Visible light induced cooperative carbonylation and (hetero)aryl migration: synthesis of multi-carbonyl compounds

Carbonylative transformation represents one of the most straightforward procedures for the synthesis of carbonyl-containing compounds. However, the carbonylative procedure toward 1,4-diketones is still limited which are key moieties with potent applications in various areas. Herein, we report a new strategy for the synthesis of multi-carbonyl compounds containing a 1,4-diketone skeleton through remote heteroaryl migration of traditionally restricted 1,3-migratory substrates utilizing carbon monoxide (CO) as the C1 synthon and diazonium compounds as the starting material.

Highly selective and additive-free Pd(OAc)2/CPP catalyzed hydroaminocarbonylation of alkynes

Herein, the synthesis of branched α,β-unsaturated amides by a hydroaminocarbonylation reaction of alkynes with various amine substrates such as aromatic amines, aliphatic amines, solid amine sources like NH4HCO3, and even strongly basic piperidines is reported, using a Pd(OAc)2/hybrid N-heterocyclic carbene-phosphine-phosphine (CPP) catalytic system. The reactions feature no additives, wide substrate scope, high selectivity (b/l > 99 : 1) and excellent yields. Mechanistic studies have disclosed that the reaction takes place via a palladium hydride pathway. CPP adopts a hybrid bidentate ligand conformation with a carbene-phosphine coordination mode, wherein one phosphorus atom remains externally accessible, potentially serving as a stabilizing auxiliary during catalytic cycles.

N-Heterocyclic carbene/palladium synergistic catalysis in organic synthesis

The cooperation of two distinct catalytic cycles to activate different reactive centers leading to a chemical transformation has been classified as synergistic catalysis. The synergistic combination of NHC with palladium catalysis has emerged as a powerful strategy in the last few years. Merging the ability of NHCs to inverse the polarity of a functional group with the unique reactivity of palladium enables transformations that cannot be accomplished by either of these catalysts alone. Despite the associated challenges, such as quenching of catalysts, reactivity mismatch etc., significant development has been achieved in the field of NHC/Pd synergistic catalysis. The recent incorporation of photoredox catalysis with NHC/Pd synergistic catalysis has further advanced this area. This review highlights the developments made in the area of NHC/Pd synergistic catalysis.

Asymmetric redox benzylation of enals enabled by NHC/Ru cooperative catalysis

The development of general methods for asymmetric benzylation of prochiral carbon nucleophiles remains a challenge in organic synthesis. The merging of ruthenium catalysis and N-heterocyclic carbene (NHC) catalysis for asymmetric redox benzylation of enals has been achieved, which opens up strategic opportunities for the asymmetric benzylation reactions. A wide range of 3,3'-disubstituted oxindoles with a stereogenic quaternary carbon center widely existing in natural products and biologically interesting molecules is successfully obtained with excellent enantioselectivities [up to 99% enantiomeric excess (ee)]. The generality of this catalytic strategy was further highlighted by its successful application in the late-stage functionalization of oxindole skeletons. Furthermore, the linear correlation between ee values of NHC precatalyst and the product elucidated the independent catalytic cycle of either the NHC catalyst or the ruthenium complex.

Rate and equilibrium constants for the addition of triazolium salt derived N-heterocyclic carbenes to heteroaromatic aldehydes

Heteroaromatic aldehydes are often used preferentially or exclusively in a range of NHC-catalysed processes that proceed through the generation of a reactive diaminoenol or Breslow Intermediate (BI), with the reason for their unique reactivity currently underexplored. This manuscript reports measurement of rate and equilibrium constants for the reaction between N-aryl triazolium NHCs and heteroaromatic aldehydes, providing insight into the effect of the NHC and heteroaromatic aldehyde structure up to formation of the BI. Variation in NHC catalyst and heteroaromatic aldehyde structure markedly affect the observed kinetic parameters of adduct formation, decay to starting materials and onward reaction to BI. In particular, large effects are observed with both 3-halogen (Br, F) and 3-methyl substituted pyridine-2-carboxaldehyde derivatives which substantially favour formation of the tetrahedral intermediate relative to benzaldehyde derivatives. Key observations indicate that increased steric hindrance leads to a reduction in both k ₂ and k _-1 for large (2,6-disubstituted)-N-Ar groups within the triazolium scaffold, and sterically demanding aldehyde substituents in the 3-position, but not in the 6-position of the pyridine-2-carboxaldehyde derivatives. As part of this study, the isolation and characterisation of twenty tetrahedral adducts formed upon addition of N-aryl triazolium derived NHCs into heteroaromatic aldehydes are described. These adducts are key intermediates in NHC-catalysed umpolung addition of heteroaromatic aldehydes and are BI precursors.

Enantioselective Cascade Annulation of α-Amino-ynones and Enals Enabled by Gold and Oxidative NHC Relay Catalysis

We report herein an unprecedented gold and oxidative NHC relay catalysis that enables highly enantioselective cascade annulation between readily available α-amino-ynones with enals. This method utilizes the in situ-generated pyrrolin-4-ones as a novel and versatile synthon, which engage with α,β-unsaturated acylazolium intermediates generated from enals by oxidative NHC catalysis to produce pyrrole-fused lactones in high yield and excellent enantioselectivity. Synthetic utility of the lactone products is also demonstrated by facile conversion to densely functionalized pyrroles and pyrrolin-4-ones in high yields with excellent stereopurity.

Recent developments on NHC-driven dual catalytic approaches

This article takes an in depth look at the early relay combination of NHC and other catalysts, and the latest progress in dual catalysis, analyzing the breakthroughs and limitations.

Direct α-Acylation of Alkenes via N-Heterocyclic Carbene, Sulfinate, and Photoredox Cooperative Triple Catalysis

N-Heterocyclic carbene (NHC) catalysis has emerged as a versatile tool in modern synthetic chemistry. Further increasing the complexity, several processes have been introduced that proceed via dual catalysis, where the NHC organocatalyst operates in concert with a second catalytic moiety, significantly enlarging the reaction scope. In biological transformations, multiple catalysis is generally used to access complex natural products. Guided by that strategy, triple catalysis has been studied recently, where three different catalytic modes are merged in a single process. In this Communication, direct α-C-H acylation of various alkenes with aroyl fluorides using NHC, sulfinate, and photoredox cooperative triple catalysis is reported. The method allows the preparation of α-substituted vinyl ketones in moderate to high yields with excellent functional group tolerance. Mechanistic studies reveal that these cascades proceed through a sequential radical addition/coupling/elimination process. In contrast to known triple catalysis processes that operate via two sets of interwoven catalysis cycles, in the introduced process, all three cycles are interwoven.

Umpolung coupling of pyridine-2-carboxaldehydes and propargylic carbonates via N-heterocyclic carbene/palladium synergetic catalysis

The umpolung cross-coupling reaction of pyridine-2-carboxaldehydes and propargylic carbonates has been developed for the first time through N-heterocyclic carbene/palladium cooperative catalysis with the judicious selection of the palladium catalyst, ligand and N-heterocyclic carbene, giving the propargylic ketones regioselectively.

Generation of azolium dienolates as versatile nucleophilic synthons via N-heterocyclic carbene catalysis

The recent advances in N-heterocyclic carbene (NHC)-catalyzed generation of azolium dienolates from different precursors and their synthetic applications for the construction of various valuable molecules are summarized comprehensively in this review.

Combined Photoredox and Carbene Catalysis for the Synthesis of Ketones from Carboxylic Acids

As a key element in the construction of complex organic scaffolds, the formation of C-C bonds remains a challenge in the field of synthetic organic chemistry. Recent advancements in single-electron chemistry have enabled new methods for the formation of various C-C bonds. Disclosed herein is the development of a novel single-electron reduction of acyl azoliums for the formation of ketones from carboxylic acids. Facile construction of the acyl azolium in situ followed by a radical-radical coupling was made possible merging N-heterocyclic carbene (NHC) and photoredox catalysis. The utility of this protocol in synthesis was showcased in the late-stage functionalization of a variety of pharmaceutical compounds. Preliminary investigations using chiral NHCs demonstrate that enantioselectivity can be achieved, showcasing the advantages of this protocol over alternative methodologies.

Construction of enantiopure imine bridged benzo[c]azepinones by a silver(i) and chiral N-heterocyclic carbene multicatalytic reaction sequence of N′-(2-alkynylbenzylidene)hydrazides and cyclopropanecarbaldehydes

A novel and efficient method for the synthesis of enantiopure bridged benzo[c]azepinone compounds has been developed by metal/N-heterocyclic carbene multicatalysis.

Dual N-heterocyclic carbene/photocatalysis: a new strategy for radical processes

Dual N-heterocyclic carbene/photocatalysis displays unique features compared with the traditional methods and indicates high potential for new radical processes.

Dehydrative Allylation between Aldehydes and Allylic Alcohols through Synergistic N-Heterocyclic Carbene/Palladium Catalysis

Dehydrative allylation between widely available aldehydes and allylic alcohols to afford β,γ-unsaturated ketones was enabled by a synergistic merger of a thiazolium N-heterocyclic carbene catalyst and a palladium-bisphosphine catalyst.

A highly efficient dual catalysis approach for C-glycosylation: addition of (o-azaaryl)carboxaldehyde to glycals

Dual catalysis by concurrent activation of glycals and (O-azaaryl)-carboxaldehydes using palladium and N-heterocyclic carbene has been developed. This activation through the formation of the Breslow intermediate and a π-allyl Pd complex is a novel and efficient approach to yield C-glycosides with yields up to 85%.