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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
Author(s) -
Guo Jiandong,
Lu Yu,
Zhao Ruihua,
Liu Zheyuan,
Menberu Wasihun,
Wang ZhiXiang
Publication year - 2018
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201800339
Subject(s) - chemistry , redox , photochemistry , alkene , catalysis , alkyne , reductive elimination , imine , singlet state , reaction mechanism , hydride , medicinal chemistry , organic chemistry , hydrogen , physics , nuclear physics , excited state
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 IV /Ti II cycling have been considered. By studying, as an example, the carboamination of alkynes with alkenes and azobenzene catalyzed by the [Ti IV ]=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 IV /Ti II redox mechanisms. The reaction first forms an azatitanacyclohexene by alkyne addition to the [Ti IV ]=NPh bond, followed by alkene insertion. The azatitanacyclohexene can either undergo C α −C γ coupling, to afford bicyclo[3.1.0]imine, or β‐H elimination, to yield a [Ti IV ]−H hydride, which then undergoes C α =C β or C γ =C δ 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 IV catalysis intrinsically favors the redox‐neutral mechanism, because a redox pathway would involve Ti II structures either in the triplet ground state or in the high‐lying open‐shell singlet state, but the involvement of triplet Ti II structures is spin‐forbidden and that of singlet Ti II structures is energetically disadvantageous.