Borrowing Hydrogen in the Activation of Alcohols

Abstract

Alcohols can be temporarily converted into carbonyl compounds by the metal-catalysed removal of hydrogen. The carbonyl compounds are reactive in a wider range of transformations than the precursor alcohols and can react in situ to give imines, alkenes, and a-functionalised carbonyl compounds. The metal catalyst, which had borrowed the hydrogen, then returns it to the transformed carbonyl compound, leading to an overall process in which alcohols can be converted into amines, compounds containing C À C bonds and b-functionalised alcohols.

A bifunctional palladium/acid solid catalyst performs the direct synthesis of cyclohexylanilines and dicyclohexylamines from nitrobenzenes

“…9 Thus, it would be desirable to have a suitable catalyst to couple aniline and cyclohexylamine and, ideally, to be able to engage the two hydrogenations in one-pot to get cyclohexylaniline 5 from nitrobenzene 2. 10 This process would avoid the cyclohexane-cyclohexanone 11 -cyclohexanol 7 → 8 + 9 50 route, 2,7,[12][13][14][15] with the corresponding low yields obtained during the oxidation of cyclohexane to produce cyclohexanone. Figure 1 shows kinetics for the hydrogenation of nitrobenzene 2 with a catalytic amount of commercially-available palladium on carbon (Pd/C), in where each point corresponds to an independent 55 batch reaction.…”

“…Production is based on two different processes: dehydroxylative amination of alcohols for manufacturing alkyl amines, 1,2 and reductive amination of ketones. 3,4 The latter includes the multiton/year production of 15 aniline 3 from nitrobenzene 2 shown in Scheme 1.…”

Direct Synthesis of Secondary Amines From Alcohols and Ammonia Catalyzed by a Ruthenium Pincer Complex

“…We have previously reported the dehydrogenative coupling of alcohols and amines (not ammonia) to form imines, catalyzed by a PNP-type Ru pincer complex [46]. A possible overall mechanistic scheme for the formation of secondary amines from primary alcohols and ammonia is as follows (Scheme 2): 1) formation of the primary amine via borrowing hydrogen strategy [29,47]; an intermediate aldehyde formed by dehydrogenation of the alcohol reacts with ammonia to form an imine a (via water elimination from a hemiaminal intermediate), which undergoes hydrogenation. 2) a similar reaction of the primary amine, in competition with ammonia, with the intermediate aldehyde, to form an alkyl imine b which undergoes hydrogenation to form the secondary amine (Path A; in case of aliphatic systems).…”

Lactic acid and hydrogen from glycerol via acceptorless dehydrogenation using homogeneous catalysts

“…[29][30][31][32] Similarly to the acceptorless dehydrogenation, substantial progress was made recently in the development of molecular organometallic catalysts for the conversion of alcohols via the borrowing hydrogen strategy, which will have great potential in the benign valorisation of biomass feedstocks. [33][34][35][36] It is worth mentioning that the occurrence of the two pathways (dehydrogenative coupling and release of H 2 or hydrogen borrowing), illustrated in scheme 2 is strongly dependent on the catalytic system. For instance it was shown for a series of Ru(N-N)(P-P) complexes that in the reactions between alcohols and amines, the ligand is a crucial factor discriminating the selectivity towards either amide or amine products [37,38].…”