Transition‐Metal‐Free Three‐Component Synthesis of Tertiary Aryl Amines from Nitro Compounds, Boronic Acids, and Trialkyl Phosphites

Acid-Promoted Amination of Cyclohexenone for the Divergent Synthesis of p-Aminophenols and Tertiary Amines

A tunable method for the selective preparation of p-aminophenol and tertiary amines from a secondary amine and cyclohexenone has been described. Nonaromatic cyclohexenones were used as an aryl source. The desired tertiary amine products were generated when using I2 as the catalyst. This approach yields single-site-selective p-aminophenol without using I2, and the 18O labeling experiments demonstrated that hydroxyl oxygen originates from O2.

Reductive coupling of nitro compounds with boronic acid derivatives: an overview

The purpose of this review is to summarize the current literature on reductive C-N coupling of nitro compounds and boronic acids, with special emphasis on the mechanistic features of the reactions. The metal-catalyzed reactions are discussed first. This is followed by electro-synthesis and organophosphorus-catalyzed reactions. Finally, the available examples of catalyst-free reactions will be covered at the end of this review.

A facile, one-pot reductive alkylation of aromatic and heteroaromatic amines in aqueous micellar media: a chemoenzymatic approach

A facile, green, selective and practical method for the catalytic N-alkylation of amines using molecular hydrogen as the reductant was developed. This procedure involves a lipase-mediated one-pot chemoenzymatic cascade wherein an amine undergoes a reductive amination with an aldehyde generated in situ. The imine formed thereby is reduced to give the corresponding amine. This process represents a convenient, environmentally benign and scalable one-pot process for the synthesis of N-alkyl amines. We report for the first time chemoenzymatic reductive alkylation in aqueous micellar media with an E-factor of 0.68.

Enabling Reductive C-N Cross-Coupling of Nitroalkanes and Boronic Acids by Steric Design of P(III)/P(V)═O Catalysts

An organophosphorus-catalyzed C-N bond-forming reductive coupling of nitroalkanes with arylboronic acids and esters is reported. The method shows excellent chemoselectivity for the nitro/boronic acid substrate pair, allowing the synthesis of N-(hetero)arylamines rich in functionalization. The identification of a sterically reduced phosphetane catalyst capable of productive coupling in the P(III)/P(V)═O redox manifold is the key enabling development. Combined experimental kinetics and computational mechanistic studies show that the sterically reduced catalyst affects post-rate-limiting steps to enable the C-N coupling event in preference to deleterious side-paths.

Visible-Light- and PPh3-Mediated Direct C-N Coupling of Nitroarenes and Boronic Acids at Ambient Temperature

We present here a metal-free, visible-light- and triphenylphosphine-mediated intermolecular, reductive amination between nitroarenes and boronic acids at ambient temperature without any photocatalyst. Mechanistically, a slow reduction of nitroarenes to a nitroso and, finally, a nitrene intermediate occurs that leads to the amination product with concomitant 1,2-aryl/-alkyl migration from a boronate complex. A wide range of nitroarenes underwent C-N coupling with aryl-/alkylboronic acids providing high yields.

Saturated Boronic Acids, Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

This review discusses recent advances in the chemistry of saturated boronic acids, boronates, and trifluoroborates. Applications of the title compounds in the design of boron-containing drugs are surveyed, with special emphasis on α-amino boronic derivatives. A general overview of saturated boronic compounds as modern tools to construct C(sp³ )-C and C(sp³ )-heteroatom bonds is given, including recent developments in the Suzuki-Miyaura and Chan-Lam cross-couplings, single-electron-transfer processes including metallo- and organocatalytic photoredox reactions, and transformations of boron "ate" complexes. Finally, an attempt to summarize the current state of the art in the synthesis of saturated boronic acids, boronates, and trifluoroborates is made, with a brief mention of the "classical" methods (transmetallation of organolithium/magnesium reagents with boron species, anti-Markovnikov hydroboration of alkenes, and the modification of alkenyl boron compounds) and a special focus on recent methodologies (boronation of alkyl (pseudo)halides, derivatives of carboxylic acids, alcohols, and primary amines, boronative C-H activation, novel approaches to alkene hydroboration, and 1,2-metallate-type rearrangements).

NaI/PPh3-Mediated Photochemical Reduction and Amination of Nitroarenes

A mild transition-metal- and photosensitizer-free photoredox system based on the combination of NaI and PPh₃ was found to enable highly selective reduction of nitroarenes. This protocol tolerates a broad range of reducible functional groups such as halogen (Cl, Br, and even I), aldehyde, ketone, carboxyl, and cyano. Moreover, the photoredox catalysis with NaI and stoichiometric PPh₃ provides also an alternative entry to Cadogan-type reductive amination when o-nitrobiarenes were used.

Ruthenium-catalysed chemoselective alkylation of nitroarenes with alkanols

The alkylation of nitroarenes with akanols catalysed by the phosphinesulfonate ruthenium complex was reported. It displays different reactivity and chemoselectivity depending on the acid–base conditions, delivering diverse anilines from nitroarenes.

An Improved PIII/PV═O-Catalyzed Reductive C-N Coupling of Nitroaromatics and Boronic Acids by Mechanistic Differentiation of Rate- and Product-Determining Steps

Experimental, spectroscopic, and computational studies are reported that provide an evidence-based mechanistic description of an intermolecular reductive C–N coupling of nitroarenes and arylboronic acids catalyzed by a redox-active main-group catalyst (1,2,2,3,4,4-hexamethylphosphetane P-oxide, i.e., 1·[O]). The central observations include the following: (1) catalytic reduction of 1·[O] to P^III phosphetane 1 is kinetically fast under conditions of catalysis; (2) phosphetane 1 represents the catalytic resting state as observed by ³¹P NMR spectroscopy; (3) there are no long-lived nitroarene partial-reduction intermediates observable by ¹⁵N NMR spectroscopy; (4) the reaction is sensitive to solvent dielectric, performing best in moderately polar solvents (viz. cyclopentylmethyl ether); and (5) the reaction is largely insensitive with respect to common hydrosilane reductants. On the basis of the foregoing studies, new modified catalytic conditions are described that expand the reaction scope and provide for mild temperatures (T ≥ 60 °C), low catalyst loadings (≥2 mol%), and innocuous terminal reductants (polymethylhydrosiloxane). DFT calculations define a two-stage deoxygenation sequence for the reductive C–N coupling. The initial deoxygenation involves a rate-determining step that consists of a (3+1) cheletropic addition between the nitroarene substrate and phosphetane 1; energy decomposition techniques highlight the biphilic character of the phosphetane in this step. Although kinetically invisible, the second deoxygenation stage is implicated as the critical C–N product-forming event, in which a postulated oxazaphosphirane intermediate is diverted from arylnitrene dissociation toward heterolytic ring opening with the arylboronic acid; the resulting dipolar intermediate evolves by antiperiplanar 1,2-migration of the organoboron residue to nitrogen, resulting in displacement of 1·[O] and formation of the target C–N coupling product upon in situ hydrolysis. The method thus described constitutes a mechanistically well-defined and operationally robust main-group complement to the current workhorse transition-metal-based methods for catalytic intermolecular C–N coupling.

How to make C–N bonds using boronic acids and their derivatives without transition metals

No need for transition-metal catalysis in amination, amidation, nitration or nitrosation reactions with boron derivatives as reagents.