The NHCs-mediated cross-coupling of aromatic aldehydes with benzyl halides: synthesis of α-aryl ketones

Research Progress on N-Heterocyclic Carbene Catalyzed Reactions for Synthesizing Ketones through Radical Mechanism

NHC-catalyzed radical cross-coupling reactions have been recently developed; they provide an efficient method to access ketones from aldehydes or carboxylic acid derivatives with sp3-hybridized carbon radical precursors. This reaction has indirectly solved the limitations in the scope of coupling partners in NHC umpolung catalyzed reactions of aldehydes. In this short review, we present some recent advances in NHC-catalyzed radical reactions, with a focus on the construction of the C–C(CO) bond.

1 Introduction

2 Oxidative Generation of NHC-Derived Ketyl Radical

2.1 NHPI Redox-Active Esters

2.2 Katritzky Pyridinium Salts

2.3 Alkyl Halides

2.4 Aryl Halides

2.5 Compounds Containing N–O Bond

2.6 Diazo Esters

2.7 Others

3 Reductive Generation of NHC-Derived Ketyl Radical

3.1 Hantzsch Esters

3.2 Sulfinates

3.3 Electron-Rich Arenes

3.4 Amines

3.5 Organoborane Reagents

4 Conclusion

Charge-Tagged N-Heterocyclic Carbenes (NHCs): Revealing the Hidden Side of NHC-Catalysed Reactions through Electrospray Ionization Mass Spectrometry

N-heterocyclic carbenes (NHCs) are key intermediates in a variety of chemical reactions. Owing to their transient nature, the interception and characterization of these reactive species have always been challenging. Similarly, the study of reaction mechanisms in which carbenes act as catalysts is still an active research field. This Minireview describes the contribution of electrospray ionization mass spectrometry (ESI-MS) to the detection of charge-tagged NHCs resulting from the insertion of an ionic group into the molecular scaffold. The use of different mass spectrometric techniques, combined with the charge-tagging strategy, allowed clarification of the involvement of NHCs in archetypal reactions and the study of their intrinsic chemistry.

N-Heterocyclic Carbene-Catalyzed Chemoselective S-O Bond Cleavage of Benzenesulfonic Carbamate

An unprecedented example of NHC-catalyzed chemoselective S-O bond cleavage of dinitrobenzenesulfonic carbamates is described. This protocol features several advantages, including mild reaction conditions, broad substrate scope, and operational simplicity, which allows it to be an attractive synthetic method for hydroxylamine synthesis. Notably, dinitrobenzenesulfonic carbamates not only work as "O" synthons but also serve as an efficient oxidant in this reaction.

N-Heterocyclic Carbene Catalysis under Oxidizing Conditions

N-heterocyclic carbene organocatalysis under oxidizing conditions provides a vast range of various synthetic procedures via diverse mechanisms. The available catalysts, bases, oxidants, and oxidizing methods afford numerous opportunities for developing this branch of organocatalysis. Furthermore, implementation of tandem reactions and cooperative catalysis in the described methodology significantly expands the possibilities of modern organic chemistry. This approach allows the synthesis of different structurally complex and often enantiomerically enriched substances, which can be interesting in terms of biological activity and natural product synthesis. Many esters, amides, thioesters, lactams, lactones, and other cyclic compounds obtained in oxidative or oxygenative reactions promoted by N-heterocyclic carbenes can be interesting precursors in advanced organic synthesis. Sophistication and broad applicability prove that the described synthetic approaches are exceptionally worthy of further development.

Theoretical study on the mechanism and enantioselectivity of NHC-catalyzed intramolecular SN2′ nucleophilic substitution: what are the roles of NHC and DBU?

A possible catalytic mechanism was proposed and studied in very detail by using the DFT method for a recently reported enantioselective intramolecular S_N2′ substitution of aldehydes with trisubstituted allylic bromides.

Recent advances in N-heterocyclic carbene catalyzed achiral synthesis

N-Heterocyclic carbenes (NHCs) have emerged as powerful and elegant organocatalysts in a variety of newly developed and unprecedented enantioselective transformations due to their unique umpolung capacity. As a supplement to conventional enantioselective organocatalysis, NHC-induced non-asymmetric catalysis has gradually attracted much interest in recent years. Herein, this review aims to reveal the recent developments in NHC-promoted non-asymmetric umpolung transformations resulting in the expeditious construction of versatile achiral natural heterocycles, carbocycles and acylated products.

Azaindole synthesis through dual activation catalysis with N-heterocyclic carbenes

A convergent, transition-metal-free synthesis of 2-aryl-azaindoles has been developed. The interception of a reactive aza-ortho-azaquinone methide intermediate by an acyl anion equivalent generated through carbene catalysis provides high yields, a wide substrate scope, and the synthesis of previously inaccessible azaindoles.

An insight into the mechanism of the aerobic oxidation of aldehydes catalyzed by N-heterocyclic carbenes

N-Heterocyclic carbene catalysis for the aerobic oxidation and esterification of aromatic aldehydes was monitored by ESI-MS (MS/MS) and the key intermediates were intercepted and characterized using the charge-tag strategy.

N-heterocyclic-carbene-catalyzed synthesis of 2-aryl indoles

A convergent and efficient transition-metal-free catalytic synthesis of 2-aryl-indoles has been developed. The interception of a highly reactive and transient aza-ortho-quinone methide by an acyl anion equivalent generated through N-hetereocyclic carbene catalysis is central to this successful strategy. High yields and a wide scope as well as the streamlined synthesis of a kinase inhibitor are reported.

NHC-catalyzed oxidative cyclization reaction for the synthesis of 3-substituted phthalides

An efficient NHC-catalyzed domino oxidation/oxa-Michael addition reaction of 2-alkenylbenzaldehydes has been developed to afford 3-substituted phthalides bearing a C3-stereogenic center with a broad substrate scope and wide functional group tolerance.

Non-asymmetric organocatalysis

Asymmetric organocatalysis is now an established methodology for the preparation of chiral compounds. However, these are not the only valuable molecules which can be conveniently obtained. Organocatalytic reactions affording achiral compounds are gaining momentum, opening unexplored pathways in the synthesis of densely functionalized aromatic moieties, olefins and useful molecules such as natural substances.

Extending NHC-catalysis: coupling aldehydes with unconventional reaction partners

Transition metal catalysis is a powerful means of effecting organic reactions, but it has some inherent drawbacks, such as the cost of the catalyst and the toxicity of the metals. Organocatalysis represents an attractive alternative and, in some cases, offers transformations unparalleled in metal catalysis. Unique transformations are a particular hallmark of N-heterocyclic carbene (NHC) organocatalysis, a versatile method for which a number of modes of action are known. The NHC-catalyzed umpolung (that is, the inversion of polarity) of electrophilic aldehydes, through formation of the nucleophilic Breslow intermediate, is probably the most important mode of action. In this Account, we discuss the reaction of Breslow intermediates with unconventional reaction partners. In two traditional umpolung reactions, the benzoin condensation and the Stetter reaction, some selectivity issues represent significant challenges, especially in intermolecular variants of these reactions. In intermolecular cross-benzoin reactions, high levels of selectivity were recently obtained, even in the hydroxymethylation of aldehydes with formaldehyde. The key to success was careful choice of the NHC catalyst and reaction conditions. Among asymmetric Stetter reactions, intermolecular versions have posed a long-standing challenge. Recently, the groups of Enders and Rovis reported the first selective and efficient systems. We have contributed to this field by developing an efficient intermolecular Stetter reaction for the formation of α-amino acid derivatives, with broad aldehyde scope and high enantiomeric excess. Moreover, tailor-made thiazolylidene catalysts allowed the unprecedented use of nonactivated olefins and alkynes as aldehyde coupling partners. The basis for this reactivity is a unique mode of NHC organocatalysis: dual activation. In a concerted but asynchronous transition state, the positively polarized proton of the Breslow intermediate activates the coupling partner (for example, an olefin), while the nucleophilic enamine moiety starts to attack the activated coupling partner. As a consequence of the concerted nature of this mechanism, excellent values for enantiomeric excess were obtained for many substrates in the intramolecular hydroacylation of alkenes. In addition, thiazolylidene catalysts have enabled the coupling of aldehydes with reactive species, for example, with arynes and with activated alkyl bromides. NHC catalysis should continue to flourish and lead to surprising developments. One remaining challenge is the asymmetric intermolecular hydroacylation of unactivated olefins. In this area, metal-based catalysts have shown promising early results, but they are far from being either general or practical. It will be interesting to see which class of catalyst, whether metal-based or NHC-based, eventually develops into the method of choice.

Enantioselective N-heterocyclic carbene-catalyzed Michael addition to α,β-unsaturated aldehydes by redox oxidation

Enantioselective N-heterocyclic carbene-catalyzed Michael addition reactions to α,β-unsaturated aldehydes by redox oxidation were realized. With 10 mol % of camphor-derived triazolium salt D, 15 mol % of DBU, 5 mol % of NaBF(4), and 100 mol % of quinone oxidant, the reactions of various dicarbonyl compounds with α,β-unsaturated aldehydes led to 3,4-dihydro-α-pyrones in good yields and excellent ee's.

On the role of CO2 in NHC-catalyzed oxidation of aldehydes

NHC-catalyzed oxidations using carbon dioxide as the stoichiometric oxidant have been carefully investigated. These studies support a secondary role of CO(2) in suppressing side reactions and exogenous oxygen as the actual oxidant.