GaCl3 -Mediated Reactions of Donor-Acceptor Cyclopropanes with Aromatic Aldehydes

The TaCl5-Mediated Reaction of Dimethyl 2-Phenylcyclopropane-1,1-dicarboxylate with Aromatic Aldehydes as a Route to Substituted Tetrahydronaphthalenes

It is found that the reaction of dimethyl 2-phenylcyclopropane-1,1-dicarboxylate with 2 equivalents each of aromatic aldehydes and TaCl5 in 1,2-dichloroethane at 23 °C for 24 h after hydrolysis gives substituted 4-phenyl-3,4-dihydronaphtalene-2,2(1H)-dicarboxylates in good yield. This represents a new type of reactions between 2-arylcyclopropane-1,1-dicarboxylates and aromatic aldehydes, yielding chlorinated tetrahydronaphthalenes with a cis arrangement of the aryl and chlorine substituents in the cyclohexene moiety. A plausible reaction mechanism is proposed.

Supercritical Gallium Trichloride in Oxidative Metal Recycling: Ga2Cl6 Dimers vs GaCl3 Monomers and Rheological Behavior

Oxidative recycling of metals is crucial for a circular economy, encompassing the preservation of natural resources, the reduction of energy consumption, and the mitigation of environmental impacts and greenhouse gas emissions associated with traditional mining and processing. Low-melting gallium trichloride appears to be a promising oxidative solvent for rare-earth metals, transuranium elements, platinum, pnictogens, and chalcogens. Typically, oxidative dissolution with GaCl3 occurs at relatively low temperatures over a few days, assuming the presence of tetrahedral Ga-Cl entities. While supercritical gallium trichloride holds the potential for advanced recycling, little is known about its structure and viscosity. Using high-energy X-ray diffraction and multiscale modeling, which includes first-principles simulations, we have revealed a dual molecular nature of supercritical gallium trichloride, consisting of tetrahedral dimers and flat trigonal monomers. The molecular geometry can be precisely tuned by adjusting the temperature and pressure, optimizing the recycling process for specific metals. The derived viscosity, consistent with the reported results in the vicinity of melting, decreases by a factor of 100 above the critical temperature, enabling fast molecular diffusion, and efficient recycling kinetics.

Three-Component Synthesis of Substituted Perhydropyrans from β-Styrylmalonates, Aldehydes, and Alkoxyaluminum Dichlorides

A three-component synthesis of substituted dimethyl dihydro-2H-pyran-3,3(4H)-dicarboxylates in up to 80% yields by the reaction of β-styrylmalonates with aromatic or aliphatic aldehydes in the presence of ROAlCl2 prepared in advance either by exposure of EtAlCl2 with air access or by mixing equimolar amounts of AlCl3 with a primary or secondary alcohol has been developed. If EtAlCl2, itself, is used, dihydro-2H-pyran-3,3(4H)-diesters are not formed at all, while dimerization of styrylmalonates by (4 + 2)-annulation-type to give substituted tetrahydronaphthalenes is the main process. The possibility of using the CH-O-Al fragment of alkoxyaluminum dichlorides in cycloaddition reactions with α-CH-functionalization has been shown for the first time.

Yb(OTf)3-Catalyzed Formal (4 + 3) Cycloaddition Reactions of 3-Benzylideneindoline-2-thiones with Donor-Acceptor Cyclopropanes

A Yb(OTf)3-catalyzed formal (4 + 3) cycloaddition reaction of donor-acceptor cyclopropanes with 3-benzylideneindoline-2-thiones as sulfur-containing 4π components has been successfully achieved. A series of functionalized 5,10-dihydro-2H-thiepino[2,3-b]indole derivatives were synthesized with good yields and moderate to good diastereoselectivity. The reaction described herein represented the inaugural (4 + 3) cycloaddition of 3-benzylideneindoline-2-thiones.

Synthesis of 2-[2-(Ethoxymethoxy)phenyl]spiro[cyclopropane-1,2′-indene]-1′,3′-dione

An 1,3-indanedione-derived donor–acceptor cyclopropane, bearing the ethoxymethyl-protected phenolic group at the ortho-position of the donor aryl substituent, has been synthesized using a reaction sequence involving the Knoevenagel condensation of 1,3-indanedione with the corresponding protected salicylaldehyde followed by the Corey–Chaykovsky cyclopropanation of the obtained adduct with dimethylsulfoxonium methylide. The structure of the synthesized cyclopropane was unambiguously proved by single-crystal X-ray diffraction data.

Dimerization/Elimination of β-Styrylmalonates under Action of TiCl4

A new type of dimerization of dimethyl (β-styryl)malonates in the presence of TiCl₄ accompanied by elimination of a methanol molecule was discovered. Selective methods for the synthesis of substituted trimethyl 4-hydroxy-[1,1'-biaryl]-3,3,5(2H)-tricarboxylates and trimethyl 7-hydroxy-9,10-dihydro-5,9-methanobenzo[8]annulene-6,8,8(5H)-tricarboxylates were developed. The regularities of the occurring processes were determined and a similar reaction of β-styrylmalonate with benzylidenemalonate in the presence of TiCl₄ was performed in the scope of the suggested mechanism.

Lewis Acid-Catalyzed Formal (4+2)-Cycloaddition between Cross-Conjugated Azatrienes and Styrylmalonates: The Way to Functionalized Quinolizidine Precursors

Quinolizidine and azaphenalene alkaloids are common in nature and exhibit a pharmaceutical activity, which stirs up increased interest in expanding the range of methods for the synthesis of the corresponding derivatives. In this work, we attempted to adapt our previously presented method for the synthesis of tetrahydropyridines to the preparation of potential precursors for these heterocycles as a separate development of a necessary intermediate stage. To this end, we studied the reactions of β-styrylmalonates with N-protected cross-conjugated azatrienes in the presence of Sn(OTf)₂. Moreover, the regioselectivity of the process involving unsymmetrically substituted azatrienes was estimated. The diene character of vinyltetrahydropyridines was studied in detail with the participation of PTAD. Finally, for the Ts-protected highly functionalized vinyltetrahydropyridines synthesized, a detosylation method to give new desired azadiene structures as precursors of the quinolizidine core was suggested.

Bi-Catalyzed 1,2-Reactivity of Spirocyclopropyl Oxindoles with Dithianediol: Access to Spiroheterocycles

The efficient Bi-catalyzed 1,2-reactivity of spirocyclopropyl oxindoles has been disclosed with dithianediols as the sulfur surrogate to furnish spiroheterocycles at moderate temperature. The procedure provides a potential approach for the construction of spirotetrahydrothiophene scaffolds with functional group diversity. The catalytic 1,2-reactivity of cyclopropanes, mechanistic studies using density functional theory studies, diastereoselectivity, and additive-free mild conditions are the important practical features.

Radical differentiation of two ester groups in unsymmetrical diazomalonates for highly asymmetric olefin cyclopropanation

Diazomalonates have been demonstrated as effective metalloradicophiles for asymmetric radical olefin cyclopropanation via Co(II)-metalloradical catalysis (MRC). Supported by D ₂-symmetric chiral amidoporphyrin ligand, Co(II)-based metalloradical system can efficiently activate unsymmetrical methyl phenyl diazomalonate (MPDM) with effective differentiation of the two ester groups for asymmetric cyclopropanation, enabling stereoselective construction of 1,1-cyclopropanediesters bearing two contiguous chiral centers, including all-carbon quaternary stereogenic center. The Co(II)-catalyzed asymmetric cyclopropanation, which operates at room temperature without slow addition of the diazo compound, is generally applicable to broad-ranging olefins and tolerates various functionalities, providing a streamlined synthesis of chiral 1,1-cyclopropanediesters in high yields with both high diastereoselectivity and enantioselectivity. Combined computational and experimental studies support the underlying stepwise radical mechanism for Co(II)-catalyzed cyclopropanation. In addition to functioning as 1,3-dipoles for forming five-membered structures, enantioenriched (E)-1,1-cyclopropanediesters serve as useful building blocks for stereoselective synthesis of different cyclopropane derivatives. In addition, the enantioenriched (E)-1,1-cyclopropanediesters can be stereoselectively converted to (Z)-diastereomers.

Cycloadditions of Donor-Acceptor Cyclopropanes and -butanes using S=N-Containing Reagents: Access to Cyclic Sulfinamides, Sulfonamides, and Sulfinamidines

We present (3+2)- and (4+2)-cycloadditions of donor-acceptor (D-A) cyclopropanes and cyclobutanes with N-sulfinylamines and a sulfur diimide, along with a one-pot, two-step strategy for the formal insertion of HNSO2 into D-A cyclopropanes. These are rare examples of cycloadditions with D-A cyclopropanes and cyclobutanes whereby the 2π component consists of two different heteroatoms, thus leading to five- and six-membered rings containing adjacent heteroatoms.

Yb(OTf)3-Mediated Annulation of Cyclopropane-1,1-dicarbonitriles with 2-Aminobenzaldehydes for Synthesis of Polysubstituted Quinolines

A Yb(OTf)₃-mediated annulation of cyclopropane-1,1-dicarbonitriles and 2-aminobenzaldehydes for the synthesis of polysubstituted quinolines in generally good yields was investigated. In the cascade reaction, the protocol includes ring opening, intermolecular nuclophilic addition, intramolecular nuclophilic addition, and demalononitrile aromatization, in which the malononitrile group serves as a deciduous directing group mediated by Yb(OTf)₃.

Sequential Knoevenagel Condensation/Cyclization for the Synthesis of Indene and Benzofulvene Derivatives

Sequential Knoevenagel condensation/cyclization leading to indene and benzofulvene derivatives has been developed. The reaction of 2-(1-phenylvinyl)benzaldehyde with malonates gave benzylidene malonates, cyclized indenes, and dehydrogenated benzofulvenes. The product selectivity depends on the reaction conditions. The reaction with piperidine, AcOH in benzene at 80 °C for 1.5 h gave a benzylidene malonate in 75% yield as a major product. The reactions with piperidine, AcOH in benzene at 80 °C for 17 h and with TiCl₄-pyridine at room temperature gave an indene derivative in 56 and 79% yields, respectively, as a major product. The reaction with TiCl₄-Et₃N gave a benzofulvene in 40% yield selectively. Indene was transformed to a benzofulvene derivative using the reagents TiCl₄-Et₃N and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The reaction of variously substituted aryl derivatives with dimethyl malonate gave indene and benzofulvene derivatives. The reactions of 2-(1-phenylvinyl)benzaldehyde with Meldrum's acid or malononitrile also gave cyclized compounds in the suitable sequential or stepwise conditions. Furthermore, the reaction of 2-arylbenzaldehydes has been investigated. The limitation and scope have been described. The reaction mechanism of the cyclization steps has been examined by DFT calculations.

Coupling of Styrylmalonates with Furan and Benzofuran Carbaldehydes: Synthesis and Chemistry of Substituted (4-Oxocyclopent-2-enyl)malonates

On the basis of the reaction of β-styrylmalonates with furfural derivatives in the presence of GaCl₃ that occurs with opening of the furan ring, an efficient diastereoselective method for the synthesis of trisubstituted cyclopentenones containing a 1,4-diketone moiety was developed. The regularities of the reaction were studied, and a number of chemical reactions of the resulting substrates were carried out. A mechanism for the formation of substituted (4-oxo-2-arylcyclopent-2-enyl)malonates was suggested.

GaCl3-Mediated Cascade [2 + 4]-Cycloaddition/[4 + 2]-Annulation of Donor-Acceptor Cyclopropanes with Conjugated Dienes: Strategy for the Construction of Benzobicyclo[3.3.1]nonane Skeleton

Structurally important benzobicyclo[3.3.1]nonane derivatives were synthesized by a gallium trichloride mediated reaction of readily available donor-acceptor cyclopropanes (DACs) with 1,3-dienes as a one-pot cascade ionic [2 + 4]-cycloaddition/Friedel-Crafts-type cyclization process. At the first stage, DACs act as sources of formal gallium 1,2-zwitterionic complexes to give 6-benzylcyclohex-3-ene-1,1-dicarboxylates that are converted under certain conditions in the presence of GaCl₃ to give benzobicyclo[3.3.1]nonanes in 30-74% yields. The process is tolerant of varying substituents at different positions of the main framework. Further, potentially useful modifications of benzobicyclo[3.3.1]nonane derivatives are demonstrated. 2-Cyclopropylbutadiene reacts with DAC at higher temperature more deeply with cleavage of three-membered rings in both cyclopropane substrates, and twofold alkylation of the Ph-substituent at ortho- and meta-positions, that leads to a 1,2,7,8,9,10-hexahydro-6H-7,10a-methanocycloocta[cd]indene skeleton. Cyclopropane-1,1-diesters containing a bulky substituent in the ester group react with isoprene in a different way to give bicyclic lactones containing a 2-oxabicyclo[2.2.2]octan-3-one moiety.

"Cyclopropanation of Cyclopropanes": GaCl3-Mediated Ionic Cyclopropanation of Donor-Acceptor Cyclopropanes with Diazo Esters as a Route to Tetrasubstituted Activated Cyclopropanes

A new ionic cyclopropanation process involving the addition of diazo esters to donor-acceptor cyclopropanes (DAC) activated by GaCl₃ has been developed. The reactions occur via 1,2-zwitterionic gallium complexes with elimination of nitrogen in all cases to give 1,1,2,3-tetrasubstituted cyclopropanes as the main products. Also, a number of related processes with the formation of various polysubstituted cyclopropanes, alkenes, and cyclobutanes, including products of multiple diazo ester addition, have been developed. Obtained by the developed method tetrasubstituted cyclopropanes are activated cyclopropanes such as DAC and can be used for further synthesis in this capacity. Their new reaction with benzaldehyde promoted by TiCl₄ and involving one of the additional functional groups has been demonstrated, which leads to five-membered lactones. The mechanisms of the occurring processes, as well as the structures and stereochemistry of a rich range of products formed, are discussed in detail.

Reactions of Styrylmalonates with Aromatic Aldehydes: Detailed Synthetic and Mechanistic Studies

Reactions of styrylmalonates with aromatic aldehydes in the presence of Lewis and Brønsted acids and their regularities have been studied in detail. Approaches to the synthesis of various polysubstituted 5,6-dihydropyran-2-ones, indenes, aryl-containing dienes and trienes, cyclopentadienes, and polycyclic lactones have been developed with chemo-, regio-, stereo-, and diastereoselectivity control. The mechanisms of these reactions were studied in depth using multinuclear NMR experiments, monitoring the reactions in the NMR tube, and ¹⁸O isotope labeling.

DFT Quest of the Active Species of the Gallium-Mediated Coupling of Methylidenemalonates and Acetylenes

In this study, three different Ga-containing systems based on GaCl₃, Ga₂Cl₆, or ionic [Ga(L)₃][GaCl₄]₃ (L = methylidenemalonate) complex were screened to elucidate the mechanism, regioselectivity, chemoselectivity, and role of Ga mediator in the reaction between two types of acetylenes (phenylacetylene and but-1-yn-1-ylbenzene) and methylidenemalonates, i.e., the 1,2-zwitterionic precursors that are similar to intermediates derived from donor-acceptor cyclopropanes (DACs). Our DFT calculation results clearly show that the ionic gallium complex [Ga(L)₃][GaCl₄]₃ represents the key mediator in the title reaction. After the formation of such a complex, the first reaction step is the nucleophilic addition of phenylacetylene or but-1-yn-1-ylbenzene to [Ga(L)₃][GaCl₄]₃, generating an unstable vinyl cation intermediate. In the phenylacetylene system, this vinyl cation intermediate accepts a chlorine atom from [GaCl₄]^- to give E-configuration intermediate. Then, the above process occurs to other two ligands of the Ga(III) complex to furnish a final product. On the other hand, in the but-1-yn-1-ylbenzene system, the vinyl cation intermediate prefers to undergo Friedel-Crafts (F-C) alkylation to generate a five-membered ring intermediate. This process is repeated on the other two methylidenemalonate ligands, giving rise to a final cyclization product. The distortion/interaction analysis shows that in the nucleophilic addition step the distortion energy of the Ga complex part is the main factor that influences the activation energy. Furthermore, the global reactivity index (GRI) analysis indicates that the Ga-complex model has the highest electrophilicity index ω, thus leading to the lowest energy barrier among three Ga-based models. In addition, DFT results reveal that the regioselectivity (E-configuration preference) and chemoselectivity (chloration or F-C alkylation) are mainly controlled by the steric effect rather than the electronic effect. The main findings of the present work provide a new way to analyze and rationalize various Ga-mediated reactions, which might also be extrapolated to organic transformations undergoing in the presence of aluminum and indium complexes.

Enantioselective Synthesis of 1-Aryl Benzo[5]helicenes Using BINOL-Derived Cationic Phosphonites as Ancillary Ligands

The synthesis of unprecedented BINOL-derived cationic phosphonites is described. Through the use of these phosphanes as ancillary ligands in AuI catalysis, a highly regio- and enantioselective assembly of appropriately designed alkynes into 1-(aryl)benzo[5]carbohelicenes is achieved. The modular synthesis of these ligands and the enhanced reactivity that they impart to AuI -centers after coordination have been found to be the key features that allow an optimization of the reaction conditions until the desired benzo[5]helicenes are obtained with high yield and enantioselectivity.

Synthesis of (Het)aryl 2-(2-hydroxyaryl)cyclopropyl Ketones

A simple general method for the synthesis of 1-acyl-2-(ortho-hydroxyaryl)cyclopropanes, which belong to the donor–acceptor cyclopropane family, has been developed. This method, based on the Corey–Chaykovsky cyclopropanation of 2-hydroxychalcones, allows for the preparation of a large diversity of hydroxy-substituted cyclopropanes, which can serve as promising building blocks for the synthesis of various bioactive compounds.

Donor-Acceptor Bicyclopropyls as 1,6-Zwitterionic Intermediates: Synthesis and Reactions with 4-Phenyl-1,2,4-triazoline-3,5-dione and Terminal Acetylenes

The bicyclopropyl system activated by incorporation of donor and acceptor groups in the presence of Lewis acids was used as a synthetic equivalent of 1,6-zwitterions. Opening of both cyclopropane rings in 2'-aryl-1,1'-bicyclopropyl-2,2-dicarboxylates (D-A bicyclopropyl, ABCDs) in the presence of GaI₃ + Bu₄N⁺GaI₄^- results in 5-iodo-5-arylpent-2-enylmalonates as products of HI formal 1,6-addition to the bicyclopropyl system. The use of GaCl₃ or GaBr₃ as a Lewis acid and terminal aryl or alkyl acetylenes as 1,6-zwitterion interceptors allows the alkyl substituent to be grown to give the corresponding acyclic 7-chloro(bromo)-hepta-2,6-dienylmalonates. The reaction of ABCDs with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) catalyzed by Yb(OTf)₃ also results in the opening of both cyclopropane rings. The reaction products are tetrahydropyridazine derivatives - (7,9-dioxo-1,6,8-triazabicyclo[4.3.0]non-3-en-2-ylmethyl)malonates - containing one more PTAD moiety in the malonyl group.

Imidazol-5-one as an Acceptor in Donor-Acceptor Cyclopropanes: Cycloaddition with Aldehydes

Spiro[imidazol-5-one-4,1'-cyclopropanes] behave as donor-acceptor (D-A) cyclopropanes in a formal cycloaddition reaction with aldehydes. The activation of such type of cyclopropanes is achieved with an equivalent of Brønsted acid. The reaction proceeds in high yields of 51-92% and demonstrates moderate diastereoselectivity at the quaternary stereocenter, which is determined by the electron-donating nature of the aldehyde partner. The ease of separation of stereoisomers allowed the creation of a library of 44 spiroannulated tetrahydrofurans with various substitution patterns.

Brønsted acid mediated intramolecular cyclopropane ring expansion/[4 + 2]-cycloaddition

A cascade reaction of 3-hydroxy-2-phenylisoindolin-1-one and cyclopropyl ketone has been developed via a BrØnsted acid-promoted ring-opening/intramolecular cross-cycloaddition/[4 + 2]-cycloaddition process. The developed methodology provides straightforward access to pentacyclic isoindolin-1-one derivatives under simple reaction conditions.

Ring-Opening Reactions of Donor-Acceptor Cyclobutanes with Electron-Rich Arenes, Thiols, and Selenols

Donor-acceptor (D-A) cyclobutanes with two geminal ester groups as acceptors are reacted with electron-rich arenes as nucleophiles to afford ring-opened products. AlCl₃ mediates this Friedel-Crafts-type reaction. A variety of donors and electron-rich arenes are used. Nucleophilic thiols and selenols also trigger this ring-opening reaction. Furthermore, a comparison of various physical parameters has been carried out for several D-A cyclobutanes.

New Dyes Based on Extended Fulvene Motifs: Synthesis through Redox Reactions of Naphthoquinones with Donor-Acceptor Cyclopropanes and Their Spectroelectrochemical Behavior

Novel dyes based on extended fulvene motifs are reported. The carbon skeleton was generated by a catalyzed addition of donor-acceptor cyclopropanes to naphthoquinone. The hydroxy group at the central ring of the tricyclic fulvene motif was converted into the triflate, which reacted efficiently with a wide range of nucleophiles, resulting in substitution and thereby providing new derivatives. The synthetic versatility allowed us to investigate the absorption, electrochemical, and UV/Vis-NIR spectroelectrochemical properties of these dyes as a function of the substituents. The dyes were shown to participate in reductive electrochemistry, the reversibility of which can be improved by appropriate selection of the substituents. Additionally, first signs of NIR electrochromism are presented, opening new avenues for the future investigations of such dyes.

Cascade Cleavage of Three-Membered Rings in the Reaction of D-A Cyclopropanes with 4,5-Diazaspiro[2.4]hept-4-enes: A Route to Highly Functionalized Pyrazolines

A new cascade process for reactions of donor-acceptor cyclopropanes (DACs) with spiro[cyclopropanepyrazolines] in the presence of EtAlCl₂ or Ga halides is reported. The action of a Lewis acid results in DAC activation and addition of the carbocationic intermediate to the azocyclopropane system of the pyrazoline with opening of the second three-membered ring and addition of a halide anion from the Lewis acid. A specific feature of this process is that one activated cyclopropane ring activates another one, and depending on the component ratio, the process can involve two DAC molecules and one pyrazoline molecule or one DAC molecule and two pyrazoline molecules. The process is tolerant to various functional groups and occurs with a wide range of substrates to give polyfunctionalized structures based on a 2-pyrazoline moiety.

Computational study on GaCl3-mediated reactions of donor–acceptor cyclopropanes with aromatic aldehydes: mechanism and role of GaCl3 and aldehydes

The mechanism and role of GaCl₃ and aldehydes for the GaCl₃-mediated reaction of donor–acceptor (D–A) cyclopropanes are clarified through our calculations.