Lanthanide formamidinates as improved catalysts for the Tishchenko reaction

摘要

The tris(formamidinato)lanthanum(III) complexes [La(o-TolForm)(3)(thf)(2)] [1; o-TolForm = N,N'-bis(o-tolyl)formamidinate], [La(XylForm)(3)(thf)] [2; XylForrn = N,N'-bis(2,6-dimethylphenyl)formamidinate], and [La(EtForm)(3)] [3, EtForm = N,N'-bis(2,6-diethylphenyl)formamidinate] are a new class of precatalysts for the Tishchenko reaction. Their catalytic activity is a result of their high Lewis acidity and the ease with which the ligand spheres can be interchanged. For the dimerization of benzaldehyde to give benzyl benzoate, which is a benchmark reaction, compound 1 is, to the best of our knowledge, the most active catalyst ever reported. On a preparative scale, the reaction can be performed in the absence of solvent. A range of aromatic, heteroaromatic, and aliphatic aldehydes was rapidly converted into the corresponding esters by using catalysts 1-3.