The solution mixtures of (S)-BINOL and Ti(OiPr)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(OiPr)2âTi(OiPr)4, is dominant in the conditions employed for the catalysis of enantioselective addition of alkylzinc reagents to aldehydes. NMR T1 and DOSY results confirmed that species I is dimeric in solution and may be formulated as [(BINOLate)Ti-(OiPr)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(OiPr)4 and variable temperature experiments. CD spectra offered ultimate proof of titanium-chelating BINOL structures.
Multiple Solution Species of Titanium(IV) 1,1'-Bi-2-naphtholate Elucidated by NMR and CD Spectroscopy
PESCITELLI, GENNARO;DI BARI, LORENZO;SALVADORI, PIERO
2004-01-01
Abstract
The solution mixtures of (S)-BINOL and Ti(OiPr)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(OiPr)2âTi(OiPr)4, is dominant in the conditions employed for the catalysis of enantioselective addition of alkylzinc reagents to aldehydes. NMR T1 and DOSY results confirmed that species I is dimeric in solution and may be formulated as [(BINOLate)Ti-(OiPr)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(OiPr)4 and variable temperature experiments. CD spectra offered ultimate proof of titanium-chelating BINOL structures.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
