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 D2h 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.