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 AlMe₃ 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, H₂NCH₂CH₂NH₂−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 °C to form the alumatrane H₂O−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 (H₂O)₂(HO)₂Al(OH)₂Al(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.