An aluminum ate base: its design, structure, function, and reaction mechanism.
An aluminum ate base, i-Bu(3)Al(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 I(2), Cu/Pd-catalyzed C-C bond formation, or direct oxidation with molecular O(2) 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 alpha-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.
Naka
Hiroshi
H
Graduate School of Pharmaceutical Sciences, Tohoku University, Aobayama, Aoba-ku, Sendai 980-8578, Japan. naka@mail.pharm.tohoku.ac.jp
Uchiyama
Masanobu
M
Matsumoto
Yotaro
Y
Wheatley
Andrew E H
AE
McPartlin
Mary
M
Morey
James V
JV
Kondo
Yoshinori
Y
D000535:Aluminum / Q000737:chemistry*; D001555:Benzene Derivatives / Q000138:chemical synthesis / Q000737:chemistry*; D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008958:Models, Molecular; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013329:Structure-Activity Relationship; D013816:Thermodynamics
D001555:Benzene Derivatives; D009942:Organometallic Compounds; D000535:Aluminum