Tricoordinate aluminum species

A mechanism for the formation of the hexacoordinate species 434 is presented in Sch. 60 [89]. Association of metal bases with the ALB catalyst 394 gives species 431 which can undergo disproportionation to give tricoordinate aluminum species 432 and the bis-alkoxide of BINOL (433). Addition of this bis-alkoxide of BINOL to ALB would then produce the hexacoordinate aluminum species 434. If this scheme is correct, it is certainly possible that the three-coordinate aluminum species 432 is the active catalyst. To test for this possibility, this species was prepared by the reaction of BINOL with trimethylaluminum and was crystallized to give crystals which were characterized by X-ray diffraction as the dimeric pentacoordinate THF adduct 435. This aluminum compound has been used previously for Mukaiyama type aldol reactions... 

Most acidity studies have been made using basic molecules such as ammonia, pyridine, and piperidine as probes. These molecules have the property that their interaction with Bronsted acid sites, Lewis acid sites, and cations and their hydrogen-bonding interactions give rise to different species detectable by infrared spectroscopy. Thus, adsorption on Bronsted acid sites gives rise to ammonium, pyridinium, and piperidinium ions with characteristic absorption frequencies of 1475, 1545, and 1610 cm"1, respectively. Adsorption on Lewis acid sites—tricoordinated aluminum... 

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