Theoretical study of the mechanisms of palladation of methylenecyclopropane and [3 + 2] cycloadditions

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.

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-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 of 5-Azaindolizine Derivatives by the Palladium-Catalyzed Intermolecular Formal [3+2] Cycloaddition of Alkylidenecyclopropanes with 1,2-Diazines

The palladium-catalyzed formal [3+2] cycloaddition reaction of alkylidenecyclopropanes with 1,2-diazines proceeded smoothly to give the corresponding 5-azaindolizine derivatives in good to allowable yields. For example, in the presence of 5 mol % of Pd(PPh(3))(4), the reaction of 1-propylhexylidenecyclopropane with phthalazine or with pyridazine proceeded at 120 degrees C without solvent, and the corresponding 2-(1-butylpentyl)pyrrolo[2,1-a]phthalazine or 6-(1-butylpentyl)pyrrolo[1,2-b]pyridazine was obtained in 61% or 49% yield, respectively.

Density Functional Study of the Insertion and Ring-Opening Mechanism of MCP over Cp2LaH and Cp2LuH Catalysts

The insertion and ring opening of methylenecyclopropane (MCP) catalyzed by Cp2LnH (Ln = La and Lu) was investigated using DFT method. Geometries and energies of the reactants, minima, and transition states for this reaction were obtained. The present study indicates the formation of Cp2LaH-MCP complex followed by 1,2-insertion through a tetrahedral transition state with subsequent ring opening at the proximal bond via hydrogen transfer transition state resulting in the formation of the final product.

Palladium-catalyzed [3+2] intramolecular cycloaddition of alk-5-ynylidenecyclopropanes: a rapid, practical approach to bicyclo[3.3.0]octenes

Readily accessible hex-5-ynylidenecyclopropane derivatives cycloisomerize to bicyclic five-membered carbocycles upon heating with catalytic amounts of a palladium complex.

Addition of heteroaromatics to alkylidenecyclopropanes catalyzed by palladium

The reaction of heteroaromatics, such as furans, thiophenes, thiazoles, and pyrroles, with alkylidenecyclopropanes proceeded smoothly in the presence of palladium catalysts, producing the corresponding alpha-allylated products in good to high yields. For example, the reaction of 3-butylpentylidenecyclopropane (1a) with 2-methylfuran (2a), ethyl 2-thiophenecarboxylate (4a), 2-isobutylthiazole (6a), and 1-methylpyrrole (11b) gave the alpha-allylated products in 70%, 66%, 77%, and 30% yield, respectively. The reaction proceeded predominantly through distal bond cleavage. The order of reactivity of heteroaromatics toward 1a is as follows: furan > thiophene approximately thiazole (5-position) > thiazole (2-position) approximately pyrrole. This methodology provides a means for introducing an allylic group to various kinds of heteroaromatics under mild conditions.