Unsymmetric BINOL-Titanium Complex for Enantioselective Alkylation

Significance

BINOL and its derivatives have emerged as very useful ligands in asymmetric reactions. Often, the use of unsubstituted BINOL (3) does not give high enantioselectivity, and thus many 3,3′-derivatives have been synthesized to obtain higher selectivities. The only problem is that by adding steps to the synthesis, the cost of ligand increases and often these ligands require high catalyst loading. The authors have found that a mono-substituted BINOL derivative effects alkylation at <1% catalyst loading. This ligand is significantly more effective than BINOL (3) and a disubstituted 3,3′-diphenyl-BINOL (2) ligand. The authors imply that the reactivity of 1 is due to an increase in the concentration of intermediate 4 due to the presence of the bulky substituent R¹ (compared to 1).