Aluminum Derivatives of a New Chiral Binucleating Ligand

A new chiral bis(β-amino alcohol) ligand, LH2, has been prepared in 40% yield by the
Cu-catalyzed coupling of 4,6-diiododibenzofuran with diphenylpyrrolidin-2-yl-methanol, the
latter of which is prepared from l-proline. The protonolysis reaction of LH2 with 2 equiv of
AliBu3 forms LAl2iBu4 (1), which was isolated in 40% yield. In the solid state, compound 1
adopts a conformation that gives the molecule approximate C2 symmetry. Each half of the
molecule features a distorted tetrahedral Al center that interacts with a chelating β-amino
alcoholate and two iBu anions. The reaction of LH2 with 2 equiv of AlMe3 failed to yield
isolable product, but the use of excess AlMe3 formed LAl4Me10 (2) in 79% isolated yield.
Compound 2 has a structure that is similar to that of 1 except that the alcoholate oxygen of
each deprotonated ligand is also coordinated to an AlMe3 group. 1H NMR spectroscopic data
of 1 in C6D6 solution are consistent with overall C2 symmetry. Thus the two iBu groups on
each Al center are inequivalent, and four resonances are observed for the diastereotopic
α-CH2 protons of these iBu ligands. The addition of 4 equiv of pyrimidine to the solution
results in rapid exchange of the two inequivalent iBu groups. A different process occurs in
the presence of a large excess of D6-methyl sulfoxide (D6-DMSO). Under these conditions,
the iBu groups no longer possess diastereotopic α-methylenes. This indicates that the iBu
groups are being removed from the chiral environment, at least temporarily.