Facile synthesis of monomeric alumatranes

Alumatranes, tricyclic neutral molecules featuring a transannular N --> Al bond, can act as Lewis acids that activate substrates in the axial coordination site. Treatment of tris(2-hydroxy-3,5-dimethylbenzyl)amine with AlMe(3) afforded dimeric (AlL)(2) 1 wherein L = tris(2-oxy-3,5-dimethylbenzyl)amine]. X-ray diffraction analysis revealed bridging between AlL monomers by two Al-O bonds. Reactions of 1 with substrates containing O or N donors generated the alumatranes THF-AlL 2, PhCHO-AlL 3, H(2)NCH(2)CH(2)NH(2)-AlL 4, and PhO-AlL](-) 5, in which the apical added ligand on the five-coordinate aluminum center causes variation in the transannular bond distance. Water coordinates with 1 at -20 degrees C to form the alumatrane H(2)O-AlL 6 that undergoes partial hydrolysis at room temperature to produce 7, which X-ray crystallography showed to be composed of four AlL fragments linked by an (H(2)O)(2)(HO)(2)Al(OH)(2)Al(OH)(2)(H(2)O)(2) framework in which the O(4)AlO(2)AlO(4) moiety is of local D(2)(h)() symmetry. According to X-ray analysis, 7 can crystallize in at least two polymorphic modifications: triclinic 7a and monoclinic 7b. The reaction of 3 with water also generated 6 and 7, depending on the reaction temperature. Dimeric 1 was found to promote the reaction of benzaldehyde with trimethylsilyl cyanide at room temperature to provide 2-trimethylsilyoxyphenylacetonitrile in 95% yield.