Mechanistic Study of the Nickel-Catalyzed α,β-Coupling of Saturated Ketones

Nickel(II)-hydrazineylpyridine catalyzed regioselective synthesis of α-benzyl substituted β-hydroxy ketones via a Fenton free radical reaction

Ni(II)-hydrazineylpyridine (Ni(II)-PyH)-catalyzed regioselective synthesis of α-benzyl substituted β-hydroxy ketones from α,β-unsaturated ketones and alcohols is reported via a Fenton free-radical reaction. This protocol enables facile access to desired products in good to excellent yields in 12 h using toluene solvent at room temperature to 100 °C. The structural analysis of the products was confirmed by 1H, 13C-NMR, GC-MS, and HRMS data. Hydrogen peroxide used in the reaction facilitates Ni-catalyst oxidation state variations by a disproportionate reaction, which makes the catalyst recyclable up to 4 catalytic cycles without loss of activity. The method has high functional group tolerance with both aliphatic and aromatic ketones and alcohols. The catalyst structure was fully characterized using IR, UV, EPR and XPS analyses. The thermal stability of the catalyst was up to 290 °C, which was confirmed via a TGA study. The green metrics of the reaction showed 90%atom economy with a turnover frequency of 165.

Catalytic Asymmetric α-Alkylation of Ketones with Unactivated Alkyl Halides

A catalytic, enantioselective method for direct α-alkylation of ketones with unactivated alkyl halides is realized by employing an α-enolizable ketone in a nickel-catalyzed C(sp3)-C(sp3) cross-coupling reaction. The key to the success is attributed to a unique bimetallic ligand. A variety of acyclic ketones and unactivated alkyl iodides can serve as suitable substrates under mild conditions to generate chiral ketones with α-quaternary carbon stereocenters in high yields with good enantioselectivities. A range of transformations based on the ketone moiety are also demonstrated to show the potential application of this method. Preliminary mechanistic studies support a dinickel-catalyzed cross-coupling mechanism.

The Deuterated "Magic Methyl" Group: A Guide to Site-Selective Trideuteromethyl Incorporation and Labeling by Using CD3 Reagents

In the field of medicinal chemistry, the precise installation of a trideuteromethyl group is gaining ever-increasing attention. Site-selective incorporation of the deuterated "magic methyl" group can provide profound pharmacological benefits and can be considered an important tool for drug optimization and development. This review provides a structured overview, according to trideuteromethylation reagent, of currently established methods for site-selective trideuteromethylation of carbon atoms. In addition to CD3 , the selective introduction of CD2 H and CDH2 groups is also considered. For all methods, the corresponding mechanism and scope are discussed whenever reported. As such, this review can be a starting point for synthetic chemists to further advance trideuteromethylation methodologies. At the same time, this review aims to be a guide for medicinal chemists, offering them the available C-CD3 formation strategies for the preparation of new or modified drugs.

Ruthenium-catalyzed room-temperature coupling of α-keto sulfoxonium ylides and cyclopropanols for δ-diketone synthesis

Previous transition metal-catalyzed synthesis processes of δ-diketones are plagued by the high cost of the rhodium catalyst and harsh reaction conditions. Herein a low-cost, room temperature ruthenium catalytic method is developed based on the coupling of α-keto sulfoxonium ylides with cyclopropanols. The mild protocol features a broad substrate scope (47 examples) and a high product yield (up to 99%). Mechanistic studies argue against a radical pathway and support a cyclopropanol ring opening, sulfoxonium ylide-derived carbenoid formation, migratory insertion C-C bond formation pathway.

Mechanistic studies on nickel-catalyzed enantioselective [3 + 2] annulation for γ-butenolide synthesis via C–C activation of diarylcyclopropenones

Detailed mechanistic studies on Ni-catalyzed C–C activation of cyclopropenones, and enantioselective [3 + 2] annulation with α-CF₃ enones or 1,2-diones toward the efficient synthesis of γ-butenolides.

Mechanistic insights into rhodium-catalyzed enantioselective allylic alkylation for quaternary stereogenic centers

Installing quaternary stereogenic carbon is an arduous task of contemporary importance in the domain of asymmetric catalysis. To this end, an asymmetric allylic alkylation of α,α-disubstituted aldehydes by using allyl benzoate in the presence of Wilkinson's catalyst [Rh(Cl)(PPh₃)₃], (R)-BINOL–P(OMe) as the external ligand, and LiHMDS as the base has been reported to offer high enantioselectivity. The mechanistic details of this important reaction remain vague, which prompted us to undertake a detailed density functional theory (SMD_(THF)/B3LYP-D3) investigation on the nature of the potential active catalyst, energetic features of the catalytic cycle, and the origin of high enantioselectivity. We note that a chloride displacement from the native Rh-phosphine [Rh(Cl)(PPh₃)₃] by BINOL–P(OMe) phosphite and an ensuing MeCl elimination can result in the in situ formation of a Rh-phosphonate [Rh(BINOL–P Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O)(PPh₃)₃]. A superior energetic span (δE) noted with such a Rh-phosphonate suggests that it is likely to serve as an active catalyst. The uptake of allyl benzoate by the active catalyst followed by the turnover determining C–O bond oxidative addition furnishes a Rh-π-allyl intermediate, which upon interception by (Z)-Li-enolate (derived from α,α-disubstituted aldehyde) in the enantiocontrolling C–C bond generates a quaternary stereogenic center. The addition of the re prochiral face of the (Z)-Li-enolate to the Rh-bound allyl moiety leading to the R enantiomer of the product is found to be 2.4 kcal mol⁻¹ more preferred over the addition through its si face. The origin of the stereochemical preference for the re face addition is traced to improved noncovalent interactions (NCIs) and less distortion in the enantiocontrolling C–C bond formation transition state than that in the si face addition. Computed enantioselectivity (96%) is in very good agreement with the experimental value (92%), so is the overall activation barrier (δE of 17.1 kcal mol⁻¹), which is in conformity with room temperature reaction conditions.

The origin of high enantioselectivity in the formation of quaternary stereogenic carbon.

Insights into NHC-catalyzed oxidative α-C(sp3)–H activation of aliphatic aldehydes and cascade [2 + 3] cycloaddition with azomethine imines

The NHC catalyst is identified to promote [2 + 3] cycloaddition by avoiding the poor FMO overlap mode in theory.