Amine-Borane Mediated Metal-Free Hydrogen Activation and Catalytic Hydrogenation

Borane-Trimethylamine Complex: A Versatile Reagent in Organic Synthesis

Borane-trimethylamine complex (Me3N·BH3; BTM) is the most stable of the amine-borane complexes that are commercially available, and it is cost-effective. It is a valuable reagent in organic chemistry with applications in the reduction of carbonyl groups and carbon-nitrogen double bond reduction, with considerable examples in the reduction of oximes, hydrazones and azines. The transfer hydrogenation of aromatic N-heterocycles and the selective N-monomethylation of primary anilines are further examples of recent applications, whereas the reduction of nitrobenzenes to anilines and the reductive deprotection of N-tritylamines are useful tools in the organic synthesis. Moreover, BTM is the main reagent in the regioselective cleavage of cyclic acetals, a reaction of great importance for carbohydrate chemistry. Recent innovative applications of BTM, such as CO2 utilization as feedstock and radical chemistry by photocatalysis, have extended their usefulness in new reactions. The present review is focused on the applications of borane-trimethylamine complex as a reagent in organic synthesis and has not been covered in previous reviews regarding amine-borane complexes.

Enhanced Activation of Coordinated Dinitrogen with p-Block Lewis Acids

This Concept article highlights recent research on Lewis acid adducts of dinitrogen complexes, including our contributions. After a reminder of the early works, it is demonstrated that such kind of species offers a new platform for dinitrogen functionalization as well as valuable models for the understanding of elementary steps of (bio)catalytic cycles. When possible, parallels regarding this mode of activation from the orbital point of view are drawn between the different systems discussed herein.

Lewis acid–base adducts of group 13 elements: synthesis, structure and reactivity toward benzaldehyde

[LGa-M(C₆F₅)₃] (M = B 1, Al 2, Ga 3) reacts with benzaldehyde in different ways including the formation of the zwitterionic species [LGa(C₆F₅){CH(Ph)(OAl(C₆F₅)₂)}] (5).

A Strategy to Control the Reactivation of Frustrated Lewis Pairs from Shelf-Stable Carbene Borane Complexes

N-Phosphine oxide substituted imidazolylidenes (PoxIms) have been synthesized and fully characterized. These species can undergo significant changes to the spatial environment surrounding their carbene center through rotation of the phosphine oxide moiety. Either classical Lewis adducts (CLAs) or frustrated Lewis pairs (FLPs) are thus formed with B(C₆F₅)₃ depending on the orientation of the phosphine oxide group. A strategy to reactivate FLPs from CLAs by exploiting molecular motions that are responsive to external stimuli has therefore been developed. The reactivation conditions were successfully controlled by tuning the strain in the PoxIm–B(C₆F₅)₃ complexes so that reactivation only occurred above ambient temperature.

Direct synthesis of a geminal zwitterionic phosphonium/hydridoborate system--developing an alternative tool for generating frustrated Lewis pair hydrogen activation systems

A convenient way to a new class of geminal Mes2PH(+)/B(C6F5)2H(-) pairs is presented. It utilizes triflic acid addition to trans-Mes2PCH=CHB(C6F5)2 followed by triflate/hydride exchange. Thermally induced ring-closure gave a phosphonium/boratacyclopropane zwitterion 8 which formed the Mes2PH(CHMe)B(C6F5)2H P/B FLP-H2 product 10 by subsequent treatment with triflic acid and a silane, or alternatively with dihydrogen at 90 °C. The product 10 is an active catalyst for the hydrogenation of a variety of unsaturated organic substrates, including a quinoline derivative. Treatment of compound 8 with HB(C6F5)2 gave a bifunctional borane 14 which selectively reduced carbon monoxide to the formyl stage.