Abstract
The first example of a manganese(iii) catalyst for the acceptorless dehydrogenation of alcohols is presented. N,N'-Bis(salicylidene)-1,2-cyclohexanediaminomanganese(iii) chloride (2) has been shown to catalyze the direct synthesis of imines from a variety of alcohols and amines with the liberation of hydrogen gas. The mechanism has been investigated experimentally with labelled substrates and theoretically with DFT calculations. The results indicate a metal-ligand bifunctional pathway in which both imine groups in the salen ligand are first reduced to form a manganese(iii) amido complex as the catalytically active species. Dehydrogenation of the alcohol then takes place by a stepwise outer-sphere hydrogen transfer generating a manganese(iii) salan hydride from which hydrogen gas is released.
Highlights
Metal-catalyzed dehydrogenation of alcohols gives rise to aldehydes and ketones, which can be further transformed into imines, amides, esters, carboxylic acids and various heterocycles in the same pot.[1]
Since this is a common ligand for manganese(III) complexes, an experiment was performed with the Jacobsen's catalyst (1, Fig. 2). This increased the yield to 79% with some unreacted benzyl alcohol remaining.[12]. This observation shows that the acceptorless dehydrogenation of alcohols can be catalyzed by manganese(III) complexes and a number of experiments were performed to optimize the
When the imination with 5% of the salan complex 8 was repeated with KOtBu as the base, a 56% yield was obtained of Nbenzylidene cyclohexylamine. This result can be explained by elimination of HCl from 8 to afford the catalytically active species 12.25 Similar eliminations of hydrogen halides under basic conditions have been described with (PNP)Mn(I), (PNP) Ru(II) and (PNNP)Fe(II) complexes to form the corresponding amido compounds.4b,26 Notably, a post analysis by LCMS of the imination with salan complex 8 showed the formation of salen complex 2 as the main manganese species together with minor amounts of some unidenti ed complexes
Summary
Metal-catalyzed dehydrogenation of alcohols gives rise to aldehydes and ketones, which can be further transformed into imines, amides, esters, carboxylic acids and various heterocycles in the same pot.[1]. This increased the yield to 79% with some unreacted benzyl alcohol remaining (entry 2).[12] This observation shows that the acceptorless dehydrogenation of alcohols can be catalyzed by manganese(III) complexes and a number of experiments were performed to optimize the The corresponding salan complex 8 furnished some conversion into the imine (entry 9) which may indicate that the Schiff base functionality in the salen ligand plays an important role in the mechanism (vide infra).
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