Stereoselective synthesis of spirocyclopentanones via N-heterocyclic carbene-catalyzed reactions of enals and dienones

An uncommon multicomponent reaction involving nucleophilic heterocyclic carbenes: facile synthesis of fully substituted cyclopentanones

An unprecedented three component reaction involving arylidene oxazolone, enal and NHC, leading to the synthesis of fully substituted cyclopentanone derivatives with three contiguous stereocenters, is reported.

Oxidative N-Heterocyclic Carbene Catalyzed Dearomatization of Indoles to Spirocyclic Indolenines with a Quaternary Carbon Stereocenter

An efficient method for the asymmetric intramolecular dearomatization of indoles by using oxidative N-heterocyclic carbene catalysis is demonstrated. Valuable optically active spirocyclic indolenines bearing an all-carbon quaternary stereocenter are obtained in excellent yields and with excellent enantioselectivity. The starting indoles are readily prepared and the reactions proceed through an intramolecular indole 3-acylation with an in situ generated acyl azolium intermediate to form a spirocyclic ketone moiety.

Enantioselective Synthesis of Spirooxindole Enols: Regioselective and Asymmetric [3+2] Cyclization of 3-Isothiocyanato Oxindoles with Dibenzylidene Ketones

A novel cinchona-alkaloid-derived organocatalyst has been developed to catalyze the asymmetric regioselective [3+2] cycloaddition of 3-isothiocyanato oxindoles with dibenzylidene ketones. A series of spirooxindole enols could be obtained in high yields with good-to-excellent diastereo- and enantioselectivities.

Insights into the Unexpected Chemoselectivity for the N-Heterocyclic Carbene-Catalyzed Annulation Reaction of Allenals with Chalcones

Lewis base N-heterocyclic carbene (NHC)-catalyzed annulation is the subject of extensive interest in synthetic chemistry, but the reaction mechanisms, especially the unexpected chemoselectivity of some of these reactions, are poorly understood. In this work, a systematic theoretical calculation has been performed on NHC-catalyzed annulation between allenals and chalcone. Multiple possible reaction pathways (A-E) leading to three different products have been characterized. The calculated results reveal that NHC is more likely to initiate the reaction by nucleophilic attack on the center carbon atom of the allene group but not the carbonyl carbon atom in allenals leading to the Breslow intermediate, which is remarkably different from the other NHC-catalyzed annulations of unsaturated aldehydes with chalcones. The computed energy profiles demonstrate that the most energetically favorable pathway (A) results in polysubstituted pyranyl aldehydes, which reasonably explains the observed chemoselectivity in the experiment. The observed chemoselectivity is demonstrated to be thermodynamically but not kinetically controlled, and the stability of the Breslow intermediate is the key for the possibility of homoenolate pathway D and enolate pathway E. This work can improve our understanding of the multiple competing pathways for NHC-catalyzed annulation reactions of unsaturated aldehydes with chalcones and provide valuable insights for predicting the chemoselectivity for this kind of reaction.

Oxidatively Initiated NHC-Catalyzed Enantioselective Synthesis of 3,4-Disubstituted Cyclopentanones from Enals

An unprecedented N-heterocyclic carbene (NHC)-catalyzed annulation of enals to form 3,4-disubstituted cyclopentanones has been discovered. Aryl enals undergo dimerization in the presence of a single-electron oxidant to form C2 symmetric cyclopentanones. A cross-reaction has also been developed, allowing for the synthesis of differentially substituted cyclopentanones. Mechanistically, the reaction is thought to proceed through radical intermediates, further establishing the synthetic utility of this class of reactivity.

Diastereoselective synthesis of cyclopentanone-fused spirooxindoles by N-heterocyclic carbene-catalyzed homoenolate annulation with isatilidenes

NHC-catalyzed reaction of enals with N-substituted isatilidenes resulting in the diastereoselective synthesis of cyclopentanone-fused spirooxindoles proceeding via the homoenolate equivalent intermediates is reported.

N-Heterocyclic carbene-catalyzed diastereoselective synthesis of β-lactone-fused cyclopentanes using homoenolate annulation reaction

NHC-catalyzed annulation of enals with 2-enoylpyridines or 2-enoylpyridine N-oxides leading to the diastereoselective synthesis of β-lactone-fused cyclopentanes is reported.

Stereoselective synthesis of ε-lactones or spiro-heterocycles through NHC-catalyzed annulation: divergent reactivity by catalyst control

NHC-catalyzed divergent annulation of enals with heterocyclic enones was developed to produce benzofuran/indole-containing ε-lactones or spiro-heterocycles in a highly diastereo- and enantioselective fashion. The chemo-selectivity controlled by the chiral catalyst backbone is particularly noteworthy.

Changing the reaction pathway by NHC/Brønsted base cooperative catalysis: highly stereoselective synthesis of multifunctional benzo[a]fluoren-11-ones from the dimerization of 2-(aroylvinyl)arylaldehydes

The unprecedented NHC/Brønsted base-cocatalyzed dimerization reaction of 2-(aroylvinyl)arylaldehydes was reported. In the presence of a triazole carbene catalyst alone, no reaction of 2-(aroylvinyl)arylaldehydes was observed. However, the combination of triazole carbene and 4-methoxyphenolate efficiently catalyzed the dimerization of 2-(aroylvinyl)arylaldehydes to proceed through a benzoin-Michael-Michael reaction cascade, producing 6-aroyl-5-(aroylmethyl)-11a-hydroxybenzo[a]fluoren-11-ones as the sole diastereomers in good yields.

NHC-catalyzed homoenolate reaction of enals and nitroalkenes: computational study of mechanism, chemoselectivity and stereoselectivity

A mechanistic study on the NHC-catalyzed homoenolate reaction of enals and nitroalkenes has been performed with the aid of DFT calculations.

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.

Enantioselective N-heterocyclic carbene catalyzed annulation reactions with imidazolidinones

Add acetic acid: A highly stereoselective N-heterocyclic carbene (NHC)-catalyzed formal [4+2] annulation between α,β-unsaturated aldehydes and imidazolidinones for the synthesis of imidazoles has been developed. Acetic acid serves as a key additive to achieve high chemoselectivity for the formal [4+2] annulation product.

A cascade reaction actuated by nucleophilic heterocyclic carbene catalyzed intramolecular addition of enals via homoenolate to α,β-unsaturated esters: efficient synthesis of coumarin derivatives

A nucleophilic heterocyclic carbene mediated intramolecular homoenolate reaction strategy for the efficient synthesis of 4-alkyl substituted coumarins is described.

Unprecedented dithiolation of enals via their NHC-catalysed umpolung reaction with organic disulfides

A novel one-pot N-heterocyclic carbene (NHC)-catalysed dithiolation of α,β-unsaturated aldehydes (enals) with organic disulfides is reported. The protocol involves homoenolate reactivity of enals, where the homoenolate attacks on the disulfide as a d(3) nucleophile followed by thioesterification to afford β-aryl/alkylsulfanyl thioesters with complete atom economy.

Exploiting Acyl and Enol Azolium Intermediates via NHeterocyclic Carbene Catalyzed Reactions of Alpha-Reducible Aldehydes

N-heterocyclic carbenes are well known for their role in catalyzing benzoin and Stetter reactions: the generation of acyl anion equivalents from simple aldehydes to react with a variety of electrophiles. However, when an aldehyde bearing a leaving group or unsaturation adjacent to the acyl anion equivalent is subjected to an NHC, a new avenue of reactivity is unlocked, leading to a number of novel transformations which can generate highly complex products from simple starting materials, many of which are assembled through unconventional bond disconnections. The field of these new reactions - those utilizing α-reducible aldehydes to access previously unexplored catalytic intermediates - has expanded rapidly in the past eight years. This review aims to provide the reader with a historical perspective on the underlying discoveries that led to the current state of the art, a mechanistic description of these reactions, and a summary of the recent advances in this area.