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

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.