Catalytic Cyclooligomerization of Enones with Three Methylene Equivalents

Chemoselective Three-Component Geminal Cross Couplings of Dihaloalkanes with Cr Catalysis: Rapid Access to Tertiary and Quaternary Alkanes via a Metal-Carbene Intermediate

Geminal cross couplings using multiple components enable the formation of several different bonds at one site in the building of tertiary and quaternary alkanes. Nevertheless, there are remaining issues of concern-cleavage of two geminal bonds and control of selectivity present challenges. We report here the geminal cross couplings of three components by reactions of dihaloalkanes with organomagnesium and chlorosilanes or alkyl tosylates by Cr catalysis, affording the formation of geminal C-C/C-Si or C-C/C-C bonds in the creation of tertiary and quaternary alkanes. The geminal couplings are catalyzed by low-cost CrCl2 , enabling the sluggishness of competitive Kumada-type side couplings and homocouplings of Grignard reagents, in achieving high chemoselectivity. Experimental and theoretical studies indicate that two geminal C-halide bonds are continuously cleaved by Cr to afford a metal carbene intermediate, which couples with a Grignard reagent, followed by silylation, in the formation of geminal C-C and C-Si bonds via a novel inner-sphere radical coupling mechanism. These three-component geminal cross couplings are value-addition to the synthesis of commercial drugs and bioactive molecules in medicinal chemistry.

Controllable methylenation with ethylene glycol as the methylene source: bridging enaminones and synthesis of tetrahydropyrimidines

Controllable methylenation using renewable ethylene glycol as the methylene source has been developed for the introduction of one or two methylene building blocks.

Electrophilic Thiocyanato Reagent Assisted Oxa-Michael/Thiocyanation of α,β-Unsaturated Ketones

A route for thiocyanation-functionalization of the electron-deficient C═C double bond was developed. Regioselective thiocyanation-etherification of α,β-unsaturated ketones was achieved. The desired products were obtained in moderate to high yields under mild conditions. It was suggested that the nucleophile was activated by the electrophilic thiocyanato reagent, and difunctionalization was achieved through a 1,4-addition/thiocyanation pathway.

Direct Access to α,β-Unsaturated Ketones via Rh/MgCl2-Mediated Acylation of Vinylsilanes

We report herein the facile and practical construction of α,β-unsaturated ketones via rhodium-catalyzed direct acylation of vinylsilanes with readily available and abundant carboxylic acids. This protocol features access to a diverse array of synthetically useful functionalities with moderate to excellent yields. More importantly, the late-stage functionalization of pharmaceuticals was also realized with synthetically useful yield.

Well-Defined, Versatile and Recyclable Half-Sandwich Nickelacarborane Catalyst for Selective Carbene-Transfer Reactions

Catalytic carbene-transfer reactions constitute a class of highly useful transformations in organic synthesis. Although catalysts based on a range of transition-metals have been reported, the readily accessible nickel(II)-based complexes have been rarely used. Herein, an air-stable nickel(II)-carborane complex is reported as a well-defined, versatile and recyclable catalyst for selective carbene transfer reactions with low catalyst loading under mild conditions. This catalyst is effective for several types of reactions including diastereoselective cyclopropanation, epoxidation, selective X-H insertions (X = C, N, O, S, Si), particularly for the unprotected substrates. This represents a rare example of carborane ligands in base metal catalysis.

Chromium-catalyzed cyclopropanation of alkenes with bromoform in the presence of 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-dihydropyrazine

Chromium-catalyzed cyclopropanation of alkenes with bromoform was achieved to produce the corresponding bromocyclopropanes. In this catalytic cyclopropanation, an organosilicon reductant, 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (1a), was indispensable for reducing CrCl3(thf)3 to CrCl2(thf)3, as well as for in situ generation of (bromomethylidene)chromium(iii) species from (dibromomethyl)chromium(iii) species. The (bromomethylidene)chromium(iii) species are proposed to react spontaneously with alkenes to give the corresponding bromocyclopropanes. This catalytic cyclopropanation was applied to various olefinic substrates, such as allyl ethers, allyl esters, terminal alkenes, and cyclic alkenes.

Nickel-Catalyzed Three-Component Cycloadditions of Enoates, Alkynes, and Aldehydes

A method for the three-component cycloaddition of enoates, alkynes, and aldehydes has been developed. Building upon previous work by this group in which stoichiometrically generated metallacycles undergo alkylation, we report a catalytic, alkylative [3 + 2] cycloaddition. From simple starting materials, structurally complex cyclopentenones may be rapidly assembled. Computational investigation of the mechanism (ωB97X-D3/cc-pVTZ//ωB97X/6-31G(d)) identified three energetically feasible pathways. Based on the relative rates of ketene formation compared to isomerization to a seven-membered metallacycle, the most likely mechanism has been determined to occur "ketene-first", with carbocyclization prior to aldol addition. Deuterium labeling studies suggest that formation of the seven-membered metallacycle becomes possible when an α-substituted enoate is used. This observed change in selectivity is due to the increased difficulty of phenoxide elimination with the inclusion of additional steric bulk of the α-substituent. The net transformation results in a [3 + 2] cycloaddition accompanied by an alkylation of the enoate substituent.