Bicyclic (alkyl)(amino)carbene (BICAAC) in a dual role: activation of primary amides and CO2 towards catalytic N-methylation

Singlet Carbenes Are Stereoinductive Main Group Ambiphiles

Stereogenic units are a critical source of molecular complexity, but their stereoselective formation via main group ambiphiles─which are suitable for derivatizing a wide scope of functionalities─is largely unexplored. Herein, using chiral cyclic (alkyl)(amino)carbenes (ChiCAACs), we study stereoinduction during the oxidative addition of E-H σ-bonds (E = C, N, O, Si, P). Through computational modeling, the relationship between stereochemical outcome and mechanism is elucidated, providing insight into when and why ChiCAACs exhibit excellent stereoselectivities. Altogether, these results demonstrate the potential for chiral main group ambiphiles to generate stereogenic units in a highly controlled manner opening avenues for applying "metal-like" reactivity in metal-free asymmetric syntheses.

Metal-Free Catalytic N-Methylation of NH-Sulfoximines Using CO2

This study reports the catalytic N-methylation of NH-sulfoximines using carbon dioxide (CO2) under metal-free conditions. A mesoionic N-heterocyclic olefin (mNHO) was used as a catalyst for the N-methylation of NH-sulfoximines in the presence of 9-borabicyclo[3.3.1]nonane (9-BBN) under mild conditions. This process was used to convert various NH-sulfoximines into N-methylsulfoximines. This protocol was also applicable for the synthesis of 13C-labeled N-methylsulfoximines using 13CO2. A mechanistic cycle was proposed by performing a series of control reactions and successfully characterizing active catalytic intermediates by either single-crystal X-ray or NMR spectroscopic studies.

Iridium-Catalyzed, Copper-Induced Reductive Cyclization of NO2-Pyrrolarenes with CO2 as a Single-Carbon Source

A new catalytic conversion type of nitro substrate with CO2 as a single-carbon source is presented, wherein a great collection of azaheterocycles is generated by a newly established iridium-catalyzed, copper-induced reductive system. This catalytic procedure handily employs poly(methylhydrosiloxane) (PMHS) as the reductant to simultaneously realize the dual reduction of the highest oxidation-state nitro and CO2 units in one operation. Elaborate mechanistic studies illustrate the essential role of the iridium catalyst in reducing the NO2 moiety as well as the double functions of the copper additive in the subsequent formylation and C-H cyclization steps.

Synthesis of N-methyl secondary amides via diboronic acid anhydride-catalyzed dehydrative condensation of carboxylic acids with aqueous methylamine

In this study, we present the first catalytic methodology for synthesizing N-methyl secondary amides via dehydrative condensation of hydroxycarboxylic acids with readily available and safe aqueous methylamine, employing diboronic acid anhydride (DBAA) as the catalyst. DBAA catalysis can also be applied to direct amidations using aqueous ethylamine or aqueous dimethylamine. Moreover, we demonstrate the applicability of this catalytic system for the concise synthesis of eight biologically active compounds containing β-amino alcohol motifs, including halostachine, synephrine, longimammine, phenylephrine, metanephrine, normacromerine, etilefrine, and macromerine.

Cyclic (Alkenyl)(Amino)Carbene (SMeCAenAC): Introducing a Member to the Cyclic (Alkyl)(Amino)Carbenes Family Featuring a Narrow Energy Gap

Herein, we report the carbene-like activity of a nonisolable, highly ambiphilic cyclic (alkenyl)(amino)carbene (SMeCAenAC, 3), which is stabilized as [(SMeCAenAC)(H)N(SiMe3)2] (4). This protected form (4) is stable in air and moisture. Compound 4 can be used as a carbene source through deamination upon heating to 130-140 °C. Moreover, density functional theory (DFT) calculations indicate that SMeCAenAC has the smallest singlet-triplet gap (37.05 kcal/mol) and a narrow highest occupied molecule orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap (3.92 eV) among the cyclic (alkyl)(amino)carbenes (CAACs). The precursor of carbene (3) can be synthesized on a multigram scale with a good yield. Moreover, the SMeCAenAC-coordinated copper complex showed excellent efficiency in the catalytic addition of phenols to electron-deficient olefins. This study also highlights that [SMeCAenAC-H]OTf can be used for metal-free catalysis, a property uniquely characteristic of an ambiphilic carbene, even though the formation of free SMeCAenAC (3) was not achieved.

Substrate-Controlled Divergent Reductive Cyclization of 2-Arylanilines Using CO2 as a Switching Reagent

Capturing CO2 is highly valued in the field of organic synthesis, especially underdeveloped dual-CO2 conversion. In this study, we detail a novel reductive cyclization of 2-indolylanilines with dual CO2 as a difunctional reagent in the presence of PMHS [poly(methylhydrosiloxane)], delivering methyl-substituted quinoxalines. Furthermore, another chemoselective cyclization with 2-pyrrolylanilines is also realized by converting mono-CO2. Mechanistic investigations shed light upon the fact that this substrate-controlled divergence mainly depends on the formation of N-diacylative intermediates.

Bicyclic (alkyl)(amino)carbenes (BICAACs): synthesis, characteristics, and applications

Carbenes in general and isolable NHCs (N-heterocyclic carbenes) in particular have been useful ligands in recent years. The emergence of CAACs [cyclic(alkyl)(amino)carbenes], BICAACs [bicyclic(alkyl)(amino)carbenes], and many other carbenes has marked revolutionary milestones in this field. These carbenes possess an intriguing blend of highly electrophilic and nucleophilic characteristics, owing to their remarkably narrow HOMO-LUMO energy gap. The isolation and characterization of these carbenes hold significance not only due to their fascinating electronic properties but have demonstrated their prowess across various domains, including isolation of transition metal complexes, medicinal applications, catalysis, and radical stabilization. While the chemistry of 5-membered NHCs and CAACs has been extensively explored, the investigation of BICAACs has just begun. This review covers the synthesis, characterization, and reactivity of BICAACs and outlines the diverse applications of BICAACs in organometallic chemistry, metal-free catalysis, and main-group chemistry.

Air and Water Stable Bicyclic (Alkyl)(Amino)Carbene Stabilized Phosphenium Cation: Reactivity and Selective Fluoride Ion Affinity

The synthesis and reactivity of an air and water stable Bicyclic (alkyl)(amino)carbene (BICAAC) stabilized phosphenium cation (1) is reported. Air and water stable phosphenium cation are rare in the literature. Compound 1 is obtained by reaction of BICAAC with Ph2PCl in THF followed by anion exchange with LiOTf. The reduction and oxidation of 1 yielded corresponding α-radical phosphine species (2) and BICAAC stabilized phosphenium oxide (3) respectively. All compounds are well characterized by single crystal X-ray diffraction studies. The Lewis acidity of compounds 1 and 3 are determined by conducting fluoride ion affinity experiments using UV-Vis spectrophotometry and multinuclei NMR spectroscopy. Compounds 1 and 3 exhibited selective binding to fluoride anion but did not interact with other halides (Cl- and Br-). Quantum chemical calculations were performed to understand the structure and nature of bonding interactions in these compounds, as well as to comprehend the specific bonding affinity to fluoride over other halide ions.

A Guide for Mono-Selective N-Methylation, N-Ethylation, and N-n-Propylation of Primary Amines, Amides, and Sulfonamides and Their Applicability in Late-Stage Modification

This review provides a comprehensive overview of mono-alkylation methodologies targeting crucial nitrogen moieties - amines, amides, and sulfonamides - found in organic building blocks and pharmaceuticals. Emphasizing the intersection of chemical precision with drug discovery, the central challenge addressed is achieving one-pot mono-selective short-chain N-alkylations (methylations, ethylations, and n-propylations), preventing undesired overalkylation. Additionally, sustainable, safe, and benign alternatives to traditional alkylating agents, including alcohols, carbon dioxide, carboxylic acids, nitriles, alkyl phosphates, quaternary ammonium salts, and alkyl carbonates, are explored. This review, categorized by the nature of the alkylating agent, aids researchers in selecting suitable methods for mono-selective N-alkylation.

DFT Study of Functional Reduction of CO2 with BH3NMe3: The Real Role of Organic Catalyst TBD

The detailed mechanism of transition metal-free-catalyzed monomethylation of 2-naphthyl acetonitrile (1a) with CO2 in the presence of triazabicyclodecene (TBD) and BH3NMe3 was investigated using density functional theory. The C-methylation process proved to generate formaldehyde followed by the formation of the product via an alcohol rather than a methoxyborane intermediate. During the reaction, CO2 is activated to form the TBD-CO2 adduct and BH3NMe3 is changed into TBD-BH2 (IM2) in the presence of TBD. IM2 plays a real reducing role within the system due to the unique coordination capability of the B atom. In addition to enhancing the nucleophilicity of 1a through deprotonation by tBuOK, our research also indicates that the generated tBuOH not only assists in proton transfer to generate an alcohol intermediate but also promotes the regeneration of TBD.

A Lewis Acid-Base Pair Catalyzed Dearomative Transformation of Unprotected Indoles via B-H Bond Activation

A sustainable and metal-free protocol has been described for the reduction of unprotected indoles. The catalytic system consists of B(C6 F5 )3 and THF as a Lewis acid-base pair that can activate the B-H bond of pincolborane (HBpin). The catalytic system encompasses a broad substrate scope. Control experiments were conducted to understand the possible catalytic intermediates involved during the present protocol.