Multiple Solution Species of Titanium(IV) 1,1‘-Bi-2-naphtholate Elucidated by NMR and CD Spectroscopy

The solution mixtures of ( S )-BINOL and Ti(O i Pr) 4, with various titanium/BINOL ratios, have been characterized by NMR and CD spectroscopies. In the presence of 1−8 metal equivalents, two main complexes (species I and II ) were detected. Species II, identified as (BINOLate)Ti(O i Pr) 2 ·Ti(O i Pr) 4, is dominant in the conditions employed for the catalysis of enantioselective addition of alkylzinc reagents to aldehydes. NMR T 1 and DOSY results confirmed that species I is dimeric in solution and may be formulated as [(BINOLate)Ti(O i Pr) 2 ] 2 . The two complexes undergo relatively fast dynamic processes at room temperature, making all naphthoate rings equivalent at NMR; spectrum decoalescence is observed at low temperature. An equilibrium process between I and II, depending on Ti(O i Pr) 4, was ascertained and quantitatively followed by titration of BINOL with Ti(O i Pr) 4 and variable-temperature experiments. CD spectra offered ultimate proof of titanium-chelating BINOL structures.