Differences between the elimination of early and late transition metals: DFT mechanistic insights into the titanium-catalyzed synthesis of pyrroles from alkynes and diazenes

Abstract

A DFT study demonstrates that titanium is capable of promoting C–N bond formation via an unconventional reductive elimination pathway featuring back-donation (REBD).

Strong Preference of the Redox‐Neutral Mechanism over the Redox Mechanism for the TiIV Catalysis Involved in the Carboamination of Alkyne with Alkene and Diazene

“…Previously, we computed the mechanisms of reaction (1) . For consistency, we adopted the same computational protocol in this study.…”

“…To release 3 a , the Ti−N and Ti−C δ covalent bonds of IM5 must break, which can be achieved by the oxidation of PhN=NPh through an interchange mechanism as represented by the formation of TS6 : PhN=NPh extracts one electron from each of the Ti−N and Ti−C δ bonds to form the two Ti−N bonds in IM6 , the remaining two electrons in the two covalent bonds forming the N=C π bond of 3 a . Previously, we have shown that IM6 can feasibly be converted into cat3 by disproportionation [reaction (3)] and we will not repeat these results in this study.…”

“…Thus, there is a crossover between singlet and triplet energy surfaces, which does not occur in steps R1–R3. Previously, we have shown that in spite of the triplet ground state of R12P , step R12 proceeds along the singlet energy surface due to weak spin–orbit coupling . The lower energy of the triplet of R12P compared with its singlet can be attributed to the interruption of the aromaticity of the pyrrole ring due to the two‐electron donation resulting from the C−N bond elimination, rather than to the +2 oxidation state of titanium in R12P , thus R12P is an exception because Ti IV complexes generally have a singlet ground state.…”

“…A similar mechanism has also been applied to elucidate the alkyne trimerizations catalyzed by various Ti IV –imido complexes including cat1 . Spurred on by the remarkable experimental advances, we previously carried out a DFT mechanistic study to gain insight into the elimination mechanism . Our study revealed that the elimination is different from the mechanism of the conventional reductive elimination.…”

Titanium redox catalysis: insights and applications of an earth-abundant base metal

“…This metallacycle then undergoes C-N reductive coupling to yield the product pyrrole and a Ti II fragment that is likely stabilized by backbonding 33 into the pyrrole 26 or an incoming diazene/alkyne 27 , in a manner similar to that seen for the Pauson-Khand reaction. Finally, reoxidation to a Ti imido via diazene disproportionation closes the catalytic cycle.…”