binol (1,1'-binaphthyl-2,2'-diol) is usually used in conjunction with oxophilic lewis acidic metals such as aluminum, titanium, zirconium, and various rare earth metals. in cases where binol itself does not provide ideal enantioselective control, it can be readily elaborated by substitution at the 3,3' positions (via lithiation) and 6,6' positions (via the 6,6'-dibromide compound prepared by electrophilic aromatic substitution) to modulate steric bulk and electronic properties. for instance, aluminum catalysts based on bulky 3,3'-disilyl substituted binol have been developed as early examples of catalytic asymmetric hetero-diels-alder reaction and claisen rearrangement, while introduction of electron-withdrawing groups at the 6,6'-positions was crucial for increasing the lewis acidity, and hence catalytic activity, of zirconium(iv) catalysts toward a mannich-type reaction. to date, however, no model for the crucial factors governing binol-directed stereoinduction has been generally accepted.