The N-alkylation of primary amines and ammonia (in situ generated from urea or aqueous ammonia) with alcohols to secondary amines was efficiently promoted by supported copper hydroxide catalysts, Cu(OH) x /Al 2 O 3 and Cu(OH) x /TiO 2 . The observed catalysis was truly heterogeneous, and the catalysts could be reused without an appreciable loss of catalytic performance.The N-alkylation of primary amines to secondary ones is of great importance because the products have been utilized as important synthons for pharmaceuticals, agricultural chemicals, and bioactive compounds.1 Frequently, alkylhalides have been utilized for the N-alkylation with stoichiometric amounts of inorganic bases.1 However, the reaction with alkylhalides produces large amounts of inorganic salts as wastes.
1An alternative environmentally-friendly approach is the Nalkylation with alcohols as alkylating reagents in the presence of appropriate transition-metal catalysts, so-called "borrowing hydrogen strategy" (or "hydrogen autotransfer strategy"):2 The dehydrogenation of an alcohol initially proceeds to afford an aldehyde (more electrophilic than an alcohol), followed by the dehydrative condensation with an amine to produce an imine. Then, the imine is hydrogenated by the transitory formed metal hydride species, giving the desired N-alkylated amine. Although many homogeneous catalysts, in particular platinum group metal complexes, have been reported to be active for the N-alkylation with alcohols, 2,3 these systems have shortcomings of the recovery and reuse of expensive catalysts and/or the indispensable use of co-catalysts such as bases and stabilizing ligands. The development of easily recoverable and recyclable heterogeneous catalysts can solve the problems of the homogeneous systems and has received a particular research interest.
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