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 AIMe(3) afforded dimeric (AIL)(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-AIL 2, PhCHO-AIL 3, H2NCH2CH2NH2- AIL 4, and [PhO-AIL](-) 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 H2O-AIL 6 that undergoes partial hydrolysis at room temperature to produce 7, which X-ray crystallography showed to be composed of four AIL fragments linked by an (H2O)(2)(HO)(2)Al(OH)(2)Al(OH)(2)-(H2O)(2) framework in which the O4AlO2AlO4 moiety is of local D-2h 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.