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Facile Synthesis of Monomeric Alumatranes

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journal contribution
posted on 25.10.2006, 00:00 by Weiping Su, Youngjo Kim, Arkady Ellern, Ilia A. Guzei, John G. Verkade
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.