An Overview the N-Heterocyclic Carbene-Catalyzed Aza-Benzoin Condensation Reaction

The importance of four-membered NHCs in stabilizing Breslow intermediates on benzoin condensation pathway

N-heterocyclic carbenes (NHCs) have been established to be effective organocatalysts for facilitating the benzoin condensation and many other reactions. These reactions involve the formation of a Breslow intermediate (BI), which exhibits umpolung chemistry. To facilitate organocatalysis, several new cyclic carbenes are being introduced, four-membered NHCs are of special interest. Whether these NHCs can exhibit catalytic influence or not, can be evaluated by exploring the potential energy surface (PES) of the benzoin condensation reaction. Quantum chemical analysis has been carried out to compare the PES of these four-membered NHCs with that of standard five-membered NHCs to explore their catalytic ability. The barrier for the first step of the reaction for the formation of BI is comparable in all the cases. But the barrier for the second step of the reaction leading to the benzoin formation from BI is estimated to be very high for the four membered NHCs. These results indicate that the probability of identifying and isolating the BI is very high in comparison to the completion of benzoin condensation reaction in the case of the four-membered NHCs.

Experimental and Theoretical Study on the Regioselective Radical Cyclization Reactions of 1-(o-Alkenylaryl)-2-amido-1-ketones for the Construction of Indeno[2,1-d][1,3]oxazin-9-ones

A simple and efficient two-step method for the construction of novel 2,4,9a-trisubstituted-4a,9a-dihydroindeno[2,1-d][1,3]oxazin-9-ones has been developed. The NHC-catalyzed aza-benzoin reaction of o-alkenyl benzaldehydes with N-acylarylimines afforded 1-(o-alkenylaryl)-2-amido-2-aryl-1-ethanones, which underwent regioselective 5-exo-trig radical cyclization to furnish the three-ring-fused heterocyclic products, generally in good yields. The synthetic method displayed good tolerance toward the nature of substituents, substitution pattern, and steric hindrance of o-alkenyl benzaldehydes. Based on this method, the synthesis of unprecedented dihydrobenzo[6,7]indeno[2,1-d][1,3]oxazin-7-ones and dihydropyrido[2',3':3,4]cyclopenta[1,2-d][1,3]oxazin-9-ones has been achieved by employing o-alkenylnaphthaldehyde and o-alkenylnicotinaldehyde as substrates. The regioselectivity between 5-exo-trig and 6-endo-trig radical cyclization reactions of different 1-(o-alkenylaryl)-2-amido-2-aryl-1-ethanones were elucidated with DFT calculations.

Synthesis of benzoin under supramolecular catalysis involving cyclodextrins in water: application for the preparation of the antiepileptic drug phenytoin

Among the cyclodextrins screened for the synthesis of 2-hydroxy-1,2-diphenylethanone (benzoin) in water, 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) exhibited the highest yield in the benzoin condensation reactions, and HP-β-CD can be recycled several times with little loss of activity through the addition of fresh VB₁. As an example of supramolecular catalysis, the methodology was applied to the “green” synthesis of the antiepileptic drug phenytoin through benzoin condensation, oxidation, and cyclization reactions in the presence of HP-β-CD, without the use of any harmful organic solvent. Moreover, the complexation behaviors of HP-β-CD with benzaldehyde and intermediates were studied by UV-vis and 2D-ROESY NMR spectroscopies to reveal the plausible mechanisms of the reactions, and HP-β-CD did not act as a simple phase transfer agent.

Benzoin was synthesized using cyclodextrins as phase transfer catalysts. As an example of supramolecular catalysis, the methodology was applied to the “green” synthesis of the antiepileptic drug Phenytoin in the presence of HP-β-CD.

Rh-Catalyzed diastereoselective addition of arylboronic acids to α-keto N-tert-butanesulfinyl aldimines: synthesis of α-amino ketones

Herein we present a diastereoselective addition of arylboronic acids to α-keto N-tert-butanesulfinyl aldimines catalyzed by a Rh(i) catalyst.

NHC-Catalyzed Aza-Benzoin Condensation of N,N'-Dipyridin-2-yl Aminals with Aldehydes

α-Amino ketones are useful compounds because of their synthetic utility and bioactivities. After observing the ability of N,N'-dipyridin-2-yl aminals to form imines in situ, the synthesis of α-amino ketones using N,N'-dipyridin-2-yl aminals was proposed. Through the NHC-catalyzed aza-benzoin reaction between aromatic/aliphatic aldehydes and N,N'-dipyridin-2-yl aminals, α-amino ketones, including aromatic, heterocyclic, and aliphatic versions, were synthesized with yields up to 99%. A direct route toward N-Boc-protected α-amino ketones from N,N,N'-tris-Boc aminals was also discovered, yielding the desired N-Boc-protected α-amino ketones in yields up to 73%.

Enantioselective N-heterocyclic carbene-catalysed intermolecular crossed benzoin condensations: improved catalyst design and the role of in situ racemisation

The enantioselective intermolecular crossed-benzoin condensation mediated by novel chiral N-heterocyclic carbenes derived from pyroglutamic acid has been investigated. A small library of chiral triazolium ions were synthesised. Each possessed a tertiary alcohol H-bond donor and a variable N-aryl substituent. It was found that increasing both the steric requirement and the electron-withdrawing characteristics of the N-aryl ring led to more chemoselective, efficient and enantioselective chemistry, however both quenching the reaction at different times and deuterium incorporation experiments involving the product revealed that this is complicated by product racemisation in situ (except in the case of benzoin itself), which explains the dependence of enantioselectivity on the electrophilicity of the reacting aldehydes common in the literature. Subsequent protocol optimisation, where one reacting partner was an o-substituted benzaldehyde, allowed a range of crossed-benzoins to be synthesised in moderate-good yields with moderate to excellent enantioselectivity.

Iminophosphorane-mediated regioselective umpolung alkylation reaction of α-iminoesters

The first regioselective umpolung alkylation of α-iminoesters with alkyl halides mediated by iminophosphorane has developed (up to 82% yield).

Organocatalytic Name Reactions Enabled by NHCs

Giving reactions the names of their discoverers is an extraordinary tradition of organic chemistry. Nowadays, this phenomenon is much rarer, although already named historical reactions are still often developed. This is also true in the case of a broad branch of N-heterocyclic carbenes catalysis. NHCs allow many unique synthetic paths, including commonly known name reactions. This article aims to gather this extensive knowledge and compare historical reactions with current developed processes. Furthermore, this review is a great opportunity to highlight some of the unique applications of these procedures in the total synthesis of biologically active compounds. Hence, this concise article may also be a source of knowledge for scientists just starting their adventure with N-heterocyclic carbene chemistry.

Imines as acceptors and donors in N-heterocyclic carbene (NHC) organocatalysis

The synthetic potential of imines as electrophiles or as a source of nucleophilic coupling partner in N-heterocyclic carbene (NHC) catalysis for the synthesis of various nitrogen heterocycles and functionalized amines is highlighted in this Feature Article. Electrophilic imines are suitable candidates for intercepting the NHC-derived acyl anions, homoenolate equivalents, and (di)enolates for the synthesis of α-amino ketones and a variety of lactam derivatives. Moreover, enamines generated from imines bearing α-hydrogen could be trapped with α,β-unsaturated acylazoliums for the synthesis of functionalized dihydropyridinones. NHCs are also useful for the umpolung of imines for the generation of aza-Breslow intermediates thus leading to the synthesis of indoles, quinolines, dihydroquinoxalines etc. A concise account of the diverse reactivity of imines in NHC catalysis has been presented.

Enantioselective NHC-Catalyzed [3+3] Annulation of α-Bromoenals with 2-Aminobenzimidazoles

A chiral carbene-catalyzed [3+3] annulation of α-bromoenals with 2-aminobenzimidazoles providing pyrimido[1,2-a]benzimidazoles has been described. This protocol features a broad scope and good functional group tolerance. Biological studies indicated that the formed pyrimido[1,2-a]benzimidazole exhibited moderate cytotoxic activity against tumor cells.

The Combination of Lewis Acid with N-Heterocyclic Carbene (NHC) Catalysis

In the last ten years, the combination of Lewis acid with N-heterocyclic carbene (NHC) catalysis has emerged as a powerful strategy in a variety of important asymmetric synthesis, due to the ready availability of starting materials, operational simplicity and mild reaction conditions. Recent findings illustrate that Lewis acid could largely enhance the efficiency and enantioselectivity, reverse the diastereoselectivity, and even influence the pathway of the same reaction partners. Herein, this review aims to reveal the recent advances in NHC-Lewis acid synergistically promoted enantioselective reactions for the expeditious assembly of versatile biologically important chiral pharmaceuticals and natural products.

Recent synthetic applications of α-amido sulfones as precursors of N-acylimino derivatives

α-Amido sulfones can be directly used as N-acylimine or N-acyliminium ion precursors in several synthetic processes aimed at the preparation of nitrogen containing compounds. This review collects the most relevant and practical utilizations of α-amido sulfones appeared in the literature after 2005.