Synthesis of α-heterosubstituted ketones through sulfur mediated difunctionalization of internal alkynes

1,1-Oxycarbonation of Terminal Alkynes via Sequential Borylation, 1,2-Migration, and Oxidation with Oxone

Alkynes are readily available and multifunctional synthetic intermediates, but their 1,1-oxofunctionalization remains challenging. Herein, we report a 1,1-oxycarbonation of terminal alkynes to construct ketones through sequential borylation, 1,2-carbon migration, and oxidation with Oxone as the proton source and oxidant. The synthetic potential of this transformation is showcased by the broad functional groups, scale-up synthesis, and diverse product transformations.

Synthesis of Aryl Thioalkynes Enabled by Electrophilic Sulfenylation of Alkynes and the Following Elimination

An unexpected deprotonative process of thiirenium ions is presented, which provides a new synthesis of aryl thioalkynes directly from terminal alkynes via the electrophilic activation of the carbon-carbon triple bonds. The conditions are well compatible with various functional-group-substituted aryl alkynes. The direct elimination from the thiirenium ion intermediate, or its tautomer, benzyl vinyl carbocation, is supported by control experiments and labeling reaction.

Exploring the Divergent Reactivity of Vinyl Radicals Emanating from Alkynes and Thiols via Photoredox Catalysis

Organic chemistry has seen a surge in visible-light-driven transformations, which offer unique reaction pathways and access to new synthetic possibilities. We aim to provide a comprehensive understanding of state-of-the-art photo-mediated alkyne functionalization, with a focus on the reactive behavior of vinyl radicals. This review outlines our contributions to the field, including developing new methods for forming carbon-carbon and carbon-heteroatom bonds.

Direct Synthesis of α-Amino Acid Derivatives by Hydrative Amination of Alkynes

α-Amino acid derivatives are key components of the molecules of life. The synthesis of α-amino carbonyl/carboxyl compounds is a contemporary challenge in organic synthesis. Herein, we report a practical method for the preparation of α-amino acid derivatives via direct hydrative amination of activated alkynes under mild conditions, relying on sulfinamides as the nitrogen source. Computational studies suggest that the reaction is enabled by a new type of sulfonium [2,3]-sigmatropic rearrangement.

Synthese von α-Aminosäurederivaten durch hydrative Aminierung von Alkinen

α‐Aminosäurederivate sind Kernbestandteile jeglichen Lebens. Die Synthese von α‐Aminocarbonyl/carboxyl Verbindungen ist jedoch weiterhin eine Herausforderung für die organische Synthese. In dieser Arbeit berichten wir von einer praktischen Herstellungsmethode für α‐Aminosäurederivate durch direkte hydrative Aminierung von aktivierten Alkinen mit Sulfinamiden unter milden Bedingungen. Computergestützte Untersuchungen legen nahe, dass eine [2,3]‐sigmatrope Sulfoniumumlagerung der zentrale Schritt der Reaktion ist.

Regio- and Stereoselective Synthesis of β-Methylthio Vinyl Triflates

Vinyl triflates are commonly employed as electrophilic vinyl sources in complex synthesis. The triflation of enolates is commonly required for the preparation of vinyl triflates, generally under strongly basic conditions. Herein, the reaction between alkynes and dimethyl(methylthio)sulfonium trifluoromethanesulfonate is presented, which leads to the development of a facile synthesis of β-methylthio vinyl triflates in a chemo-, regio-, and stereoselective manner under neutral and extremely simple conditions.

Organocatalytic asymmetric synthesis of α-amino esters from sulfoxonium ylides

Described here is the first organocatalytic asymmetric N–H insertion reaction of α-carbonyl sulfoxonium ylides. Without a metal catalyst, this reaction represents an attractive complement to the well-established carbene insertion reactions. As a stable surrogate of diazocarbonyl compounds, sulfoxonium ylides reacted with a range of aryl amines to provide efficient access to α-aryl glycines with excellent enantiocontrol in the presence of a suitable chiral phosphoric acid catalyst. The high stability and weak basicity of sulfoxonium ylides not only enable this protocol to be user-friendly and practically useful, but also preclude catalyst decomposition, which is crucial to the excellent amenability to electron-poor amine nucleophiles. Detailed mechanistic studies indicated that the initial protonation is reversible and the C–N bond formation is rate-determining.

An organocatalytic asymmetric N–H insertion reaction of α-carbonyl sulfoxonium ylides has been developed to provide efficient access to α-amino esters without involving a metal carbenoid intermediate.

Recent Progress in Enolonium Chemistry under Metal-Free Conditions

Umpolung approach through inversion of the polarity of conventional enolates, has opened up an unprecedented opportunity in the cross-coupling via alkylation. The enolonium equivalents can be accessed either by hypervalent iodine reagents, activation/oxidation of amides, or the oxidation of alkynes. Under umpolung conditions, highly basic conditions required for classical enolate chemistry can be avoided, and they can couple with unmodified nucleophiles such as heteroatom donors and electron-rich arenes.

Palladium-catalyzed benzylic C(sp3)-H carbonylative arylation of azaarylmethyl amines with aryl bromides

A highly selective palladium-catalyzed carbonylative arylation of weakly acidic benzylic C(sp3)-H bonds of azaarylmethylamines with aryl bromides under 1 atm of CO gas has been achieved. This work represents the first examples of use of such weakly acidic pronucleophiles in this class of transformations. In the presence of a NIXANTPHOS-based palladium catalyst, this one-pot cascade process allows a range of azaarylmethylamines containing pyridyl, quinolinyl and pyrimidyl moieties and acyclic and cyclic amines to undergo efficient reactions with aryl bromides and CO to provide α-amino aryl-azaarylmethyl ketones in moderate to high yields with a broad substrate scope and good tolerance of functional groups. This reaction proceeds via in situ reversible deprotonation of the benzylic C-H bonds to give the active carbanions, thereby avoiding prefunctionalized organometallic reagents and generation of additional waste. Importantly, the operational simplicity, scalability and diversity of the products highlight the potential applicability of this protocol.

Sulfur-Mediated Reactions through Sulfonium Salts and Ylides

This Account discusses several new reaction methods developed in our group that utilize sulfur-mediated reactions through sulfonium salts and ylides, highlighting the interplay of rational design and serendipity. Our initial goal was to convert aliphatic C–H bonds into C–C bonds site-selectively, and without the use of transition-metal catalysts. While a proof-of-concept has been achieved, this target is far from being ideally realized. The unexpected discovery of an anti-Markovnikov rearrangement and subsequent studies on difunctionalization of alkynes were much more straightforward, and eventually led to the new possibility of asymmetric N–H insertion of sulfonium ylides through Brønsted acid catalysis.

1 Introduction

2 Allylic/Propargylic C–H Functionalization

3 Anti-Markovnikov Rearrangement

4 Difunctionalization of Alkynes

5 Asymmetric N–H Insertion of Sulfonium Ylides

6 Conclusion

Synthetic Routes Towards the Synthesis of Geminal α-Difunctionalized Ketones

The importance of gem-difunctionalized ketones is represented by their broad applications across chemical boundaries over recent years. The interesting reactivities that this class of compounds possess have made them ideal building blocks to access high-value organic molecules. Furthermore, the gem-difunctionalized ketone moiety has featured in numerous bioactive molecules. For these reasons, a plethora of routes to access such significant molecules have been developed by research groups worldwide - this account looks at delineating the synthesis of gem-difunctionalized ketones from carbonyl substrates, diazo compounds, sulfur ylides and alkynyl reactants.

Recent advances in the synthesis of α-amino ketones

Due to the importance of the amino ketone motif in synthetic and medicinal chemistry, the number of protocols developed in recent years has considerably increased. This review serves to collate and critically evaluate novel methodologies published since 2011 towards this high value synthon. The chapters are divided by the requisite functionality in the starting material, and an emphasis is placed on discussing functional group compatibility and resultant product substitution patterns. Throughout, applications to medicinal targets are highlighted and mechanistic details are presented, and we further provide a short outlook for future development and emerging potential within this area.

Selective synthesis of α-organylthio esters and α-organylthio ketones from β-keto esters and sodium S-organyl sulfurothioates under basic conditions

We described herein a selective method to prepare α-organylthio esters and α-organylthio ketones by the reaction of β-keto esters with sodium S-benzyl sulfurothioate or sodium S-alkyl sulfurothioate (Bunte salts) under basic conditions in toluene as the solvent at 100 °C. When 4 equivalents of a base were used, a series of differently substituted α-thio esters were obtained with up to 90% yield. On the other hand, employing 2 equivalents of a base, α-thio ketones were achieved after 18 h under air. Furthermore, after a shorter reaction time, the isolation of keto-enol tautomers was possible, revealing them as significant intermediates for the mechanism elucidation.

Metal-free catalytic hydrocarboxylation of hexafluorobut-2-yne

An efficient method for stereoselective synthesis of trifluorinated enol esters catalyzed by base was introduced.

Trifluoromethyl Sulfoxides: Reagents for Metal-Free C-H Trifluoromethylthiolation

Trifluoromethyl sulfoxides are a new class of trifluoromethylthiolating reagent. The sulfoxides engage in metal-free C-H trifluoromethylthiolation with a range of (hetero)arenes. The method is also applicable to the functionalization of important compound classes, such as ligand derivatives and polyaromatics, and in the late-stage trifluoromethylthiolation of medicines and agrochemicals. The isolation and characterization of a sulfonium salt intermediate supports an interrupted Pummerer reaction mechanism.

Cu(I)-Catalyzed Three-Component Cyclization for the Construction of Functionalized Thiazoles

A novel and straightforward strategy for the synthesis of functionalized thiazoles from thioamides, ynals, and alcohols via a copper(I)-catalyzed three-component reaction has been described. Through the formation of new C-S, C-N, and C-O bonds in one pot, it is easy to produce various valuable thiazoles fixed with aryl or heteroaryl groups. In addition, the reaction also exhibits other unique advantages, such as high step economics, good functional group tolerance, and good regioselectivity.

Synthesis of Epoxides from Alkyl Bromides and Alcohols with in Situ Generation of Dimethyl Sulfonium Ylide in DMSO Oxidations

Direct conversion of the readily available alkyl bromides and alcohols to value-added epoxides using dimethyl sulfoxide (DMSO) under mild reaction conditions has been developed. Benzyl and allyl bromides, and activated and unactivated alcohols all proceeded smoothly to give epoxides in high to excellent yield. Dimethyl sulfide, generated by DMSO oxidations, was in situ elaborated to form the substituted dimethyl sulfonium ylide species that participates in the Corey-Chaykovsky epoxidation in a domino and one-pot fashion, respectively.

Sulfoxide-mediated oxidative cross-coupling of phenols

A metal-free, oxidative coupling of phenols with various nucleophiles, including arenes, 1,3-diketones and other phenols, is reported. Cross-coupling is mediated by a sulfoxide which inverts the reactivity of the phenol partner. Crucially, the process shows high selectivity for cross-versus homo-coupling and allows efficient access to a variety of aromatic scaffolds including biaryls, benzofurans and, through an iterative procedure, aromatic oligomers.

A metal-free, oxidative coupling of phenols with various nucleophiles, including arenes, 1,3-diketones and other phenols, is reported.

Direct electrooxidation of alkynes to benzoin bis-ethers

Electrosynthesis of benzoin bis-ethers from alkynes and alcohols under metal-free and external oxidant free conditions.

Sulfur-Mediated Allylic C-H Arylation, Epoxidation, and Aziridination

Transition-metal-free, sulfur-mediated allylic C-H arylation, epoxidation, and aziridination were realized through one-pot procedures. The reaction design involved initial addition between olefins and triflic anhydride activated sulfoxides, followed by subsequent reactions of the allylic sulfur ylides generated under basic conditions with arylboronic acids, aldehydes, or aldimines, to give allylic arylation, epoxidation, or aziridination products, respectively.

Synthesis of γ-substituted carbonyl compounds from DMSO-mediated oxidation of enynamides: mechanistic insights and carbon- and hetero-functionalizations

1,3-Enynamides underwent oxygenative coupling with carbon- and heteroatom nucleophiles with high remote selectivity. Kinetic analysis revealed a continuum mechanism between concerted S_N2′′ and via a carbocation, depending on the nucleophiles used.

Oxidative coupling of 1,3-enynamides using DMSO as a terminal oxidant has been developed. Carbon as well as unmodified heteroatom nucleophiles, including aliphatic alcohols, thiols, and hydrazides, could be efficiently alkylated at the γ-position in a highly regioselective fashion. The kinetic analysis suggested a nucleophile-dependent mechanism ranging from a concerted S_N2′′ to a carbocationic mechanism. Thus, the remote site-selectivity was ascribed to the partial positive charge developing at the terminal carbocationic center.

Synthesis of N-Alkylpyridin-4-ones and Thiazolo[3,2-a]pyridin-5-ones through Pummerer-Type Reactions

N-Alkylated 4-pyridones were obtained through a one-pot procedure involving either normal or interrupted Pummerer reactions between triflic anhydride-activated sulfoxides and 4-fluoropyridine derivatives, followed by hydrolysis. On the other hand, triflic anhydride-activated benzyl 6-fluoro-2-pyridyl sulfoxide could react with alkenes or alkynes to afford thiazolo[3,2-a]pyridin-5-ones, via the pyridinium salt intermediates.

Synthesis of polysubstituted cyclic 1,2-diketones enabled by iterative sulfoxide-mediated arylation

A metal-free iterative C–H arylation of cyclic 1,2-diketones with aryl sulfoxides has been developed.