Nickel-Catalyzed [3+2+2] Cycloadditions between Alkynylidenecyclopropanes and Activated Alkenes

Palladium-catalyzed selective C-C bond cleavage of keto-vinylidenecyclopropanes: construction of structurally rich dihydrofurans and tetrahydrofurans

Palladium-catalyzed selective cleavage of the distal C-C bond and proximal C-C bond of keto-vinylidenecyclopropanes by altering the sterically bulky phosphine ligands has been realized. The proximal C-C bond cleavage can be achieved by using dtbpf as a phosphine ligand, affording bicyclic products containing dihydrofuran skeletons in good yields along with broad substrate scope. In proximal C-C bond cleavage reactions, the eight-membered cyclic palladium intermediate plays a key role in the reaction. The [3 + 2] cycloaddition of keto-vinylidenecyclopropanes through the distal C-C bond cleavage can be effectively accomplished with t BuXPhos as a phosphine ligand and ZnCl2 as an additive, delivering bicyclic products containing tetrahydrofuran skeletons in good yields. The further transformation of these bicyclic products has been demonstrated, and the reaction mechanisms of two different C-C bond cleavage reactions have been investigated by control experiments and DFT calculations.

Palladium catalyzed stereoselective intramolecular [3 + 2] cycloaddition reactions of (E) & (Z)-ene-vinylidenecyclopropanes

A palladium-catalyzed ring-opening cyclization of (E) & (Z)-ene-vinylidenecyclopropanes has been developed via an intramolecular [3 + 2] cycloaddition process in the presence of a sterically bulky biaryl phosphine ligand, stereoselectively affording fused cis- & trans-bicyclo[4.3.0] skeletal products in good yields with a broad substrate scope and good functional tolerance. A plausible reaction mechanism was proposed on the basis of previous work and the DFT calculations.

Enantio- and Regioselective Cascade Hydroboration of Methylenecyclopropanes for Facile Access to Chiral 1,3- and 1,4-Bis(boronates)

Chiral 1, n-bis(boronate) plays a crucial role in organic synthesis and medicinal chemistry. However, their catalytic and asymmetric synthesis has long posed a challenge in terms of operability and accessibility from readily available substrates. The recent discovery of the C═C bond formation through β-C elimination of methylenecyclopropanes (MCP) has provided an exciting opportunity to enhance molecular complexity. In this study, the catalyzed asymmetric cascade hydroboration of MCP is developed. By employing different ligands, various homoallylic boronate intermediate are obtained through the hydroboration ring opening process. Subsequently, the cascade hydroboration with HBpin or B2pin2 resulted in the synthesis of enantioenriched chiral 1,3- and 1,4-bis(boronates) in high yields, accompanied by excellent chemo- and enantioselectivities. The selective transformation of these two distinct C─B bonds also demonstrated their application potential in organic synthesis.

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.

Palladium-catalyzed Enantio- and Regioselective Ring-Opening Hydrophosphinylation of Methylenecyclopropanes

Transition metal-catalyzed hydrofunctionalization of methylenecyclopropanes (MCP) has presented a considerable challenge due to the difficult manipulation of regioselectivity and complicated reaction patterns. Herein, we report a straightforward Pd-catalyzed ring-opening hydrophosphinylation reaction of MCP via highly selective C-C bond cleavage. This allows for rapid and efficient access to a wide range of chiral allylic phosphine oxides in good yields and high enantioselectivities. Additionally, density functional theory (DFT) calculations were performed to elucidate the reaction mechanism and the origin of product enantioselectivity.

Palladium- and Brønsted acid-catalyzed enantio-, site- and E/Z-selective addition of alkylidenecyclopropanes with imines

Transition-metal catalyzed functionalization of ACPs has been widely investigated in cycloaddition and 1,3-difunctionalization reactions. However, the transition metal catalyzed nucleophilic reactions of ACPs have rarely been reported. In this article, an enantio-, site- and E/Z-selective addition of ACPs with imines for the synthesis of dienyl substituted amines has been developed via palladium- and Brønsted acid co-catalysis. A range of synthetically valuable dienyl substituted amines were effectively prepared with good to excellent yields and excellent enantio- and E/Z-selectivities.

Palladium-Catalyzed Tandem Cycloisomerization/Cross-Coupling of Carbonyl- and Imine-Tethered Alkylidenecyclopropanes

Pd0 catalysts featuring phosphorus-based monodentate ligands can detour the reactivity of carbonyl-tethered alkylidenecyclopropanes (ACPs) from standard (3+2) cycloadditions towards tandem cycloisomerization/cross-coupling processes. This new reactivity lies on the formation of key π-allyl oxapalladacyclic intermediates, which are subsequently trapped with external nucleophilic partners, instead of undergoing canonical C-O reductive eliminations. Importantly, the use of imine-tethered ACP's is also feasible. Therefore, the method provides a straightforward and stereoselective entry to a wide variety of highly functionalized cyclic alcohols and amines.

Thermally-Induced Intramolecular [4+2] Cycloaddition of Allylamino- or Allyloxy-Tethered Alkylidenecyclopropanes

A thermally-induced intramolecular [4+2] cycloaddition reaction of allylamino- or allyloxy-tethered alkylidenecyclopropanes has been reported in this paper, giving a new protocol for the rapid construction of polycyclic skeleton molecules in moderate to excellent yields with a broad substrate scope. On the basis of control experiments and DFT calculations, we disclosed that the reaction proceeded through a [4+2] cycloaddition and trace of water assisted 1,3-H shift process to give the target product.

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.

Highly Enantioselective Cobalt-Catalyzed (3+2) Cycloadditions of Alkynylidenecyclopropanes

Low-valent cobalt complexes equipped with chiral ligands can efficiently promote highly enantioselective (3+2) cycloadditions of alkyne-tethered alkylidenecyclopropanes. The annulation allows to assemble bicyclic systems containing five-membered rings in good yields and with excellent enantiomeric ratios. We also present a mechanistic discussion based on experimental and computational data, which support the involvement of Co^I /Co^III catalytic cycles.

Visible light mediated synthesis of 4-aryl-1,2-dihydronaphthalene derivatives via single-electron oxidation or MHAT from methylenecyclopropanes

A new synthetic strategy of a single-electron oxidation and MHAT of methylenecyclopropanes (MCPs) for the rapid construction of 4-aryl-1,2-dihydronaphthalene derivatives by merging photoredox catalysis and cobalt catalysis has been developed.

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.

Visible-light-promoted divergent functionalizations of methylenecyclopropanes

Visible-light-induced monofunctionalization and difunctionalization of MCPs via ring-opening and cyclization processes have been developed for the syntheses of difluoromethyl compounds, alkyl compounds, halides, and sulfonyl compounds.

[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.

Transition-Metal-Catalyzed Cycloaddition Reactions to Access Seven-Membered Rings

The efficient and selective synthesis of functionalized seven-membered rings remains an important pursuit within synthetic organic chemistry, as this motif appears in numerous drug-like molecules and natural products. Use of cycloaddition reactions remains an attractive approach for their construction within the perspective of atom and step economy. Additionally, the ability to combine multiple components in a single reaction has the potential to allow for efficient combinatorial strategies of diversity-oriented synthesis. The inherent entropic penalty associated with achieving these transformations has impressively been overcome with development of catalysis, whereby the reaction components can be pre-organized through activation by transition-metal-catalysis. The fine-tuning of metal/ligand combinations as well as reaction conditions allows for achieving chemo-, regio-, diastereo- and enantioselectivity in these transformations. Herein, we discuss recent advances in transition-metal-catalyzed construction of seven-membered rings via combination of 2-4 components mediated by a variety of metals. An emphasis is placed on the mechanistic aspects of these transformations to both illustrate the state of the science and to highlight the unique application of novel processes of transition-metals in these transformations.

Conversion of anilines to chiral benzylic amines via formal one-carbon insertion into aromatic C-N bonds

Insertion of atoms into aromatic carbon-nitrogen bonds is an appealing method for the synthesis of nitrogen-containing molecules and it has the advantage of the availability and abundance of anilines. However, the direct cleavage of aromatic carbon-nitrogen bonds is challenging due to the particularly inert and stable nature of these bonds. Here we report a formal, enantioselective one-carbon insertion into an aromatic carbon-nitrogen bond via an aromaticity dissembly-reconstruction process to directly convert anilines to chiral α-branched benzylic amines. The process involves oxidative dearomatization of para-substituted anilines, chiral sulfur ylide-mediated asymmetric aziridination, and subsequent rearrangement. Chiral sulfur ylides serve as one-carbon insertion units.

Atom insertion into aromatic carbon-nitrogen (C-N) bonds is useful for the synthesis of nitrogen-containing molecules, but challenging due to the inert nature of these bonds. Here, the authors report one-carbon insertion into aromatic C-N bonds to directly convert anilines to chiral benzylic amines.

Rhodium-catalyzed ene-cycloisomerization of allylic-sulfide-tethered alkylidenecyclopropanes: DFT analysis of origins of regio- and diastereo-selectivities

A unconventional reaction mechanism can be responsible for the regio- and diastereo-selectivities of rhodium-catalyzed ene-cycloisomerization of allylic-sulfide-tethered alkylidenecyclopropanes.

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.

Fulvene Synthesis by Rhodium(I)-Catalyzed [2+2+1] Cycloaddition: Synthesis and Catalytic Activity of Tunable Cyclopentadienyl Rhodium(III) Complexes with Pendant Amides

It has been established that a Rh^I+ /segphos complex catalyzes the [2+2+1] cycloaddition of 1,6-diynes with cyclopropylideneacetamides to give substituted fulvenes in good yields. The reductive complexation of the product fulvenes with RhCl₃ in EtOH furnished the corresponding dinuclear cyclopentadienyl Rh^III complexes bearing a pendant amide moiety. These Rh^III complexes were highly active catalysts for oxidative annulation and cyclization through C(sp² )-H and C(sp³ )-H functionalization.

"Cut and Sew" Transformations via Transition-Metal-Catalyzed Carbon-Carbon Bond Activation

The transition metal-catalyzed "cut and sew" transformation has recently emerged as a useful strategy for preparing complex molecular structures. After oxidative addition of a transition metal into a carbon-carbon bond, the resulting two carbon termini can be both functionalized in one step via the following migratory insertion and reductive elimination with unsaturated units, such as alkenes, alkynes, allenes, CO and polar multiple bonds. Three- or four-membered rings are often employed as reaction partners due to their high ring strains. The participation of non-strained structures generally relies on cleavage of a polar carbon-CN bond or assistance of a directing group.

Recent Methodologies That Exploit C-C Single-Bond Cleavage of Strained Ring Systems by Transition Metal Complexes

In this review, synthetic and mechanistic aspects of key methodologies that exploit C-C single-bond cleavage of strained ring systems are highlighted. The focus is on transition-metal-catalyzed processes that are triggered by C-C bond activation and β-carbon elimination, with the review concentrating on developments from mid-2009 to mid-2016.

Recent advances in the chemical transformations of functionalized alkylidenecyclopropanes (FACPs)

Classification of functionalized alkylidenecyclopropanes (FACPs) and recent chemical transformations for the synthesis of novel and useful polycyclic and heterocyclic compounds.

A practical deca-gram scale ring expansion of (R)-(−)-carvone to (R)-(+)-3-methyl-6-isopropenyl-cyclohept-3-enone-1

A route to enantiopure (R)-(+)-3-methyl-6-isopropenyl-cyclohept-3-enone-1, an intermediate for terpenoids, has been developed and includes a highly chemo- and regioselective Tiffeneau–Demjanov reaction.

Nickel-catalyzed intramolecular [3 + 2 + 2] cycloadditions of alkylidenecyclopropanes. A straightforward entry to fused 6,7,5-tricyclic systems

A highly diastereo- and chemoselective intramolecular nickel-catalyzed cycloaddition of alkene- and alkyne-tethered alkynylidenecyclopropanes is reported. The method constitutes the first fully intramolecular [3 + 2 + 2] alkylidenecyclopropropane cycloaddition occurring via a proximal cleavage of the cyclopropane and makes it possible to build relevant 6,7,5-tricyclic frameworks in a single-pot reaction. Importantly, the reaction outcome is highly dependent on the characteristics of the nickel ligands.