Transition Metal-Catalyzed Cycloadditions of Cyclopropanes for the Synthesis of Carbocycles: C–C Activation in Cyclopropanes

Deacylative arylation and alkynylation of unstrained ketones

Ketones are ubiquitous in bioactive natural products, pharmaceuticals, chemical feedstocks, and synthetic intermediates. Hence, deacylative coupling reactions enable the versatile elaboration of a plethora of chemicals to access complex drug candidates and natural products. Here, we present deacylative arylation and alkynylation strategies for the synthesis of a wide range of alkyl-tethered arenes and alkynes from cyclic ketones and methyl ketones under dual nickel/photoredox catalysis. This reaction begins by generating a pre-aromatic intermediate (PAI) through the condensation of the ketone and N'-methylpicolino-hydrazonamide (MPHA), followed by the oxidative cleavage of the PAI α-C─C bond to form an alkyl radical, which is subsequently intercepted by a Ni complex, facilitating the formation of diverse C(sp3)-C(sp2)/C(sp) bonds with remarkable generality. This protocol features a one-pot reaction capability, high regioselectivity and ring-opening efficiency, mild reaction conditions, and a broad substrate scope with excellent functional group compatibility.

Cobalt(I)-Catalyzed (3 + 2 + 2) Cycloaddition between Alkylidenecyclopropanes, Alkynes, and Alkenes

Cobalt(I) catalysts equipped with bisphosphine ligands can be used to promote formal (3 + 2 + 2) intramolecular cycloadditions of enynylidenecyclopropanes of type 1. The method provides synthetically appealing 5,7,5-fused tricyclic systems in good yields and with complete diastereo- and chemoselectivity. Interestingly, its scope differs from that of previously reported annulations based on precious metal catalysts, specifically rhodium and palladium. Noticeably, density functional theory calculations confirm that the mechanism of the reaction is also different from those proposed for these other catalysts.

Biocatalytic One-Carbon Ring Expansion of Aziridines to Azetidines via a Highly Enantioselective [1,2]-Stevens Rearrangement

We report enantioselective one-carbon ring expansion of aziridines to make azetidines as a new-to-nature activity of engineered "carbene transferase" enzymes. A laboratory-evolved variant of cytochrome P450BM3, P411-AzetS, not only exerts unparalleled stereocontrol (99:1 er) over a [1,2]-Stevens rearrangement but also overrides the inherent reactivity of aziridinium ylides, cheletropic extrusion of olefins, to perform a [1,2]-Stevens rearrangement. By controlling the fate of the highly reactive aziridinium ylide intermediates, these evolvable biocatalysts promote a transformation which cannot currently be performed using other catalyst classes.

Atroposelective Kinetic Resolution of 8H-Indeno[1,2-c]thiophen-8-ols via Pd-Catalyzed C-C Bond Cleavage Reaction

The present work demonstrates a palladium-catalyzed kinetic resolution/ring-opening reaction of 8H-indeno[1,2-c]thiophen-8-ols. The reaction proceeds in a highly regioselective manner, and both optically active thiophene-phenyl atropisomers and stereogenic 8H-indeno[1,2-c]thiophen-8-ols were obtained with high enantiomeric excesses. The synthetic applications of the obtained thiophenyl atropisomers were briefly investigated.

Synthesis of Planar Chiral 2-Aryl Aroylferrocenes via Palladium-Catalyzed C-C Bond-Cleavage/Ring-Opening Reaction

A palladium-catalyzed ring-opening reaction of optically active ferrocenyl tertiary alcohols for the construction of planar chiral ketones is reported. The stereochemistry of the hydroxyl group in ferrocenyl alcohols markedly affects reaction: ferrocenyl alcohols with a β-hydroxyl group show better reactivity and chemoselectivity than the corresponding α-hydroxyl analogues. The treatment of α-hydroxyl substrates with trifluoroacetic acid successfully realizes the inversion of the orientation of hydroxyl group to the corresponding β-analogues.

Rhodium-Catalyzed Dehydrogenative Cycloisomerization of Dienylcyclopropane to Highly Substituted Toluene

A rhodium-catalyzed dehydrogenative cycloisomerization of dienylcyclopropane compounds is reported, which provides a straightforward approach to a variety of highly substituted toluene derivatives in 67-85% yields. The dienylcyclopropane-imides are produced by a single-step formal three-component olefination procedure. Preliminary mechanistic studies indicated that an electron-withdrawing group as R plays a critical role in completing this transformation.

Co-Catalyzed Asymmetric Intramolecular [3+2] Cycloaddition of Yne-Alkylidenecyclopropanes and its Reaction Mechanism

Developing new transition metal-catalyzed asymmetric cycloadditions for the synthesis of five-membered carbocycles (FMCs) is a research frontier in reaction development due to the ubiquitous presence of chiral FMCs in various functional molecules. Reported here is our discovery of a highly enantioselective intramolecular [3+2] cycloaddition of yne-alkylidenecyclopropanes (yne-ACPs) to bicyclo[3.3.0]octadiene and bicyclo[4.3.0]nonadiene molecules using a cheap Co catalyst and commercially available chiral ligand (S)-Xyl-BINAP. This reaction avoids the use of precious Pd and Rh catalysts, which are usually the choices for [3+2] reactions with ACPs. The enantiomeric excess in the present reaction can be up to 92 %. Cationic cobalt(I) species was suggested by experiments as the catalytic species. DFT calculations showed that this [3+2] reaction starts with oxidative cyclometallation of alkyne and ACP, followed by ring opening of the cyclopropyl (CP) group and reductive elimination to form the cycloadduct. This mechanism is different from previous [3+2] reactions of ACPs, which usually start from CP cleavage, not from oxidative cyclization.

Cycloaddition of cyclopropanes for the elaboration of medium-sized carbocycles

The stereocontrolled formation of medium-sized carbocycles is a major goal in modern organic chemistry due to their widespread occurrence in natural products and pharmaceutically active ingredients. One approach consists in the use of cycloaddition reactions which notably results in high selectivities and atom-economy. To this end, cyclopropanes are ideal substrates since they can provide readily functionalized three- or five-carbon synthons. Herein we report advances made in cycloaddition reactions of cyclopropanes towards the synthesis of medium-sized carbocycles via transition metal catalysis or Lewis acid catalysis.

[C^N]-Alkenyl Gold(III) Complexes by Proximal Ring-Opening of (2-Pyridyl)alkylidenecyclopropanes: Mechanistic Insights

Pyridine-substituted alkylidenecyclopropanes (Py-ACPs) react with gold(III) salts under mild reaction conditions through an unprecedented, proximal ring-opening pathway, to generate highly appealing, catalytically active pyridine alkenyl [C^N]-gold(III) species. Mechanistic studies reveal that the activation of the C-C bond of the ACP takes place through an unusual concerted, σ-bond metathesis type-process.

Silylium-Ion-Promoted (5+1) Cycloaddition of Aryl-Substituted Vinylcyclopropanes and Hydrosilanes Involving Aryl Migration

A transition‐metal‐free (5+1) cycloaddition of aryl‐substituted vinylcyclopropanes (VCPs) and hydrosilanes to afford silacyclohexanes is reported. Catalytic amounts of the trityl cation initiate the reaction by hydride abstraction from the hydrosilane, and further progress of the reaction is maintained by self‐regeneration of the silylium ions. The new reaction involves a [1,2] migration of an aryl group, eventually furnishing 4‐ rather than 3‐aryl‐substituted silacyclohexane derivatives as major products. Various control experiments and quantum‐chemical calculations support a mechanistic picture where a silylium ion intramolecularly stabilized by a cyclopropane ring can either undergo a kinetically favored concerted [1,2] aryl migration/ring expansion or engage in a cyclopropane‐to‐cyclopropane rearrangement.

Catalytic asymmetric cycloaddition reactions of enoldiazo compounds

This review describes catalytic asymmetric cycloaddition reactions of silyl-protected enoldiazo compounds for the construction of highly functionalized carbo- and heterocycles which possess one or more chiral center(s). The enoldiazo compound or its derivative, donor-acceptor cyclopropene, form electrophilic vinylogous metal carbene intermediates that combine stepwise with nucleophilic dipolar reactants to form products from [3 + 1]-, [3 + 2]-, [3 + 3]-, [3 + 4]-, and [3 + 5]-cycloaddition, generally in high yield and with exceptional stereocontrol and regioselectivity.

Dirhodium(II)-catalyzed [3 + 2] cycloaddition of N-arylaminocyclopropane with alkyne derivatives

Dirhodium(II) complex-catalyzed [3 + 2] reactions between N-arylaminocyclopropanes and alkyne derivatives are described. The cycloaddition products proved to be versatile synthetic intermediates. trans-Cyclic β-amino acids and derivatives thereof can be conveniently synthesized using this cycloaddition protocol.

Umpolung α-Silylation of Cyclopropyl Acetates via Low-Temperature Catalytic C-C Activation

We report a redox-neutral, catalytic C-C activation of cyclopropyl acetates to produce silicon-containing five-membered heterocycles in a highly region-and chemoselective fashion. The umpolung α-selective silylation leading to dioxasilolanes is opposed to contemporary β-selective C-C functionalization protocols of cyclopropanols. Lewis base activation of dioxasilolanes as α-silyl carbinol equivalents undergoes the unconventional [1,2]-Brook rearrangement to form tertiary alcohols. Notably, mechanistic studies indicate that an electrophilic metal-π interaction harnessing highly fluorinated Tp (CF 3 ) 2 Rh(nbd) catalyst permitted a low-temperature C-C activation.

Ring-opening formal hetero-[5+2] cycloaddition of 1-tosyl-2,3-dihydro-1H-pyrroles with terminal alkynes: entry to 1-tosyl-2,3-dihydro 2,3-dihydro-1H-azepines

Lewis acid-catalyzed formal hetero-[5+2] cycloaddition of 2,3-dihydro-1H-pyrroles with alkynes through C(sp²)–N bond cleavage toward 2,3-dihydro-1H-azepines is described.

Palladium-catalyzed intramolecular transfer hydrogenation & cycloaddition of p-quinamine-tethered alkylidenecyclopropanes to synthesize perhydroindole scaffolds

A facile Pd-catalyzed intramolecular transfer hydrogenation and cycloaddition of p-quinamine-tethered alkylidenecyclopropanes has been developed, giving rigid tricyclic perhydroindole skeletons in moderate to good yields.

Manganese-catalyzed regiospecific sp3C–S bond formation through C–C bond cleavage of cyclobutanols

A manganese-catalyzed regioselective sp³C–S bond formation through C–C bond cleavage of cyclobutanols is described.

Rh-catalysed [5 + 1] cycloaddition of allenylcyclopropanes and CO: reaction development and application to the formal synthesis of (−)-galanthamine

A Rh-catalyzed [5 + 1] cycloaddition of allenylcyclopropanes and CO has been developed to synthesize functionalized 2-methylidene-3,4-cyclohexenones. This cycloaddition reaction has been utilized as a key step in the formal synthesis of natural product (−)-galanthamine.