Synthesis of Aliphatic and Aromatic Amines

Numerous examples of the successful use of metal catalysts for the transformation of alcohols to amines can be found elsewhere [1,3,4,11,13-15], although most of the original information is available in the patent literature. 

Bimetallic Cu/Ni colloids stabilized by Ca and Ba stearates were applied in a simple four-neck round-bottomed flask for the amination of linear and branched oxo-alcohols with dimethylamine [16]. 78-92% yields of tertiary amines were achieved within 3-5 h at 210 °C. 

Transalkylation of amines with alcohols is usually an undesired side-reaction which reduces amination selectivity. Several metals, e. g. Co, Fe, Ni, or Ru, are good catalysts of interchange between alcohols and amines, including tertiary amines [3]. This reaction can be used for the synthesis of asymmetric tertiary amines. Triethylamine, for example, can be transalkylated with dodecanol over a Ni-Cr alloy catalyst, at 250 °C and 15 bar, to form dodecyldimethylamine in 96 % yield. 

Amination of phenols with ammonia is a simple route to aromatic amines. Transformation of 2,6-dimethylphenoI in a continuous fixed-bed reactor at 210 °C over Pd/C yielded 90% 2,6-dimethylaniline after only 6 s contact time [17]. 2,6-Dimethylcyclohexanone and 2,6-dimethylcyclohexylamine were identified as key intermediates at low contact times the concentration of the latter reached 25 %. Compared with the performance of other metals, Pd-based catalysts have outstanding activity and selectivity in the amination of phenols [4]. 

Special multicomponent catalysts were necessary for N-aUcylation of sterically hindered anilines with alkoxy alcohols [9]. Doping Cu-chromite with Pd accelerated the hydrogenation of the imine intermediate this was found to be the rate-limiting step for all the catalysts. Alkali cations (Na and Ba ) increased the activity whereas acidic centers (e. g. Cr and Al ) favored hydrogenolysis of the ether bond. For N-alkylation with secondary alkoxy alcohols a Pt/Si02 catalyst doped with Sn + and Ca ions afforded the best performance. Yields of mono-alkylated anilines varied in the range 22-93 % depending on the steric hindrance in the reactants. 

---