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

Content Provider	Royal Society of Chemistry (RSC)
Author	Dang, Yanfeng Guo, Jiandong Deng, Xi Qu, Shuanglin Wang, Zhi-Xiang Song, Chunyu Lu, Yu
Copyright Year	2017
Abstract	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 via an unconventional elimination pathway, passing through a comparatively stable masked TiII complex (i.e., IM4) rather than pyrrole directly. The formation of IM4 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 IM4 rather than becoming a lone pair on a Ti center as adopted in the catalysis of late TMs. Because of its stability, IM4 requires additional oxidation by diazene to liberate pyrrole. The triplet counterpart (IM4T) of IM4 is more stable than IM4, but the elimination is unlikely to reach IM4T, because the process is spin-forbidden and the spin–orbit coupling is weak. Alternatively, one may consider the forming pyrrole in IM4 as a redox-active ligand, reserving the two electrons resulting from the formal reductive elimination and then releasing the electrons when IM4 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 IM4-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.
Starting Page	2413
Ending Page	2425
Page Count	13
File Format	HTM / HTML PDF
ISSN	20416520
Volume Number	8
Issue Number	3
Journal	Chemical Science
DOI	10.1039/c6sc04456e
Language	English
Publisher	Royal Society of Chemistry
Access Restriction	Subscribed
Subject Keyword	Lone pair Oxidative addition Ti Ligand Pyrrole Titanium Covalent bond Diazene Electronegativity Spin\u2013orbit interaction
Content Type	Text
Resource Type	Article
Subject	Chemistry

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