Aluminum Alkyl Complexes Supported by Bidentate N,N Ligands: Synthesis, Structure, and Catalytic Activity for Guanylation of Amines

The reactions of AlMe 3 or AlEt 3 with 2-pyridyl- or indolyl-substituted imines were studied, leading to the formation of different organoaluminum complexes. While the reactions of the iminopyridine Cy[N═CMe-2-(C 5 H 4 N)] 2 ( L 1 ) derived from 1-(pyridin-2-yl)ethanone and trans -1,2-cyclohexanediamine with AlEt 3 gave the aluminum complex Cy[NC(Me)(Et)-2-(C 5 H 4 N)AlEt 2 ] 2 ( 1 ), in which the two ketimine groups of the ligand were transformed into the amido functionality through the addition of two ethyl groups, the reaction of L 1 with AlMe 3 afforded the aluminum complex Cy[NC(═CH 2 )-2-(C 5 H 4 N)AlMe 2 ] 2 ( 2 ) via a sp 3 C–H activation with elimination of two methane molecules. The reactions of indolyl-2-aldimines (2-(RN═CH)C 8 H 5 NH (R = t Bu ( L 2 H ), C 6 H 5 ( L 3 H ), 2,6-Me 2 C 6 H 3 ( L 4 H )) with AlMe 3 or AlEt 3 afforded only the deprotonated indolyl aluminum complexes [2-(RN═CH)C 8 H 5 N]AlMe 2 (R = t Bu ( 3 ), C 6 H 5 ( 4 ), 2,6-Me 2 C 6 H 3 ( 5 )) and [2-(2,6-Me 2 C 6 H 3 N═CH)C 8 H 5 N]AlEt 2 ( 6 ), respectively. The structures of complexes 2 – 6 were characterized by spectral methods and X-ray crystallographic analyses. These aluminum complexes showed a high catalytic activity in the addition of amines to carbodiimides to form guanidines. The mechanism of the catalytic process was studied by control experiments and 1 H NMR monitoring. Together with the isolation of the complex [2-(2,6-Me 2 C 6 H 3 N═CH)C 8 H 5 N][CyN═C(4-MeC 6 H 3 N)(NHCy)]AlMe ( 7 ), a probable mechanism for the guanylation reaction was proposed.