Catalytically excited hydrogen as reducing agent

In most cases reductive alkylation involving hydrogen and a catalyst is carried out in an autoclave. The catalysts usually used for hydrogenations of this type are Raney nickel (more rarely Raney cobalt) and platinum metals. The hydrogen pressure and the reaction temperature needed depend largely on the activity of the carbonyl compound and the catalyst, and generalization is impossible. With nickel catalysts, hydrogen pressures of the order of 20 to 150 atm and reaction temperatures between 40° and 150° are required, but there are as many cases where reaction takes place smoothly at room temperature with pressures of 1-3 atm. 

The greatest importance of reductive alkylation is for preparation of primary amines from aldehydes and ketones. This is mostly effected in alcoholic solution with Raney nickel and an excess of ammonia. 

Formaldehyde is unsuitable for alkylation of ammonia as it yields urotropine. Lower aliphatic aldehydes are added only gradually to the reaction mixture to avoid aldol condensations. Higher aliphatic aldehydes and aromatic aldehydes react smoothly and special precautions are generally unnecessary. 

Heptylamine 995 Heptanal (113 g, 1 mole), methanol (150 ml), Raney nickel (10 g), and anhydrous ammonia (50 ml) are mixed in a stirring autoclave and then heated at 90° under a hydrogen pressure of 100 atm. Hydrogen uptake is complete in about 30 min. The catalyst is filtered off and ammonia and methanol are distilled off through a 20-cm column until the bath-temperature reaches 100°. The residue is freed from water by azeotropic distillation with benzene. After removal of the benzene, heptylamine (93.5 g, 81 %) distils at 50°/14 mm. 

Benzylamine 996 A solution of ammonia (51 g, 3 moles) in cold ethanol (300 ml) is treated with benzaldehyde (3 moles) and Raney nickel (10 g). Under an initial hydrogen pressure of 90 atm hydrogenation begins at 40° and is complete after 30 min at a final temperature of 70°. Distillation of the filtered product gives benzylamine (287 g, 89.4%), b.p. 70-80°/8 mm, and dibenzylamine (21.7 g, 7.1 %), b.p. 140-150°/7 mm. 

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