MULTIDENTATE LEWIS-ACIDS - ADDUCTS OF MONODENTATE AND BIDENTATE TITANIUM TRICHLOROALKOXIDES WITH KETONES

Like TiCl4, titanium trichloroisopropoxide (4) is a strong Lewis acid able to form 1:2 adducts with ketones. Low-temperature H-1 and C-13 NMR spectra indicate that these adducts adopt a mer octahedral geometry. Exchange of free and bound pinacolone occurs by a dissociative mechanism with DELTA-G double-ended-dagger 219 = 9.2 kcal/mol. Potentially bidentate titanium trichloroalkoxides 19-21 can be prepared by treating the bis(trimethylsilyl) ethers of trans-1,2-cyclohexanediols with 2 equiv of TiCl4. Compounds 19-21 form crystalline 1:2 ketone adducts even in the presence of excess ketone. An X-ray crystallographic study has shown that the close proximity of the OTiCl3 groups favors symmetric adducts 30 (X = Cl) with bridging chlorides, not adducts 31 with bridging, doubly activated carbonyl groups. Similar chloride-bridged structures are adopted in solution. Low-temperature H-1 and C-13 NMR spectra establish that unsymmetric 1:3 adducts 36 (X = Cl) are formed in solution in the presence of additional ketone. Symmetrization of the 1:3 pinacolone complex derived from bidentate titanium trichloroalkoxide 21 occurs by a normal dissociative mechanism with DELTA-G double-ended-dagger 223 = 10.4 kcal/mol. Since this process is slower than exchange in 1:2 adducts of monodentate analogue 4, symmetric intermediates or transition states 38 (X = Cl) with a single bridging carbonyl oxygen do not offer a low-energy intramolecular pathway for exchange. Slow exchange in the 1:3 adduct provides evidence that the two OTiCl3 groups in compounds 19-21 cooperate by forming a single chloride bridge that enhances the Lewis acidity of one site at the expense of the other.