Tetranuclear BINOL-titanium complex in selective direct aldol additions

The extremely robust and water-stable tetranuclear complex Ti(4)(micro-BINOLato)(6)(micro(3)-OH)(4) (1) catalyzes the direct aldol addition with high degrees of regioselectivity at the sterically more encumbered alpha-side of unsymmetrical ketones. The formation of quaternary stereocenters is described. Oxygen-containing ene components can also be used as starting material in these reactions. When used with aliphatic aldehydes, acetals 18 or acetals of aldol adducts 20 were observed. As few as 0.2 mol % loadings with this catalyst 1 were enough to complete the reactions. Mechanistical aspects of the reactions are discussed.