Three asymmetric guanidinato metal complexes: Synthesis, crystal structures and their use as pre-catalysts in the Meerwein–Ponndorf–Verley reduction

The guanidinatolithium compound [{(C2H5)2N}C(NC6H5)(NAr)LiOEt2]2 (Ar = 2,6-Me2C6H3) (1), is readily accessible in high yield upon insertion reaction of lithium diethyl amide with unsymmetric carbodiimine ArNCNC6H5. The reaction of 1 with AlCl3 in Et2O afforded the guanidinatoaluminum compound [{(C2H5)2N}C(NC6H5)(NAr)]2AlCl (2). Compound 2 reacted with equivalent MeLi to give [{(C2H5)2N}C(NC6H5)(NAr)]2AlCH3 (3), which was also prepared by the treatment of Et2NH with sequentially trimethylaluminum and carbodiimine ArNCNC6H5 in hexane. Compounds 1–3 were characterized by 1H, 13C NMR spectra and single crystal X-ray diffraction analysis. In addition, 1, 2 and 3 were used pre-catalyst to catalyze the Meerwein–Ponndorf–Verley reduction, and both of 2 and 3 exhibited good to excellent catalytic activity.

Graphical abstractThe synthesis of three asymmetric guanidinato metal complexes is described. The molecular structures of three guanidinato metal complexes were confirmed by a single crystal X-ray diffraction study. Further, the catalytic efficiency of the complexes has been evaluated for the Meerwein–Ponndorf–Verley reduction of a series of aromatic and aliphatic aldehydes to the corresponding alcohols.Figure optionsDownload full-size imageDownload as PowerPoint slide