From Five to Five: Titanium Ketimine Complexes with Monoaza-butadiene η4-Coordination Mode and Hidden η2-Imine Reactivity
Fischer M., Schmidtmann M., Beckhaus R.
In the reactions of LTiCl3 (L: Cp∗, Cp), Mg, and PhN=C(p-tolyl)2 (1) the formation of η4-ketimine complexes Cp∗Ti(Cl)(η4-PhN=C(p-tolyl)2) (2a) and CpTi(Cl)( η4-PhN=C(p-tolyl)2) (2b) are observed. Their "nonclassic" five-membered titanium monoazabutadiene envelope coordination modes, involving one of the p-tolyl substituents, is confirmed by single crystal X-ray diffraction analysis of 2b. In reactions of 2 with aldehydes, ketones, alkynes, carbodiimides, isocyanates, isothiocyanates, and imines five-membered titanacycles are formed in a regioselective manner. This behavior is in agreement with a hidden η2-imine reactivity. All reaction products are fully characterized, including single crystal X-ray diffraction studies. For the PhC≡CH insertion products (4b, 4c) the formation of Ti-C(Ph) units are observed. By insertion of the isocyanate CyNCO the formation of a Ti-N bond in 6a is preferred in comparison to the insertion of the isothiocyanate PhNCS, where a Ti-S bond is formed (7a). By reacting 2a with aldimine PhN=C(H)(p-tolyl) the nonsymmetric titanaimidazolidine derivative 8a is formed by subsequent ketimine-aldimine coupling. By derivatization of 2a with LiN(Me)Cy the formation of a the titanadihydropyrrole 9a is observed, caused by a 1,3-H-shift. 2 appears to be inert toward a broad range of terminal olefins. Reacting 2a with allyl ethyl ether results in a spontaneous ether cleavage reaction to 10a.