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

Early transition metals (TMs), such as titanium, generally resist undergoing reductive elimination to form C-X bonds due to their weak electronegativity. By analyzing the mechanism of the titanium-catalyzed synthesis of pyrroles from alkynes and diazenes, the present study revealed that titanium is able to promote C-N bond formation an unconventional elimination pathway, passing through a comparatively stable masked Ti complex (, ) rather than pyrrole directly. The formation of originates from the bilateral donation and back-donation between Ti and the pyrrole ligand. Formally, it could be considered that the two electrons resulting from the unconventional reductive elimination are temporarily buffered by back-donation to a symmetry-allowed unoccupied π-orbital of the pyrrole ring in rather than becoming a lone pair on a Ti center as adopted in the catalysis of late TMs. Because of its stability, requires additional oxidation by diazene to liberate pyrrole. The triplet counterpart ( ) of is more stable than , but the elimination is unlikely to reach , because the process is spin-forbidden and the spin-orbit coupling is weak. Alternatively, one may consider the forming pyrrole in as a redox-active ligand, reserving the two electrons resulting from the formal reductive elimination and then releasing the electrons when is oxidized by diazene. These insights allow us to propose the conditions for early TMs to undergo a similar elimination, whereby the forming product will have symmetry-allowed frontier molecular orbitals to form donation and back-donation bonding with a TM center and a substrate possessing a comparatively strong oxidizing ability to oxidize an -like intermediate for product release. These insights may provide another way of constructing C-X bonds through a similar reductive elimination pathway, using early TM catalysts.