Oxidative organocatalysed enantioselective coupling of indoles with aldehydes that forms quaternary carbon stereocentres

Catalytic Asymmetric α-Functionalization of α-Branched Aldehydes

Aldehydes constitute a main class of organic compounds widely applied in synthesis. As such, catalyst-controlled enantioselective α-functionalization of aldehydes has attracted great interest over the years. In this context, α-branched aldehydes are especially challenging substrates because of reactivity and selectivity issues. Firstly, the transient trisubstituted enamines and enolates resulting upon treatment with an aminocatalyst or a base, respectively, would exhibit attenuated reactivity; secondly, mixtures of E- and Z-configured enamines/enolates may be formed; and third, effective face-discrimination on such trisubstituted sp2 carbon intermediates by the incoming electrophilic reagent is not trivial. Despite these issues, in the last 15 years, several catalytic approaches for the α-functionalization of prostereogenic α-branched aldehydes that proceed in useful yields and diastereo- and enantioselectivity have been uncovered. Developments include both organocatalytic and metal-catalyzed approaches as well as dual catalysis strategies for forging new carbon–carbon and carbon–heteroatom (C-O, N, S, F, Cl, Br, …) bond formation at Cα of the starting aldehyde. In this review, some key early contributions to the field are presented, but focus is on the most recent methods, mainly covering the literature from year 2014 onward.

Organocatalyzed Cross-Nucleophile Couplings: Umpolung of Catalytic Enamines

ConspectusThe concept of umpolung, or polarity reversal, introduced by Seebach and Corey nearly half a century ago, ushered a new paradigm into synthetic chemistry. Novel connections were able to be forged among functional groups that were typically inaccessible. Conceptually, an umpolung reaction is identified only upon retrosynthetic analysis. Stoichiometric examples have served as a platform to develop and refine elegant methodologies into catalytic processes. The advent of these unconventional arrangements of canonical synthons into new points of diversity has expanded the repertoire of the synthetic toolbox. Within this context, asymmetric organocatalyzed methodologies remain rare, and there are even fewer aminocatalyzed variants.Recent years have witnessed a renaissance in α-functionalizations of aldehydes, specifically in the context of oxidative umpolung strategies. Unlike previous open-shell approaches, application of a quinone-based oxidant in conjunction with an aminocatalyst leads to a discrete, substitutionally labile quinone adduct. These have proven to be valuable building blocks toward polar reactivity─auguring the advent of new avenues to construct tetrasubstituted tertiary stereocenters through the application of conventional nucleophiles to form C-C, C-N, C-O, and C-S bonds through an organocatalyzed cross-nucleophile coupling (organo-CNC) reaction. The resulting nonepimerizable stereocenter demonstrates high optical fidelity and provides a significant advancement in many applications that suffer from racemization, such as in vivo studies.This strategy harnesses a trifunctional aminocatalyst to promote an unusual S_N2 reaction at a highly congested center. The selection of the quinone oxidant and nucleophile converges to a continuum of reactivity ranging from enantioselective oxidation to stereoselective substitution. A remarkable aspect of these developments is the identification of an asymmetric S_N2 dynamic kinetic resolution (S_N2-DKR) manifold. These organo-CNC reactions are highly modular and demonstrate complete stereocontrol from the catalyst with minimal influence from incoming chiral nucleophiles. Leveraging this facet, these technologies have been extended to peptidic bioconjugations bearing bio-orthogonoal linker molecules.This Account aims to highlight the progress, from an internal perspective, toward directing the initial result into established methodologies. Within this construct, the underlying principles of each reaction will be disseminated with specific content on inherent challenges and opportunity. Combined, these will serve as an instructive tool to stimulate applications in cross-disciplinary interfaces.

Chiral Primary Amine Catalyzed Enantioselective Tandem Reactions Based on Heyns Rearrangement: Synthesis of α-Tertiary Amino Ketones

Herein, we disclose a new catalytic asymmetric tandem reaction based on the Heyns rearrangement for the synthesis of chiral α-amino ketones with readily available substrates. The rearrangement is different from the Heyns rearrangement in that the α-amino ketones were obtained without the shift of the carbonyl group. The key to success is using chiral primary amine as a catalyst by mimicking glucosamine-6-phosphate synthase in catalyzing the efficient Heyns rearrangement in organisms.

Investigation of the Organocatalytic Chlorination of 2-Phenylpropanal

Results of an examination of the organocatalytic enantioselective α-chlorination of 2-phenylpropanal are described. Synthetic investigation including the screening of primary and secondary aminocatalysts, many different reaction conditions, and other α-branched aldehydes show that especially primary aminocatalysts can catalyze the formation of the α-chloro branched aldehydes in good yields, but only with moderate enantioselectivities. In order to try to understand the challenge in obtaining high enantioselectivity for the aminocatalytic α-chlorination of α-branched aldehydes a series of experimental investigations were performed employing 2-phenylpropanal as a model system. These investigations have been coupled with computational investigations, which provided important insight into the moderate enantioselectivity of this chlorination reaction. Analysis of the reaction showed, that the lack of control over the selectivity of formation of the (E)- and (Z)-enamine intermediate, and the clustering of reaction barriers of possible reaction pathways help to rationalize difficulties in producing high enantioselectivity.

Metal-free, Oxidative α-Coupling of Aldehydes with Amine Nucleophiles for the Preparation of Congested C(sp3)-N Bonds

This article discloses the direct α-amination of α-branched aldehydes applying nitrogen-based nucleophiles. Under organocatalyzed, oxidative conditions α-branched aldehydes are umpoled to their electrophilic synthons and, subsequently, displaced by a variety of nucleophilic amines to form tetrasubstituted tertiary centers. A similar strategy has been previously employed to form congested C-C, C-O, and C-S bonds; however, unsatisfactory results were received when extending the methodology to include C-N bonds. Initially, intramolecular α-amination reactions were undertaken to foster dihydroquinoxaline-type products. A solvent exchange to the polar, aprotic solvent, MeNO₂, proved critical to facilitate intermolecular α-C-N bond formation with a wide range of amine coupling partners (N-heterocycles, N,N-diaryl amines, and anilines). Application of the solvent exchange to the enantioselective S_N2-DKR manifold provided distinct regimes leading to refinement in yield and enantioselectivity.

Organocatalytic enantioselective reactions involving prochiral carbocationic intermediates

Since the discovery of carbocations in 1901, the past 120 years have witnessed many marvelous advances in the chemistry of carbocations. The state-of-the-art research in this field is to overcome the intrinsic instability and high reactivity of the prochiral carbocationic intermediates to develop catalytic asymmetric reactions. Such transformations enable the facile synthesis of structurally diverse value-added products from readily available starting materials such as alkenes, alcohols, and carbonyl derivatives, and enjoy high and even perfect atom-economy in most cases. Notably, such allows catalytic stereoconvergent synthesis from racemic substrates and can realize regioselectivity in olefin functionalization reactions complementary to radical processes. With the rapid developments in modern asymmetric organocatalysis, a variety of highly enantioselective protocols evolving prochiral carbocationic intermediates have been achieved by employing three strategies, namely chiral ion-pairing, chiral nucleophile, or chiral carbenium ion strategy. This feature article aims to summarize the exciting advances in this emerging area and briefly showcase the possible mechanisms. The advantages and limitations of each strategy are presented as well as their synthetic applications in the synthesis of natural products or bioactive compounds.

Enantioselective α-Etherification of Branched Aldehydes via an Oxidative Umpolung Strategy

Saturated carbonyl compounds are, via their enolate analogues, inherently nucleophilic at the α-position. In the presence of a benzoquinone oxidant, the polarity of the α-position of racemic α-branched aldehydes is inverted, allowing for an enantioselective etherification using readily available oxygen-based nucleophiles and an amino acid-derived primary amine catalyst. A survey of benzoquinone oxidants identified p-fluoranil and DDQ as suitable reaction partners. p-Fluoranil enables the preparation of α-aryloxylated aldehydes using phenol nucleophiles in up to 91 % ee, following either a one-step or a two-step, one-pot protocol. DDQ allows for a more general etherification protocol in combination with a broader range of alcohol nucleophiles with enantioselectivities up to 95 % ee. Control experiments and isolation of a key quinol intermediate supports a mechanism proceeding via an S_N 2 dynamic-kinetic resolution. These studies provide the basis for an aminocatalytic umpolung concept that allows for the asymmetric construction of tertiary ethers in the α-position of aldehydes.

Catalytic Aerobic Cross-Dehydrogenative Coupling of Azlactones en Route to α,α-Disubstituted α-Amino Acids

We developed a catalytic aerobic method to synthesize α,α-disubstituted α-amino acids through cross-dehydrogenative coupling of azlactones. Combining an iron catalyst with a bisoxazolidine ligand resulted in high catalytic performance, and cross-coupling with an indole proceeded smoothly under aerobic conditions. A wide variety of α-aryl and aliphatic amino acid derived azlactones were applied to the present catalysis. In addition, a quaternary carbon could be constructed using oxindole and benzofuranone under aerobic conditions.

New Trends in Enantioselective Cross-Dehydrogenative Coupling

The development of cross-dehydrogenative coupling in recent years has simplified the synthesis of many materials, as a result of facile C–H activation, which, together with its greater atom economy and environmental friendliness, has made an impact on modern organic chemistry. Indeed, many C–C and C–X (X = N, O, P, S, B, or Si) coupling reactions can now be performed directly between two C–H bonds or a C–H and an X–H bond, simply by adding catalytic amounts of a metal salt to a mixture of the two and an oxidant to accept the two hydrogen atoms released. Chiral organocatalysts or chiral ligands have been joined to promote enantioselective processes, resulting in the development of efficient reaction cascades that provide products in high yields and high levels of asymmetric induction through cooperative catalysis. In recent years, photochemical oxidation and electrochemistry have widened even more the scope of cross-dehydrogenative coupling (CDC). In this review, we summarized the recent literature in this subject, hoping that it will inspire many new synthetic strategies.