Amine‐Borane Dehydrogenation and Transfer Hydrogenation Catalyzed by α‐Diimine Cobaltates

Abstract

Anionic α‐diimine cobalt complexes, such as [K(thf)1.5{(DippBIAN)Co(η4‐cod)}] (1; Dipp=2,6‐diisopropylphenyl, cod=1,5‐cyclooctadiene), catalyze the dehydrogenation of several amine‐boranes. Based on the excellent catalytic properties, an especially effective transfer hydrogenation protocol for challenging olefins, imines, and N‐heteroarenes was developed. NH3BH3 was used as a dihydrogen surrogate, which transferred up to two equivalents of H2 per NH3BH3. Detailed spectroscopic and mechanistic studies are presented, which document the rate determination by acidic protons in the amine‐borane.

Catalytic Transfer Hydrogenation of Arenes and Heteroarenes

“…First, average deuterium incorporation in the product was studied using different deuterated ammonia boranes and trifluoroethanols (cf. Table S7 in the Supporting Information) [34] . The results seem to indicate that the protic hydrogen from the solvent and the hydridic hydrogen from borane are primarily involved in the hydrogen transfer.…”

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

“…Another example of TH and hydrogenation of olefins was reported by Wolf and co‐workers [37] . the authors used a catalytic amount of a reduced cobaltate anion [K(thf) 1.5 {( IPr BIAN)Co(COD)}] (BIAN=bis(iminoacenaphthene)diamine) to perform dehydrogenation of H 3 N⋅BH 3 , TH of disubstituted olefins and imines with H 3 N⋅BH 3 and hydrogenation of tri‐ and tetrasubstituted alkenes (Scheme 11).…”

Fast and Efficient Nickel(II)‐catalysed Transfer Hydrogenation of Quinolines with Ammonia Borane

“…The produced H 2 gas from AB dehydrogenation is released in situ, facilitating catalyst-mediated hydrogenation of an unsaturated compound. Co (II)-, Cu(I)-, Bi(I)-, Pd(II)-and borane catalysts dominate the field of catalytic TH of unsaturated compounds with AB as H 2 source, i. e. nitriles, [9] nitroarenes, [10] N-heteroaromatics, [11] alkynes, [12] alkenes, [11] imines, [13] and pyridines. [14] These reactions are, however, still limited to long reaction times, elevated temperatures and excessive amounts of AB.…”

“…To our knowledge these conditions are the best reported for the TH of any substrate using AB operating under homogeneous conditions. Previous studies either required longer reaction times, [11] higher temperatures, [12c] more than 1 equivalent of AB [9a] or phosphine-containing ligands. [9a] Further evaluation revealed that no dehydrogenation of 1,2,3,4-tetrahydroquinoline was observed when 1,2,3,4-tetrahydroquinoline was added to a methanol/toluene solution of the pre-catalyst at elevated (110°C) temperatures.…”

“…Quinoxaline, showed a low conversion at first, however increasing the reaction time to 1 hour led to a very high yield, comparable to previous studies and in a much shorter reaction time. [11] NMR studies were conducted to elucidate the possible mechanism during which quinoline undergoes TH with AB. AB was subjected to dehydrogenation using NiCl 2 • 6H 2 O and L1.…”