Proton-Coupled Electron Transfer: Transition-Metal-Free Selective Reduction of Chalcones and Alkynes Using Xanthate/Formic Acid

CS2/KOH System-Promoted Stereoselective Synthesis of (E)-Alkenes from Diarylalkynes and a "Hidden" Zinin-Type Reduction of Nitroarenes into Arylamines

In this work, we present the CS2/KOH system as a practical and efficient reductive medium for obtaining (E)-alkenes from alkynes through a highly stereoselective semireduction reaction. This cost-effective system enabled successful semireduction reactions of diverse alkynes using water as a hydrogen source, yielding moderate to excellent yields. The versatility of this protocol is further demonstrated through the synthesis of relevant compounds such as pinosylvin and resveratrol precursors, along with the notable anticancer agent DMU-212. Furthermore, during the reaction scope investigation, we serendipitously disclosed that this reductive system was also able to promote a Zinin-type reaction to reduce nitroarenes into arylamines.

Chemoselective Reduction of α,β-Unsaturated Carbonyl Compounds via a CS2/t-BuOK System: Dimethyl Sulfoxide as a Hydrogen Source

A novel and practical approach to access saturated ketones from unsaturated ketone derivatives via a CS₂/t-BuOK system in dimethyl sulfoxide (DMSO) is reported. The in situ generation of xanthate salt through the reaction of carbon disulfide and potassium tert-butoxide is essential to this transformation. Deuterium-labeling experiments demonstrated that DMSO can act as a hydrogen donor.

Switching the chemoselectivity of perakine reductase for selective reduction of α,β-unsaturated ketones by Arg127 mutation

The chemoselectivity of perakine reductase (PR) was engineered through rational design. We identified Arg127 as a control site of chemoselectivity. Mutation of Arg127 switched the chemoselectivity of PR between CO and CC or led to non-selectivity towards α,β-unsaturated ketones, leading to the production of allylic alcohols, saturated ketones, or a mixture of both. This study provides an example for developing novel reductases for α,β-unsaturated ketones.

Rh(III)-Catalyzed Enone Carbonyl/Ketone-Directed Aerobic C-H Olefination of Aromatics with Unactivated Olefins

A Rh(III)-catalyzed weak enone carbonyl/ketone-assisted aerobic oxidative C-H olefination of aromatics with unactivated alkenes has been developed. This protocol involves cross-dehydrogenative Heck-type olefination reaction of various substituted biologically relevant chalcones and aromatic ketones such as acetophenones and chromones with various functionalized unactivated olefins in moderate to good yields. Further, ortho-alkylation of chalcones with norbornene is also demonstrated. A possible reaction mechanism involving weak chelation-assisted C-H activation/insertion/β-hydride elimination was proposed and supported by the deuterium labeling studies.

Methanol as a hydrogen source: room-temperature highly-selective transfer hydrogenation of α,β-unsaturated ketones

The described system offers an ideal, user-friendly protocol for the chemoselective homogeneous hydrogenation of α,β-unsaturated ketones at room temperature using methanol as a liquid organic hydrogen carrier. Excellent yields were achieved with an in situ-prepared phosphine-free Cp*Ir(III)/bipyridonate complex. Chemoselective reduction with other reducible functionalities and late-stage functionalization were also explored.

Transition Metal-Free Iodine-Catalyzed Denitrative C-S Cross-Coupling: An Atypical Route to Access Thiochromane Derivatives

An iodine-catalyzed denitrative C-S cross-coupling reaction has been developed to attain thiochromanones from 2'-nitrochalcones and xanthate. The strategy was extended for a three-component synthesis of thiochromenes via intermolecular C-S cross-coupling followed by aldol reaction. The reaction proceeds via activation of the keto group of chalcone through a halogen bond complex with iodine/denitrative C-S bond formation with xanthate/sulfa-Michael addition to chalcones. The methodology was also demonstrated for chemoselective reduction of chalcones. The protocol was also employed to synthesize biologically important 3'-hydroxythioflavone and thiochromenones.

Thermodynamics of the elementary steps of organic hydride chemistry determined in acetonitrile and their applications

This review focuses on the thermodynamics of the elementary step of 421 organic hydrides and unsaturated compounds releasing or accepting hydride or hydrogen determined in acetonitrile as well as their potential applications.

Selective deoxygenative alkylation of alcohols via photocatalytic domino radical fragmentations

The delivery of alkyl radicals through photocatalytic deoxygenation of primary alcohols under mild conditions is a so far unmet challenge. In this report, we present a one-pot strategy for deoxygenative Giese reaction of alcohols with electron-deficient alkenes, by using xanthate salts as alcohol-activating groups for radical generation under visible-light photoredox conditions in the presence of triphenylphosphine. The convenient generation of xanthate salts and high reactivity of sequential C–S/C–O bond homolytic cleavage enable efficient deoxygenation of primary, secondary and tertiary alcohols with diverse functionality and structure to generate the corresponding alkyl radicals, including methyl radical. Moreover, chemoselective radical monodeoxygenation of diols is achieved via selective formation of xanthate salts.

The generation of alkyl radicals through deoxygenation of abundant alcohols via photoredox catalysis is of interest. In this study, the authors report a one-pot strategy for visible-light-promoted photoredox coupling of alcohols with electron-deficient alkenes, assisted by carbon disulfide and triphenylphosphine.

Electrocatalytic Reduction of C-C π-Bonds via a Cobaltocene-Derived Concerted Proton-Electron Transfer Mediator: Fumarate Hydrogenation as a Model Study

Reductive concerted proton-electron transfer (CPET) is poorly developed for the reduction of C-C π-bonds, including for activated alkenes that can succumb to deleterious pathways (e.g., a competing hydrogen evolution reaction or oligomerization) in a standard electrochemical reduction. We demonstrate herein that selective hydrogenation of the C-C π-bond of fumarate esters can be achieved via electrocatalytic CPET (eCPET) using a CPET mediator comprising cobaltocene with a tethered Brønsted base. High selectivity for electrocatalytic hydrogenation is observed only when the mediator is present. Mechanistic analysis sheds light on two distinct kinetic regimes based on the substrate concentration: low fumarate concentrations operate via rate-limiting CPET followed by an electron-transfer/proton-transfer (ET/PT) step, whereas high concentrations operate via CPET followed by a rate-limiting ET/PT step.

Copper-catalyzed domino synthesis of multi-substituted benzo[b]thiophene through radical cyclization using xanthate as a sulfur surrogate

The Cu-catalyzed domino synthesis of multi-substituted benzo[b]thiophene through radical cyclization of 2-iodophenyl ketones was developed using xanthate as a sulfur surrogate. This method was extended to obtain tetracyclic Lupinalbin analogues through double C-S/C-O bond formation by changing the substituents. The products were converted to a HTI photoswitch, benzothiophene-fused flavone.

Borane-Catalyzed, Chemoselective Reduction and Hydrofunctionalization of Enones Enabled by B-O Transborylation

The use of stoichiometric organoborane reductants in organic synthesis is well established. Here these reagents have been rendered catalytic through an isodesmic B-O/B-H transborylation applied in the borane-catalyzed, chemoselective alkene reduction and formal hydrofunctionalization of enones. The reaction was found to proceed by a 1,4-hydroboration of the enone and B-O/B-H transborylation with HBpin, enabling catalyst turnover. Single-turnover and isotopic labeling experiments supported the proposed mechanism of catalysis with 1,4-hydroboration and B-O/B-H transborylation as key steps.

Chemoselective Transfer Hydrogenation of α,β-Unsaturated Ketones Catalyzed by Iridium Complexes

Efficient chemoselective transfer hydrogenation of the C=C bond of α,β-unsaturated ketones has been developed, using the iridium complexes containing pyridine-imidazolidinyl ligands as catalysts and formic acid as a hydrogen source. In comparison with organic solvents or H2O as solvent, the mixed solvents of H2O and MeOH are critical for a high catalytic chemoselective transformation. This chemoselective transfer hydrogenation can be carried out in air, which is operationally simple, allowing a wide variety of α,β-unsaturated substrates with different functional groups (electron-donating and electron-withdrawing substituents) leading to chemoselective transfer hydrogenation in excellent yields. The practical application of this protocol is demonstrated by a gram-scale transformation.

Palladium-catalyzed cross-coupling reactions on a bromo-naphthalene scaffold in the search for novel human CC chemokine receptor 8 (CCR8) antagonists

The naphthalene sulfonamide scaffold is known to possess CCR8 antagonistic properties. In order to expand the structure-activity relationship study of this compound class, a variety of palladium-catalyzed cross-coupling reactions was performed on a bromo-naphthalene precursor yielding a diverse library. These compounds displayed CCR8 antagonistic properties in binding and calcium mobilization assays, with IC₅₀ values in the 0.2 - 10 µM range. The decreased activity, when compared to the original lead compound, was rationalized by homology molecular modeling.

Radical condensation between benzylic alcohols and acetamides to form 3-arylpropanamides

A new radical condensation reaction is developed where benzylic alcohols and acetamides are coupled to generate 3-arylpropanamides with water as the only byproduct. The transformation is performed with potassium tert-butoxide as the only additive and gives rise to a variety of 3-arylpropanamides in good yields. The mechanism has been investigated experimentally with labelled substrates, trapping experiments and spectroscopic measurements. The findings indicate a radical pathway where potassium tert-butoxide is believed to serve a dual role as both base and radical initiator. The radical anion of the benzylic alcohol is proposed as the key intermediate, which undergoes coupling with the enolate of the amide to form the new C–C bond. Subsequent elimination to the corresponding cinnamamide and olefin reduction then affords the 3-arylpropanamides.

Benzylic alcohols and acetamides are coupled into 3-arylpropanamides by a new radical condensation through the radical anion of the alcohol.

Copper-catalyzed double C–S bond formation for the synthesis of 2-acyldihydrobenzo[b]thiophenes and 2-acylbenzo[b]thiophenes

Efficient domino methods are developed for the synthesis of 2,3-dihydrobenzo[b]thiophenes and 2-acylbenzo[b]thiophenes from 2-iodoketones using in situ generated iodine from by-product KI with a Cu-catalyst and xanthate.

Cascade cyclization versus chemoselective reduction: a solvent-controlled product divergence

A convenient controllable cascade cyclization and partial reduction of enones for the divergent construction of two types of valuable compounds including polysubstituted thiophenes and saturated ketones are developed.

Cu-Catalyzed one-pot synthesis of thiochromeno-quinolinone and thiochromeno-thioflavone via oxidative double hetero Michael addition using in situ generated nucleophiles

A copper catalyzed three-component synthesis of π-conjugated tetracyclic thiochromeno-quinolinone and thiochromeno-thioflavone was established via oxidative double hetero Michael addition using in situ generated nucleophiles.

Chemoselective and metal-free reduction of α,β-unsaturated ketones by in situ produced benzeneselenol from O-(tert-butyl) Se-phenyl selenocarbonate

The carbon–carbon double bond of arylidene acetones and chalcones can be selectively reduced with benzeneselenol generated in situ by reacting O-(tert-butyl) Se-phenyl selenocarbonate with hydrochloric acid in ethanol. This mild, metal-free and experimentally simple reduction procedure displays considerable functional-group compatibility, products are obtained in good to excellent yields, and the use of toxic Se/CO mixture and NaSeH, or the smelly and air-sensitive benzeneselenol, is avoided.

The carbon–carbon double bond of arylidene acetones and chalcones can be selectively reduced with benzeneselenol generated in situ by reacting O-(tert-butyl) Se-phenyl selenocarbonate with hydrochloric acid in ethanol.