Domino Sequence of Ketimization and Electrophilic Amination for an Inverse Aza Intramolecular Morita-Baylis-Hillman Adduct

Morita-Baylis-Hillman (MBH) reaction, typically catalyzed by a Lewis base, is a popular and useful method for C-C bond formation. Unfortunately, it is limited by a slow reaction rate and has sensitivity toward steric and electronic parameters. Despite tremendous efforts, the versatility of the reaction keeps the quest open for new mechanistic and catalytic pathways. Here, we have reported a Bro̷nsted acid-catalyzed, electrophilic amination (Umpolung of imine) as a method for an inverse Aza Intramolecular MBH adduct in the form of 2-acylindole. Umpolung of imine with nitrogen acting as an electrophilic center has been achieved. Interestingly, the reaction was also shown to occur under catalyst-free conditions also. The expected products of ketimine formation, 6π electrocyclization, or quinoline formation were least/not observed. A large number of examples have demonstrated the reaction strength. β-aryl-substituted acrylate and acrylamide (cinnamates and cinnamides), which are extremely sluggish in conventional MBH chemistry, are the highlights of the developed methodology. The annulated product exhibited keto-enol tautomerism, which was proven by 1H NMR integrals. As an application, another tandem reaction in the form of Michael addition on a highly complex amine was carried out to provide spiro-annulated indole.

Phosphine-promoted intramolecular Rauhut-Currier/Wittig reaction cascade to access (hetero)arene-fused diquinanes

We describe the first phosphine-promoted intramolecular Rauhut-Currier reaction that triggers an intramolecular Wittig process assembling new classes of diquinanes. The one-pot strategy provides ready access to simple diquinanes and various (hetero)arene-fused diquinanes incorporated with up to two contiguous all-carbon quaternary centers under metal-free and neutral conditions. We showcased the generality of the method on a broad range of substrates and demonstrated its synthetic utility in accessing various advanced intermediates relevant to natural product synthesis and material science.

Phosphine-catalyzed Rauhut-Currier reaction of γ-alkyl allenoate and subsequent trapping using the Diels-Alder reaction

We have disclosed a Rauhut-Currier reaction of γ-alkyl-substituted allenoate, catalyzed by L-valine-derived amide phosphine, to form trisubstitued allenoate, which was trapped by maleimide or DMAD via the Diels-Alder reaction. Exo-bicyclic succinimide derivatives including three continuous stereocenters with an exo-carbon-carbon double bound were constructed in up to quantitative yields with high stereospecificity.

Enantioselective Desymmetrization Triggered by Iminium-Enamine Activation: Access to Complex Cyclohepta[b]indoles

The expeditious construction of complex molecules having multiple stereocentres is highly desirable in organic chemistry. In the present communication, we report the development of an organocatalytic asymmetric desymmetrization of prochiral enal-tethered cyclohexadienones via the C3-selective Friedel-Crafts alkylation of indoles triggered by LUMO-lowering iminium activation/HOMO-raising enamine activation. The reaction provides access to bicyclic enones, which further undergo acid-mediated intramolecular annulation from C2-position to afford highly strained cyclohepta[b]indoles with five contiguous stereocentres and three new C-C bonds in excellent enantioselectivity and diastereoselectivity.

Solvent-Mediated Enantioselective Rauhut-Currier Cyclization via Iminium and Enamine Activation

In this work, we have developed an unconventional and highly enantioselective solvent-promoted Rauhut-Currier cyclization of enal-tethered cyclohexadienone by exploiting the reactivity of a simple Jørgensen-Hayashi catalyst through the merging of iminium and enamine activation. This asymmetric desymmetrization reaction has broad substrate scope in good yields with high to excellent enantioselectivity. DFT calculations suggest that the elimination of the alkoxy group is the rate-limiting step and that it proceeds through proton abstraction by solvent instead of a direct 1,3-proton shift.

Chiral Aziridine Phosphines as Highly Effective Promoters of Asymmetric Rauhut–Currier Reaction

A series of chiral enantiomerically pure aziridines containing a phosphine moiety were synthesized and successfully applied as organocatalysts in asymmetric intramolecular Rauhut–Currier reactions of p-quinone derivatives. The desired chiral phenols were achieved in high chemical yields and with satisfactory values of enantiomeric excess (up to 98% ee, in some cases). The stereochemical course of the title reaction may be controlled by the use of an appropriate enantiomer of the catalyst. The individual enantiomers of the organocatalyst led to the formation of specific enantiomers of the chiral product.

Recent Advances in Enantioselective Organocatalytic Reactions Enabled by NHCs Containing Triazolium Motifs

N-Heterocyclic carbenes (NHCs) containing triazolium motifs have emerged as a powerful tool in organocatalysis. Recently, various NHC pre-catalyst mediated organic transformations have been developed successfully. This article aims to compile the current state of knowledge on NHC-triazolium catalysed enantioselective name reactions and introduce newly developed catalytic methods. Furthermore, this review article framework provides an excellent opportunity to highlight some of the unique applications of these catalytic procedures in the natural product synthesis of biologically active compounds, notably the wide range of preparation of substituted chiral alcohols, and their derivatives. This article provides an overview of chiral NHC triazolium-catalyst libraries synthesis and their catalytic application in enantioselective reactions.

Enantioselective Rauhut-Currier Reaction with β-Substituted Acrylamides Catalyzed by N-Heterocyclic Carbenes

β-Substituted acrylamides have low electrophilicity and are yet to be exploited in the enantioselective Rauhut-Currier reaction. By exploiting electron-withdrawing protection of the amide and moderate nucleophilicity N-heterocyclic carbenes, such substrates have been converted to enantioenriched quinolones. The reaction proceeds with complete diastereoselectivity, good yield, and modest enantioselectivity. Derivatizations are reported, as are computational studies, supporting decreased amide bond character with electron-withdrawing protection of the nitrogen.

An N-heterocyclic carbene-catalyzed switchable reaction of 9-(trimethylsilyl)fluorene and aldehydes: chemoselective synthesis of dibenzofulvenes and fluorenyl alcohols

An N-heterocyclic carbene-catalyzed synthesis of dibenzofulvenes and fluorenyl alcohols was developed. In the presence of 10 mol% NHC (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and 4 Å molecular sieves, 9-(trimethylsilyl)fluorene undergoes an olefination reaction with aldehydes to produce dibenzofulvenes in 43-99% yields. However, on reducing the NHC loading to 1 mol% and with the addition of water, 9-(trimethylsilyl)fluorene selectively undergoes nucleophilic addition with aldehydes to afford fluorenyl alcohols in 40-95% yields.

Enantioselective Cyclopropanation/[1,5]-Hydrogen Shift to Access Rauhut-Currier Product

A Michael addition initiated cyclopropanation/[1,5]-hydrogen shift has been developed for the enantioselective synthesis of Rauhut-Currier products. The reaction of α-alkyl diazoesters and in situ generated o-quinone methides proceeds in the presence of chiral oxazaborolidinium ion, providing Z-stereocontrolled Rauhut-Currier products in high yields (up to 96%) with excellent Z/E selectivities (>20:1) and enantioselectivities (up to >99% ee). The synthetic utility was illustrated by conversion of the product to 3,4-dihydrocoumarins with two adjacent chiral stereocenters.

Mechanistic study on the NHC-catalyzed [3+4] annulation of enals and thiazolones

Reaction mechanisms and origins of regio- and stereo-selectivities of NHC catalyzed [3+4] annulation of enals and thiazolones.

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.

Direct Access to Highly Enantioenriched α-Branched Acrylonitriles through a One-Pot Sequential Asymmetric Michael Addition/Retro-Dieckmann/Retro-Michael Fragmentation Cascade

A highly enantioselective synthesis of α-branched acrylonitriles is reported featuring a one-pot sequential asymmetric Michael addition/retro-Dieckmann/retro-Michael fragmentation cascade. The method, which relies on a solid, bench-stable, and commercially available acrylonitrile surrogate, is practical, scalable, and highly versatile and provides a direct access to a wide range of enantioenriched nitrile-containing building blocks. Most importantly, the method offers a new tool to incorporate an acrylonitrile moiety in an asymmetric fashion.