Nickel‐Catalyzed Intramolecular Coupling of Sulfones via the Extrusion of Sulfur Dioxide

Unimolecular Fragment Coupling: A New Bond-Forming Methodology via the Deletion of Atom(s)

Unimolecular fragment coupling (UFC) is defined as a reaction format, wherein atom(s) located in the middle of a molecule are extruded, and the remaining fragments are coupled. UFC is a potentially powerful strategy that is an alternative to transition-metal-catalyzed cross-coupling because the target chemical bond is formed in an intramolecular fashion, which is inherently beneficial for chemoselectivity and stereoselectivity issues. In this Perspective, we will present an overview of the recent advances in UFC reactions, which encompass those proceeding through the elimination of CO2, CO, SO2, isocyanates, N2, or single atoms primarily via transition metal catalysis.

Reactivity of phenoxathiin-based thiacalixarenes towards C-nucleophiles

A starting thiacalix[4]arene can be easily transformed into oxidized phenoxathiin-based macrocycles 9 and 9', representing an unusual structural motif in calixarene chemistry. The presence of electron-withdrawing groups (SO2, SO) and the considerable internal strain caused by the condensed heterocyclic moiety render these molecules susceptible to nucleophilic attack. The reaction with various organolithium reagents provides a number of different products resulting from the cleavage of either the calixarene skeleton or the phenoxathiin group or both ways simultaneously. This enables the preparation of thiacalixarene analogues with unusual structural features, including systems containing a biphenyl fragment as a part of the macrocyclic skeleton. The above-described transformations, unparalleled in classical calixarene chemistry, clearly demonstrate the synthetic potential of this thiacalixarene subgroup.

Cu(II)-Mediated Sulfonylation of (Hetero)arenes with TosMIC Using Monodentate Directing Groups

Monodentate chelation-assisted direct ortho-C-H sulfonylation of (hetero)arenes using TosMIC as the novel sulfonylating reagent has been developed. A broad range of substrates, including indolines, indoles, 2-phenylpyridines, and others were well tolerated to afford the corresponding products in moderate to good yields. Mechanistic studies revealed that the sulfonyl radical might be involved. Inspired by the above discovery, preliminary para-C-H sulfonylation of naphthalene substrate was also successfully realized. The current protocol featured with cheap metal catalysis, good functional group compatibility, and operational convenience.

Electroreductive Desulfurative Transformations with Thioethers as Alkyl Radical Precursors

Thioethers are highly prevalent functional groups in organic compounds of natural and synthetic origin but remain remarkably underexplored as starting materials in desulfurative transformations. As such, new synthetic methods are highly desirable to unlock the potential of the compound class. In this vein, electrochemistry is an ideal tool to enable new reactivity and selectivity under mild conditions. Herein, we demonstrate the efficient use of aryl alkyl thioethers as alkyl radical precursors in electroreductive transformations, along with mechanistic details. The transformations proceed with complete selectivity for C(sp3 )-S bond cleavage, orthogonal to that of established transition metal-catalyzed two-electron routes. We showcase a hydrodesulfurization protocol with broad functional group tolerance, the first example of desulfurative C(sp3 )-C(sp3 ) bond formation in Giese-type cross-coupling and the first protocol for electrocarboxylation of synthetic relevance with thioethers as starting materials. Finally, the compound class is shown to outcompete their well-established sulfone analogues as alkyl radical precursors, demonstrating their synthetic potential for future desulfurative transformations in a one-electron manifold.

Regiodivergent synthesis of sulfone-tethered lactam-lactones bearing four contiguous stereocenters

Sulfone-tethered lactones/amides/amines display a diverse spectrum of biological activities, including anti-psychotic and anti-hypertensive. Sulfones are also widely present in functional materials and fragrances. We therefore reasoned that a regiodivergent and stereocontrolled strategy that merges the sulfone, lactone, and lactam motifs would likely lead to the discovery of new pharmacophores and functional materials. Here, we report mild conditions for the sulfonyllactonization of γ-lactam-tethered 5-aryl-4(E)-pentenoic acids. The annulation is highly modular, chemoselective, and diastereoselective. With respect to regioselectivity, trisubstituted alkenoic acids display a preference for 5-exo-trig cyclization whereas disubstituted alkenoic acids undergo exclusive 6-endo-trig cyclization. The lactam-fused sulfonyllactones bear angular quaternary as well as four contiguous stereocenters. The products are post-modifiable, especially through a newly developed Co-catalyzed reductive cross-coupling protocol.

Catalyst-Free Electrochemical Sulfonylation of Organoboronic Acids

A simple and efficient electrochemical sulfonylation of organoboronic acids with sodium arylsulfinate salts has been reported for the first time. A variety of aryl, heteroaryl, and alkenylsulfones were obtained in good to excellent yields via a simple electrochemical sulfonylation of various arylboronic acids, heterocyclic boronic acids, or alkenylboronic acids with sodium arylsulfinate at room temperature in 5 h under the catalyst-free and additive-free conditions. A plausible mechanism has been proposed based on various radical-trapping and CV control experiments.

Ni-catalyzed C-S bond construction and cleavage

Sulfur-containing compounds have attracted considerable interest due to their wide-ranging applications in pharmaceuticals, agriculture, natural products, and organic materials. The development of efficient and rapid methods for the construction and transformation of sulfur-containing compounds is of great importance. Since nickel is inexpensive and has a variety of valence states, strong nucleophilicity and low energy barriers for oxidative addition, the construction and transformation of sulfur-containing compounds by nickel-catalyzed cross-coupling have become important strategies. In addition, sulfur-containing compounds have also been playing increasingly important roles in the field of cross-coupling due to their thermodynamically stable but dynamic activity. This review will focus on nickel-catalyzed construction and transformation of various sulfide-containing compounds, such as sulfides, disulfides, and hypervalent sulfur-containing compounds.

Beyond classical sulfone chemistry: metal- and photocatalytic approaches for C-S bond functionalization of sulfones

The exceptional versatility of sulfones has been extensively exploited in organic synthesis across several decades. Since the first demonstration in 2005 that sulfones can participate in Pd-catalysed Suzuki-Miyaura type reactions, tremendous advances in catalytic desulfitative functionalizations have opened a new area of research with burgeoning activity in recent years. This emerging field is displaying sulfone derivatives as a new class of substrates enabling catalytic C-C and C-X bond construction. In this review, we will discuss new facets of sulfone reactivity toward further expanding the flexibility of C-S bonds, with an emphasis on key mechanistic features. The inherent challenges confronting the development of these strategies will be presented, along with the potential application of this chemistry for the synthesis of natural products. Taken together, this knowledge should stimulate impactful improvements on the use of sulfones in catalytic desulfitative C-C and C-X bond formation. A main goal of this article is to bring this technology to the mainstream catalysis practice and to serve as inspiration for new perspectives in catalytic transformations.

Palladium-Catalyzed Unimolecular Fragment Coupling of N-Allylamides via Elimination of Isocyanate

Transition metal-catalyzed unimolecular fragment coupling (UFC) is defined as processes that forge new chemical bonds through the extrusion of molecules, such as CO and CO₂, and the subsequent recombination of the remaining fragments. Herein, we report on a new UFC reaction that involves the palladium-catalyzed elimination of an isocyanate fragment from an amide, with the formation of carbon-carbon and carbon-heteroatom bonds. An organometallic intermediate that is relevant to the catalytic reaction was characterized by X-ray crystallography. This UFC reaction enables the late-stage transformation of an amide functionality, allowing amides to be used as a convertible directing or protecting group.

Unified Approach to Benzo[d]thiazol-2-yl-Sulfonamides

In this paper, we report a unified approach to N-substituted and N,N-disubstituted benzothiazole (BT) sulfonamides. Our approach to BT-sulfonamides starts from simple commercially available building blocks (benzo[d]thiazole-2-thiol and primary and secondary amines) that are connected via (a) a S oxidation/S-N coupling approach, (b) a S-N coupling/S-oxidation sequence, or via (c) a S-oxidation/S-F bond formation/SuFEx approach. The labile N-H bond in N-monoalkylated BT-sulfonamides (pK_a (BTSO₂N(H)Bn) = 3.34 ± 0.05) further allowed us to develop a simple weak base-promoted N-alkylation method and a stereoselective microwave-promoted Fukuyama-Mitsunobu reaction. N-Alkyl-N-aryl BT-sulfonamides were accessed with the help of the Chan-Lam coupling reaction. Developed methods were further used in stereo and chemoselective transformations of podophyllotoxin and several amino alcohols.

A Titanium-Catalyzed Reductive α-Desulfonylation

A titanium(III)-catalyzed desulfonylation gives access to functionalized alkyl nitrile building blocks from α-sulfonyl nitriles, circumventing traditional base-mediated α-alkylation conditions and strong single electron donors. The reaction tolerates numerous functional groups including free alcohols, esters, amides, and it can be applied also to the α-desulfonylation of ketones. In addition, a one-pot desulfonylative alkylation is demonstrated. Preliminary mechanistic studies indicate a catalyst-dependent mechanism involving a homolytic C-S cleavage.

Room-Temperature Palladium(II)-Catalyzed Direct 2-Arylation of Indoles with Tetraarylstannanes

A palladium(II)-catalyzed direct 2-arylation of indoles by tetraarylstannanes with oxygen (balloon) as the oxidant at room temperature has been developed. Various tetraarylstannanes can be employed as aryl sources for 2-arylation of indoles in up to 89% yield, providing a practical and efficient catalytic protocol for accessing 2-arylindoles.

Transition-metal mediated carbon-sulfur bond activation and transformations: an update

Carbon-sulfur bond cross-coupling has become more and more attractive as an alternative protocol to establish carbon-carbon and carbon-heteroatom bonds. Diverse transformations through transition-metal-catalyzed C-S bond activation and cleavage have recently been developed. This review summarizes the advances in transition-metal-catalyzed cross-coupling via carbon-sulfur bond activation and cleavage since late 2012 as an update of the critical review on the same topic published in early 2013 (Chem. Soc. Rev., 2013, 42, 599-621), which is presented by the categories of organosulfur compounds, that is, thioesters, thioethers including heteroaryl, aryl, vinyl, alkyl, and alkynyl sulfides, ketene dithioacetals, sulfoxides including DMSO, sulfones, sulfonyl chlorides, sulfinates, thiocyanates, sulfonium salts, sulfonyl hydrazides, sulfonates, thiophene-based compounds, and C[double bond, length as m-dash]S functionality-bearing compounds such as thioureas, thioamides, and carbon disulfide, as well as the mechanistic insights. An overview of C-S bond cleavage reactions with stoichiometric transition-metal reagents is briefly given. Theoretical studies on the reactivity of carbon-sulfur bonds by DFT calculations are also discussed.

Nickel-Catalyzed Intramolecular Desulfitative C-N Coupling: A Synthesis of Aromatic Amines

A nickel-catalyzed intramolecular C-N coupling reaction via SO₂ extrusion is presented. The use of a catalytic amount of BPh₃ allows the transformation to take place under much milder conditions (60 °C) than previously reported C-N coupling reactions by CO or CO₂ extrusion (160-180 °C). In addition, this method displays good functional group tolerance and versatility, as it can be applied to the synthesis of dialkyl aryl amines, alkyl diaryl amines, and triaryl amines. The robustness of the desulfitative C-N coupling is demonstrated by three high-yielding gram-scale reactions.