Strategies for Using Quaternary Ammonium Salts as Alternative Reagents in Alkylations

Alkylation reactions are pivotal in organic chemistry, with wide-ranging utilization across various fields of applied synthetic chemistry. However, conventional reagents employed in alkylations often pose substantial health and exposure risks. Quaternary ammonium salts (QAS) present a promising alternative for these transformations offering significantly reduced hazards as they are non-cancerogenic, non-mutagenic, non-flammable, and non-corrosive. Despite their potential, their use in direct organic transformations remains relatively unexplored. This review outlines strategies for utilizing QAS as alternative reagents in alkylation reactions, providing researchers with safer approaches to chemical synthesis.

Cobalt-Catalyzed Deaminative Amino- and Alkoxycarbonylation of Aryl Trialkylammonium Salts Promoted by Visible Light

Catalytic carbonylations of aryl electrophiles via C(sp2 )-N cleavage remains a significant challenge. Herein, we demonstrate an aminocarbonylation of aniline-derived trialkylammonium salts promoted by visible light with a simple cobalt catalyst. The reaction proceeds under mild conditions suitable for late-stage functionalization and is amenable to telescoped carbonylations directly from anilines. A range of alkylamines are successful partners, and alkoxycarbonylation is also demonstrated. Mechanistic studies and DFT calculations support a novel mechanism for catalytic carbonylations of aryl electrophiles involving a key visible light-induced carbonyl photodissociation.

Ruthenium-Catalyzed Isomerization of ortho-Silylanilines to Their para Isomers

The catalytic ortho to para transposition of a silyl group in aniline derivatives is described. [RuCl₂(p-cymene)]₂/BINAP in conjunction with a Cu(OAc)₂ additive serves as a potent catalytic system. This method is also applicable to the isomerization of 2-silylpyrrole derivatives to the corresponding 3-silyl isomers.

Palladium-Catalyzed Stereoselective Difunctionalization of Bicyclic Alkenes with Organoammonium Salts and Organoboronic Compounds

A palladium-catalyzed difunctionalization of bicyclic alkenes with organoammonium salts and organoboronic compounds was reported. A wide range of functionalized cyclic products, including those bearing functional groups, were produced stereoselectively in good to excellent yields. The gram-scale experiment, one-pot operation, and synthetic application of β-borylated products further demonstrated the synthetic value of this new reaction in organic synthesis.

Trialkylammonium salt degradation: implications for methylation and cross-coupling

Trialkylammonium (most notably N,N,N-trimethylanilinium) salts are known to display dual reactivity through both the aryl group and the N-methyl groups. These salts have thus been widely applied in cross-coupling, aryl etherification, fluorine radiolabelling, phase-transfer catalysis, supramolecular recognition, polymer design, and (more recently) methylation. However, their application as electrophilic methylating reagents remains somewhat underexplored, and an understanding of their arylation versus methylation reactivities is lacking. This study presents a mechanistic degradation analysis of N,N,N-trimethylanilinium salts and highlights the implications for synthetic applications of this important class of salts. Kinetic degradation studies, in both solid and solution phases, have delivered insights into the physical and chemical parameters affecting anilinium salt stability. ¹H NMR kinetic analysis of salt degradation has evidenced thermal degradation to methyl iodide and the parent aniline, consistent with a closed-shell S_N2-centred degradative pathway, and methyl iodide being the key reactive species in applied methylation procedures. Furthermore, the effect of halide and non-nucleophilic counterions on salt degradation has been investigated, along with deuterium isotope and solvent effects. New mechanistic insights have enabled the investigation of the use of trimethylanilinium salts in O-methylation and in improved cross-coupling strategies. Finally, detailed computational studies have helped highlight limitations in the current state-of-the-art of solvation modelling of reaction in which the bulk medium undergoes experimentally observable changes over the reaction timecourse.

The dual reactivity of N,N,N-trimethylanilinium salts towards arylation and methylation is decoupled in this mechanistic investigation to enable more strategic application of these salts in either reaction class.

Palladium-catalyzed direct reductive cross-coupling of aryltrimethylammonium salts with aryl bromides

Palladium-catalyzed direct reductive cross-coupling of aryltrimethylammonium salts with aryl bromides proceeded efficiently in a one-pot manner in the presence of Mg turnings, LiCl, and TMEDA in THF to afford the corresponding biaryl compounds.

Chemical transformations of quaternary ammonium salts via C–N bond cleavage

The reaction of quaternary ammonium salts via C–N bond cleavage to construct C–C, C–H and C–heteroatom bonds is summarized.

Synthesis and characterization of new square planar heteroleptic cationic complexes [Ni(ii) β-oxodithioester-dppe]+; their use as a catalyst for Chan–Lam coupling

Six new structurally characterized heteroleptic cationic [Ni(ii) β-oxodithioester-dppe]⁺PF₆⁻ complexes as efficient catalysts for Chan–Lam coupling reaction affording N-arylated products with significant functional group tolerance.

Double C-N bond cleavages of N-alkyl 4-oxopiperidinium salts: access to unsymmetrical tertiary sulfonamides

In this paper, regiospecific, double intraannular C-N bond cleavages of N-alkyl 4-oxopiperidinium salts are presented. The reaction sequence involves a charge-transfer complex, in situ formed between sulfonyl chloride and N-methylmorpholine, which induces S-Cl bond homolysis of sulfonyl chloride, yielding a reactive sulfonyl radical that further induces the double C-N bond cleavages of N-alkyl 4-oxopiperidinium salt. The secondary amine thus produced was trapped by sulfonyl chloride to yield the desired sulfonamide product. The key feature of this protocol is that two intraannular C-N bonds of the 4-oxopiperidine ring are cleaved in one step under metal- and oxidant-free conditions.

Non-transition Metal-Mediated Diverse Aryl-Heteroatom Bond Formation of Arylammonium Salts

Aryl–heteroatom (C–X) bonds ubiquitously exist in organic, medicinal, and material chemistry, but a universal method to construct diverse C–X bonds is lacking. Here we report our discovery of a convenient and efficient approach to construct various C–X bonds using arylammonium salts as the substrate via an S_NAr process. This strategy features mild reaction condition, no request of transition metal catalyst, and easy formation of various C–X bonds (C–S, C–Si, C–Sn, C–Ge, C–Se, C–N). The method was successfully applied to a late-stage functionalization of an existing antibiotic drug, to a Clickable reaction of NBD-based ammonium salt as turn-on fluorescent probe to recognize L-cysteine and homocysteine, and to the synthesis of a DNA encoded library (DEL) bearing different C–X bonds.

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An efficient approach to construct various C–heteroatom bonds

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Readily accessible ammonium salts as substrates

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No request of transition metal catalyst and broad functional group compatibility

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Great applicability in late-stage functionalization, fluorescent probe, and DEL

Catalytic reduction of aryl trialkylammonium salts to aryl silanes and arenes

Aryl trialkylammonium salts serve as versatile substrates for nickel-catalyzed reductions, allowing access to functionalized arenes and aryl silanes.

A new approach for the reduction of aryl ammonium salts to arenes or aryl silanes using nickel catalysis is reported. This method displays excellent ligand-controlled selectivity based on the N-heterocyclic carbene (NHC) ligand employed. Utilizing a large NHC in non-polar solvents generates aryl silanes, while small NHCs in polar solvents promote reduction to arenes. Several classes of aryl silanes can be accessed from simple aniline building blocks, including those useful for cross-couplings, oxidations, and halogenations. The reaction conditions are mild, functional group tolerant, and provide efficient access to a variety of benzene derivatives.

Asymmetric synthesis via stereospecific C-N and C-O bond activation of alkyl amine and alcohol derivatives

This perspective showcases our development of benzylic and allylic amine and alcohol derivatives as electrophiles for stereospecific, nickel-catalyzed cross-coupling reactions, as well as the prior art that inspired our efforts. The success of our effort has relied on the use of benzyl ammonium triflates as electrophiles for cross-couplings via C-N bond activation and benzylic and allylic carboxylates for cross-couplings via C-O bond activation. Our work, along with others' exciting discoveries, has demonstrated the potential of stereospecific, nickel-catalyzed cross-couplings of alkyl electrophiles in asymmetric synthesis, and enables efficient generation of both tertiary and quaternary stereocenters.

Formal group insertion into aryl C‒N bonds through an aromaticity destruction-reconstruction process

Given the abundance and the ready availability of anilines, the selective insertion of atoms into the aryl carbon–nitrogen bonds will be an appealing route for the synthesis of nitrogen-containing aromatic molecules. However, because aryl carbon–nitrogen bonds are particularly inert, anilines are normally activated by conversion to more reactive intermediates such as aryldiazonium salts to achieve functionalization of the aryl carbon–nitrogen bonds, but the nitrogen atom is usually not incorporated into products, instead being discarded. The selective insertion of groups into aryl carbon–nitrogen bonds remains an elusive challenge and an unmet need in reaction design. Here we show an aromaticity destruction-reconstruction process that selectively inserts a trimethylenemethane (TMM) group into the aromatic carbon–nitrogen bond of anilines concomitant with a benzylic carbon–hydrogen bond functionalization. This process provides a transformative mode for anilines, and the insertion products are versatile precursor to various nitrogen-containing aromatic molecules through simple conversions.

Activation of C–N bonds of anilines requires transformation of the amino group into a more reactive functionality. Here, the authors report an aromaticity destruction-reconstruction process that converts abundant anilines into valuable amines through group insertion into the C–N bond and benzylic C–H functionalization.

Palladium-catalyzed carbonylation of benzylic ammonium salts to amides and esters via C–N bond activation

An efficient palladium-catalyzed carbonylation reaction of readily available quaternary ammonium salts with CO is reported for the first time to afford arylacetamides and arylacetic acid esters via benzylic C–N bond cleavage.

Nickel-Catalysed Synthesis of 17-Arylandrosta-5,16-Dienes

An efficient Ni-catalysed coupling reaction between arylboronic acids and 17-trifluoromethanesulfonyl- 3β-acetoxyandrosta-5,16-diene which was obtained by the sulfonylation of 3β-acetoxylandrost-5-ene-17-one was developed to afford 17-aryl- 3β-acetoxy-androsta-5,16-dienes in moderate to good yields (52–85%). The structure of the 3,4-dimethoxyaryl product was confirmed by X-ray crystallography.

Breaking Amides using Nickel Catalysis

Amides have been widely studied for decades, but their synthetic utility has remained limited in reactions that proceed with rupture of the amide C-N bond. Using Ni catalysis, we have found that amides can now be strategically employed in several important transformations: esterification, transamidation, Suzuki-Miyaura couplings, and Negishi couplings. These methodologies provide exciting new tools to build C-heteroatom and C-C bonds using an unconventional reactant (i.e., the amide), which is ideally suited for use in multi-step synthesis. It is expected that the area of amide C-N bond activation using nonprecious metals will continue to flourish and, in turn, will promote the growing use of amides as synthons in organic synthesis.

Palladium catalyzed Suzuki cross-coupling of benzyltrimethylammonium salts via C–N bond cleavage

A Pd catalyzed Suzuki cross-coupling of a benzyltrimethylammonium salt is described. This reaction offers a highly efficient approach to diarylmethanes and also paves the way for the application of benzyltrimethylammonium salts in Pd catalyzed cross-coupling reactions.

Theoretical study on the reaction mechanism of “ligandless” Ni-catalyzed hydrodesulfurization of aryl sulfide

The reaction mechanism of Ni(COD)₂ catalyzed hydrodesulfurization of aryl sulfide PhSMe with HSiMe₃ has been predicted to have two competitive reaction pathways, with or without PhSMe spectator ligand, by using density functional theory methods.

Nickel-catalyzed Chan–Lam cross-coupling: chemoselective N-arylation of 2-aminobenzimidazoles

A complementary set of Ni- and Cu-based catalyst systems for the chemoselective N-arylation of 2-aminobenzimidazoles have been developed.

Nickel-catalyzed C–N bond reduction of aromatic and benzylic quaternary ammonium triflates

A nickel-catalyzed, efficient C–N bond reduction of aromatic and benzylic ammonium triflates has been developed using sodium isopropoxide as a reducing agent.

Conversion of amides to esters by the nickel-catalysed activation of amide C-N bonds

Amides are common functional groups that have been studied for more than a century. They are the key building blocks of proteins and are present in a broad range of other natural and synthetic compounds. Amides are known to be poor electrophiles, which is typically attributed to the resonance stability of the amide bond. Although amides can readily be cleaved by enzymes such as proteases, it is difficult to selectively break the carbon-nitrogen bond of an amide using synthetic chemistry. Here we demonstrate that amide carbon-nitrogen bonds can be activated and cleaved using nickel catalysts. We use this methodology to convert amides to esters, which is a challenging and underdeveloped transformation. The reaction methodology proceeds under exceptionally mild reaction conditions, and avoids the use of a large excess of an alcohol nucleophile. Density functional theory calculations provide insight into the thermodynamics and catalytic cycle of the amide-to-ester transformation. Our results provide a way to harness amide functional groups as synthetic building blocks and are expected to lead to the further use of amides in the construction of carbon-heteroatom or carbon-carbon bonds using non-precious-metal catalysis.

Highly efficient pincer nickel catalyzed cross-coupling of aryltrimethylammonium triflates with arylzinc reagents

N,N,P-Pincer nickel complexes [Ni(Cl){N(2-R₂PC₆H₄)(2′-Me₂NC₆H₄)}] highly effectively catalyze the cross-coupling of aryltrimethylammonium triflates with arylzinc reagents.