Abstract
In order to design improved chiral ruthenium catalysts for asymmetric olefin metathesis, enantiomeric catalysts incorporating C1-symmetric N-Heterocyclic carbenes (NHC) ligands with syn-related substituents on the backbone were synthesized starting from meso-1,2-diphenylethylenediamine. The absolute configuration of the enantiomers of the desymmetrized meso diamine was assigned by optical rotation analysis and in silico calculations, and was found to be maintained in their respective ruthenium catalysts by comparison of the relative electronic circular dichroism (ECD) spectra. The catalytic behaviour of the enantiomeric ruthenium complexes was investigated in model asymmetric metathesis transformations and compared to that of analogous complexes bearing C1-symmetric NHC ligands with an anti backbone. Modest enantioselectivities were registered and different catalyst properties depending on the nature of stereochemical relationship of substituents on the backbone were observed.
Highlights
In the last two decades, N-Heterocyclic carbenes (NHCs) have become indispensable tools in both transition-metal-based catalysis and organocatalysis
The employment of stable, easy-handling chiral NHC Ru-based catalysts has resulted in significant advancements in asymmetric olefin metathesis, a powerful methodology for carbon-carbon bond formation leading to enantioenriched molecules [7,8,9,10,11]
Installation of a 2-isopropylphenyl group on one of the nitrogen atoms of meso-1,2-diphenylethylenediamine (A) through a Pd-catalyzed cross-coupling reaction resulted in a racemic mixture (B) which was separated in its constituent enantiomers by enantioselective high-performance liquid chromatography (HPLC)
Summary
In the last two decades, N-Heterocyclic carbenes (NHCs) have become indispensable tools in both transition-metal-based catalysis and organocatalysis. Asymmetric transformations mediated by metal catalysts bearing NHCs with chiral architectures have attracted increasing interest, and intensive research efforts have permitted the obtainment of highly stereoselective catalysts [5,6]. A number of highly enantioselective catalysts bearing C2 - and C1 -symmetric NHC ligands derived from chiral 1,2-diamines were reported by Grubbs [12,13,14] and Collins [15,16,17,18] (Figure 1). Ru-catalysts containing C1 -symmetric bidentate or backbone-monosubstituted NHC ligands were introduced by Hoveyda [19,20,21,22] and Blechert [23,24], respectively, and both these classes of catalysts provided high enantiomeric excesses in model asymmetric reactions
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