Construction of quaternary stereogenic carbon centers through copper-catalyzed enantioselective allylic cross-coupling with alkylboranes

Enantioselective construction of triaryl-substituted all-carbon quaternary stereocenters via organocatalytic arylation of oxindoles with azonaphthalenes

Azonaphthalenes have been verified as a class of effective arylation reagents in a variety of asymmetric transformations. Here a highly efficient approach to construct triaryl-substituted all-carbon quaternary stereocenters through chiral phosphoric acid-catalyzed enantioselective arylation of 3-aryl-2-oxindoles with azonaphthalenes is disclosed. This chemistry is scalable and displays excellent functional group tolerance, furnishing a series of 3,3-disubstituted 2-oxindole derivatives in good yields with excellent enantiocontrol. Preliminary mechanistic data suggest that the initially formed direct addition intermediate undergoes intramolecular annulation under acidic reaction conditions.

Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

Since the discovery of persistent carbenes by the isolation of 1,3-di-l-adamantylimidazol-2-ylidene by Arduengo and coworkers, we witnessed a fast growth in the design and applications of this class of ligands and their metal complexes. Modular synthesis and ease of electronic and steric adjustability made this class of sigma donors highly popular among chemists. While the nature of the metal-carbon bond in transition metal complexes bearing N-heterocyclic carbenes (NHCs) is predominantly considered to be neutral sigma or dative bonds, the strength of the bond is highly dependent on the energy match between the highest occupied molecular orbital (HOMO) of the NHC ligand and that of the metal ion. Because of their versatility, the coordination chemistry of NHC ligands with was explored with almost all transition metal ions. Other than the transition metals, NHCs are also capable of establishing a chemical bond with the main group elements. The advances in the catalytic applications of the NHC ligands linked with a second tether are discussed. For clarity, more frequently targeted catalytic reactions are considered first. Carbon-carbon coupling reactions, transfer hydrogenation of alkenes and carbonyl compounds, ketone hydrosilylation, and chiral catalysis are among highly popular reactions. Areas where the efficacy of the NHC based catalytic systems were explored to a lesser extent include CO2 reduction, C-H borylation, alkyl amination, and hydroamination reactions. Furthermore, the synthesis and applications of transition metal complexes are covered.

Catalytic Asymmetric Allylic Substitution with Copper(I) Homoenolates Generated from Cyclopropanols

By using copper(I) homoenolates as nucleophiles, which are generated through the ring-opening of 1-substituted cyclopropane-1-ols, a catalytic asymmetric allylic substitution with allyl phosphates is achieved in high to excellent yields with high enantioselectivity. Both 1-substituted cyclopropane-1-ols and allylic phosphates enjoy broad substrate scopes. Remarkably, various functional groups, such as ether, ester, tosylate, imide, alcohol, nitro, and carbamate are well tolerated. Moreover, the present method is nicely extended to the asymmetric construction of quaternary carbon centers. Some control experiments argue against a radical-based reaction mechanism and a catalytic cycle based on a two-electron process is proposed. Finally, the synthetic utilities of the product are showcased by means of the transformations of the terminal olefin group and the ketone group.

Enantioselective Tertiary Electrophile (Hetero)Benzylation: Pd-Catalyzed Substitution of Isoprene Monoxide with Arylacetates*

The enantioselective generation of quaternary carbon centers remains challenging but is of growing importance for the preparation of functional molecules. Metal catalyzed allylic alkylations of tertiary electrophiles can provide access to these substructures but remain generally incompatible with organometallic benzyl nucleophiles. Here we demonstrate that electron-deficient arylacetates can serve as benzyl nucleophile surrogates to generate enantioenriched acyclic molecules containing a quaternary carbon center via a two-step substitution-decarboxylation process using isoprene monoxide. Products are often obtained in >90 % ee using a commercially available catalyst. An array of electron-withdrawing functional groups on the arylacetate moiety are tolerated. The lactone generated by the initial substitution reaction can be used in further stereoselective transformations to prepare molecules with acyclic vicinal quaternary stereocenters.

Copper-Catalyzed Enantioconvergent Radical Suzuki-Miyaura C(sp3)-C(sp2) Cross-Coupling

A copper-catalyzed enantioconvergent Suzuki-Miyaura C(sp³)-C(sp²) cross-coupling of various racemic alkyl halides with organoboronate esters has been established in high enantioselectivity. Critical to the success is the use of a chiral cinchona alkaloid-derived N,N,P-ligand for not only enhancing the reducing capability of copper catalyst to favor a stereoablative radical pathway over a stereospecific S_N2-type process but also providing an ideal chiral environment to achieve the challenging enantiocontrol over the highly reactive radical species. The reaction has a broad scope with respect to both coupling partners, covering aryl- and heteroarylboronate esters, as well as benzyl-, heterobenzyl-, and propargyl bromides and chlorides with good functional group compatibility. Thus, it provides expedient access toward a range of useful enantioenriched skeletons featuring chiral tertiary benzylic stereocenters.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Asymmetric synthesis via stereospecific C-N and C-O bond activation of alkyl amine and alcohol derivatives

This perspective showcases our development of benzylic and allylic amine and alcohol derivatives as electrophiles for stereospecific, nickel-catalyzed cross-coupling reactions, as well as the prior art that inspired our efforts. The success of our effort has relied on the use of benzyl ammonium triflates as electrophiles for cross-couplings via C-N bond activation and benzylic and allylic carboxylates for cross-couplings via C-O bond activation. Our work, along with others' exciting discoveries, has demonstrated the potential of stereospecific, nickel-catalyzed cross-couplings of alkyl electrophiles in asymmetric synthesis, and enables efficient generation of both tertiary and quaternary stereocenters.

Difunctionalization of ketones via gem-bis(boronates) to synthesize quaternary carbon with high selectivity

A transformation of gem-bis(boronate) compounds with different electrophiles through a tertiary boronate intermediate to concurrently introduce aldehyde and allylic groups, which provides an efficient protocol to difunctionalize ketones, was reported.

Highly efficient and recyclable water-soluble fullerene-supported PdCl2 nanocatalyst in Suzuki–Miyaura cross-coupling reaction

A water-soluble fullerene-supported PdCl₂ nanocatalyst [C₆₀-TEG_S/PdCl₂] was prepared by coordination of water-soluble fullerene nanoparticles with palladium chloride. In pure water, the catalytic activity of nanocatalyst [C₆₀-TEG_S/PdCl₂] for Suzuki–Miyaura cross-coupling reaction was investigated under different reaction conditions. The results showed that biphenyl compounds could be synthesized in high yields at room temperature using 0.01 mol% of [C₆₀-TEG_S/PdCl₂] as the catalyst and K₂CO₃ as the base with the reaction time of 4 h. The catalyst was recycled five times, and the yield clearly did not decrease.

The water-soluble fullerene-supported PdCl₂ nanocatalyst [C₆₀-TEG_S/PdCl₂] exhibits high activity and recycling in pure water toward the Suzuki–Miyaura cross-coupling reaction.

Acyclic Quaternary Carbon Stereocenters via Enantioselective Transition Metal Catalysis

Whereas numerous asymmetric methods for formation of quaternary carbon stereocenters in cyclic systems have been documented, the construction of acyclic quaternary carbon stereocenters with control of absolute stereochemistry remains a formidable challenge. This Review summarizes enantioselective methods for the construction of acyclic quaternary carbon stereocenters from achiral or chiral racemic reactants via transition metal catalysis.

Stereospecific, Nickel-Catalyzed Suzuki-Miyaura Cross-Coupling of Allylic Pivalates To Deliver Quaternary Stereocenters

Recognizing the importance of all-carbon, quaternary stereocenters in complex molecule synthesis, a stereospecific, nickel-catalyzed cross-coupling of allylic pivalates with arylboroxines to deliver products equipped with quaternary stereocenters and internal alkenes was developed. The enantioenriched allylic pivalate starting materials are readily prepared, and a variety of functional groups can be incorporated on both the allylic pivalate and the arylboroxine. Additional advantages include the use of a commercially available and air-stable Ni(II) salt and BISBI ligand, mild reaction conditions, and high yields and ee's. The observed stereoinversion of this reaction is consistent with an open transition state in the oxidative addition step.

Enantioselective Conjunctive Cross-Coupling of Bis(alkenyl)borates: A General Synthesis of Chiral Allylboron Reagents

Palladium-catalyzed conjunctive cross-coupling is used for the synthesis of enantioenriched allylboron reagents. This reaction employs nonsymmetric bis(alkenyl)borates as substrates and appears to occur by a mechanism that involves selective activation of the less substituted alkene followed by migration of the more substituted alkene during the course of a Pd-induced metalate rearrangement.

Copper(i)-catalysed asymmetric allylic reductions with hydrosilanes

The copper(i)-catalysed asymmetric allylic reduction enables a regio- and stereoselective transfer of a hydride nucleophile in an S_N2′-fashion onto allylic bromides.

Stereospecific Cross Couplings To Set Benzylic, All-Carbon Quaternary Stereocenters in High Enantiopurity

Asymmetric preparation of all-carbon quaternary stereocenters is an important goal. Despite advances in formation of highly enantioenriched products with quaternary stereocenters proximal to a functional group, methods to install quaternary stereocenters isolated from functional groups are limited. Transition metal catalysis offers a potential solution, but prior cross couplings are limited to allylic substrates or deliver little to no enantiomeric enrichment. We report a stereospecific, nickel-catalyzed Suzuki-Miyaura arylation of tertiary benzylic acetates to deliver products with diaryl and triaryl quaternary stereocenters in high yields and ee's. This reaction employs an inexpensive, air-stable Ni(II) salt and commercially available phosphine ligand to transform tertiary alcohol derivatives, which are easily available in exceptional ee, into valuable products with stereoretention.

Diastereodivergent combined carbometalation/zinc homologation/C-C fragmentation reaction as an efficient tool to prepare acyclic allylic quaternary carbon stereocenters

A new strategy has been developed to construct enantiomerically enriched acyclic allylic quaternary carbon stereocenters in a single-pot operation through a combined carbometalation/zinc homologation/fragmentation sequence. Proper tuning of the reaction conditions enables the synthesis of the two enantiomers starting from a single enantiomer of the starting material.

Catalytic SN2'- and Enantioselective Allylic Substitution with a Diborylmethane Reagent and Application in Synthesis

A catalytic method for the site- and enantioselective addition of commercially available di-B(pin)-methane to allylic phosphates is introduced (pin=pinacolato). Transformations may be facilitated by an NHC-Cu complex (NHC=N-heterocyclic carbene) and products obtained in 63-95 % yield, 88:12 to >98:2 S(N)2'/S(N)2 selectivity, and 85:15-99:1 enantiomeric ratio. The utility of the approach, entailing the involvement of different catalytic cross-coupling processes, is highlighted by its application to the formal synthesis of the cytotoxic natural product rhopaloic acid A.

Copper-catalyzed stereoselective conjugate addition of alkylboranes to alkynoates

A copper-catalyzed conjugate addition of alkylboron compounds (alkyl-9-BBN, prepared by hydroboration of alkenes with 9-BBN-H) to alkynoates to form β-disubstituted acrylates is reported. The addition occurred in a formal syn-hydroalkylation mode. The syn stereoselectivity was excellent regardless of the substrate structure. A variety of functional groups were compatible with the conjugate addition.

The copper-catalysed Suzuki–Miyaura coupling of alkylboron reagents: disproportionation of anionic (alkyl)(alkoxy)borates to anionic dialkylborates prior to transmetalation

A novel Cu-catalyzed Suzuki–Miyaura cross-coupling of alkylboron reagents with aryl and heteroaryl iodides is reported.

Recent advances in copper-catalyzed asymmetric coupling reactions

Copper-catalyzed (or -mediated) asymmetric coupling reactions have received significant attention over the past few years. Especially the coupling reactions of aryl or alkyl halides with nucleophiles became a very powerful tool for the formation of C–C, C–N, C–O and other carbon–heteroatom bonds as well as for the construction of heteroatom-containing ring systems. This review summarizes the recent progress in copper-catalyzed asymmetric coupling reactions for the formation of C–C and carbon–heteroatom bonds.

Stereoselective formation of quaternary stereogenic centers via alkylation of α-substituted malonate-imidazolidinones

A new stereoselective alkylation methodology is presented for formation of chiral, nonracemic quaternary centers via a chiral auxiliary protocol involving α-alkylated malonate imidazolidinones. Based on two X-ray structures of quaternized products, the diastereoselectivity observed may be rationalized via a transition-state involving an s-transC-N conformation of the C-N bond of the auxiliary, with the metal cation (K(+)) chelated into the malonate six-membered hole as a Z-enolate. A deprotection protocol involving ethanethiolate exchange of the imide to the corresponding thioester, followed by a standard Fukuyama reduction and a borohydride reduction, furnishes α,α'-quaternized β-hydroxypropionates in high ee overall.