An Aluminum Ate Base: Its Design, Structure, Function, and Reaction Mechanism

An aluminum ate base, i-Bu3Al(TMP)Li, has been designed and developed for regio- and
chemoselective direct generation of functionalized aromatic aluminum compounds. Direct alumination
followed by electrophilic trapping with I2, Cu/Pd-catalyzed C−C bond formation, or direct oxidation with
molecular O2 proved to be a powerful tool for the preparation of 1,2- or 1,2,3-multisubstituted aromatic
compounds. This deprotonative alumination using i-Bu3Al(TMP)Li was found to be effective in aliphatic
chemistry as well, enabling regio- and chemoselective addition of functionalized allylic ethers and carbamates
to aliphatic and aromatic aldehydes. A combined multinuclear NMR spectroscopy, X-ray crystallography,
and theoretical study showed that the aluminum ate base is a Li/Al bimetallic complex bridged by the
nitrogen atom of TMP and the α-carbon of an i-Bu ligand and that the Li exclusively serves as a recognition
point for electronegative functional groups or coordinative solvents. The mechanism of directed ortho
alumination reaction of functionalized aromatic compounds has been studied by NMR and in situ FT-IR
spectroscopy, X-ray analysis, and DFT calculation. It has been found that the reaction proceeds with facile
formation of an initial adduct of the base and aromatic, followed by deprotonative formation of the
functionalized aromatic aluminum compound. Deprotonation by the TMP ligand rather than the isobutyl
ligand was suggested and reasoned by means of spectroscopic and theoretical study. The remarkable
regioselectivity of the ortho alumination reaction was explained by a coordinative approximation effect
between the functional groups and the counter Li+ ion, enabling stable initial complex formation and creation
of a less strained transition state structure.