Intermolecular Pummerer Coupling with Carbon Nucleophiles in Non-Electrophilic Media

α-Functionalisation of Cyclic Sulfides Enabled by Lithiation Trapping

A general and straightforward procedure for the lithiation trapping of cyclic sulfides such as tetrahydrothiophene, tetrahydrothiopyran and a thiomorpholine is described. Trapping with a wide range of electrophiles is demonstrated, leading to more than 50 diverse α-substituted saturated sulfur heterocycles. The methodology provides access to a range of α-substituted cyclic sulfides that are not easily synthesised by the currently available methods.

Acyl Azolium-Photoredox-Enabled Synthesis of β-Keto Sulfides

α-Heteroatom functionalization is a key strategy for C-C bond formation in organic synthesis, as exemplified by the addition of a nucleophile to electrophilic functional groups, such as iminium ions; oxocarbenium ions; and their sulfur analogues, sulfenium ions. We envisioned a photoredox-enabled radical Pummerer-type reaction realized through the single-electron oxidation of a sulfide. Following this oxidative event, α-deprotonation would afford α-thio radicals that participate in radical-radical coupling reactions with azolium-bound ketyl radicals, thereby accessing a commonly proposed mechanistic intermediate of the radical-radical coupling en route to functionalized additive Pummerer products. This system provides a complementary synthetic approach to highly functionalized sulfurous products, including modification of methionine residues in peptides, and beckons further exploration in C-C bond formations previously limited in the standard two-electron process.

D. dos Santos AC, Kohlhepp SV, Mendoza A. Direct Addition of Grignard Reagents to Aliphatic Carboxylic Acids Enabled by Bulky turbo ‐Organomagnesium Anilides

The synthesis of ketones through addition of organometallic reagents to aliphatic carboxylic acids is a straightforward strategy that is limited to organolithium reagents. More desirable Grignard reagents can be activated and controlled with a bulky aniline‐derived turbo‐Hauser base. This operationally simple procedure allows the straightforward preparation of a variety of aliphatic and perfluoroalkyl ketones alike from functionalized alkyl, aryl and heteroaryl Grignard reagents.

Synthesis of carbinoxamine via α-C(sp3)–H 2-pyridylation of O, S or N-containing compounds enabled by non-D–A-type super organoreductants and sulfoxide- or sulfide HAT reagents

The radical cations of tertiary amines (R₃N) have been well-established as the precursors of HAT reagents in photochemical transformations. Similarly, thiols and thioacids bearing SH groups have also been widely applied as HAT reagents. Despite the fact that sulfoxides (R₂SO) and sulfides (RSR) also bear lone pairs of electrons, these compounds have been barely reported as HAT reagents in photocatalysis. On the other hand, the α-C–H 4-pyridylation of O or N-containing compounds has been documented, whereas 2-pyridylation remains challenging. However, the antihistamine and anticholinergic agent carbinoxamine is an ether bearing 2-pyridyl, which has not been obtained by the existing α-photoarylation of ether. In this work, we report the discovery of a non-donor–acceptor (D–A) type organic photoreductant CBZ6 and sulfoxide/sulfide synergistically catalyzed general α-C(sp³)–H arylation of ethers, thioethers and amines. By using as low as 1 mol% of CBZ6 as a recyclable organic photoreductant and sulfoxides or sulfides as a new type of HAT reagent, the 2- or 4-pyridylation of O, N, or S-containing compounds has been accomplished. This is the first base-free version of α-C–H 2-/4-pyridylation of O, N, or S-containing compounds. It is the first example of sulfoxides or sulfides working as HAT reagents. It is also the first general method for photocatalytic HAT 2-pyridylation of various ethers, amines or thioethers.

We report the discovery of a non-donor–acceptor (D–A) type organic photoreductant CBZ6 and sulfoxide/sulfide synergistically catalyzed general α-C(sp³)–H arylation of ethers, thioethers and amines.

Photoredox/Nickel Dual Catalytic Cross-Coupling of Potassium Thiomethyltrifluoroborates with Aryl and Heteroaryl Bromides

The cross-coupling of S-aryl and S-alkyl potassium thiomethyltrifluoroborates with aryl and heteroaryl bromides is reported via photoredox/nickel dual catalysis. The transformation is achieved under mild conditions with commercially available or readily prepared, air stable reagents and affords benzylthioether products in moderate to good yields with good functional group tolerance. A practical and improved synthesis of potassium thiomethyltrifluoroborates is also reported that affords access to previously undescribed reagents.

Computational and Experimental Study of Turbo-Organomagnesium Amide Reagents: Cubane Aggregates as Reactive Intermediates in Pummerer Coupling

The dynamic equilibria of organomagnesium reagents are known to be very complex, and the relative reactivity of their components is poorly understood. Herein, a combination of DFT calculations and kinetic experiments is employed to investigate the detailed reaction mechanism of the Pummerer coupling between sulfoxides and turbo-organomagnesium amides. Among the various aggregates studied, unprecedented heterometallic open cubane structures are demonstrated to yield favorable barriers through a concerted anion-anion coupling/ S-O cleavage step. Beyond a structural curiosity, these results introduce open cubane organometallics as key reactive intermediates in turbo-organomagnesium amide mixtures.

α-C–H difluoroalkylation of alkyl sulfoxides via intermolecular Pummerer reaction

A simple and practical intermolecular Pummerer reaction using difluoroenol silyl ethers as nucleophiles is described. The protocol allows for highly efficient α-difluoroalkylation of a wide spectrum of alkyl sulfoxides.

Scalable Synthesis of Esp and Rhodium(II) Carboxylates from Acetylacetone and RhCl3·xH2O

Rhodium(II) carboxylates are privileged catalysts for the most challenging carbene-, nitrene-, and oxo-transfer reactions. In this work, we address the strategic challenges of current organic and inorganic synthesis methods to access these rhodium(II) complexes through an oxidative rearrangement strategy and a reductive ligation reaction. These studies illustrate the multiple benefits of oxidative rearrangement in the process-scale synthesis of congested carboxylates over nitrile anion alkylation reactions, and the impressive effect of inorganic additives in the reductive ligation of rhodium(III) salts.

Metal-Free Synthesis of Phenol-Aryl Selenides via Dehydrogenative C-Se Coupling of Aryl Selenoxides with Phenols

Herein, we disclose the synthesis of diaryl selenides through an unexpected C-Se coupling between aryl benzyl selenoxides and phenols. The synthetic significance of the method is that it provides a mild, rapid, and metal-free access to organoselenides in high yields with excellent functional group tolerance. This coupling of aryl benzyl selenoxides reveals a completely new reaction possibility compared with aryl sulfoxides. We also probed the reaction mechanism of this unexpected transformation through experimental studies and revealed a special Se(IV)-Se(III)-Se(II) reaction pathway.

Sulfur(IV)-Mediated Unsymmetrical Heterocycle Cross-Couplings

Despite the tremendous utilities of metal-mediated cross-couplings in modern organic chemistry, coupling reactions involving nitrogenous heteroarenes remain a challenging undertaking - coordination of Lewis basic atoms into metal centers often necessitate elevated temperature, high catalyst loading, etc. Herein, we report a sulfur (IV) mediated cross-coupling amendable for the efficient synthesis of heteroaromatic substrates. Addition of heteroaryl nucleophiles to a simple, readily-accessible alkyl sulfinyl (IV) chloride allows formation of a trigonal bipyramidal sulfurane intermediate. Reductive elimination therefrom provides bis-heteroaryl products in a practical and efficient fashion.

Cu(I)-Catalyzed Enantioselective Alkynylation of Thiochromones

A highly efficient asymmetric synthesis of chiral thiochromanones is developed via Cu(I)/phosphoramidite catalyzed asymmetric alkynylation of thiochromones under mild reaction conditions. The catalyst system is tolerant of various thiochromone precursors and terminal alkynes. The established asymmetric transformation provides different enatiomeric-enriched thiochromanones with more molecular complexity and enables access to chiral thioflavanones, a subgroup of flavonoid by further functionalization.

i-Pr2NMgCl·LiCl Enables the Synthesis of Ketones by Direct Addition of Grignard Reagents to Carboxylate Anions

The direct preparation of ketones from carboxylate anions is greatly limited by the required use of organolithium reagents or activated acyl sources that need to be independently prepared. Herein, a specific magnesium amide additive is used to activate and control the addition of more tolerant Grignard reagents to carboxylate anions. This strategy enables the modular synthesis of ketones from CO₂ and the preparation of isotopically labeled pharmaceutical building blocks in a single operation.

Pummerer Synthesis of Chromanes Reveals a Competition between Cyclization and Reductive Chlorination

The competition between an unprecedented reductive chlorination and the Pummerer reaction was studied and applied to the synthesis of benzofused oxygen heterocycles including 3-aminochromanes and in the intramolecular chlorination of activated aromatic rings. The use of (COCl)₂ as a Pummerer activator showed substantial activity, producing α-chlorinated sulfides that can undergo Pummerer-Friedel-Crafts cyclization. If the aromatic ring has electron-donating groups in position three, then the reaction follows a different pathway, yielding the reductive chlorination products, where the chlorine atom comes from a sulfonium salt.

Bond-Forming and -Breaking Reactions at Sulfur(IV): Sulfoxides, Sulfonium Salts, Sulfur Ylides, and Sulfinate Salts

Organosulfur compounds have long played a vital role in organic chemistry and in the development of novel chemical structures and architectures. Prominent among these organosulfur compounds are those involving a sulfur(IV) center, which have been the subject of countless investigations over more than a hundred years. In addition to a long list of textbook sulfur-based reactions, there has been a sustained interest in the chemistry of organosulfur(IV) compounds in recent years. Of particular interest within organosulfur chemistry is the ease with which the synthetic chemist can effect a wide range of transformations through either bond formation or bond cleavage at sulfur. This review aims to cover the developments of the past decade in the chemistry of organic sulfur(IV) molecules and provide insight into both the wide range of reactions which critically rely on this versatile element and the diverse scaffolds that can thereby be synthesized.

Mechanism and regioselectivity of the anionic oxidative rearrangement of 1,3-diketones towards all-carbon quaternary carboxylates

The oxidative rearrangement of 1,3-diketones is an underexplored alternative to enolate chemistry in the synthesis of all-carbon quaternary carboxylates. The mechanistic investigation of this reaction has resulted in a mild base mediated protocol, whose regioselectivity has been studied in challenging acyclic substrates.

Transition-Metal-Free Cross-Coupling of Benzothiophenes and Styrenes in a Stereoselective Synthesis of Substituted (E,Z)-1,3-Dienes

A transition metal-free one-pot stereoselective approach to substituted (E,Z)-1,3-dienes was developed by using an interrupted Pummerer reaction/ligand-coupling strategy. Readily available benzothiophene S-oxides, which can be conveniently prepared by oxidation of the parent benzothiophenes, undergo Pummerer coupling with styrenes. Reaction of the resultant sulfonium salts with alkyllithium/magnesium reagents generates underexploited hypervalent sulfurane intermediates that undergo selective ligand coupling, resulting in dismantling of the benzothiophene motif and the formation of decorated (E,Z)-1,3-dienes.

Palladium-Catalyzed Enantioselective Alkenylation of Sulfenate Anions

A novel approach to synthesize enantio-enriched alkenyl/aryl sulfoxides is achieved by using CsF to generate sulfenate anions and conducting the catalytic enantioselective alkenylation with [Pd(allyl)Cl]₂/(2 R)-1-[(1 R)-1-[bis(1,1-dimethylethyl)phosphino]ethyl]-2-(diphenylphosphino)ferrocene (SL-J002-1). A wide variety of sulfoxides bearing sensitive functional groups are produced with high yields (up to 94%) and enantioselectivities (up to 92%).

General Cyclopropane Assembly by Enantioselective Transfer of a Redox-Active Carbene to Aliphatic Olefins

Asymmetric cyclopropane synthesis currently requires bespoke strategies, methods, substrates, and reagents, even when targeting similar compounds. This approach slows down discovery and limits available chemical space. Introduced herein is a practical and versatile diazocompound and its performance in the first unified asymmetric synthesis of functionalized cyclopropanes. The redox-active leaving group in this reagent enhances the reactivity and selectivity of geminal carbene transfer. This effect allowed the asymmetric cyclopropanation of various olefins, including unfunctionalized aliphatic alkenes, that enables the three-step total synthesis of (-)-dictyopterene A. This unified synthetic approach delivers high enantioselectivities that are independent of the stereoelectronic properties of the functional groups transferred. Our results demonstrate that orthogonally differentiated diazocompounds are viable and advantageous equivalents of single-carbon chirons.