Organolanthanide complexes amidines

Traditionally, aluminum alkoxides have been used as homogeneous catalysts for the Tishchenko reaction. Other catalysts such as boric acid and a few transition-metal complexes have also been used. However, these catalysts are either reactive only imder extreme reaction conditions or very slow. Recently, great effort has been focused on the development of organolanthanide complexes as catalysts because of their high Lewis acidity, easily tunable coordination sphere around the metal, and environmentally friendly property. Series of organolanthanide complexes, such as alkyls, amides, and amidinates, were found to be highly efficient catalysts for the Tishchenko reaction. The proposed catalytic mechanism is provided in Scheme 11, and the precatalyst reacts with aldehyde to give lanthanide alkoxide as the catalytic active species. 

Insertion of di-t-butylcarbodiimide into organolanthanite complexes (La = Er, Y, Gd) to give organolanthanide amidinates is also observed. Likewise, diisopropylcarbodiimide undergoes insertion into lanthanocene amides. However, diisopropylcarbodiimide does not react with lanthanocene guanidate complexes In the reaction of Et2 Y(N-i-Pr2 )2 with... 

An interesting organolanthanide-catalyzed reaction which has been studied in recent years is the addition of terminal alkynes to carbodiimides leading to the novel class of ,A -disubstituted propiolamidines. It was found that half-sandwich rare earth metal complexes bearing silylene-linked cyclopentadienyl-amido ligands can act as excellent catalysts in this addition reaction. As illustrated in Scheme 58, a rare earth amidinate species has been confirmed to be a true catalytic species [68]. 

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