Strong Preference of the Redox-Neutral Mechanism over the Redox Mechanism for the Ti IV Catalysis Involved in the Carboamination of Alkyne with Alkene and Diazene.

Titanium catalysis generally prefers redox-neutral mechanisms. Yet it has been reported that titanium could promote bond formations in a way similar to reductive elimination. Accordingly, redox catalytic cycles involving TiIV /TiII cycling have been considered. By studying, as an example, the carboamination of alkynes with alkenes and azobenzene catalyzed by the [TiIV ]=NPh imido complex, we performed DFT computations to gain an understanding of how the "abnormal" catalysis takes place, thereby allowing us to clarify whether the catalysis really follows TiIV /TiII redox mechanisms. The reaction first forms an azatitanacyclohexene by alkyne addition to the [TiIV ]=NPh bond, followed by alkene insertion. The azatitanacyclohexene can either undergo Cα -Cγ coupling, to afford bicyclo[3.1.0]imine, or β-H elimination, to yield a [TiIV ]-H hydride, which then undergoes Cα =Cβ or Cγ =Cδ insertion to give an α,β- or β,γ-unsaturated imine, respectively. Both the geometric and electronic structures indicate that the catalytic cycles proceed through redox-neutral mechanisms. The alternative redox mechanisms (e.g., by N-H or C-H reductive elimination) are substantially less favorable. We concluded that electronically, the TiIV catalysis intrinsically favors the redox-neutral mechanism, because a redox pathway would involve TiII structures either in the triplet ground state or in the high-lying open-shell singlet state, but the involvement of triplet TiII structures is spin-forbidden and that of singlet TiII structures is energetically disadvantageous.