The carbon–sulfur bond formation reaction is of paramount importance for functionalized materials design, as well as for biochemical applications. The use of expensive metal-based catalysts and the consequent contamination with trace metal impurities are challenging drawbacks of the existing methodologies. Here, we describe the first environmentally friendly metal-free photoredox pathway to the thiol–yne click reaction. Using Eosin Y as a cheap and readily available catalyst, C–S coupling products were obtained in high yields (up to 91%) and excellent selectivity (up to 60 : 1). A 3D-printed photoreactor was developed to create arrays of parallel reactions with temperature stabilization to improve the performance of the catalytic system.

/pdf/visible-light-mediated-metal-free-thiol-yne-click-reaction-2qicaubnac.pdf

Visible light mediated metal-free thiol–yne click reaction

Dehydrogenative aromatization is one of the attractive alternative methods for directly synthesizing primary anilines from NH3 and cyclohexanones. However, the selective synthesis of primary anilines is quite difficult because the desired primary aniline products and the cyclohexanone substrates readily undergo condensation affording the corresponding imines (i.e., N-cyclohexylidene-anilines), followed by hydrogenation to produce N-cyclohexylanilines as the major products. In this study, primary anilines were selectively synthesized in the presence of supported Pd nanoparticle catalysts (e.g., Pd/HAP, HAP=hydroxyapatite, Ca10 (PO4 )6 (OH)2 ) by utilizing competitive adsorption unique to heterogeneous catalysis; in other words, when styrene was used as a hydrogen acceptor, which preferentially adsorbs on the Pd nanoparticle surface in the presence of N-cyclohexylidene-anilines, various structurally diverse primary anilines were selectively synthesized from readily accessible NH3 and cyclohexanones. The Pd/HAP catalyst was reused several times though its catalytic performance gradually declined.

Selective Synthesis of Primary Anilines from NH 3 and Cyclohexanones by Utilizing Preferential Adsorption of Styrene on the Pd Nanoparticle Surface

The [4+2] cycloaddition of 3-alkoxyfurans with N-substituted maleimides provides the first general route for preparing endo-cantharimides. Unlike the corresponding reaction with 3H furans, the reaction can tolerate a broad range of 2-substitued furans including alkyl, aromatic, and heteroaromatic groups. The cycloaddition products were converted into a range of cantharimide products with promising lead-like properties for medicinal chemistry programs. Furthermore, the electron-rich furans are shown to react with a variety of alternative dienophiles to generate 7-oxabicyclo[2.2.1]heptane derivatives under mild conditions. DFT calculations have been performed to rationalize the activation effect of the 3-alkoxy group on a furan Diels–Alder reaction.

/pdf/irreversible-endo-selective-diels-alder-reactions-of-kwmnaj63ig.pdf

Irreversible endo-selective diels-alder reactions of substituted alkoxyfurans: a general synthesis of endo-cantharimides.

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. 

Ni-catalyzed C–S bond construction and cleavage

Transition metal-mediated catalysis routinely enables substrates of multiple π-systems to be efficiently coupled with various carbon nucleophiles along with simultaneous ring formation. This transformation, however, remains unexplored in connection with pericyclic processes. Reported here is a protocol for cycloalkene synthesis based on the merger of rhodium catalysis and a retro-ene reaction. The approach allows alkyne-tethered hydrazones and organoboronic acids to undergo a cascade of addition–cyclization–rearrangement reactions to provide cycloalkene products. The process is initiated by the rhodium-catalyzed addition–cyclization and completed with the allylic diazene rearrangement. The reaction can also be rendered asymmetric by using chiral diene ligands for the rhodium catalyst, whereby enantioselective addition to the C═N bond establishes the C—N stereocenter whose chirality is transferred to an allylic C—H center via suprafacial rearrangement.

Rhodium-Catalyzed Tandem Addition-Cyclization-Rearrangement of Alkynylhydrazones with Organoboronic Acids.

A pericyclic approach for the synthesis of six-membered ring structures is described. The method employs 1,3-dienes with a 1-sulfur substituent in a tandem sequence of Diels–Alder and retro-ene reactions. In this pairing of [4 + 2] cycloaddition and 1,5-sigmatropic rearrangement, 1-sulfenyl-1,3-dienes engage in Diels–Alder reactions with electron-deficient dienophiles. Subsequently, the sulfenyl group of the cycloadducts is oxidized and unmasked to form allylic sulfinic acids, which undergo sterospecific reductive transposition via sulfur dioxide extrusion. The sequence can also include an inverse electron demand Diels–Alder reaction by using a 1-sulfonyl-1,3-diene. This combination of two pericyclic events offers novel stereocontrolled access to cyclohexenes that are inaccessible via a direct [4 + 2] cycloaddition route.

Sinae Kim

Papers

Tandem Diels-Alder and retro-ene reactions of 1-sulfenyl- and 1-sulfonyl-1,3-dienes as a traceless route to cyclohexenes.

Concise Synthesis of the Calicheamicin Oligosaccharide (I) Using the Sulfoxide Glycosylation Method.

Tandem Diels—Alder and Retro-Ene Reactions of 1-Sulfenyl- and 1-Sulfonyl-1,3-dienes as a Traceless Route to Cyclohexenes.