Copper-catalyzed enantioselective conjugate addition of organometallic reagents to challenging Michael acceptors

Broadly Applicable Copper(I)-Catalyzed Alkyne Semihydrogenation and Hydrogenation of α,β-Unsaturated Amides Enabled by Bifunctional Iminopyridine Ligands

A highly active bifunctional catalyst consisting of a copper(I)/N-heterocyclic carbene complex and a basic 2-iminopyridine subunit allows for copper hydride chemistry under low H2 pressure, achieving efficient catalysis reaching 1 bar (balloon pressure). The bifunctional catalyst tolerates a remarkable variety of functional groups in catalytic alkyne semihydrogenations. Furthermore, this catalyst design gives rise to a high reactivity that allows for the catalytic hydrogenation of α,β-unsaturated amides (a substrate class hitherto unreactive in copper hydride catalysis) at a low H2 pressure for the first time. In this manner, late-stage modification and isotope labeling of α,β-unsaturated amides, common subunits in biologically active compounds, can be realized through catalytic hydrogenation using a first-row transition metal catalyst based on abundant copper. Preliminary mechanistic experiments indicate that the bifunctional catalyst operates via an iminopyridine-mediated proximity effect. We hypothesize that the coordination of an alcohol as a proton source on the copper(I) complex facilitates the overall reactions through a rapid proto-decupration step.

Asymmetric Conjugate Addition of Vinyl Arene-Derived Chiral Benzylcopper Nucleophiles

We report the copper-catalyzed asymmetric conjugate addition of vinyl arene-derived nucleophiles to α,β-unsaturated diesters. Due to the high electrophilicity of α,β-unsaturated diesters toward nucleophilic copper catalysts, inclusion of vinyl arenes as pronucleophiles is challenging due to chemoselectivity issues in a multicomponent reaction compared with those of other successful examples of dienes, allenes, and enynes. In this method, chemoselective in situ generation of a chiral benzylic copper intermediate and its stereoselective addition to α,β-unsaturated diesters provided organoboron adducts with contiguous stereogenic centers with good levels of diastereoselectivity and enantioselectivity (≤85:15 diastereomeric ratio and 99% enantiomeric excess, respectively).

Asymmetric tandem conjugate addition and reaction with carbocations on acylimidazole Michael acceptors

We present here a stereoselective tandem reaction based on the asymmetric conjugate addition of dialkylzinc reagents to unsaturated acylimidazoles followed by trapping of the intermediate zinc enolate with carbocations. The use of a chiral NHC ligand provides chiral zinc enolates in high enantiomeric purities. These enolates are reacted with highly electrophilic onium compounds to afford densely substituted acylimidazoles. DFT calculations helped to understand the reactivity of the zinc enolates derived from acylimidazoles and allowed their comparison with metal enolates obtained by other conjugate addition reactions.

Enolates ambushed - asymmetric tandem conjugate addition and subsequent enolate trapping with conventional and less traditional electrophiles

Metal enolates are useful intermediates and building blocks indispensable in many organic synthetic transformations. Chiral metal enolates obtained by asymmetric conjugate additions of organometallic reagents are structurally complex intermediates that can be employed in many transformations. In this review, we describe this burgeoning field that is reaching maturity after more than 25 years of development. The effort of our group to broaden possibilities to engage metal enolates in reactions with new electrophiles is described. The material is divided according to the organometallic reagent employed in the conjugate addition step, and thus to the particular metal enolate formed. Short information on applications in total synthesis is also given.

Iron-Catalyzed Coupling of Alkenes and Enones: Sakurai-Michael-type Conjugate Addition of Catalytic Allyliron Nucleophiles

The iron-catalyzed coupling of alkenes and enones through allylic C(sp3)-H functionalization is reported. This redox-neutral process employs a cyclopentadienyliron(II) dicarbonyl catalyst and simple alkene substrates to generate catalytic allyliron intermediates for 1,4-addition to chalcones and other conjugated enones. The use of 2,4,6-collidine as the base and a combination of triisopropylsilyl triflate and LiNTf2 as Lewis acids was found to facilitate this transformation under mild, functional group-tolerant conditions. Both electronically unactivated alkenes as well as allylbenzene derivatives could be employed as pronucleophilic coupling partners, as could a range of enones bearing electronically varied substituents.

Catalytic Asymmetric Conjugate Reduction

Enantioselective reduction reactions are privileged transformations for the construction of trisubstituted stereogenic centers. While these include established synthetic strategies, such as asymmetric hydrogenation, methods based on the enantioselective addition of hydridic reagents to electrophilic prochiral substrates have also gained importance. In this context, the asymmetric conjugate reduction (ACR) of α,β-unsaturated compounds has become a convenient approach for the synthesis of chiral compounds with trisubstituted stereocenters in α-, β-, or γ-position to electron-withdrawing functional groups. Because such activating groups are diverse and amenable of further derivatizations, ACRs provide a general and powerful synthetic entry towards a variety of valuable chiral building blocks. This Review provides a comprehensive collection of catalytic ACR methods involving transition-metal, organic, and enzymatic catalysis since its first versions dating back to the late 1970s.

Copper Hydride-Catalyzed Enantioselective Olefin Hydromethylation

The enantioselective installation of a methyl group onto a small molecule can result in the significant modification of its biological properties. While hydroalkylation of olefins represents an attractive approach to introduce alkyl substituents, asymmetric hydromethylation protocols are often hampered by the incompatibility of highly reactive methylating reagents and a lack of general applicability. Herein, we report an asymmetric olefin hydromethylation protocol enabled by CuH catalysis. This approach leverages methyl tosylate as a methyl source compatible with the reducing base-containing reaction environment, while a catalytic amount of iodide ion transforms the methyl tosylate in situ into the active reactant, methyl iodide, to promote the hydromethylation. This method tolerates a wide range of functional groups, heterocycles, and pharmaceutically relevant frameworks. Density functional theory studies suggest that after the stereoselective hydrocupration, the methylation step is stereoretentive, taking place through an SN2-type oxidative addition mechanism with methyl iodide followed by a reductive elimination.

Direct decarboxylative Giese reactions

The quest to find milder and more sustainable methods to generate highly reactive, carbon-centred intermediates has led to a resurgence of interest in radical chemistry. In particular, carboxylic acids are seen as attractive radical precursors due their availability, low cost, diversity, and sustainability. Moreover, the corresponding nucleophilic carbon-radical can be easily accessed through a favourable radical decarboxylation process, extruding CO₂ as a traceless by-product. This review summarizes the recent progress on using carboxylic acids directly as convenient radical precursors for the formation of carbon-carbon bonds via the 1,4-radical conjugate addition (Giese) reaction.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Asymmetric copper-catalyzed conjugate additions of organometallic reagents in the syntheses of natural compounds and pharmaceuticals

Access to enantiopure complex molecular structures is crucial for the development of new drugs as well as agents used in crop-protection. In this regard, numerous asymmetric methods have been established. Copper-catalyzed 1,4-additions of organometallic reagents are robust C-C bond formation strategies applicable in a wide range of circumstances. This review analyses the syntheses of natural products and pharmaceutical agents, which rely on the application of asymmetric Cu-catalyzed conjugate additions of various organometallic reagents. A wide range of available organometallics, e.g. dialkylzinc, trialkylaluminum, Grignard, and organozirconium, can now be used in conjugate additions to address various synthetic challenges present in targeted natural compounds. Furthermore, efficient catalysts allow high levels of stereofidelity over a diverse array of starting Michael acceptors.