Regiocontrolled Oxidative C–C Coupling of Dienol Ethers and 1,3-Dicarbonyl Compounds

Enantioselective electrochemical nickel-catalyzed vinylogous radical reactions

Highly functionalized structural motifs with extended chiral carbon chains are prevalent in a wide range of bioactive compounds and play critical roles in the production of various functionalized molecules. Here, we describe a nickel-catalyzed asymmetric radical-based electrochemical functionalization of silyl polyenolates at α-, γ-, ε-, and η-positions. Driven by electric current, this methodology provides a sustainable route to access enantioenriched dicarbonyls via vinylogous radical pathways. It demonstrates excellent functional groups tolerance, mild reaction conditions, broad substrate compatibility, formation of quaternary stereocenters at remote positions, and high levels of regio- and enantioselectivity (up to 98% enantiomeric excess). Mechanistic investigations indicate that ferrocene-based electron transfer mediators are pivotal in the anodic oxidation process, facilitating the generation of nickel-bound α-carbonyl radicals while suppressing the undesired oxidation of silyl polyenolates, thus guiding the selection of mediators for electrocatalytic systems. The versatility of catalytic asymmetric electrosynthesis is highlighted by the preparation of valuable enantioenriched building blocks and the total synthesis of (-)-ethosuximide.

Regio- and Stereoselective Electro-Mediated Carboalkoxylation of 1,3-Dienes

1,3-Dienes are versatile raw materials for building molecular complexity. We report herein mild conditions for the regio- and stereoselective [only the (E) isomer obtained] 1,4-carboalkoxylation of 1,3-dienes. This electrochemical multicomponent reaction provides an eco-efficient and straightforward access to a diverse range of (E)-polyfunctionalized allyl ether products, without requiring any metal catalyst.

Fe-Catalyzed Structurally Divergent γ-Polyhaloalkylation of Siloxydienes

Regioselective γ-polyhaloalkylation is achieved using tetrahalomethanes or α,α,α-trihaloalkyl compounds and siloxydienes via Fe(II) catalysis. A range of siloxydienes are functionalized in good yields with high stereoselectivity under mild reaction conditions. Structural divergence is observed as either haloalkylated or haloalkenylated products are formed on the basis of the substitution pattern of the siloxydiene. The halogenated products show utility in further synthetic transformations, selective reduction, and cross-coupling reactions.

Iron-Catalyzed Gamma-Gamma Dimerization of Siloxydienes

We report the oxidative dimerization reaction of siloxydienes derived from simple enones that creates a new gamma-gamma (γ-γ) C-C bond using catalytic iron and benzoyl peroxide as the terminal oxidant in acetonitrile solvent at ambient temperature. The reaction shows a broad substrate scope including cyclic and acyclic siloxydienes derived from ketones, aldehydes, and esters, which are converted to 1,8-dicarbonyl compounds under mild catalytic reaction conditions in 19-89 % yield across 30 examples. The method is suitable for the coupling of sterically demanding carbon centers, including the formation of vicinal quaternary centers. Conceptually, the dienol ether serves as a precursor to a conjugated radical cation, which undergoes highly site selective γ-dimerization reactions. The γ-γ dimerization strategy is applied to the synthesis of a bioactive analogue of honokiol.

Cu/Pd-catalyzed arylboration of a 1-silyl-1,3-cyclohexadiene for stereocontrolled and diverse cyclohexane/ene synthesis

The synthesis and Cu/Pd-catalyzed arylboration of 1-silyl-1,3-cyclohexadiene is described. This diene is significant as it allows for synthesis of polyfunctional cyclohexane/enes. To achieve high levels of diastereoselectivity, the use of a pyridylidene Cu-complex was employed. In addition, through the use of a chiral catalyst, an enantioselective reaction was possible. Due to the presence of the silyl and boron substituents, the products can be easily diversified into a range of valuable cyclohexane/ene products.

Formal γ-C(sp3)-H Activation of Ketones via Microwave-Promoted and Iminyl-Radical-Mediated 1,5-Hydrogen Atom Transfer

Microwave irradiation of O-phenyloximes triggers N-O homolysis and 1,5-hydrogen atom transfer (HAT), resulting in formal γ-C-H functionalization of ketones after trapping of the radical intermediate and in situ imine hydrolysis. The Lewis acid InCl3·H2O facilitated HAT, enabling functionalization of benzylic and nonbenzylic secondary carbon atoms. Functionalization of primary carbons was feasible but afforded low yields, requiring ClCH2CO2H instead of InCl3·H2O as an additive. C-O and C-C bond formation could both be accomplished by this method.

Metal and Oxidant-Free Brønsted Acid-Mediated Cascade Reaction to Substituted Benzofurans

Substituted hydroxy-benzofurans are easily accessible by treatment of resorcinols and 1,2-diaza-1,3-dienes (DDs) under acidic conditions. The reaction happens through an uncommon Michael reaction between aromatic derivatives as aromatic C(sp²)-H nucleophiles and DDs as acceptors. Also, the behavior of different phenols and 2-naphthol was investigated.

Study of Meldrum's Acid Cyclization Reactions

On the basis of the cyclization reactions reported by Danishefsky et al. of Meldrum's acid hydroxylethyl and anilinoethyl derivatives, the cyclization of the sulfamidomethylene and ureidomethylene derivatives was attempted without success. To understand the lack of reactivity of these compounds versus the successful cyclization of the ethyl derivatives, the corresponding mechanisms of reaction for both processes have been explored by means of MP2/6-311+G(d,p) calculations in an aqueous environment. The conformational analysis of all of these structures revealed that, while for the ethyl derivatives the minimum energy conformation corresponds to that of the cyclization initiating structure, for the methylene analogues the entrance channel conformations are substantially less stable than the energy minimum. Intramolecular hydrogen bonds were found in all of the energy minima as well as in the cyclization initiating conformations as determined by analysis of their electron density. The potential energy surfaces for the successful and unsuccessful cyclization processes were obtained at room temperature and 100 °C. Comparison of both processes allows rationalization that the lack of reactivity of the methylene derivatives can be thermodynamically explained based not only on the strength of the intramolecular hydrogen bond formed in their energy minima but also by the energy penalty needed to reach the entrance channel conformation and by the calculated energy barriers.

Catalytic asymmetric synthesis of diphenylbutazone analogues

The asymmetric Michael addition of diphenylbutazone and its analogues to α,β-unsaturated 2-acyl imidazoles has been developed with a chiral-at-metal Rh(iii) complex as a catalyst.