N-Heterocyclic carbene-mediated redox condensation of alcohols

N-Heterocyclic carbenes (NHCs) with a variety of oxidants promote the Mitsunobu-type coupling reactions of alcohols with phenols, carboxylic acids, and phthalimide.

From the N-Heterocyclic Carbene-Catalyzed Conjugate Addition of Alcohols to the Controlled Polymerization of (Meth)acrylates

Among various N-heterocyclic carbenes (NHCs) tested, only 1,3-bis(tert-butyl)imidazol-2-ylidene (NHC(tBu) ) proved to selectively promote the catalytic conjugate addition of alcohols onto (meth)acrylate substrates. This rather rare example of NHC-catalyzed 1,4-addition of alcohols was investigated as a simple means to trigger the polymerization of both methyl methacrylate and methyl acrylate (MMA and MA, respectively). Well-defined α-alkoxy poly(methyl (meth)acrylate) (PM(M)A) chains, the molar masses of which could be controlled by the initial [(meth)acrylate]0/[ROH]0 molar ratio, were ultimately obtained in N,N-dimethylformamide at 25 °C. A hydroxyl-terminated poly(ethylene oxide) (PEO-OH) macro-initiator was also employed to directly access PEO-b-PMMA amphiphilic block copolymers. Investigations into the reaction mechanism by DFT calculations revealed the occurrence of two competitive concerted pathways, involving either the activation of the alcohol or that of the monomer by NHC(tBu) .

Oxa-Michael addition polymerization of acrylates catalyzed by N-heterocyclic carbenes

N-heterocyclic carbenes (NHCs) smoothly catalyze the oxa-Michael addition polymerization of hydroxyl functionalized acrylate monomers at room temperature via a zwitterionic intermediate.

Transfer hydrogenation promoted by N-heterocyclic carbene and water

N-Heterocyclic carbenes (NHCs) promote the transfer hydrogenation of various activated CC, CN, and NN bonds with water as the proton source.

N-Heterocyclic carbene-catalyzed annulation of cyclic β-enamino esters with enals: access to functionalized indolo[2,3-a]quinolizidines

A novel synthetic approach to functionalized indolo[2,3-a]quinolizidines is developed via an N-heterocyclic carbene-catalyzed annulation of cyclic β-enamino esters with enals.

Cooperative N-heterocyclic carbene/Brønsted acid catalysis for the tail-to-tail (co)dimerization of methacrylonitrile

The first tail-to-tail dimerization of methacrylonitrile (MAN) has been realized by the cooperative use of N-heterocyclic carbene (NHC) and Brønsted acid catalysts, producing 2,5-dimethylhex-2-enedinitrile with the E/Z ratio of 24:76. Although the NHC alone was not effective for the catalysis, the addition of alcohols resulted in the significant increase of the dimer yield up to 82% in the presence of 5 mol % NHC. Detailed experimental studies including the ESI-MS analysis of the intermediates, stoichiometric (co)dimerizations, and deuterium-labeling experiments revealed the mechanistic aspects of the proton transfer, isomerization, umpolung, and rate-limiting steps, allowing us to observe several mechanistic differences between the dimerization of MAN and that of methyl methacrylate. The stoichiometric reactions in the presence and absence of an alcohol suggest that the alcohol additives play a role in promoting the intermolecular proton transfers from the deoxy-Breslow intermediate to the regenerated NHC in the second half of the catalytic cycle. In addition, the codimerizations of MAN with n-butyl methacrylate (n-BuMA) have been studied. While the dimerization of n-BuMA was sluggish in the presence of an alcohol, the catalytic activity for the codimerization was enhanced by the cooperative systems.

N-heterocyclic carbene catalyzed carba-, sulfa-, and phospha-Michael additions with NHC·CO₂ adducts as precatalysts

N-heterocyclic carbene catalyzed Michael additions have been revisited with 1,3-dialkyl- or 1,3-diarylimidazol(in)ium-2-carboxylates, that is, NHC·CO2 adducts, as the source of the free NHC catalysts in solution. Using these precatalysts, a number of efficient carba-, sulfa-, and phospha-Michael additions were achieved very conveniently, without the need for an external strong base to generate the NHC by deprotonation of an azolium salt. To further expand the scope of the procedure, some NHC-catalyzed sulfa-Michael/aldol organocascades were also investigated.

Cooperative N-heterocyclic carbene (NHC)–Lewis acid-mediated regioselective umpolung formal [3 + 2] annulations of alkynyl aldehydes with isatins

A novel synthesis of spirooxindoles has been developed by cooperative NHC–Lewis acid-mediated formal [3 + 2] annulations of alkynyl aldehydes with isatins.

Synthesis of 3,3-disubstituted indoline-2-thiones catalysed by an N-heterocyclic carbene

A catalytic method has been developed for construction of indoline-2-thiones containing an all-carbon quaternary centre at the C-3 position.