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

Abstract

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 Ti /Ti cycling have been considered. By studying, as an example, the carboamination of alkynes with alkenes and azobenzene catalyzed by the [Ti ]=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 Ti /Ti redox mechanisms. The reaction first forms an azatitanacyclohexene by alkyne addition to the [Ti ]=NPh bond, followed by alkene insertion. The azatitanacyclohexene can either undergo C -C coupling, to afford bicyclo[3.1.0]imine, or β-H elimination, to yield a [Ti ]-H hydride, which then undergoes C =C or C =C 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 Ti catalysis intrinsically favors the redox-neutral mechanism, because a redox pathway would involve Ti structures either in the triplet ground state or in the high-lying open-shell singlet state, but the involvement of triplet Ti structures is spin-forbidden and that of singlet Ti structures is energetically disadvantageous.

Modern applications of low-valent early transition metals in synthesis and catalysis

“…It is proposed that the α,β-unsaturated imines are formed through N-H reductive elimination, which has not previously been observed on Ti. Computational studies have proposed a more complex insertion-type pathway for the formation of this product, although experimental evidence for either pathway is currently lacking 320 .…”

Cp2Ti(II) Mediated Rearrangement of Cyclopropyl Imines

“…This formal C−C oxidative addition is the microscopic reverse of the α,γ-coupling seen in the Ticatalyzed carboamination reaction (Figure 1B, top). The structure of 3 was confirmed by 1 H NMR, 13 C NMR, and Xray crystallography (Figure 4, Figures S4 and S5). Complex 3 has a Ti−N bond distance of 1.997(2) Å, slightly longer than a typical Ti−NR 2 LX-type amide bond (CSD average = 1.972), and consistent with a Ti−N single bond.…”

Carbodiimide Synthesis via Ti-Catalyzed Nitrene Transfer from Diazenes to Isocyanides

“…Azobenzene’s synergistic “redox-neutral” buffering of electron density serves the same purpose as an ancillary redox noninnocent ligand and was previously proposed by Wang for several other Ti redox catalytic reactions. , Interestingly, previous computations for pyrrole product release from Ti II in the related catalytic formal [2 + 2 + 1] synthesis of pyrroles did not require this type of buffering . While the reasons for this difference are currently unclear, a contributing factor could be the stability of the released product: in the case of pyrrole release, reformation of the aromatic ring (loss of Ti-pyrrole backbonding) could be enough of a driving force to eject Ti II even in the absence of azobenzene as a redox buffer; on the contrary, in carbodiimide synthesis (and the related carboamination reactions calculated by Wang), formation of a single CN π bond is likely not enough of a driving force to eject free Ti II . Nonetheless, it is also important to note that the other (non-azobenzene bound) pathways are low enough in energy (∼25 kcal/mol, Table ) that they also may contribute to the overall reaction rate to some extent and that azobenzene is not absolutely critical for catalysis.…”