Steric Effects in Aluminum Compounds Containing Monoanionic Potentially Bidentate Ligands: Effect of the Steric Bulk at the α-Carbon

Reaction of [(ⁱBu)₂AlH]₂ with l-ephedrine yields [(ⁱBu)₂A1{μ-OC(H)(Ph)CH(Me)N(H)Me}]₂ (1). Reaction of Al(^tBu)₃ with (2S,3R)-(+)-4-(dimethylamino)-l,2-diphenyl-3-methyl-2-butanol (Chirald^®) yields (^tBu)₂Al[OC(CH₂Ph)(Ph)CH(Me)CH₂NMe₂] (2). The molecular structures of compounds 1 and 2 have been determined by X-ray crystallography. Compound 2 is monomeric with a six membered chelate heterocyclic ring. In contrast, compound 1 exists as a bridged dimer. The formation of monomeric chelate species for compound 2, rather than the alkoxide bridged dimer as is found for 1 is found to be due to the steric bulk of the aluminum alkyl and substitution at the ligand’s α-carbon. Crystal data: (1) orthorhombic, P2₁2₁2₁, a=10.987(2), b=18.795(4), c =19.270(4) Å, V=3979(1) ų, Z=4, R=0.1056, R_w=0.2454. (2) tetragonal, 14, a=22.236(3), c=10.784(1) Å, V=5332.0(5) ų, Z=8, R=0.0485, R_w=0.0537.