Nickel-Catalyzed Borylation of Aryl- and Benzyltrimethylammonium Salts via C–N Bond Cleavage

Selective C-N Borylation of Alkyl Amines Promoted by Lewis Base

An efficient method for the metal-free deaminative borylation of alkylamines, using bis(catecholato)diboron as the boron source, to directly synthesize various alkyl potassium trifluoroborate salts is introduced. The key to this high reactivity is the utilization of pyridinium salt activated alkylamines, with a catalytic amount of a bipyridine-type Lewis base as a promoter. This transformation shows good functional-group compatibility (e.g., it is unimpeded by the presence of a ketone, indole, internal alkene, or unactivated alkyl chloride) and can serve as a powerful synthetic tool for borylation of amine groups in complex compounds. Mechanistic experiments and computations suggest a mechanism in which the Lewis base activated B₂ cat₂ unit intercepts an alkyl radical generated by single-electron transfer (SET) from a boron-based reductant.

Photoinduced Deaminative Borylation of Alkylamines

An operationally simple deaminative borylation reaction of primary alkylamines has been developed. The formation of electron-donor-acceptor complexes between N-alkylpyridinium salts and bis(catecholato)diboron enables photoinduced single-electron transfer and fragmentation to carbon-centered radicals, which are subsequently borylated. The mild conditions allow a diverse range of readily available alkylamines to be efficiently converted into synthetically valuable alkylboronic esters under catalyst-free conditions.

Vinylation of Benzylic Amines via C-N Bond Functionalization of Benzylic Pyridinium Salts

Cross-couplings of benzylic pyridinium salts and vinylboronic acids or esters have been developed. Via these benzylic pyridinium intermediates, benzylic amines can be engaged in these cross-couplings via C-N bond functionalization. This method boasts mild reaction conditions and excellent tolerance for heteroaryl substituents and a range of functional groups.

Transforming Benzylic Amines into Diarylmethanes: Cross-Couplings of Benzylic Pyridinium Salts via C-N Bond Activation

A nickel-catalyzed cross-coupling of benzylic pyridinium salts with arylboronic acids was developed. Coupled with chemoselective pyridinium formation, this method allows benzyl primary amines to be efficiently converted to di(hetero)arylmethanes. Excellent heteroaryl and functional group tolerance is observed, and a one-pot procedure enables benzylic amines to be converted to diarylmethanes directly.

Chemoselective Benzylation of Aldehydes Using Lewis Base Activated Boronate Nucleophiles

A benzylation of aldehydes using primary and secondary benzylboronic acid pinacol esters is reported. Activation of the boronic ester with s-butyllithium rendered it nucleophilic toward aldehydes. The activated nucleophile chemoselectively transfers the benzyl group over the s-butyl group, providing excellent yields of the benzylated products. ¹¹B NMR experiments were performed to study the mechnism of this transformation.

Cross-coupling polycondensation via C-O or C-N bond cleavage

π-Conjugated polymers are widely used in optoelectronics for fabrication of organic photovoltaic devices, organic light-emitting diodes, organic field effect transistors, and so on. Here we describe the protocol for polycondensation of bifunctional aryl ethers or aryl ammonium salts with aromatic dimetallic compounds through cleavage of inert C-O/C-N bonds. This reaction proceeds smoothly in the presence of commercially available Ni/Pd catalyst under mild conditions, affording the corresponding π-conjugated polymers with high molecular weight. The method is applicable to monomers that are unreactive in other currently employed polymerization procedures, and opens up the possibility of transforming a range of naturally abundant chemicals into useful functional compounds/polymers.

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 decarbonylative borylation of aroyl fluorides

A transformation of aroyl fluorides with diboron via nickel-catalysed carbon–fluorine bond cleavage and a sequential decarbonylation, which provides an efficient protocol to functionalize arylboronates, has been reported.

Organozinc-Mediated Direct C-C Bond Formation via C-N Bond Cleavage of Ammonium Salts

We report a direct cross-coupling reaction between diarylzinc (Ar₂ Zn) and aryltrimethylammonium salts (ArNMe₃⁺ ⋅^- OTf) in the presence of LiCl, via C-N bond cleavage. The reaction takes place smoothly upon heating in THF without any external catalyst, enabling an efficient and chemoselective formation of biaryl products. Mechanistic studies indicate that the reaction proceeds through a single electron transfer route.

NH4I/tert-Butyl Hydroperoxide-Promoted Oxidative C–N Cleavage of Tertiary Amines Leading to Nitroaromatic Compounds

A NH4I/ tert-butyl hydroperoxide-promoted oxidation of tertiary N-aryl- N,N-dialkylamines in DMSO has been developed to access nitroaromatic compounds. This methodology involves sequential N-dealkylation reactions in one-pot and a radical pathway is proposed.

Decarboxylative borylation

The widespread use of alkyl boronic acids and esters is frequently hampered by the challenges associated with their preparation. We describe a simple and practical method to rapidly access densely functionalized alkyl boronate esters from abundant carboxylic substituents. This broad-scope nickel-catalyzed reaction uses the same activating principle as amide bond formation to replace a carboxylic acid moiety with a boronate ester. Application to peptides allowed expedient preparations of α-amino boronic acids, often with high stereoselectivity, thereby facilitating synthesis of the alkyl boronic acid drugs Velcade and Ninlaro as well as a boronic acid version of the iconic antibiotic vancomycin. The reaction also enabled the discovery and extensive biological characterization of potent human neutrophil elastase inhibitors, which offer reversible covalent binding properties.

Nickel-catalysed retro-hydroamidocarbonylation of aliphatic amides to olefins

Amide and olefins are important synthetic intermediates with complementary reactivity which play a key role in the construction of natural products, pharmaceuticals and manmade materials. Converting the normally highly stable aliphatic amides into olefins directly is a challenging task. Here we show that a Ni/NHC-catalytic system has been established for decarbonylative elimination of aliphatic amides to generate various olefins via C-N and C-C bond cleavage. This study not only overcomes the acyl C-N bond activation in aliphatic amides, but also encompasses distinct chemical advances on a new type of elimination reaction called retro-hydroamidocarbonylation. This transformation shows good functional group compatibility and can serve as a powerful synthetic tool for late-stage olefination of amide groups in complex compounds.

Harnessing Alkyl Amines as Electrophiles for Nickel-Catalyzed Cross Couplings via C-N Bond Activation

We developed a strategy to harness alkyl amines as alkylating agents via C-N bond activation. This Suzuki-Miyaura cross coupling of alkylpyridinium salts, readily formed from primary amines, is the first example of a metal-catalyzed cross coupling via C-N bond activation of an amine with an unactivated alkyl group. This reaction enjoys broad scope and functional group tolerance. Primary and secondary alkyl groups can be installed. Preliminary studies suggest a NiI/NiIII catalytic cycle.

Convenient and General Zinc-Catalyzed Borylation of Aryl Diazonium Salts and Aryltriazenes under Mild Conditions

A convenient and general zinc‐catalyzed borylation of aryl diazonium salts and aryltriazenes has been developed. With bis‐ (pinacolato)diboron as the borylation reagent, aryldiazonium tetrafluoroborate salts and aryltriazenes were transformed into the corresponding arylboronates in moderate to excellent yields under mild conditions. As a convenient and practical methodology, no additional ligands, base, or any other additives are required here.

Rhodium-Catalyzed Decarbonylative Borylation of Aromatic Thioesters for Facile Diversification of Aromatic Carboxylic Acids

Transformation of aromatic thioesters into arylboronic esters was achieved efficiently using a rhodium catalyst. The broad functional-group tolerance and mild conditions of the method have allowed for the two-step decarboxylative borylation of a wide range of aromatic carboxylic acids, including commercially available drugs.

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.

Stille coupling via C-N bond cleavage

Cross-coupling is a fundamental reaction in the synthesis of functional molecules, and has been widely applied, for example, to phenols, anilines, alcohols, amines and their derivatives. Here we report the Ni-catalysed Stille cross-coupling reaction of quaternary ammonium salts via C–N bond cleavage. Aryl/alkyl-trimethylammonium salts [Ar/R–NMe₃]⁺ react smoothly with arylstannanes in 1:1 molar ratio in the presence of a catalytic amount of commercially available Ni(cod)₂ and imidazole ligand together with 3.0 equivalents of CsF, affording the corresponding biaryl with broad functional group compatibility. The reaction pathway, including C–N bond cleavage step, is proposed based on the experimental and computational findings, as well as isolation and single-crystal X-ray diffraction analysis of Ni-containing intermediates. This reaction should be widely applicable for transformation of amines/quaternary ammonium salts into multi-aromatics.

The Stille reaction is a long-standing method for the construction of carbon-carbon bonds by coupling aryl halides with organotin compounds. Here the authors report a Stille coupling between aryl ammonium salts and arylstannanes catalysed by nickel.

Palladium-catalyzed regioselective and stereo-invertive ring-opening borylation of 2-arylaziridines with bis(pinacolato)diboron: experimental and computational studies

A palladium catalyzed regioselective borylative ring opening reaction of 2-arylaziridines to give β-amino-β-arylethylborates was developed. The reaction reported herein represents the first example of ring-opening borylation of non-vinylic aziridines and direct borylative C(sp3)-N bond cleavage of neutral organic substrates. NMR studies and density functional theory (DFT) calculations suggested that the active intermediate for the reaction is a PdL2 complex [L = P(t-Bu)2Me]. The multi-component artificial force-induced reaction method (MC-AFIR) located the transition states for the regioselectivity-determining aziridine ring opening that proceeds in an SN2 fashion, and explained the selectivity of the reaction. The full catalytic cycle consists of a selectivity-determining aziridine ring opening (oxidative addition), a proton transfer, phosphine ligand dissociation from the catalyst, boron-boron bond cleavage, and reductive elimination. Water is important to the drive the transmetalation step. The calculated overall mechanism and selectivity are consistent with the experimental results.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B-C bonds and other processes.