N-Heterocyclic Carbene-Catalyzed Synthesis of Ynones via C–H Alkynylation of Aldehydes with Alkynyliodonium Salts—Evidence for Alkynyl Transfer via Direct Substitution at Acetylenic Carbon

Thermocatalytic epoxidation by cobalt sulfide inspired by the material's electrocatalytic activity for oxygen evolution reaction

New discoveries in catalysis by earth-abundant materials can be guided by leveraging knowledge across two sub-disciplines of heterogeneous catalysis: electrocatalysis and thermocatalysis. Cobalt sulfide has been reported to be a highly active electrocatalyst for the oxygen evolution reaction (OER). Under these oxidative conditions, cobalt sulfide forms oxidized surfaces that outperform directly prepared cobalt oxide in OER catalysis. We postulated that the catalytic activity of oxidized cobalt sulfide for OER could reflect a more general ability to catalyze O-transfer reactions. Herein, we show that cobalt sulfide (CoS x ) indeed catalyzes the epoxidation of cyclooctene, a thermal O-transfer reaction. Similarly to OER, the surface-oxidized CoS x formed under reaction conditions outperformed the directly prepared cobalt oxide, hydroxide, and oxyhydroxide for epoxidation catalysis. Another notable phenomenological parallel to OER was revealed by the electron paramagnetic resonance (EPR) analysis of all spent Co-based catalysts that showed significant structural changes and the formation of paramagnetic Co(ii) and Co(iv) species. Mechanistic investigations suggest that a higher density of Co(ii) and/or an easier formation of high-valent Co species in the case of surface-oxidized cobalt sulfide is responsible for its high activity as an epoxidation catalyst. Our results provide important insight into the surface chemistry of Co-based catalysts and show the potential of oxidized CoS x as an earth-abundant catalyst for O-transfer reactivity beyond OER. This work highlights the utility of bridging electrocatalysis and thermocatalysis for the development of more sustainable chemical processes.

Stereoselective Unsymmetrical 1,1-Diborylation of Alkynes with a Neutral sp2 -sp3 Diboron Reagent

The incorporation of two distinct boryl groups at the same carbon center in organic molecules has attracted growing research interest due to its potential for facilitating controlled, precise synthesis through stepwise dual carbon-boron bond transformations. Here we report a method to access unsymmetrical 1,1-diborylalkene (UDBA) stereoselectively via the reaction of readily available alkynes with a neutral sp2 -sp3 diboron reagent (NHC)BH2 -Bpin (NHC=N-heterocyclic carbene). Attributing to the chemically easily distinguishable nature of the sp2 and sp3 boryl moieties, controllable stepwise derivatization of the resultant UDBAs is realized. This process leads to various multifunctionalized olefins and organoborons, such as acylboranes, which are difficult to prepare by other methods.

Mechanisms and Stereoselectivities in the NHC-Catalyzed [4 + 2] Annulation of 2-Bromoenal and 6-Methyluracil-5-carbaldehyde

To disclose the reaction mechanism and selectivity in the NHC-catalyzed reaction of 2-bromoenal and 6-methyluracil-5-carbaldehyde, a systematic computational study has been performed. According to DFT computations, the catalytic cycle is divided into eight elementary steps: nucleophilic attack of the NHC on 2-bromoenal, 1,2-proton transfer, C-Br bond dissociation, 1,3-proton transfer, addition to 6-methyluracil-5-carbaldehyde, [2 + 2] cycloaddition, NHC dissociation, and decarboxylation. The Bronsted acid DABCO·H+ plays a crucial role in proton transfer and decarboxylation steps. The addition to 6-methyluracil-5-carbaldehyde determines both chemoselectivity and stereoselectivity, leading to R-configured carbocycle-fused uracil, in agreement with experimental results. NCI analysis indicates that the CH···N, CH···π, and LP···π interactions should be the key factor for determining the stereoselectivity. ELF analysis shows the main role of the NHC in promoting C-Br bond dissociation. The mechanistic insights obtained in the present work may guide the rational design of potential NHC catalysts.

Ruthenium-Catalyzed C-C Coupling of Terminal Alkynes with Primary Alcohols or Aldehydes: α,β-Acetylenic Ketones (Ynones) via Oxidative Alkynylation

The first metal-catalyzed oxidative alkynylations of primary alcohols or aldehydes to form α,β-acetylenic ketones (ynones) are described. Deuterium labelling studies corroborate a novel reaction mechanism in which alkyne hydroruthenation forms a transient vinylruthenium complex that deprotonates the terminal alkyne to form the active alkynylruthenium nucleophile.

Recent progress in alkynylation with hypervalent iodine reagents

Although alkynes are one of the smallest functional groups, they are among the most versatile building blocks for organic chemistry, with applications ranging from biochemistry to material sciences. Alkynylation reactions have traditionally relied on the use of acetylenes as nucleophiles. The discovery and development of ethynyl hypervalent iodine reagents have allowed to greatly expand the transfer of alkynes as electrophilic synthons. In this feature article the progress in the field since 2018 will be presented. After a short introduction on alkynylation reactions and hypervalent iodine reagents, the developments in the synthesis of alkynyl hypervalent iodine reagents will be discussed. Their recent use in base-mediated and transition-metal catalyzed alkynylations will be described. Progress in radical-based alkynylations and atom-economical transformations will then be presented.

Metal-Free S-Arylation of Phosphorothioate Diesters and Related Compounds with Diaryliodonium Salts

We developed a direct metal-free S-arylation of phosphorothioate diesters using diaryliodonium salts. The method allows for the preparation under simple conditions of a broad range of S-aryl phosphorothioates, including complex molecules (e.g., dinucleotide or TADDOL derivatives), as well as other related organophosphorus compounds arylated at a chalcogen. The reaction proceeds with a full retention of the stereogenic center at the phosphorus atom, opening convenient access to P-chiral products. The mechanism of the reaction was established using DFT calculations.

Hypervalent iodine(iii) promoted C–H/C–H amination/annulation tandem reactions: synthesis of benzimidazoles from simple anilines and aldehydes

A novel hypervalent iodine mediated cascade transformation of anilines and aldehydes to benzimidazoles was developed.

Accessing Boron-Doped Pentaphene Analogues from 12-Boradibenzofluorene

Borole-doped polycyclic aromatic hydrocarbons (PAHs) have garnered attention in recent years due to their attractive photophysical properties and potential utility in electronic devices. In this work, a borole-doped PAH, 12-boradibenzofluorene, is synthesized and formal intermolecular nitrene and oxygen atom insertion reactions were employed to access 1,2-azaborine- and 1,2-oxaborine-containing analogues of the carbonaceous PAH pentaphene. Iodosobenzene is established as a versatile reagent for oxygen atom insertion reactions into a variety of borole species to access 1,2-oxaborine systems.

N-Heterocyclic Carbene-Catalyzed Cyclization of Aldehydes with α-Diazo Iodonium Triflate: Facile Access to 2,5-Disubstituted 1,3,4-Oxadiazoles

Herein, we report a novel organocatalytic process for synthesis of complex 1,3,4-oxadiazoles from readily accessible aldehydes. By exploiting the nucleophilicity of the putative Breslow intermediate and the inherent electrophilicity of α-diazo iodonium triflate, we have found that N-heterocyclic carbene catalyst promotes efficient cyclization of various aldehydes and α-diazo iodonium triflates. The reaction proceeds under mild conditions with a wide range of functional group tolerance. The heterocyclic products can be readily further functionalized, rendering the protocol highly valuable.

Tale of the Breslow intermediate, a central player in N-heterocyclic carbene organocatalysis: then and now

N-Heterocyclic carbenes (NHCs) belong to the popular family of organocatalysts used in a wide range of reactions, including that for the synthesis of complex natural products and biologically active compounds. In their organocatalytic manifestation, NHCs are known to impart umpolung reactivity to aldehydes and ketones, which are then exploited in the generation of homoenolate, acyl anion, and enolate equivalents suitable for a plethora of reactions such as annulation, benzoin, Stetter, Claisen rearrangement, cycloaddition, and C-C and C-H bond functionalization reactions and so on. A common thread that runs through these NHC catalyzed reactions is the proposed involvement of an enaminol, also known as the Breslow intermediate, formed by the nucleophilic addition of an NHC to a carbonyl group of a suitable electrophile. In the emerging years of NHC catalysis, enaminol remained elusive and was largely considered a putative intermediate owing to the difficulties encountered in its isolation and characterization. However, in the last decade, synergistic efforts utilizing an array of computational and experimental techniques have helped in gaining important insights into the formation and characterization of Breslow intermediates. Computational studies have suggested that a direct 1,2-proton transfer within the initial zwitterionic intermediate, generated by the action of an NHC on the carbonyl carbon, is energetically prohibitive and hence the participation of other species capable of promoting an assisted proton transfer is more likely. The proton transfer assisted by additives (such as acids, bases, other species, or even a solvent) was found to ease the kinetics of formation of Breslow intermediates. These important details on the formation, in situ detection, isolation, and characterization of the Breslow intermediate are scattered over a series of reports spanning well over a decade, and we intend to consolidate them in this review and provide a critical assessment of these developments. Given the central role of the Breslow intermediate in organocatalytic reactions, this treatise is expected to serve as a valuable source of knowledge on the same.

Synthesis of Ketones by C-H Functionalization of Aldehydes with Boronic Acids under Transition-Metal-Free Conditions

A method for the synthesis of ketones from aldehydes and boronic acids via a transition-metal-free C-H functionalization reaction is reported. The method employs nitrosobenzene as a reagent to drive the simultaneous activation of the boronic acid as a boronate and the activation of the C-H bond of the aldehyde as an iminium species that triggers the key C-C bond-forming step via an intramolecular migration from boron to carbon. These findings constitute a practical, scalable, and operationally straightforward method for the synthesis of ketones.

Mechanistic study on the NHC-catalyzed [3+4] annulation of enals and thiazolones

Reaction mechanisms and origins of regio- and stereo-selectivities of NHC catalyzed [3+4] annulation of enals and thiazolones.