Highly Active Dinuclear Titanium(IV) Complexes for the Catalytic Formation of a Carbon-Heteroatom Bond.

A series of mononuclear titanium(IV) complexes with the general composition κ-[R{NHPhP(X)}Ti(NMe)] [R = CH, X = Se (3b); R = trans-CH, X = S (4a), Se (4b)] and [{κ-N(PPhSe)}Ti(NMe)] (6b) and two dinuclear titanium(IV) complexes, [CH{(NPhPS)(N)}Ti(NMe)] (3c) and [{κ-N(PPhSe)}Ti(NMe)] (6c), are reported. Dinuclear titanium(IV) complex 6c acts as an efficient catalyst for the chemoselective addition of an E-H bond (E = N, O, S, P, C) to heterocumulenes under mild conditions. The catalytic addition of aliphatic and aromatic amines, alcohol, thiol, phosphine oxide, and acetylene to the carbodiimides afforded the corresponding hydroelemented products in high yield at mild conditions with a broader substrate scope. The catalytic efficiency of the dinuclear complex depends on the cooperative effect of the Ti ions, the systematic variation of the intermetallic distance, and the ligand's steric properties of the complex, which enhances the reaction rate. Most interestingly, this is the first example of catalytic insertion of various E-H bonds into the carbodiimides using a single-site catalyst because only the titanium-mediated insertion of E-H into a C═N unsaturated bond is reported to date. The amine and alcohol insertion reaction with the carbodiimides showed first-order kinetics with respect to the titanium(IV) catalyst as well as substrates. A most plausible mechanism for hydroelementation reaction is also proposed, based on the spectroscopic data of the controlled reaction, a time-course study, and the Hammett plot.