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The Key Role of the Nonchelating Conformation of the Benzylidene Ligand on the Formation and Initiation of Hoveyda–Grubbs Metathesis Catalysts
Author(s) -
Bieszczad Bartosz,
Barbasiewicz Michał
Publication year - 2015
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.201501959
Subject(s) - chemistry , ligand (biochemistry) , metathesis , tricyclohexylphosphine , ruthenium , associative substitution , salt metathesis reaction , catalysis , stereochemistry , chelation , conformational isomerism , organic chemistry , polymerization , molecule , phosphine , biochemistry , receptor , polymer
Abstract Experimental studies of Hoveyda–Grubbs metathesis catalysts reveal important consequences of substitution at the 6‐position of the chelating benzylidene ligand. The structural modification varies conformational preferences of the ligand that affects its exchange due to the interaction of the coordinating site with the ruthenium center. As a consequence, when typical S‐chelated systems are formed as kinetic trans ‐Cl 2 products, for 6‐substituted benzylidenes the preference is altered toward direct formation of thermodynamic cis ‐Cl 2 isomers. Activity data and reactions with tricyclohexylphosphine (PCy 3 ) support also a similar scenario for O‐chelated complexes, which display fast trans ‐Cl 2 ⇄cis‐Cl 2 equilibrium observed by NMR EXSY studies. The presented conformational model reveals that catalysts, which cannot adopt the optimal nonchelating conformation of benzylidene ligand, initiate through a high‐energy associative mechanism.