Catalytic, Enantioselective β-Protonation through a Cooperative Activation Strategy

Enantioselective Relay Coupling of Perfluoroalkyl and Vinylogous Ketyl Radicals

β-Chiral carboxylic acids and their derivatives are highly valuable structural motifs in the fields of asymmetric synthesis and medicinal chemistry. However, the introduction of a sterically demanding sidechain to the β-carbon, such as an all-carbon quaternary center, remains a significant challenge in classical polar processes. Recently, N-heterocyclic carbene (NHC) mediated coupling reactions involving persistent ketyl radicals have emerged as a promising strategy to assemble highly crowded carbon-carbon bonds. Nevertheless, achieving enantioselectivity in these reactions remains highly challenging. In this work, we report our recent progress in controlling enantioselectivity for relay coupling of perfluoroalkyl and persistent vinylogous ketyl radicals. We developed a chiral bifunctional NHC-squaramide catalyst that achieves high facial selectivity in a critical bond-forming event involving the coupling of a congested tertiary carbon radical and vinylogous ketyl radical. Chiral carboxylates bearing an all-carbon quaternary center at the β-position can be prepared in good yield and excellent enantiomeric excess. Results from density functional theory (DFT) calculations and nuclear Overhauser effect (NOE) experiments indicate that the N,N'-diaryl squaramide motif adopts an unusual syn-syn conformation, enabling hydrogen bonding interactions with the enolate oxygen, thereby rigidifying the overall conformation of the transition state.

Enantioselective SN 2 Alkylation of Homoenolates by N-Heterocyclic Carbene Catalysis

The functionalization of the β-carbon of enals with electrophiles is a signature umpolung reactivity of N-heterocyclic carbene (NHC) derived homoenolates. However, only a limited number of electrophiles are shown to be compatible, with most of them being π-electrophiles. In this study, the successful enantioselective β-alkylation of homoenolates is reported using Csp3 electrophiles through an SN 2 strategy. The protocol shows a broad scope regarding alkyl electrophiles, delivering good yields, and excellent enantioselectivities (up to 99% ee). It enables the installation of drug-like structural motifs in either enals or alkylating agents, demonstrating its potential as a valuable tool for late-stage modification. Furthermore, a concise synthetic route is presented to chiral pyrroloindoline-type skeletons. Preliminary mechanistic studies support a direct SN 2 mechanism.

NHC-catalyzed enantioselective access to β-cyano carboxylic esters via in situ substrate alternation and release

A carbene-catalyzed asymmetric access to chiral β-cyano carboxylic esters is disclosed. The reaction proceeds between β,β-disubstituted enals and aromatic thiols involving enantioselective protonation of enal β-carbon. Two main factors contribute to the success of this reaction. One involves in situ ultrafast addition of the aromatic thiol substrates to the carbon-carbon double bond of the enal substrate. This reaction converts almost all enal substrate to a Thiol-click Intermediate, significantly reducing aromatic thiol substrates concentration and suppressing the homo-coupling reaction of enals. Another factor is an in situ release of enal substrate from the Thiol-click Intermediate for the desired reaction to proceed effectively. The optically enriched β-cyano carboxylic esters from our method can be readily transformed to medicines that include γ-aminobutyric acids derivatives such as Rolipram. In addition to synthetic utilities, our control of reaction outcomes via in situ substrate modulation and release can likely inspire future reaction development.

Highly enantioselective δ-protonation and formal [3 + 3] annulation promoted by N-heterocyclic carbene

A chiral NHC catalyst reacts with α,β-γ,δ-diunsaturated aldehydes to generate an extended Breslow intermediate. Upon δ-protonation and tautomerization, the resulting α,β-unsaturated acyl azolium undergoes [3 + 3] annulation with enamines to afford various dihydropyridinones.

Metal-free thioesterification of α,β-unsaturated aldehydes with thiols

For the first time, the synthesis of thioesters starting from enals and thiols has been performed in the presence of a bulky N-heterocyclic carbene (NHC) as a catalyst.

Poly(imidazolium) Carbosilane Dendrimers: Synthesis, Catalytic Activity in Redox Esterification of α,β-Unsaturated Aldehydes and Recycling via Organic Solvent Nanofiltration

Three series of poly(ionic) carbosilane dendrimers peripherally functionalized with imidazolium groups substituted on N-3 with methyl, isopropyl and 2,6-diisopropylphenyl (Dipp) were prepared up to the 3rd generation together with model monovalent imidazolium iodides and used as N-heterocyclic carbene (NHC) precursors. Catalytic activity of model and dendritic NHCs generated in situ by deprotonation with DBU was tested in redox esterification of α,β-unsaturated aldehydes and the influence of substitution, dendrimer generation, temperature and substrate structure on the reaction outcome was evaluated. Dipp substituted NHCs showed high activity and selectivity in the reaction with primary alcohols. Effectiveness of organic solvent nanofiltration for the recycling of dendritic NHCs was demonstrated on the 1st generation Dipp substituted catalyst in model redox esterification of cinnamaldehyde with benzyl alcohol. A marked increase in both activity and selectivity in the first four reaction runs was observed and this improved performance was preserved in the following catalytic cycles.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Enantio- and Diastereoselective Hydrofluorination of Enals by N-Heterocyclic Carbene Catalysis

In contrast to well-established asymmetric hydrogenation reactions, enantioselective protonation is an orthogonal approach for creating highly valuable methine chiral centers under redox-neutral conditions. Reported here is the highly enantio- and diastereoselective hydrofluorination of enals by an asymmetric β-protonation/α-fluorination cascade catalyzed by N-heterocyclic carbenes (NHCs). The two nucleophilic sites of a homoenolate intermediate, generated from enals and an NHC, are sequentially protonated and fluorinated. The results show that controlling the relative rates of protonation, fluorination, and esterification is crucial for this transformation, and can be accomplished using a dual shuttling strategy. Structurally diverse carboxylic acid derivatives with two contiguous chiral centers are prepared in a single step with excellent d.r. and ee values.

Enantioselective Hydroamidation of Enals by Trapping of a Transient Acyl Species

An enantioselective synthesis of β-chiral amides through asymmetric and redox-neutral hydroamidation of enals is reported. In this reaction, a chiral N-heterocyclic carbene (NHC) catalyst reacts with enals to generate the homoenolate intermediate. Upon highly enantioselective β-protonation through proton-shuttle catalysis, the resulting azolium intermediate reacts with imidazole to yield the key β-chiral acyl species. This transient intermediate provides access to diversified β-chiral carbonyl derivatives, such as amides, hydrazides, acids, esters, and thioesters. In particular, β-chiral amides can be prepared in excellent yield and ee (40 chiral amides, up to 95 % yield and 99 % ee). This modular strategy overcomes the challenge of disruption of the highly selective proton-shuttling process by basic amines.

Enantioselective cooperative proton-transfer catalysis using chiral ammonium phosphates

Chiral phosphorate anions are shown to be highly enantioselective templates for proton-transfer catalysis.

N-Heterocyclic carbene-catalyzed sulfa-Michael addition of enals

An efficient N-heterocyclic carbene (NHC) catalyzed sulfa-Michael addition (SMA) between enals and thiols has been developed.