Oxidative N-Heterocyclic Carbene Catalysis

N-Heterocyclic carbene (NHC) catalysis is a by now consolidated organocatalytic platform for a number of synthetic (asymmetric) transformations via diverse reaction modes/intermediates. In addition to the typical umpolung processes involving acyl anion/homoenolate equivalent species, implementation of protocols under oxidative conditions greatly expands the possibilities of this methodology. Oxidative NHC-catalysis allows for oxidative and oxygenative transformations through specific manipulations of Breslow-type species depending upon the oxidant used (external oxidant or O₂ /air), the derived NHC-bound intermediates paving the way to non-umpolung processes through activation of carbon atoms and heteroatoms. This review is intended to update the state of the art in oxidative NHC-catalyzed reactions that appeared in the literature from 2014 to present, with a strong focus to crucial intermediates and their mechanistic implications.

Impact of counteranions on N-heterocyclic carbene gold(i)-catalyzed cyclization of propargylic amide

N-Heterocyclic carbene (NHC) Au(i)-catalyzed organic synthesis has recently been receiving increasing attention, especially with the activation of alkynes. In contrast, counteranions, being widely problematic in Au(i)-catalyzed transformations, are commonly considered as innocent partners and are not respectably included in a computational model. Herein, we report density functional theory (DFT) investigations of the Au(i)-catalyzed cyclization of propargylic amides to exploit the mechanistic effect of several counteranions to shed some light for further future developments. Among the counteranions used in this study, NTf₂ ^-, ClO₄ ^-, TsO^-, TFA^-, TfO^-, MsO^-, and SbF₆ ^-, both the cyclization and protodeauration step favor the 5-exo-dig product over the 6-endo-dig product when the alkyne moiety is terminated with hydrogen. These anions reveal a crucial influence on the energy profile through lowering the barriers of the reaction. Mechanistically, the results obtained from all counteranions show that the protodeauration is slower than the cyclization. By using an energetic span model, the results clearly indicate that the rate-determining state is the protodeauration step for all counteranions, and thus protodeauration is the turnover-limiting step. The turnover frequency (TOF) results for the formation of the 5-exo-dig product show cyclization reactivity in the order of MsO^- > TFA^- > ClO₄ ^- > NTf₂ ^- > TfO^- > TsO^- ≫ SbF₆ ^-, whereas an order of TFA^- > MsO^- > NTf₂ ^- > TfO^- ≈ ClO₄ ^- > SbF₆ ^- ⋙ TsO^- is calculated for the protodeauration, suggesting that SbF₆ ^- and TsO^- are disfavored due to their slow protodeauration. In this regard, and for the 6-endo-dig pathway, our conclusions demonstrate an order of TfO^- > TFA^- > MsO^- > NTf₂ ^- > ClO₄ ^- > TsO^- ⋙ SbF₆ ^- for the cyclization and TFA^- > TsO^- > MsO^- > TfO^- > NTf₂ ^- > ClO₄ ^- ⋙ SbF₆ ^- for the protodeauration, advocating that the anions SbF₆ ^-, NTf₂ ^- and ClO₄ ^- are unlikely partners for the 6-endo-dig pathway because of their slow protodeauration. Finally, the findings here advise that any engineering of the counteranion to increase the efficiency of catalytic system would be more effective on the protodeauration step rather than the cyclization step.

A mechanistic study on the gold(i)-catalyzed cyclization of propargylic amide: revealing the impact of expanded-ring N-heterocyclic carbenes

Density functional theory (DFT) was applied to understand the mechanistic pathway of the gold(i)-catalyzed cyclization of propargylic amide, and to reveal the impact of expanded-ring N-heterocyclic carbenes.

Theoretical investigation on the Cu(i)-catalyzed N-carboxamidation of indoles with isocyanates to form indole-1-carboxamides: effects of solvents

The solvents act as the hydrogen-bond acceptor to facilitate intermolecular addition, and then play the proton-shuttle to assist H1⁺-shift. The stronger electron-donating property of solvent is favorable for the present Cu(i)-catalyzed reactions.

Gold-Catalyzed Homogeneous (Cyclo)Isomerization Reactions

Gold is currently one of the most used metals in organometallic catalysis. The ability of gold to activate unsaturated groups in different modes, together with its tolerance to a wide range of functional groups and reaction conditions, turns gold-based complexes into efficient and highly sought after catalysts. Natural products and relevant compounds with biological and pharmaceutical activity are often characterized by complex molecular structures. (Cyclo)isomerization reactions are often a useful strategy for the generation of this molecular complexity from synthetically accessible reactants. In this review, we collect the most recent contributions in which gold(I)- and/or gold(III)-catalysts mediate intramolecular (cyclo)isomerization transformations of unsaturated species, which commonly feature allene or alkyne motifs, and organize them depending on the substrate and the reaction type.

Theoretical investigation on the mechanism of Cu(ii)-catalyzed synthesis of 4-quinolones: effects of additives HOTf vs. HOTs

Hydrogen-bond donor/proton-donor ability is revealed to be the primary factor that controls the catalytic capability of additives (HOTf vs. HOTs).

Insights into the mechanisms of Cu(i)-catalyzed heterocyclization of α-acyl-α-alkynyl ketene dithioacetals to form 3-cyanofurans: the roles of NH4OAc

NH₄OAc is decomposed into NH₃ and HOAc, and both NH₃ and HOAc as the proton shuttle can prompt catalytic reactions.

Insights into the mechanisms of Ag-catalyzed synthesis of CF3-substituted heterocycles via [3+2]-cycloaddition from α-trifluoromethylated methyl isocyanides: effects of DBU and exploration of diastereoselectivity

The diastereoselectivity of [3+2]-cycloaddition can be reasonably explained by the analysis of steric hindrance using DFT calculations.