Nickel-Catalyzed Three-Component [3+2+2] Cocyclization of Ethyl Cyclopropylideneacetate and Alkynes—Selective Synthesis of Multisubstituted Cycloheptadienes

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.

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.

Visible Light-Promoted Green and Sustainable Approach for One-Pot Synthesis of 4,4'-(Arylmethylene)bis(1H-pyrazol-5-ols), In Vitro Anticancer Activity, and Molecular Docking with Covid-19 Mpro

A visible light-promoted, efficient, green, and sustainable strategy has been adopted to unlatch a new pathway toward the synthesis of a library of medicinally important 4,4'-(arylmethylene)bis(1H-pyrazol-5-ols) moieties using substituted aromatic aldehydes and sterically hindered 3-methyl-1-phenyl-2-pyrazoline-5-one in excellent yield. This reaction shows high functional group tolerance and provides a cost-effective and catalyst-free protocol for the quick synthesis of biologically active compounds from readily available substrates. Synthesized compounds were characterized by spectroscopic techniques such as IR, 1HNMR, 13CNMR, and single-crystal XRD analysis. All the synthesized compounds were evaluated for their antiproliferative activities against a panel of five different human cancer cell lines and compared with Tamoxifen using MTT assay. Compound 3m exhibited maximum antiproliferative activity and was found to be more active as compared to Tamoxifen against both the MCF-7 and MDA-MB-231 cell lines with an IC50 of 5.45 and 9.47 μM, respectively. A molecular docking study with respect to COVID-19 main protease (Mpro) (PDB ID: 6LU7) has also been carried out which shows comparatively high binding affinity of compounds 3f and 3g (-8.3 and -8.8 Kcal/mole, respectively) than few reported drugs such as ritonavir, remdesivir, ribacvirin, favipiravir, hydroxychloroquine, chloroquine, and olsaltamivir. Hence, it reveals the possibility of these compounds to be used as effective COVID-19 inhibitors.

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.

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.

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.

Gold-catalyzed dual C-C bond cleavage of biphenylenes bearing a pendant alkyne at ambient temperature

We report a catalytic skeletal rearrangement of biphenylenes with a pendant alkyne moiety at room temperature by a cationic gold catalyst, which involves the cleavage of two bonds: the C-C bond of biphenylene and the C(sp)-C(sp2 or sp3) bond. Experimental and theoretical studies revealed that the reaction mechanism included π-activation of the alkyne, ring expansion and 1,2-carbon shift.

Palladium-catalyzed hydroalkylation of methylenecyclopropanes with simple hydrazones

A palladium-catalyzed hydroalkylation reaction of methylenecyclopropanes via highly selective C–C σ-bond scission was achieved under mild conditions, in which simple hydrazones served as carbanion equivalents. This method featured good functional group compatibility, affording high yields of C-alkylated terminal alkenes.

A palladium-catalyzed hydroalkylation of methylenecyclopropanes via selective C–C σ-bond scission was achieved, in which simple hydrazones served as carbanion equivalents. This method affords high yields of C-alkylated terminal alkenes with good functional group compatibility.

Rhodium-Catalyzed Enantioselective [2 + 2 + 1] Cycloaddition of 1,6-Enynes with Cyclopropylideneacetamides

It has been established that a cationic rhodium(I)/( R)-tol-BINAP or ( R)-BINAP complex catalyzes the enantioselective [2 + 2 + 1] cycloaddition of 1,6-enynes, possessing monosubstituted alkene units, with cyclopropylideneacetamides at room temperature through the elimination of ethylene to give bicyclic (cyclopent-2-en-1-ylidene)acetamides with high enantioselectivity.

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.

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.

Regio- and stereoselective synthesis of a library of bioactive dispiro-oxindolo/acenaphthoquino andrographolides via 1,3-dipolar cycloaddition reaction under microwave irradiation

Dispiro-pyrrolidino/pyrrolizidino fused oxindoles/acenaphthoquinones have been derived from andrographolide via azomethine ylide cycloaddition to the conjugated double-bond under microwave (MW) irradiation. The reactions are chemo-, stereo-, and regioselective in nature. Change in amino acid from sarcosine/N-benzyl glycine to l-proline changes the regiochemistry. A representative library of 40 compounds along with in vitro anticancer evaluation is reported.