Article (Scientific journals)
Ruthenium catalysts bearing a benzimidazolylidene ligand for the metathetical ring-closure of tetrasubstituted cycloolefins
Borguet, Yannick; Zaragoza, Guillermo; Demonceau, Albert et al.
2015In Journal of the Chemical Society. Dalton Transactions, 44, p. 9744-9755
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Abstract :
[en] Deprotonation of 1,3-di(2-tolyl)benzimidazolium tetrafluoroborate with a strong base afforded 1,3-di(2-tolyl)benzimidazol-2-ylidene (BTol), which dimerized progressively into the corresponding dibenzotetraazafulvalene. The complexes [RhCl(COD)(BTol)] (COD is 1,5-cyclooctadiene) and cis-[RhCl(CO)2(BTol)] were synthesized to probe the steric and electronic parameters of BTol. Comparison of the percentage of buried volume (%VBur) and of the Tolman electronic parameter (TEP) of BTol with those determined previously for 1,3-dimesitylbenzimidazol-2-ylidene (BMes) revealed that the two N-heterocyclic carbenes displayed similar electron donicities, yet the 2-tolyl substituents took a slightly greater share of the rhodium coordination sphere than the mesityl groups, due to a more pronounced tilt. The anti,anti conformation adopted by BTol in the molecular structure of [RhCl(COD)(BTol)] ensured nonetheless a remarkably unhindered access to the metal center, as evidenced by steric maps. Second-generation ruthenium-benzylidene and isopropoxybenzylidene complexes featuring the BTol ligand were obtained via phosphine exchange from the first generation Grubbs and Hoveyda-Grubbs catalysts, respectively. The atropisomerism of the 2-tolyl substituents within [RuCl2([double bond, length as m-dash]CHPh)(PCy3)(BTol)] was investigated by using variable temperature NMR spectroscopy, and the molecular structures of all four possible rotamers of [RuCl2([double bond, length as m-dash]CH-o-OiPrC6H4)(BTol)] were determined by X-ray crystallography. Both complexes were highly active at promoting the ring-closing metathesis (RCM) of model [small alpha],[small omega]-dienes. The replacement of BMes with BTol was particularly beneficial to achieve the ring-closure of tetrasubstituted cycloalkenes. More specifically, the stable isopropoxybenzylidene chelate enabled an almost quantitative RCM of two challenging substrates, viz., diethyl 2,2-bis(2-methylallyl)malonate and N,N-bis(2-methylallyl)tosylamide, within a few hours at 60 °C.
Disciplines :
Chemistry
Author, co-author :
Borguet, Yannick
Zaragoza, Guillermo
Demonceau, Albert ;  Université de Liège > Département de chimie (sciences) > Chimie macromoléculaire et catalyse organique
Delaude, Lionel  ;  Université de Liège > Département de chimie (sciences) > Chimie organométallique et catalyse homogène
Language :
English
Title :
Ruthenium catalysts bearing a benzimidazolylidene ligand for the metathetical ring-closure of tetrasubstituted cycloolefins
Publication date :
2015
Journal title :
Journal of the Chemical Society. Dalton Transactions
ISSN :
1472-7773
eISSN :
1364-5447
Publisher :
Royal Society of Chemistry
Volume :
44
Pages :
9744-9755
Peer reviewed :
Peer Reviewed verified by ORBi
Available on ORBi :
since 25 May 2015

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