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

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.

Automated summary

This review discusses the use of H3N.BH3 and its analogues as sacrificial hydrogen donors in transfer hydrogenation reactions, distinguishing between classical TH, nonclassical TH, and hydrogenation pathways. Experimental and computational analyses can be employed to verify or discount mechanisms, displaying how the operating pathway can be disrupted by various factors, leading to nontrivial assignment of the mechanism.