Amine–Boranes as Transfer Hydrogenation and Hydrogenation Reagents: A Mechanistic Perspective

Abstract

Transfer hydrogenation (TH) has historically been dominated by Meerwein–Ponndorf–Verley (MPV) reactions. However, with growing interest in amine–boranes, not least ammonia–borane (H3N⋅BH3), as potential hydrogen storage materials, these compounds have also started to emerge as an alternative reagent in TH reactions. In this Review we discuss TH chemistry using H3N⋅BH3 and their analogues (amine–boranes and metal amidoboranes) as sacrificial hydrogen donors. Three distinct pathways were considered: 1) classical TH, 2) nonclassical TH, and 3) hydrogenation. Simple experimental mechanistic probes can be employed to distinguish which pathway is operating and computational analysis can corroborate or discount mechanisms. We find that the pathway in operation can be perturbed by changing the temperature, solvent, amine–borane, or even the substrate used in the system, and subsequently assignment of the mechanism can become nontrivial.

A metal-free protocol for the preparation of amines using ammonia borane under mild conditions

“…[43][44][45] Thus, in the last decades ammonia borane has been employed as a reducing agent for CvO, CvN and CvC bonds, but only a few reports are known for reductive amination reactions. [46][47][48] Ramachandran group 49 and Hu group 50 described reductive amination procedure employing boron amine complex as a reductant. However, a sub-stoichiometric promoter or heating conditions are necessary to obtain amine product in good yields.…”

pH-Responsive Delivery of H2 through Ammonia Borane-Loaded Hollow Polydopamine for Intervertebral Disc Degeneration Therapy

“…To achieve efficient delivery and endogenous stimuluscontrolled long-term release in the IVD, we recently developed a pH-responsive ammonia borane (AB) as a precursor for H2 production. AB has a good H2 storage capacity (10 wt.%) and can be decomposed into H2 in an aciddependent manner [26]. Both normal and degenerated discs have an acidic environment due to the accumulation of byproducts of glycolysis, such as lactic acid.…”

Amino‐Modified Polymer Immobilized Ionic Liquid Stabilized Ruthenium Nanoparticles: Efficient and Selective Catalysts for the Partial and Complete Reduction of Quinolines

“…Moreover, the transfer hydrogenation of 3‐substituted quinolines occurred with complete selectivity for the partially reduced 1,2‐dihydroquinolines, which are potentially versatile synthons to chiral tetrahydroquinolines [1] as well as reducing agents for use in transfer hydrogenations [29] or direct alkylation of anilines using alcohols via borrowing hydrogen [30] . This is the first example of a metal NP catalysed hydrogenation of quinoline using dimethylamine borane as the hydrogen source and although examples of metal‐free and metal NP catalysed reductions of quinoline with amine borane have been reported, the TOFs are rather low, and a large excess of hydrogen donor is required [31a–f] …”