All-carbon quaternary stereogenic centers in acyclic systems through the creation of several C-C bonds per chemical step

Nickel-catalyzed formation of quaternary carbon centers using tertiary alkyl electrophiles

This review provides a comprehensive summary of recent advances in nickel-catalyzed reactions employing tertiary alkyl electrophiles for the construction of quaternary carbon centers.

Chemistry of 3-hydroxy-2-aryl acrylate: syntheses, mechanisms, and applications

3-Hydroxy-2-aryl acrylate is important scaffold which is widely used for the synthesis of pharmacologically active compounds. This review summarises the synthetic methods of the 3-hydroxy-2-aryl acrylate including mechanisms and applications.

Mechanistic insights into rhodium-catalyzed enantioselective allylic alkylation for quaternary stereogenic centers

Installing quaternary stereogenic carbon is an arduous task of contemporary importance in the domain of asymmetric catalysis. To this end, an asymmetric allylic alkylation of α,α-disubstituted aldehydes by using allyl benzoate in the presence of Wilkinson's catalyst [Rh(Cl)(PPh₃)₃], (R)-BINOL–P(OMe) as the external ligand, and LiHMDS as the base has been reported to offer high enantioselectivity. The mechanistic details of this important reaction remain vague, which prompted us to undertake a detailed density functional theory (SMD_(THF)/B3LYP-D3) investigation on the nature of the potential active catalyst, energetic features of the catalytic cycle, and the origin of high enantioselectivity. We note that a chloride displacement from the native Rh-phosphine [Rh(Cl)(PPh₃)₃] by BINOL–P(OMe) phosphite and an ensuing MeCl elimination can result in the in situ formation of a Rh-phosphonate [Rh(BINOL–P Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O)(PPh₃)₃]. A superior energetic span (δE) noted with such a Rh-phosphonate suggests that it is likely to serve as an active catalyst. The uptake of allyl benzoate by the active catalyst followed by the turnover determining C–O bond oxidative addition furnishes a Rh-π-allyl intermediate, which upon interception by (Z)-Li-enolate (derived from α,α-disubstituted aldehyde) in the enantiocontrolling C–C bond generates a quaternary stereogenic center. The addition of the re prochiral face of the (Z)-Li-enolate to the Rh-bound allyl moiety leading to the R enantiomer of the product is found to be 2.4 kcal mol⁻¹ more preferred over the addition through its si face. The origin of the stereochemical preference for the re face addition is traced to improved noncovalent interactions (NCIs) and less distortion in the enantiocontrolling C–C bond formation transition state than that in the si face addition. Computed enantioselectivity (96%) is in very good agreement with the experimental value (92%), so is the overall activation barrier (δE of 17.1 kcal mol⁻¹), which is in conformity with room temperature reaction conditions.

The origin of high enantioselectivity in the formation of quaternary stereogenic carbon.

Diastereo-, Enantio-, and anti-Selective Formation of Secondary Alcohol and Quaternary Carbon Stereocenters by Cu-Catalyzed Additions of B-Substituted Allyl Nucleophiles to Carbonyls

A general method for the synthesis of secondary homoallylic alcohols containing α-quaternary carbon stereogenic centers in high diastereo- and enantioselectivity (up to >20:1 dr and >99:1 er) is disclosed. Transformations employ readily accessible aldehydes, allylic diboronates, and a chiral copper catalyst and proceed by γ-addition of in situ generated enantioenriched boron-stabilized allylic copper nucleophiles. The catalytic protocol is general for a wide variety of aldehydes as well as a variety of 1,1-allylic diboronic esters. Hammett studies disclose that diastereoselectivity of the reaction is correlated to the electronic nature of the aldehyde, with dr increasing as aldehydes become more electron poor.

Carbamoyl Fluoride-Enabled Enantioselective Ni-Catalyzed Carbocarbamoylation of Unactivated Alkenes

A carbamoyl fluoride-enabled enantioselective Ni-catalyzed carbocarbamoylation of unactivated alkenes was developed, providing a broad range of chiral γ-lactams bearing an all-carbon quaternary center in 45-96% yield and 38-97% ee.

Biocatalytic Construction of Quaternary Centers by Aldol Addition of 3,3-Disubstituted 2-Oxoacid Derivatives to Aldehydes

The congested nature of quaternary carbons hinders their preparation, most notably when stereocontrol is required. Here we report a biocatalytic method for the creation of quaternary carbon centers with broad substrate scope, leading to different compound classes bearing this structural feature. The key step comprises the aldol addition of 3,3-disubstituted 2-oxoacids to aldehydes catalyzed by metal dependent 3-methyl-2-oxobutanoate hydroxymethyltransferase from E. coli (KPHMT) and variants thereof. The 3,3,3-trisubstituted 2-oxoacids thus produced were converted into 2-oxolactones and 3-hydroxy acids and directly to ulosonic acid derivatives, all bearing gem-dialkyl, gem-cycloalkyl, and spirocyclic quaternary centers. In addition, some of these reactions use a single enantiomer from racemic nucleophiles to afford stereopure quaternary carbons. The notable substrate tolerance and stereocontrol of these enzymes are indicative of their potential for the synthesis of structurally intricate molecules.

Forming All-Carbon Quaternary Stereocenters by Organocatalytic Aminomethylation: Concise Access to β2,2 -Amino Acids

The asymmetric synthesis of β^2,2 -amino acids remains a formidable challenge in organic synthesis. Here a novel organocatalytic enantioselective aminomethylation of ketenes with stable and readily available N,O-acetals is reported, providing β^2,2 -amino esters bearing an all-carbon quaternary stereogenic center in high enantiomeric ratios with a catalytic amount of chiral phosphoric acid. Typically, this transformation probably proceeds through an asymmetric counter-anion-directed catalysis. As a result, a concise, practical, and atom-economic protocol toward rapidly access to β^2,2 -amino acids has been developed.

Chemoselective Homologation-Deoxygenation Strategy Enabling the Direct Conversion of Carbonyls into (n+1)-Halomethyl-Alkanes

The sequential installation of a carbenoid and a hydride into a carbonyl, furnishing halomethyl alkyl derivatives, is reported. Despite the employment of carbenoids as nucleophiles in reactions with carbon-centered electrophiles, sp³-type alkyl halides remain elusive materials for selective one-carbon homologations. Our tactic levers on using carbonyls as starting materials and enables uniformly high yields and chemocontrol. The tactic is flexible and is not limited to carbenoids. Also, diverse carbanion-like species can act as nucleophiles, thus making it of general applicability.

Stereoselective Access to Fully Substituted Aldehyde-Derived Silyl Enol Ethers by Iridium-Catalyzed Alkene Isomerization

An in situ generated cationic Ir-catalyst isomerizes simple allylic silyl ethers into valuable, fully substituted aldehyde-derived silyl enol ethers. Importantly, by judicious choice of substrate, either of the two possible stereoisomers of a given enolate derivative is accessible with complete stereoselectivity. One-pot isomerization-aldol and isomerization-allylation processes illustrate the synthetic utility of this method.

Opportunities for Tapping into Three-Dimensional Chemical Space through a Quaternary Carbon

A quaternary carbon bears four other carbon substituents or combination of four non-hydrogen substituents at four vertices of a tetrahedron. The spirocyclic quaternary carbon positioned at the center of a bioactive molecule offers conformational rigidity, which in turn reduces the penalty for conformational entropy. The quaternary carbon is a predominant feature of natural product structures and has been associated with more effective and selective binding to target proteins compared to planar compounds with a high sp² count. The presence of a quaternary carbon stereocenter allows the exploration of novel chemical space to obtain new molecules with enhanced three-dimensionality. These characteristics, coupled to an increasing awareness to develop sp³-rich molecules, boosted utility of quaternary carbon stereocenters in bioactive compounds. It is hoped that this Perspective will inspire the chemist to utilize quaternary carbon stereocenters to enhance potency, selectivity, and other drug-like properties.

Catalytic enantioselective construction of vicinal quaternary carbon stereocenters

This review summarizes the advances in the catalytic enantioselective construction of vicinal quaternary carbon stereocenters, introduces major synthetic strategies and discusses their advantages and limitations, highlights the application of known protocols in the total synthesis of natural products, and outlines the synthetic opportunities.

This review summarizes the advances in catalytic enantioselective construction of vicinal quaternary carbon stereocenters, introduces major synthetic strategies and discusses their advantages and limitations, and outlines the synthetic opportunities.

Visible-Light-Mediated Intermolecular Radical Conjugate Addition for the Construction of Vicinal Quaternary Carbon Centers

The visible light-driven organophotoredox catalysis is reported for the construction of vicinal quaternary carbon centers. Intermolecular conjugate addition of alkyl radicals, derived from 2,2-disubstituted dihydroquinazolinones, to Michael acceptors under blue light irradiation and rhodamine B catalysis allows the facile assembly of diverse, vicinal secondary/quaternary, tertiary/quaternary, and quaternary/quaternary carbon centers at room temperature. Our method provides a synthetically versatile protocol since both 2,2-disubstituted dihydroquinazolinones and Michael acceptors can be conveniently prepared from readily available ketones.

Synthesis of Quaternary Carbon Stereogenic Centers by Diastereoselective Conjugate Addition of Boron-Stabilized Allylic Nucleophiles to Enones

A method for the site-selective and diastereoselective conjugate addition of boron-stabilized allylic nucleophiles to α,β-unsaturated ketones is disclosed. Transformations involve easily prepared γ,γ-disubstituted allyldiboron reagents and proceed in the presence of a fluoride activator at 80 °C. Reactions proceed with a wide variety of enones and allyldiboron reagents efficiently to deliver ketone products that contain otherwise difficult-to-access vicinal β-tertiary and γ-quaternary carbon stereogenic centers and an alkenylboron moiety. The utility of the method is highlighted by several transformations, including cross-coupling and carbocyclizations.

Highly Enantioselective Synthesis of Indazoles with a C3-Quaternary Chiral Center Using CuH Catalysis

C3-substituted 1H-indazoles are useful and important substructures in many pharmaceuticals. Methods for direct C3-functionalization of indazoles are relatively rare, compared to reactions developed for the more nucleophilic N1 and N2 positions. Herein, we report a highly C3-selective allylation reaction of 1H-N-(benzoyloxy)indazoles using CuH catalysis. A variety of C3-allyl 1H-indazoles with quaternary stereocenters were efficiently prepared with high levels of enantioselectivity. Density functional theory (DFT) calculations were performed to explain the reactivity differences between indazole and indole electrophiles, the latter of which was used in our previously reported method. The calculations suggest that the indazole allylation reaction proceeds through an enantioselectivity-determining six-membered Zimmerman-Traxler-type transition state, rather than an oxidative addition/reductive elimination sequence, as we proposed in the case of indole alkylation. The enantioselectivity of the reaction is governed by both ligand-substrate steric interactions and steric repulsions involving the pseudoaxial substituent in the six-membered allylation transition state.

Enantioselective synthesis of mixed 3,3'-bisindoles via a phosphine-catalyzed umpolung γ-addition of 3'-indolyl-3-oxindoles to allenoates

An enantioselective umpolung γ-addition reaction of 3'-indolyl-3-oxindoles to allenoates catalyzed by amino acid-derived bifunctional phosphine catalysts has been developed. A wide range of chiral mixed 3,3'-bisindole scaffolds containing an all-carbon quaternary stereogenic center were obtained in high yields and with excellent enantioselectivities. 3,3'-Bisindoles are valuable synthetic intermediates, the employment of which led to formal total syntheses of (+)-Chimonanthine, (+)-Folicanthine and (-)-Calycanthine, as well as facile creation of useful pyrrolidinoindoline core structure.

Enantioselective Assembly of Cycloenones with a Nitrile-Containing All-Carbon Quaternary Center from Malononitriles Enabled by Ni Catalysis

Chiral nitriles are valuable molecules in modern organic synthesis and drug discovery. Selectively differentiating the two nitrile groups of widely available malononitrile derivatives is a straightforward yet underdeveloped route to construct enantioenriched nitriles. Here we report an enantioselective nickel-catalyzed desymmetrization of malononitriles for the generation of nitrile-containing all-carbon quaternary stereocenters. This protocol involves a nickel-catalyzed addition of aryl boronic acids to alkynes, followed by a selective nitrile insertion, providing unprecedented access to enantioenriched 5-7-membered α-cyano-cycloenones with a fully substituted olefin from a broad range of substrates. The synthetic utility of these nitrile products is demonstrated by gram-scale synthesis and conversion to several useful functional groups.

Construction of 1-Tetralols Bearing Two Contiguous Quaternary Chiral Centers through a Rhodium-Catalyzed Enantioselective Desymmetrization Cascade Reaction

A novel and efficient access to polyfunctionnalized chiral 1-tetralols, bearing two contiguous quaternary carbon stereocenters, has been developed from various and easily accessible alkynyl-1,3-diketones, through a cascade process including a regioselective alkyne insertion, a 1,4-Rh shift, and a nucleophilic addition step via the desymmetrization of the 1,3-diketone moiety thanks to an appropriate rhodium-chiral diene complex in the presence of arylboronic acids.

Cu-Catalyzed Diastereo- and Enantioselective Reactions of γ,γ-Disubstituted Allyldiboron Compounds with Ketones

A catalytic diastereo- and enantioselective method for the preparation of complex tertiary homoallylic alcohols containing a vicinal quaternary carbon stereogenic center and a versatile alkenylboronic ester is disclosed. Transformations are promoted by 5 mol % of a readily available copper catalyst bearing a bulky monodentate phosphoramidite ligand, which is essential for attaining both high dr and er. Reactions proceed with a wide variety of ketones and allylic 1,1-diboronate reagents, which enables the efficient preparation of diverse array of molecular scaffolds.

Enantioselective Construction of Quaternary Stereogenic Centers by the Addition of an Acyl Anion Equivalent to 1,3-Dienes

We report the enantioselective formation of quaternary stereogenic centers by the intermolecular addition of malononitrile, an acyl anion equivalent, and related pronucleophiles to several 1,3-disubstituted acyclic 1,3-dienes in the presence of a Pd-PHOX catalyst. Products are obtained in up to 88% yield and 99:1 er and in most cases are formed as a single regioisomer. The products' malononitrile unit undergoes oxidative functionalization to afford β,γ-unsaturated carbonyls bearing internal olefins and α-quaternary stereogenic centers.

Construction of Quaternary Carbon Center by Catalytic Asymmetric Alkylation of 3-Arylpiperidin-2-ones Under Phase-Transfer Conditions

A highly enantioselective synthesis of δ-lactams having a chiral quaternary carbon center at the α-position has been developed through an asymmetric alkylation of 3-arylpiperidin-2-ones under phase-transfer conditions. In this transformation, a 2,2-diarylvinyl group on the δ-lactam nitrogen atom plays a crucial role as a novel protecting group and an achiral auxiliary for improving both yield and enantioselectivity of the reaction.

Cross-dehydrogenative coupling enables enantioselective access to CF3-substituted all-carbon quaternary stereocenters

A cross-dehydrogenative coupling strategy for enantioselective access to acyclic CF₃-substituted all-carbon quaternary stereocenters has been established. By using catalytic DDQ with MnO₂ as an inexpensive terminal oxidant, asymmetric cross coupling of racemic δ-CF₃-substituted phenols with indoles proceeded smoothly, providing CF₃-bearing all-carbon quaternary stereocenters with excellent chemo- and enantioselectivities. The generality of the strategy is further demonstrated by efficient construction of all-carbon quaternary stereocenters bearing other polyfluoroalkyl and perfluoroalkyl groups such as CF₂Cl, C₂F₅, and C₃F₇.

A cross-dehydrogenative coupling (CDC) strategy for enantioselective access to acyclic CF₃-substituted all-carbon quaternary stereocenters has been established.

Diastereo- and Enantioselective Synthesis of Homoallylic Amines Bearing Quaternary Carbon Centers

A Cu-catalyzed method for the efficient enantio- and diastereoselective synthesis of chiral homoallylic amines bearing a quaternary carbon and an alkenylboron is disclosed. Transformations are promoted by a readily prepared (phosphoramidite)-Cu complex and involve bench-stable γ,γ-disubstituted allyldiborons and benzyl imines; products are obtained in up to 82% yield, >20:1 dr, and >99:1 er. Reactions proceed via stereodefined boron-stabilized allylic Cu species formed by an enantioselective transmetalation. Utility of the 1-amino-3-alkenylboronate products is highlighted by a variety of synthetic transformations.

A Spiroalkylation Method for the Stereoselective Construction of α-Quaternary Carbons and Its Application to the Total Synthesis of (R)-Puraquinonic Acid

Cyclic α-quaternary carbon stereocenters were prepared from biselectrophillic substrates and an easily prepared chiral bicyclic sulfonyl lactam. This was achieved in two steps by spiroalkylation, employing biphasic reaction conditions with a phase-transfer catalyst, followed by reduction and alkylation with a series of alkyl halide electrophiles. The products of this method were isolated in good yields with with high levels of diastereoselectivity. This methodology was employed in the enantioselective total synthesis of (R)-puraquinonic acid (1) for a late-stage installation of the α-quaternary carbon stereocenter. This enabled the shortest synthesis of 1 to date, an eight-pot sequence providing an overall yield of 14%.

Highly Selective and Catalytic Generation of Acyclic Quaternary Carbon Stereocenters via Functionalization of 1,3-Dienes with CO2

The catalytic asymmetric functionalization of readily available 1,3-dienes is highly important, but current examples are mostly limited to the construction of tertiary chiral centers. The asymmetric generation of acyclic products containing all-carbon quaternary stereocenters from substituted 1,3-dienes represents a more challenging, but highly desirable, synthetic process for which there are very few examples. Herein, we report the highly selective copper-catalyzed generation of chiral all-carbon acyclic quaternary stereocenters via functionalization of 1,3-dienes with CO₂. A variety of readily available 1,1-disubstituted 1,3-dienes, as well as a 1,3,5-triene, undergo reductive hydroxymethylation with high chemo-, regio-, E/Z-, and enantioselectivities. The reported method features good functional group tolerance, is readily scaled up to at least 5 mmol of starting diene, and generates chiral products that are useful building blocks for further derivatization. Systemic mechanistic investigations using density functional theory calculations were performed and provided the first theoretical investigation for an asymmetric transformation involving CO₂. These computational results indicate that the 1,2-hydrocupration of 1,3-diene proceeds with high π-facial selectivity to generate an (S)-allylcopper intermediate, which further induces the chirality of the quaternary carbon center in the final product. The 1,4-addition of an internal allylcopper complex, which differs from previous reports involving terminal allylmetallic intermediates, to CO₂ kinetically determines the E/Z- and regioselectivity. The rapid reduction of a copper carboxylate intermediate to the corresponding silyl-ether in the presence of Me(MeO)₂SiH provides the exergonic impetus and leads to chemoselective hydroxymethylation rather than carboxylation. These results provide new insights for guiding further development of asymmetric C-C bond formations with CO₂.

Regio- and Diastereoselective Copper-Catalyzed Carbometalation of Cyclopropenylsilanes

We therein report a regio- and diastereoselective copper-catalyzed carbometalation of enantioenriched readily available cyclopropenylsilanes as a synthetic tool to access a large family of stereodefined fully-(hexa)-substituted cyclopropanes.

Diastereoselective ring opening of fully-substituted cyclopropanes via intramolecular Friedel-Crafts alkylation

We herein disclose a diastereoselective ring opening of non-donor-acceptor cyclopropanes via an intramolecular Friedel-Crafts alkylation en route to functionalized dihydronaphthalene scaffolds possessing quaternary carbon stereocentres. The transformation proceeds through a selective bond breaking at the most alkylated carbon centre with a pure retention of configuration. Mechanistic investigations and computational studies revealed that alkoxy functionality is the key for selective bond breaking leading to a complete retention of configuration.

Enantioselective Dearomatization of Indoles by an Azoalkene-Enabled (3+2) Reaction: Access to Pyrroloindolines

The enantioselective dearomatization of indoles by an organocatalytic (3+2) reaction has been established. The reaction makes use of simple indole derivatives as substrates, and employs azoalkenes reaction partners. A wide range of pyrroloindolines containing an all-carbon quaternary stereogenic center were readily prepared in high yields and with excellent enantioselectivities.

Construction of All-Carbon Chiral Quaternary Centers through CuI -Catalyzed Enantioselective Reductive Hydroxymethylation of 1,1-Disubstituted Allenes with CO2

A catalytic enantioselective construction of all-carbon chiral quaternary centers through reductive hydroxymethylation of 1,1-disubstituted allenes with CO₂ has been developed. In the presence of a copper/Mandyphos catalyst, CO₂ is transformed into an alcohol oxidation level by an asymmetric reductive C-C bond formation with allenes by using hydrosilane (HSi(OMe)₂ Me) as a reductant. The resulting chiral homoallylic alcohols are versatile synthetic intermediates and can be conveniently converted into a variety of useful chiral chemicals.

Regio- and Stereoselective Synthesis of Fully Substituted Silyl Enol Ethers of Ketones and Aldehydes in Acyclic Systems

The regio- and stereoselective preparation of fully substituted and stereodefined silyl enol ethers of ketones and aldehydes through an allyl-Brook rearrangement is reported. This fast and efficient method proceeds from a mixture of E and Z isomers of easily accessible starting materials.