Anilines and cyclohexylamines are widely utilized as precursors for polymers, active pharmaceutical ingredients, fine chemicals, dyes and agrochemicals. Traditionally, these primary amines are obtained from arenes via multi-step processes, which display clear technological disadvantages. Phenols, on the other hand, are very attractive alternative starting materials since the overall transformation with NH3 into anilines or cyclohexylamines only produce water as a by-product. Moreover, phenols are efficiently produced at low price and are also highly prevalent in renewable lignocellulosic biomass. Solid acidic catalysts have been used for the gas-phase transformation of phenol into aniline, but require high temperatures and are quickly deactivated. More recently, an alternative approach in the liquid phase was proposed in which phenol is partially hydrogenated to cyclohexanone, which easily reacts with amines into an imine intermediate and (de)hydrogenated further into either N-substituted cyclohexylamines or anilines. However, the production of primary amines remained a difficult challenge as by-product formation into secondary amines could drastically decrease product selectivity. Herein, we report the first direct amination of phenol with NH3 into primary amines in the liquid phase through an efficient one-step process. Both commercially available and self-prepared supported (noble) metals were able to catalyze this reaction. While palladium showed excellent selectivity towards anilines, rhodium and nickel displayed a remarkable activity and selectivity towards cyclohexylamines. As reactive imine intermediates are formed, it was shown that carefully controlling both H2 and NH3 partial pressures as well as reaction temperature were crucial to suppress by-product formation, viz. undesired secondary amines. As a result, aniline and cyclohexylamine yields were maximized (≥ 95%) in both cases. Finally, the scope for the direct amination of phenols was successfully expanded by converting a wide range of C-substituted phenols into their corresponding primary amines.

Direct liquid-phase amination of phenols into anilines and cyclohexylamines

2019 Fall National Meeting

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