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((OPr)-Pr-i)(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((OPr)-Pr-i)(2).Ti((OPr)-Pr-i)(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((OPr)-Pr-i)(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(OiPr)4, was ascertained and quantitatively followed by titration of BINOL with Ti((OPr)-Pr-i)(4) and variable-temperature experiments. CD spectra offered ultimate proof of titanium-chelating BINOL structures.