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Coupling and Dearomatization of Pyridines at a Transient η 2 ‐Cyclopropene/Bicyclobutane Zirconocene Complex
Author(s) -
Romero Nuria,
Dufrois Quentin,
Vendier Laure,
Dinoi Chiara,
Etienne Michel
Publication year - 2017
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.201703371
Subject(s) - cyclopropene , cyclopropane , chemistry , pyridine , stereospecificity , ligand (biochemistry) , reductive elimination , medicinal chemistry , coupling reaction , stereochemistry , photochemistry , catalysis , organic chemistry , ring (chemistry) , biochemistry , receptor
This paper reports on stereospecific coupling reactions between an η 2 ‐cyclopropene ligand and pyridine derivatives, which are preferred to alternative C−H bond activation reactions. The dicyclopropylzirconocene complex [Cp 2 Zr( c ‐C 3 H 5 ) 2 ] ( 1 ) eliminates cyclopropane to generate the η 2 ‐cyclopropene/bicyclobutane intermediate [Cp 2 Zr(η 2 ‐ c ‐C 3 H 4 )] ( A ). A does not activate any pyridine C−H bonds, but rather pyridine inserts into a Zr−C bond of A , yielding an azazirconacycle with a dearomatized pyridyl group [Cp 2 Zr{κ 2 ‐N,C 8 ‐(2‐ c ‐C 3 H 4 )‐C 5 H 5 N}] ( 2 ). Kinetic data, isotopelabelling experiments, and DFT calculations indicate that the rate‐determining step of this stereospecific reaction is cyclopropane elimination, and that the stability of the intermediate [Cp 2 Zr(η 2 ‐ c ‐C 3 H 4 )(NC 5 H 5 )] ( A‐py ) governs the selectivity of the reaction. Complex 2 tautomerizes to [Cp 2 Zr{κ 2 ‐N,C 8 ‐(2‐C 3 H 5 )‐C 5 H 4 N}] ( 6 ) through a base‐catalyzed proton migration accompanied by cyclopropyl opening and restoration of conjugation within the zirconacycle.