Highly enantioselective carbonyl-ene reaction catalyzed by chiral titanium complexes: Toward structural elucidation of catalytically active species

We have developed highly enantioselective carbonyl-ene reactions catalyzed by a chiral titanium complex prepared from optically pure binaphthol (BINOL) and (i-PrO)₂TiCl₂ in the presence of molecular sieves (MS) 4 A. During the course of the structural elucidation of catalytically active "BINOL-Ti", MS 4 A employed in the catalyst preparation step was found to function not only as a base to trap HCl but also as H₂O donor eventually providing the active "BINOL-Ti". The effect of counter cations of zeolites was also investigated in the catalyst preparation step. A sufficient content of Na cation in the zeolites is the key for efficient formation of the active "BINOL-Ti". Direct transformation of BINOL-Ti(Oi-Pr)₂ under the influence of the hydrated MS 4 A into the active catalyst was examined, where BINOL-Ti(Oi-Pr)₂ was assembled into the active "BINOL-Ti" primarily composed of μ₃-oxo (Ti₃O) species via the di-μ₃-oxo bridging tetranuclear titanium complex. Although the exact structure of the active "BINOL-Ti" has not yet been determined, we proposed a plausible pathway to the formation of active "BINOL-Ti" from BINOL and (i-PrO)₂TiCl₂ under the influence of hydrated MS 4 A.