Synthesis of Methylene- and Alkylidenecyclopropane Derivatives

A general catalytic synthetic strategy for highly strained methylenecyclobutanes and spiromethylenecyclobutanes

Highly strained methylenecyclobutanes (MCBs) are intriguing scaffolds in synthetic chemistry and drug discovery, but there is no such strategy that enables the synthesis of structurally diverse MCBs with defined stereochemistry. We report a general synthetic strategy for (boromethylene)cyclobutanes (BMCBs) and spiro-BMCBs by a challenging Cu-catalyzed highly chemo-, stereo-, and regioselective borylative cyclization of aliphatic alkynes. This strategy not only enables the installation of various functionalities at each site on the MCB skeleton with unambiguous stereochemistry but also introduces a versatile boromethylene unit that is readily transformable to a wide range of new functional groups; these features significantly expand the structural diversity of MCBs and are particularly valuable in drug discovery. The concise and divergent total syntheses of four cyclobutane-containing natural products were achieved from one common BMCB obtained by this strategy. The origin of the high regioselectivity in the borylcupration of alkynes and the high efficiency of the strained ring cyclization was also studied.

Rh(iii)-Catalyzed dienylation and cyclopropylation of indoles at the C4 position with alkylidenecyclopropanes

Herein, we report a Rh(iii)-catalyzed C–H functionalization of indoles at the C4 position with alkylidenecyclopropanes (ACPs).

A visible-light mediated ring opening reaction of alkylidenecyclopropanes for the generation of homopropargyl radicals

Classical cyclopropylcarbinyl radical clock reactions have been widely applied to conduct mechanistic studies for probing radical processes for a long time; however, alkylidenecyclopropanes, which have a similar molecular structure to methylcyclopropanes, surprisingly have not yet attracted researcher's attention for similar ring opening radical clock processes. In recent years, photocatalytic NHPI ester activation chemistry has witnessed significant blooming developments and provided new synthetic routes for cross-coupling reactions. Herein, we wish to report a non-classical ring opening radical clock reaction using innovative NHPI esters bearing alkylidenecyclopropanes upon photoredox catalysis, providing a brand-new synthetic approach for the direct preparation of a variety of alkynyl derivatives. The potential synthetic utility of this protocol is demonstrated in the diverse transformations and facile synthesis of bioactive molecules or their derivatives and medicinal substances.

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.

Synthesis and applications of methyleneaziridines

Methyleneaziridines (MAs) are a special subset of aziridines featuring an exocyclic C–C double bond on the three-membered ring. They have found great potential in organic synthesis. In this review, the structural characterization of MAs, synthetic methods, chemical transformations and mechanisms, especially the advances achieved over the past decade are comprehensively summarized.

This review surveys the structural characterization, synthetic methods, chemical transformations and mechanisms of methyleneaziridines, especially the advances achieved over the past decade.

Cu(I)-Catalyzed addition-cycloisomerization difunctionalization reaction of 1,3-enyne-alkylidenecyclopropanes (ACPs)

A copper(i)-catalyzed three-component addition-cycloisomerization difunctionalization reaction of 1,3-enyne-ACPs with Togni I reagent and TMSCN under mild reaction conditions has been developed, affording 3-trifluoroethylcyclopenta[b]naphthalene-4-carbonitrile derivatives. The reaction proceeded through a copper(i)-catalyzed 1,4-addition of conjugated 1,3-enynes via a radical relay process and aromatic cycloisomerization of allene-ACP intermediates.

A Chiral Naphthyridine Diimine Ligand Enables Nickel-Catalyzed Asymmetric Alkylidenecyclopropanations

A novel class of chiral naphthyridine diimine ligands (NDI*) readily accessible from C₂ -symmetric 2,6-di-(1-arylethyl)anilines is described. The utility of these ligands, particularly one with fluorinated aryl side arms, is demonstrated by a reductive Ni-catalyzed enantioselective alkylidene transfer reaction from 1,1-dichloroalkenes to olefins. This transformation provides direct access to a broad range of synthetically valuable alkylidenecyclopropanes in high yields and enantioselectivities.

Superbase-promoted selective carbon-carbon bond cleavage driven by aromatization

A novel selective carbon-carbon single bond cleavage has been disclosed through the copper-catalyzed reaction of 1-alkyl-3-alkylindolin-2-imine hydrochlorides with substituted 1-(bromomethyl)-2-iodobenzenes leading to fused N-heterocycles. Mechanistic studies showed that the intrinsic drive of aromatization and the action of the superbase derived from sodium tert-butoxide and dimethylsulfoxide were the key factors leading to the carbon-carbon single bond cleavage. Furthermore, the obtained N-heterocycles are indoloquinoline derivatives with wide biological activities.

Sigmatropic rearrangements of cyclopropenylcarbinol derivatives. Access to diversely substituted alkylidenecyclopropanes

Cyclopropenes constitute useful precursors of other classes of compounds incorporating a three-membered ring. Although the transformation of substituted cyclopropenes into alkylidenecyclopropanes can be accomplished through different strategies, this review is focusing specifically on the use of [2,3]- and [3,3]-sigmatropic rearrangements involving cyclopropenylcarbinol derivatives as substrates. These sigmatropic rearrangements, which have been developed in recent years, allow a remarkably efficient and stereoselective access to a wide variety of heterosubstituted and/or functionalized alkylidenecyclopropanes which would not be readily accessible by other strategies. The different [2,3]- and [3,3]-sigmatropic rearrangements of cyclopropenylcarbinol derivatives disclosed to date, as well as the analysis of their substrate scope and some applications of the products arising from those reactions, are presented in this review.

Thermally-induced intramolecular [2 + 2] cycloaddition of acrylamide-tethered alkylidenecyclopropanes

An efficient and highly regio- and diastereoselective synthetic method to cyclobutane-containing spiro[2.3]hexane fused with six-membered hetero-cycles has been disclosed via a thermally-induced intramolecular [2 + 2] cycloaddition of acrylamide-tethered alkylidenecyclopropanes. The DFT calculations indicate that this intramolecular cycloaddition proceeds in a concerted manner and account for the product selectivity. These reactions exhibited excellent yields and functional group tolerance under metal free conditions.

Preparation and Utility of N-Alkynyl Azoles in Synthesis

Heteroatom-substituted alkynes have attracted a significant amount of interest in the synthetic community due to the polarized nature of these alkynes and their utility in a wide range of reactions. One specific class of heteroatom-substituted alkynes combines this utility with the presence of an azole moiety. These N-alkynyl azoles have been known for nearly 50 years, but recently there has been a tremendous increase in the number of reports detailing the synthesis and utility of this class of compound. While much of the chemistry of N-alkynyl azoles mirrors that of the more extensively studied N-alkynyl amides (ynamides), there are notable exceptions. In addition, as azoles are extremely common in natural products and pharmaceuticals, these N-alkynyl azoles have high potential for accessing biologically important compounds. In this review, the literature reports of N-alkynyl azole synthesis, reactions, and uses have been assembled. Collectively, these reports demonstrate the growth in this area and the promise of exploiting N-alkynyl azoles in synthesis.

Rh(i)-Catalyzed stereoselective intramolecular cycloaddition reactions of ene-vinylidenecyclopropanes for the construction of fused 6,5-bicyclic skeletons with a quaternary all-carbon stereocenter

Rh(i)-catalyzed asymmetric [3 + 2] cycloadditions of ene-VDCPs are demonstrated, producing cyclic ring structures with an all-carbon quaternary stereocenter in good yields with excellent enantioselectivities.

Rh(i)-Catalyzed intramolecular [3 + 2] cycloaddition reactions of yne-vinylidenecyclopropanes

A Rh(i)-catalyzed intramolecular [3 + 2] cycloaddition reaction of yne-vinylidenecyclopropanes has been developed, providing fused [6.5]-bicyclic products in moderate to good yields.

[3,3]-Sigmatropic Rearrangement of Cyclopropenylcarbinyl Cyanates: Access to Alkylidene(aminocyclopropane) Derivatives

Cyclopropenylcarbinyl cyanates, generated in situ by dehydration of the corresponding carbamates, underwent an efficient and stereoselective [3,3]-sigmatropic rearrangement leading to the corresponding alkylidene(isocyanatocyclopropanes), which could be converted into various alkylidene(aminocyclopropane) derivatives in a one-pot manner. This transformation complements the repertoire of sigmatropic rearrangements involving cyclopropenylcarbinol derivatives and in particular, the previously reported Overman rearrangement of cyclopropenylcarbinyl trichloroacetimidates.

Pd(II)-Catalyzed Cyclization-Oxidation of Urea-Tethered Alkylidenecyclopropanes

A Pd(OAc)₂-catalyzed intramolecular oxidative cyclization of urea-tethered alkylidenecyclopropanes with urea as a nitrogen source through a Pd(II)/Pd(IV) catalytic cycle has been presented, giving the corresponding cyclobuta[ b]indoline derivatives in moderate to good yields with a broad substrate scope. The reaction proceeds through a ring expansion of alkylidenecyclopropane along with the nucleophilic attack of nitrogen atom onto the in situ generated palladium carbenoid species as well as an oxidation process.

Zirconocene-Mediated Selective C-C Bond Cleavage of Strained Carbocycles: Scope and Mechanism

Several approaches using organozirconocene species for the remote cleavage of strained three-membered ring carbocycles are described. ω-Ene polysubstituted cyclopropanes, alkylidenecyclopropanes, ω-ene spiro[2.2]pentanes, and ω-ene cyclopropyl methyl ethers were successfully transformed into stereodefined organometallic intermediates, allowing an easy access to highly stereoenriched acyclic scaffolds in good yields and, in most cases, excellent selectivities. DFT calculations and isotopic labeling experiments were performed to delineate the origin of the obtained chemo- and stereoselectivities, demonstrating the importance of microreversibility.

A facile method for the synthesis of trifluoromethylthio-/chloro-homoallylic alcohols from methylenecyclopropanes

Trifluoromethylthiolated/chloro-homoallylic alcohols have been directly accessed by the reaction of (PhSO₂)₂NSCF₃/Cl with methylenecyclopropanes (MCPs) in DMF in moderate to good yields.

Unusual reactivity of rhodium carbenes with allenes: an efficient asymmetric synthesis of methylenetetrahydropyran scaffolds

The reaction of a rhodium vinyl carbene with an O-tethered allene stereoselectively produces the 4-methylenetetrahydropyran core through a rhodium trimethylenemethane-like intermediate.

Iron-catalyzed or iodine-induced intramolecular halocyclization of N-vinyl-tethered methylenecyclopropanes: facile access to halogenated 1,2-dihydroquinolines

Iron(iii) catalyst or I₂ acts as a Lewis acid to activate enamines, affording the corresponding halogenated 1,2-dihydroquinolines in good yields.

Palladium-catalyzed oxidative cyclization of aniline-tethered alkylidenecyclopropanes with O2: a facile protocol to selectively synthesize 2- and 3-vinylindoles

A facile and versatile palladium catalyzed oxidative cyclization of aniline-tethered alkylidenecyclopropanes with O₂ to selectively construct important synthetic intermediates 2- and 3-vinylindoles in moderate to good yields has been developed.

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.

Visible-Light-Induced Trifluoromethylation of Isonitrile-Substituted Methylenecyclopropanes: Facile Access to 6-(Trifluoromethyl)-7,8-Dihydrobenzo[k]phenanthridine Derivatives

A new visible-light-induced trifluoromethylation of isonitrile-substituted methylenecyclopropanes is developed. A range of substituted 6-(trifluoromethyl)-7,8-dihydrobenzo[k]phenanthridine derivatives are readily furnished by this newly developed tandem reaction with moderate to good yields. This reaction allows the direct formation of two six-membered rings and three new C-C bonds, including the C-CF3 bond, under visible light irradiation.

Copper mediated carbometalation reactions

Since the first discovery of carbocupration of alkynes in the 1970s a tremendous amount of research has been carried out in this field. The exceptionally high selectivities obtained attribute to the great synthetic value of carbocupration reactions. This tutorial review will present the most important features of carbocupration of alkynes and highlight the most relevant reviews. Then a comprehensive review of copper mediated carbometalation of cyclopropenes will follow. The latter method has received much attention over the last decade as it allows the highly selective construction of poly-substituted cyclopropanes which can be transformed into acyclic derivatives bearing one or multiple tertiary or quaternary carbon stereocenters.

Unprecedented Oxycyanation of Methylenecyclopropanes for the Facile Synthesis of Benzoxazine Compounds Containing a Cyano Group

A novel intramolecular oxycyanation of methylenecyclopropanes is reported that proceeds through oxidative cleavage of the N-CN bond and subsequent palladium transfer from N to O of the amide group. A range of substituted benzo[d][1,3]oxazines with a cyano group are readily furnished by this newly developed oxycyanation reaction. Tris(4-trifluoromethylphenyl)phosphine as a ligand has been found to be crucial to effectively promote the transformation with high chemo- and regioselectivity. Moreover, the reaction outcome can be significantly affected by the electronic effect of the acyl group attached to the nitrogen atom of methylenecyclopropanes. When R(3) is a chloromethyl group, the pyrrolo[2,3-b]quinoline derivative is obtained by thermal-induced [3+2] cycloaddition of methylenecyclopropane to the methanediimine intermediate.

Rh(ii)-Catalyzed formation of pyrrolo[2,3-b]quinolines from azide-methylenecyclopropanes and isonitriles

Azide-methylenecyclopropanes underwent an intermolecular cyclization with isonitriles catalyzed by a Rh^IIcomplex to produce a series of pyrrolo[2,3-b]quinolines in good yields. Moreover, synthetic applications to construct useful heterocycles have also been achieved.

Thermally induced formal [3+2] cyclization of ortho-aminoaryl-tethered alkylidenecyclopropanes: facile synthesis of furoquinoline and thienoquinoline derivatives

Thermally induced formal [3+2] cycloaddition reactions of alkylidenecyclopropanes with the in situ generation of isocyanates and isothiocyanates have been developed.

Copper-catalyzed regio- and enantioselective aminoboration of alkylidenecyclopropanes: the synthesis of cyclopropane-containing β-aminoalkylboranes

A Cu-catalyzed aminoboration of alkylidenecyclopropanes with B₂pin₂ and hydroxylamines produced cyclopropane-containing β-aminoalkylboranes in moderate to good yields.

An isocyanide insertion approach to substituted pyrrolo[2,3-b]quinolines under metal-free and azide-free conditions

An isocyanide insertion approach to construct the pyrrolo[2,3-b]quinoline skeleton by an I₂/CHP mediated reaction of aminome-thylenecyclo-propanes (amino-MCPs) is developed.

Gold(i)-catalyzed cycloisomerization of vinylidenecyclopropane-enes via carbene or non-carbene processes

Gold catalyzed cycloisomerization of aromatic ring tethered vinylidenecyclopropane-enes provides a divergent synthetic protocol for the construction of O-containing fused heterocycles through controllable carbene or non-carbene related processes. The carbene induced process features a new amphiphilic strategy to generate a gold carbene via a rearrangement of vinylidenecyclopropane. Whereas, the electronic effect of the ortho-substituents switches the reaction mode onto the non-carbene related process, from which five- or six-membered rings are selectively produced through allyl-migration.

Gold-catalyzed intermolecular anti-markovnikov hydroamination of alkylidenecyclopropanes

The cationic gold phosphine complex [{PCy2 (o-biphenyl)}Au(NCMe)](+) SbF6 (-) (Cy=cyclohexyl) catalyzes the intermolecular, anti-Markovnikov hydroamination reaction of monosubstituted and cis- and trans-disubstituted alkylidenecyclopropanes (ACPs) with imidazolidin-2-ones and other nucleophiles. This reaction forms 1-cyclopropyl alkylamine derivatives in high yield and with high regio- and diastereoselectivity. NMR spectroscopic analysis of gold π-ACP complexes and control experiments point to the sp hybridization of the ACP internal alkene carbon atom as controlling the regiochemistry of the ACP hydroamination reaction.