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

The reactions of AlMe3 or AlEt3 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-(C5H4N)]2 (L-1) derived from 1-(pyridin-2-yl)ethanone and trans-1,2-cyclohexanediamine with AlEt3 gave the aluminum complex Cy[NC(Me)(Et)-2-(C5H4N)AlEt2](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 AlMe3 afforded the aluminum complex Cy[NC(=CH2)-2-(C5H4N)AlMe2](2) (2) via a sp(3) C-H activation with elimination of two methane molecules. The reactions of indolyl-2-aldimines (2-(RN=CH)C8H5NH (R = Bu-t (L2H), C6H5 (L3H), 2,6-Me2C6H3 ((LH)-H-4)) with AlMe3 or AlEt3 afforded only the deprotonated indolyl aluminum complexes [2-(RN=CH)C8H5N]AlMe2 (R = Bu-t (3), C6H5 (4), 2,6-Me2C6H3 (5)) and [2-(2,6-Me2C6H3N=CH)C8H5N]AlEt2 (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 H-1 NMR monitoring. Together with the isolation of the complex [2-(2,6-Me2C6H3N=CH)C8H5N] [CyN=C(4-MeC6H3N)(NHCy)]AlMe (7), a probable mechanism for the guanylation reaction was proposed.