Borane-Catalyzed Transfer Hydrogenations of Pyridines with Ammonia Borane

Abstract

With the use of ammonia borane as a hydrogen source, a borane catalyzed metal-free transfer hydrogenation of pyridines was successfully realized for the first time to furnish a variety of piperidines in 44-88% yields with moderate to excellent cis-selectivities. The ease in handling without requiring high pressure H makes this transfer hydrogenation practical and useful.

Dehydrogenation of Amine–Boranes Using p‐Block Compounds

“…Du and co‐workers also used a FLP strategy for the transfer hydrogenation of pyridines . Inspired by Baker and Dixon, they found that the combination of a 2,6‐substituted pyridine with B(C 6 F 5 ) 3 can abstract dihydrogen from ammonia–borane, giving piperidines with excellent cis ‐selectivity, along with the formation of borazine, cyclotriborazane, and polyborazylenes as dehydrogenated products.…”

“…Indeed, 76 can react with another equivalent of methylamine-borane to form oligomeric borane adduct 79 (Scheme38), which can also be formed by reacting 74 with H 3 B·NHMeBH 2 ·NH 2 Me, and providese videncef or dehydrogenation through ac hain-growth mechanism.A ddition of a third equivalent of MeNH 2 ·BH 3 resulted in the formation of the cyclic trimer (NHMeBH 2 ) 3 and regenerationo ft he catalyst. In situ NMR measurements indicated that the addition of am ethylamine-borane unit occurs through an end-growth dehydrogenative mechanism, insteado fi nsertion of MeNH 2 ·BH 3 into the PÀBb ond of the adduct (79). Additionally, 74 can further dehydrogenate (NHMeBH 2 ) 3 under catalytic conditions (1 mol %o f74)p roducing trimethylborazine at 55 8C.…”

“…Du and co-workers also used aF LP strategy for the transfer hydrogenation of pyridines. [79] Inspired by Baker and Dixon, [48] they found that the combination of a2 ,6-substituted pyridine with B(C 6 F 5 ) 3 can abstract dihydrogen from ammonia-borane, giving piperidines with excellent cis-selectivity,a longw itht he formationo fb orazine, cyclotriborazane, and polyborazylenes as dehydrogenated products.A fter optimization, av ariety of 2,6-diarylpyridines were successfully hydrogenated to the corresponding products in 63-88 %y ield with high cis-selectivity (97:3-99:1 dr;S cheme 43). It was also found that 2-aryl-6methylpyridines can be applied for transfer hydrogenation and severals ubstrates were successfully hydrogenated with moderate to good yields (56-88%)a nd good selectivity (86:14-99:1 dr).…”

Transfer Hydrogenation of Azo Compounds with Ammonia Borane Using a Simple Acyclic Phosphite Precatalyst

“…Although rapid H 2 ‐release from AB has been widely demonstrated by metal‐based catalysts,, latest focus on the hydrogen transfer from AB (or amine‐boranes in general) has been directed to organocatalyzed dehydrogenation using, for example, p ‐block‐based systems. [8a], [8b], [8c], [8d], [8e], [8f], have addressed the use of pnictogen‐based compounds for TH reactions from AB , e.g. a bismuth‐ and several phosphorus‐based catalysts .…”

Reduction of phenylacetylenes using Raney Ni–Al alloy, Al powder in the presence of noble metal catalysts in water

“…5 Also a Hantzsch amido dihydropyridine has been reported as a transfer hydrogenation reagent for α,β-unsaturated ketones while pyridine derivatives were reduced via borane-catalyzed transfer hydrogenation reaction with ammoniaborane. 6,7 Recently, use of a copper catalyst as a monophasic catalytic system and silica-supported copper nanoparticles have been employed for the selective semireduction of alkynes. 8,9 In another study, a nonclasical…”