Aluminum Catalysts from Axially Chiral Bis-Phenols

3 Aluminum Catalysts from Axially Chiral Bis-Phenols 

Atropisomers of conformationally restricted bis-phenols have been popular ligands in several applications (Sch. 5, 8, 9, 11-13 Tables 4 and 5). Aluminum compounds prepared from bis-l,T-binaphth-2,2 -ol (BINOL) 40 and the derivatives 97 were examined as chiral catalysts in the reaction of methyl acrylate and cyclopentadiene by Maruoka, Concepcion and Yamamoto [50] and by Ketter, Glahsl and Hermann [47]. Four catalysts prepared from four derivatives of the 3,3 -bis-triarylsilyl derivatives of 97 and trimethylaluminum were examined in both toluene and dichloromethane the results are summarized in Sch. 26 [50]. Slightly higher asymmetric induction was observed in toluene and for the f-butyldiphenysilyl derivative 97b. The catalyst prepared 

Aluminmn catalysts derived from the three BINOL derivatives outlined in Table 14 have been used in the asymmetric cycloaddition of the A-crotyloxazolidinone 175 and cyclopentadiene. These reactions are slower and require the use of stoichiometric amounts of catalyst. Although the dienophiles 175 are bidentate and should lead to a more conformationally restrained dienophile-Lewis acid complex, asymmetric induction is quite low. 

Catalyst Solvent Temp (°C) Time (h) % yield endo/exo % ee 

Obtaining optimum asymmetric induction in the cycloaddition of methacrolein with the catalyst prepared from the VAPOL ligand involved the slow addition of the dienophile. Initially 10 % of the dienophile was added, the remainder being added over 3 h with a syringe pump (Table 15). If the dienophile was added in one portion asymmetric induction in the cycloadduct 3 at the end of the reaction was only 87.7 % ee (Table 15, entry 3). The amount of asymmetric induction was also found to be a function of concentration, and increased from 91.4 to 97.8 % ee when the concentration of dienophile was reduced from 1.0 to 0.05 m (Table 15, entries 1 and 2). The amount of induction was also found to be function of the substrate-to-catalyst ratio— selectivity was lower at higher ratios (Table 15, entries 1 and 3). Finally, it was observed that as5nmnetric induction was less at the early stages of the reaction that at the end. When the reaction was stopped after 30 % completion the induction was 81.1 % ee whereas at 100 % completion it was 87.7 % ee (Table 15, entries 3 and 5). 

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