-sulfenyl oxazoles are formed by α-selective reaction under Brønsted acid conditions from the same substrates. The nature of stabilising gold-sulfur interactions have been investigated by natural bond orbital analysis, showing that the S→Au interactions are significantly stronger in the intermediate that favours the 5-sulfenyl oxazoles. A kinetic survey identifies catalyst inhibition processes. This study into the regiodivergent methods includes the development of telescoped annulation-oxidation protocols for regioselective access to oxazole sulfoxides and sulfones.

DFT Study on the Mechanisms and Selectivities in Rh (III)-Catalyzed [5 + 1] Annulation of 2-Alkenylanilides and 2-Alkylphenols with Allenyl Acetates

The mechanisms and regio-, chemo-, and stereoselectivity were theoretically investigated in the Rh(III)-catalyzed [5 + 1] annulation of 2-alkenylanilides and 2-alkylphenols with allenyl acetates. Two different reactants, 2-alkenylanilides and 2-alkylphenols, were selected as model systems in the density functional theory calculations. The obtained theoretical results show that both these reactants exhibit similar steps, namely, (1) N-H/O-H deprotonation and C-H activation, (2) allenyl acetate migratory insertion, (3) β-oxygen elimination, (4) intramolecular nucleophilic addition of the nitrogen/oxygen-rhodium bond resulting in [5 + 1]-annulation, and (5) protonation with the formation of the desired product and regeneration of the Rh(III) catalyst. The theoretical evidence suggests that the selectivity is determined at the step of allenyl acetate's migratory insertion. Moreover, the regioselectivity is driven by electronic effects, while the interaction energies (C-H···π and C-H···O interactions) play a more imperative role in controlling the stereoselectivity. The obtained theoretical results not only well rationalize the experimental observations but also provide important mechanistic insights for related types of [5 + 1]-annulation reactions.

DFT study on isothiourea-catalyzed C-C bond activation of cyclobutenone: the role of the catalyst and the origin of stereoselectivity

The organocatalytic C-C bond activation strategy stands out as a new reaction mode for the release of ring strain and expands the scope of organocatalysts. Thus, disclosing the role of the organocatalyst in the C-C bond cleavage process would be of interest. Here, an isothiourea-catalyzed C-C bond activation/cycloaddition reaction of cyclobutenone is selected as a computational model to uncover the role of the catalyst. Based on the calculations, the electrocyclic cleavage of cyclobutenone is calculated to be energetically more favorable than the isothiourea-catalyzed C-C bond cleavage, which is different from the NHC-catalyzed C-C bond activation of cyclobutenone. The computational results show that the isothiourea promotes the reaction by increasing the nucleophilicity of vinyl ketene and thus lowers the energy barrier of the cycloaddition process. Moreover, NCI and AIM analyses are performed to disclose the origin of stereoselectivity.

Elucidating the mechanism and origin of stereoselectivity in the activation/transformation of an acetic ester catalyzed by an N-heterocyclic carbene

The activation of an ester by N-heterocyclic carbene (NHC) organocatalysis is an efficient and important approach for generating an NHC-bound enolate intermediate, an important active intermediate in the transformation of carbonyl compounds. Herein, we perform a theoretical study on the NHC-catalyzed activation and transformation reaction of an acetic ester in which the NHC-bound enolate intermediate is a key intermediate. Multiple activation and transformation pathways are proposed and analyzed to identify an energetically favorable pathway. The use of different substrates for the reaction is considered. When a chalcone substrate is used, [4+2] cycloaddition between the enolate intermediate and the chalcone is identified to be both the rate- and stereoselectivity-determining step for the reaction, with the R-configured product being generated as the major isomer. Noncovalent interaction (NCI) and atoms-in-molecules (AIM) analyses are performed to identify the origin of the stereoselectivity of the reaction, and a local reactivity analysis is conducted to explore substrate and catalyst effects on the reaction.