Efficient and chemoselective hydroboration of organic nitriles promoted by TiIV catalyst supported by unsymmetrical acenaphthenequinonediimine ligand

Insight into the MOtBu (M=Na, K)-Mediated Dehydrogenation of Dimethylamine-Borane and Transfer Hydrogenation of Nitriles to Primary Amines

Selective synthesis of primary amines from nitriles is challenging in synthetic chemistry due to the possible en-route generation of various amines and imines. Herein, we report a practical and operationally simple MOtBu-mediated (M=Na, K) transfer hydrogenation of nitriles to the corresponding primary amines with a relatively unexplored sacrificial hydrogen source (dimethylamine borane). The strategy encompasses a broad substrate scope under transition metal-free conditions and does not require any solvent. The mechanistic investigation was performed with the aid of control experiments and spectroscopic studies. The GC analysis of the reaction mixture exhibited the evolution of the H2 gas. Additionally, detailed computational calculations were undertaken to shed light on the possible intermediates and transition states involved during the present protocol.

Iron complexes supported by silyl-NHC chelate ligands: synthesis and use for double hydroboration of nitriles

Iron complexes bearing new silyl-NHC bidentate ligands were synthesised by treating Fe3(CO)12 with a mixture of N-(hydrosilyl)methyl imidazolium salts and a base. These complexes showed high performance in the catalytic double hydroboration of nitrile with pinacolborane (HBpin) to produce N,N-bis(boryl)amine by a combination of UV irradiation and mild heating (60 °C). The product yields for the hydroboration of aromatic and aliphatic nitriles reached 85%-95% (NMR) using an iron complex (5 mol%). Reducing the loading amount of the iron complex to 0.5 mol% still afforded the products in high yields. An analogous ruthenium complex, which was similarly synthesised using Ru3(CO)12, showed lower activity. Stoichiometric reactions of the iron complex with nitriles afforded Fe(0)-N-silylimine complexes, which may be dormant states in nitrile hydroboration. A catalytic mechanism including Fe(0) N-silylimine species is proposed.

Metal/Non-Metal Catalyzed Activation of Organic Nitriles

Nitrile activation is a prominent topic in recent developments in chemistry, especially in organic, inorganic, biological chemistry, as well as in the natural synthesis of products and in the pharmaceutical industry. The activation of nitriles using both metal and non-metal precursors has attracted several researchers, who are exploring newer ways to synthesize novel compounds. Nitrile activation can be achieved by combining various catalytic double hydroelementation reactions, such as hydrosilylation, hydroboration, and hydrogenation of organonitriles using silanes, pinacolborane, and other sources of hydrogen. These methodologies have garnered considerable attention since they are effective in the reduction of organonitriles, whose end products are extensively applied in synthetic organic chemistry. In this review, we summarize the development of selective hydroborylation, hydrosilylation, dihydroborysilylation, and hydrogenation of organonitriles, as well as their reaction mechanisms and the role of metal complexes in the catalytic cycles. This review article explains various synthetic methodologies applied toward the reduction of organonitriles into corresponding amines.

Applications of catalysis in hydroboration of imines, nitriles, and carbodiimides

The catalytic hydroboration of imines, nitriles, and carbodiimides is a powerful method of preparing amines which are key synthetic intermediates in the synthesis of many value-added products. Imine hydroboration has perennially featured in notable reports while nitrile and carbodiimide hydroboration have gained attention recently. Initial developments in catalytic hydroboration of imines and nitriles employed precious metals and typically required harsh reaction conditions. More recent advances have shifted toward the use of base metal and main group element catalysis and milder reaction conditions. In this survey, we review metal and nonmetal catalyzed hydroboration of these unsaturated organic molecules and group them into three distinct categories: precious metals, base metals, and main group catalysts. The TON and TOF of imine hydroboration catalysts are reported and summarized with a brief overview of recent advances in the field. Mechanistic and kinetic studies of some of these protocols are also presented.

Catalytic Hydroboration and Reductive Amination of Carbonyl Compounds by HBpin using a Zinc Promoter

In this paper, the chemoselective hydroboration of aldehydes and ketones, catalyzed by Zinc(II) complexes [ k 2 -(PyCH=NR)ZnX 2 ] [R = CPh 3 , X = Cl ( 1 ) and R = Dipp (2,6-diisoropylphenyl) and X = I ( 2 )], in the presence of pinacolborane (HBpin) in ambient temperature and solvent-free conditions, which produced corresponding boronate esters in high yield, is reported. Zinc metal complexes 1 and 2 were derived in 80-90% yield from the reaction of iminopyridine [PyCH=NR] with anhydrous zinc dichloride in dichloromethane at room temperature. The solid-state structures of both zinc complexes were confirmed using X-ray crystallography. Zinc complex 1 was also used as a competent pre-catalyst in the reductive amination of carbonyl compounds with HBpin under mild and solvent-free conditions to afford a high yield (up to 97%) of the corresponding secondary amines. The wider substrate scope of both reactions was explored. Catalytic protocols using zinc as a pre-catalyst demonstrated an atom-economic and green method with diverse substrates bearing excellent functional group tolerance. Computational studies established a plausible mechanism for catalytic hydroboration.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Hydroboration of Nitriles, Esters, and Carbonates Catalyzed by Simple Earth-Abundant Metal Triflate Salts

During the past decade earth-abundant metals have become increasingly important in homogeneous catalysis. One of the reactions in which earth-abundant metals have found important applications is the hydroboration of unsaturated C-C and C-X bonds (X=O or N). Within these set of transformations, the hydroboration of challenging substrates such as nitriles, carbonates and esters still remain difficult and often relies on elaborate ligand designs and highly reactive catalysts (e. g., metal alkyls/hydrides). Here we report an effective methodology for the hydroboration of challenging C≡N and C=O bonds that is simple and applicable to a wide set of substrates. The methodology is based on using a manganese(II) triflate salt that, in combination with commercially available potassium tert-butoxide and pinacolborane, catalyzes the hydroboration of nitriles, carbonates, and esters at room temperature and with near quantitative yields in less than three hours. Additional studies demonstrated that other earth-abundant metal triflate salts can facilitate this reaction as well, which is further discussed in this report.

Catalytic Nitrile Hydroboration: A Route to N,N-Diborylamines and Uses Thereof

Catalytic reduction of nitriles is considered as an attractive and atom-economical route to a diversity of synthetically valuable primary amines. Compared to other methods, dihydroboration approach has been developed relatively recently but has already attracted the attention of many research groups due to reasonably mild reaction conditions, selectivity control and the access to N,N-diborylamines, which emerged as powerful reagents for C-N bond forming reactions. Early developments in catalytic dihydroboration of nitriles implied precious metal catalysts along with harsh conditions and prolonged reaction times, whereas recent advances mostly rely on base and main group metal catalytic systems with significantly improved profiles. This minireview aims to provide an overview of advances and challenges of dihydroboration of nitriles with d-, f- and main group metal catalysts. Mechanistic features of different catalytic systems, functional group tolerance and scope of the methods are also presented. The synthetic utility of N,N-diborylamies, beyond simple protodeborylation, is discussed in the aspect of N-arylation, imine and amide synthesis.

Lithium amide catalyzed hydroboration of nitriles

An operationally facile hydroboration of nitriles is reported that utilizes the stable and inexpensive catalyst LiN(SiMe₃)₂. The reaction displayed good tolerance of functional groups and also converted carbonyl derivatives.

Scope and mechanism of nitrile dihydroboration mediated by a β-diketiminate manganese hydride catalyst

Nitrile insertion allows for manganese-catalyzed nitrile dihydroboration at 80 °C.