Gold-Catalyzed Regio- and Stereoselective Alkenylation of Quinoline N-Oxides with Allenamides

A gold-catalyzed cycloaddition/ring opening of allenamides with quinoline N-oxides has been developed, which provides C2-alkenylated quinolines with high E selectivity in moderate to high yields. It is noted that quinoline N-oxides with a C8 or C7 substituent are crucial for this catalytic reaction.

The Catalytic Asymmetric Allylic Alkylation of Acyclic Enolates for the Construction of Quaternary and Tetrasubstituted Stereogenic Centres

To facilitate the discovery and development of new pharmaceuticals, the demand for novel stereofunctionalised building blocks has never been greater. Whilst molecules bearing quaternary and tetrasubstituted stereogenic centres are ideally suited to explore untapped areas of chemical space, the asymmetric construction ofsterically congested carbon centres remains a longstanding challenge in organic synthesis. The enantioselective assembly of acyclic stereogenic centres is even more demanding due to the need to restrict a much wider range of geometries and conformations of the intermediates involved. In this context, the catalytic asymmetric allylicalkylation (AAA) of acyclic prochiral nucleophiles, namely enolates, has become an indispensable tool to access a range of linearα-quaternary andα-tetrasubstituted carbonyl compounds. However, unlike the AAA of cyclic enolates with a fixed enolate geometry, to achieve high levels of stereocontrol in the AAA of acyclic enolates, the stereoselectivity of enolisation must be considered. The aim of this review is to offer acomprehensivediscussion of catalytic AAA reactions of acyclic prochiral enolates and their analogues to generate congested quaternary and tetrasubstituted chiral centres using metal, non-metal and dual catalysis, with particular focus given to the control of enolate geometry and its impact on the stereochemical outcome of the reaction.

Enantioselective merged gold/organocatalysis

Gold complexes, because of their unique carbophilic nature, have evolved as efficient catalysts for catalyzing various functionalization reactions of C-C multiple bonds. However, the realization of enantioselective transformations via gold catalysis remains challenging due to the geometrical constraints and coordination behaviors of gold complexes. In this context, merged gold/organocatalysis has emerged as one of the intriguing strategies to achieve enantioselective transformations which could not be possible by using a single catalytic system. Historically, in 2009, this field started with the merging of gold with axially chiral Brønsted acids and chiral amines to achieve enantioselective transformations. Since then, based on the unique reactivity profiles offered by each catalyst, several reports utilizing gold in conjunction with various chiral organocatalysts such as amines, Brønsted acids, N-heterocyclic carbenes, hydrogen-bonding and phosphine catalysts have been documented in the literature. This article demonstrates an up-to-date development in this field, especially focusing on the mechanistic interplay of gold catalysts with chiral organocatalysts.

Metal-Catalyzed Enantioconvergent Transformations

Enantioconvergent catalysis has expanded asymmetric synthesis to new methodologies able to convert racemic compounds into a single enantiomer. This review covers recent advances in transition-metal-catalyzed transformations, such as radical-based cross-coupling of racemic alkyl electrophiles with nucleophiles or racemic alkylmetals with electrophiles and reductive cross-coupling of two electrophiles mainly under Ni/bis(oxazoline) catalysis. C-H functionalization of racemic electrophiles or nucleophiles can be performed in an enantioconvergent manner. Hydroalkylation of alkenes, allenes, and acetylenes is an alternative to cross-coupling reactions. Hydrogen autotransfer has been applied to amination of racemic alcohols and C-C bond forming reactions (Guerbet reaction). Other metal-catalyzed reactions involve addition of racemic allylic systems to carbonyl compounds, propargylation of alcohols and phenols, amination of racemic 3-bromooxindoles, allenylation of carbonyl compounds with racemic allenolates or propargyl bromides, and hydroxylation of racemic 1,3-dicarbonyl compounds.

Sugar-Assisted Kinetic Resolutions in Metal/Chiral Amine Co-Catalyzed α-Allylations and [4+2] Cycloadditions: Highly Enantioselective Synthesis of Sugar and Chromane Derivatives

Functionalized triose-, furanose and chromane-derivatives were synthesized by the titled reactions. The sugar-assisted kinetic resolution/C-C bond-forming cascade processes generate a functionalized sugar derivative with a quaternary stereocenter in a highly enantioselective fashion (up to >99 % ee) by using a simple combination of metal and chiral amine co-catalysts. Notably, the interplay between the chiral sugar substrate and the chiral amino acid derivative allowed for the construction of a functionalized sugar product with high enantioselectivity (up to 99 %) also when using a combination of racemic amine catalyst (0 % ee) and metal catalyst.

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.

Synergistic Strategies in Aminocatalysis

Synergistic catalysis offers the unique possibility of simultaneous activation of both the nucleophile and the electrophile in a reaction. A requirement for this strategy is the stability of the active species towards the reaction conditions and the two concerted catalytic cycles. Since the beginning of the century, aminocatalysis has been established as a platform for the stereoselective activation of carbonyl compounds through HOMO‐raising or LUMO‐lowering. The burgeoning era of aminocatalysis has been driven by a deep understanding of these activation and stereoinduction modes, thanks to the introduction of versatile and privileged chiral amines. The aim of this review is to cover recent developments in synergistic strategies involving aminocatalysis in combination with organo‐, metal‐, photo‐, and electro‐catalysis, focusing on the evolution of privileged aminocatalysts architectures.

Acyclic Quaternary Carbon Stereocenters through Transition-Metal-Catalyzed Enantioselective Functionalization of Unsaturated Hydrocarbons

Acyclic quaternary carbon stereocenters occur frequently in natural products, bioactive molecules, and pharmaceutical compounds. Construction of a carbon stereogenic center attached to four different carbons with defined spatial arrangement is a daunting challenge in asymmetric catalysis. Significant efforts have been directed towards the stereoselective construction of such acyclic quaternary carbon stereocenters. In particular, catalytic generation of acyclic quaternary carbon stereocenters through functionalization of unsaturated hydrocarbons is an extremely attractive approach because unsaturated hydrocarbons are easily accessible both in industry and in organic synthesis. In this short review, we summarize the recent advances achieved in this research area, with the aim to inspire future development.

1 Introduction

2 Acyclic Quaternary Carbon Stereocenters through Functionalization of Allenes

3 Acyclic Quaternary Carbon Stereocenters through Functionalization of Dienes

4 Acyclic Quaternary Carbon Stereocenters through Functionalization of Mono-alkenes

5 Acyclic Quaternary Carbon Stereocenters through Functionalization of Alkynes

6 Summary and Outlook

Synergistic Peptide and Gold Catalysis: Enantioselective Addition of Branched Aldehydes to Allenamides

The combination of a peptide catalyst and a gold catalyst is presented for enantioselective addition reactions between branched aldehydes and allenamides. The two catalysts act in concert to provide γ,δ‐enamide aldehydes bearing a fully substituted, benzylic stereogenic center – a structural motif common in many natural products and therapeutically active compounds – with good yields and enantioselectivities. The reaction tolerates a variety of alkyl and alkoxy substituted aldehydes and the products can be elaborated into several chiral building blocks bearing either 1,4‐ or 1,5‐ functional group relationships. Mechanistic studies showed that the conformational features of the peptide are important for both the catalytic efficiency and stereochemistry, while a balance of acid/base additives is key for ensuring formation of the desired product over undesired side reactions.

Synergistic Copper/Enamine Catalysis for the Regio-, Stereo-, and Enantioselective Intermolecular α-Addition of Aldehydes to Allenamides

We herein describe an intermolecular enantioselective α-addition of aldehydes to allenamides using a dual copper/enamine catalytic system. Highly enantioselective addition of aldehydes was obtained thanks to secondary amine catalysts. The process was found to be highly regio-, stereo-, and enantioselective under mild conditions.

Construction of Enantioenriched γ,γ-Disubstituted Butenolides Enabled by Chiral Amine and Lewis Acid Cascade Cocatalysis

Herein we report a cascade cocatalysis strategy for the facile construction of chiral γ,γ-disubstituted butenolides. The synthetic manifold employs simple alkynoic acids instead of the preformed silyloxy furans or 5-substituted furan-2(3H)-ones. In situ formed 5-substituted furan-2(3H)-ones by AgNO₃ or Ph₃PAuCl/AgOTf catalyzed cyclization of alkynoic acids can smoothly engage in the subsequent chiral diphenylprolinol TMS-ether catalyzed Michael and Michael-aldol reactions. The cascade process serves as a general approach to chiral quaternary γ,γ-disubstituted butenolides.

Asymmetric Reactions Enabled by Cooperative Enantioselective Amino- and Lewis Acid Catalysis

Organocatalysis-the branch of catalysis featuring small organic molecules as the catalysts-has, in the last decade, become of central importance in the field of asymmetric catalysis, so much that it is now comparable to metal catalysis and biocatalysis. Organocatalysis is rationalized and classified by a number of so-called activation modes, based on the formation of a covalent or not-covalent intermediate between the organocatalyst and the organic substrate. Among all the organocatalytic activation modes, enamine and iminium catalysis are widely used for the practical preparation of valuable products and intermediates, both in academic and industrial contexts. In both cases, chiral amines are employed as catalysts. Enamine activation mode is generally employed in the reaction with electrophiles, while nucleophiles require the iminium activation mode. Commonly, in both modes, the reaction occurs through well-organized transitions states. A large variety of partners can react with enamines and iminium ions, due to their sufficient nucleophilicity and electrophilicity, respectively. However, despite the success, organocatalysis still suffers from narrow scopes and applications. Multicatalysis is a possible solution for these drawbacks because the two different catalysts can synergistically activate the substrates, with a simultaneous activation of the two different reaction partners. In particular, in this review we will summarize the reported processes featuring Lewis acid catalysis and organocatalytic activation modes synergically acting and not interfering with each other. We will focus our attention on the description of processes in which good results cannot be achieved independently by organocatalysis or Lewis acid catalysis. In these examples of cooperative dual catalysis, a number of new organic transformations have been developed. The review will focus on the possible strategies, the choice of the Lewis acid and the catalytic cycles involved in the effective reported combination. Additionally, some important key points regarding the rationale for the effective combinations will be also included. π-Activation of organic substrates by Lewis acids, via formation of electrophilic intermediates, and their reaction with enamines will be also discussed in this review.

Enamine/Transition Metal Combined Catalysis: Catalytic Transformations Involving Organometallic Electrophilic Intermediates

The concept of merging enamine activation catalysis with transition metal catalysis is an important strategy, which allows for selective chemical transformations not accessible without this combination. The amine catalyst activates the carbonyl compounds through the formation of a reactive nucleophilic enamine intermediate and, in parallel, the transition metal activates a wide range of functionalities such as allylic substrates through the formation of reactive electrophilic π-allyl-metal complex. Since the first report of this strategy in 2006, considerable effort has been devoted to the successful advancement of this technology. In this chapter, these findings are highlighted and discussed.

Chiral AuI - and AuIII -Isothiourea Complexes: Synthesis, Characterization and Application

During an investigation into the potential union of Lewis basic isothiourea organocatalysis and gold catalysis, the formation of gold-isothiourea complexes was observed. These novel gold complexes were formed in high yield and were found to be air- and moisture stable. A series of neutral and cationic chiral gold(I) and gold(III) complexes bearing enantiopure isothiourea ligands was therefore synthesized and fully characterized. The steric and electronic properties of the isothiourea ligands was assessed through calculation of their percent buried volume and the synthesis and analysis of novel iridium(I)-isothiourea carbonyl complexes. The novel gold(I)- and gold(III)-isothiourea complexes have been applied in preliminary catalytic and biological studies, and display promising preliminary levels of catalytic activity and potency towards cancerous cell lines and clinically relevant enzymes.

Enantioselective Construction of Acyclic Quaternary Carbon Stereocenters: Palladium-Catalyzed Decarboxylative Allylic Alkylation of Fully Substituted Amide Enolates

We report a divergent and modular protocol for the preparation of acyclic molecular frameworks containing newly created quaternary carbon stereocenters. Central to this approach is a sequence composed of a (1) regioselective and -retentive preparation of allyloxycarbonyl-trapped fully substituted stereodefined amide enolates and of a (2) enantioselective palladium-catalyzed decarboxylative allylic alkylation reaction using a novel bisphosphine ligand.

Tryptophan/copper-catalyzed aromatization reaction of chiral cyclohexanones to phenols

By merging organocatalysis with copper catalysis, a highly efficient stereospecific approach for the synthesis of chiral phenols from cyclohexanones has been developed for the first time. The aromatization reaction proceeds through the in situ formation of enone intermediates and further subsequent bromination/dehydrobromination reactions. And a series of functionalized phenol derivatives are obtained in good yields (up to 89%) and good to excellent enantioselectivities (up to 99% ee).

Enantioselective Terminal Addition to Allenes by Dual Chiral Primary Amine/Palladium Catalysis

We herein describe a synergistic chiral primary amine/achiral palladium catalyzed enantioselective terminal addition to allenes with α-branched β-ketocarbonyls and aldehydes. The reactions afford allylic adducts bearing acyclic all-carbon quaternary centers with high regio- and enantioselectivity. A wide range of allenes including those aliphatic or 1,1'-disubstituted could be employed, thus expanding the scope of typical asymmetric allylic alkylation reactions.

Enantioselective [2 + 2] cycloaddition of N-allenamides with cyclic N-sulfonylketimines: access to polysubstituted azetidines bearing quaternary stereocenters

A Ni(ClO₄)₂-catalyzed enantioselective [2 + 2] cycloaddition of N-allenamides with cyclic N-sulfonylketimines was developed, which regioselectively occurred at the proximal C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds of the N-allenamides.

A Ni(ClO₄)₂-catalyzed enantioselective [2 + 2] cycloaddition of N-allenamides with cyclic N-sulfonylketimines was developed, which regioselectively occurred at the proximal CC bonds of the N-allenamides. Broad substrate scope of N-allenamides and cyclic N-sulfonylketimines was observed. A range of fused polysubstituted azetidines bearing quaternary stereocenters were afforded in good yields and with excellent enantioselectivities (up to 99%).

Combinations of Aminocatalysts and Metal Catalysts: A Powerful Cooperative Approach in Selective Organic Synthesis

The cooperation and interplay between organic and metal catalyst systems is of utmost importance in nature and chemical synthesis. Here innovative and selective cooperative catalyst systems can be designed by combining two catalysts that complement rather than inhibit one another. This refined strategy can permit chemical transformations unmanageable by either of the catalysts alone. This review summarizes innovations and developments in selective organic synthesis that have used cooperative dual catalysis by combining simple aminocatalysts with metal catalysts. Considerable efforts have been devoted to this fruitful field. This emerging area employs the different activation modes of amine and metal catalysts as a platform to address challenging reactions. Here, aminocatalysis (e.g., enamine activation catalysis, iminium activation catalysis, single occupied molecular orbital (SOMO) activation catalysis, and photoredox activation catalysis) is employed to activate unreactive carbonyl substrates. The transition metal catalyst complements by activating a variety of substrates through a range of interactions (e.g., electrophilic π-allyl complex formation, Lewis acid activation, allenylidene complex formation, photoredox activation, C-H activation, etc.), and thereby novel concepts within catalysis are created. The inclusion of heterogeneous catalysis strategies allows for "green" chemistry development, catalyst recyclability, and the more eco-friendly synthesis of valuable compounds.

Rhodium-Catalyzed Regio- and Stereoselective [2 + 2] Cycloaddition of Allenamides

A highly regio- and stereoselective Rh-catalyzed intermolecular head-to-head [2 + 2] cycloaddition of allenamides was developed. The intermolecular cycloadducts, trans-dimethylenecyclobutane-1,2-diamine derivatives, were achieved in good yields with high regioselectivity and stereoselectivity.

Asymmetric dual catalysis via fragmentation of a single rhodium precursor complex

A strategy for dual transition metal catalysis and organocatalysis is reported via disintegration of a single rhodium complex. Conveniently, the chiral-at-metal rhodium precatalyst can be synthesized in just two steps starting from rhodium trichloride without the need for any chromatography.