37877-95-5Relevant articles and documents
Effective modulation of the donor properties of n-heterocyclic carbene ligands by "Through-space" communication within a planar chiral scaffold
Fuerstner, Alois,Alcarazo, Manuel,Krause, Helga,Lehmann, Christian W.
, p. 12676 - 12677 (2007)
A high yielding approach to planar chiral carbene ligands is described, in which an imidazo[1,5-a]pyridine-3-ylidene unit is embedded into a cyclophane scaffold. As evident from the IR data of the corresponding rhodium complex 18 as the parent member of this family, these new ligands turned out to be exceptionally strong electron donors, rivaling or even outperforming the other diamino-stabilized five-membered N-heterocyclic carbenes (NHC) known to date. If the remote ring of the cyclophane is substituted by four fluorine atoms, however, the donor capacity is significantly reduced by the through-space interaction of the tetrafluorobenzene moiety with the underneath carbene entity. Since X-ray data suggest that the steric demand of the fluorinated and the nonfluorinated cyclophane-2-ylidenes are virtually identical, these results illustrate that the electronic properties of an NHC ligand can be tuned over a wide range without the need to change its constitution or steric attributes. The synthesis route leading to the planar chiral imidazopyridinium salts serving as the immediate carbene precursors involves a previously unrecognized 6π-electrocyclic ring opening reaction upon exposure of the pyridine-N-oxide 12 to N,N-dimethyl-carbamoyl chloride and TMSCN, as evident from the crystal structure analysis of the resulting azatriene adduct 13. Copyright
A new planar chiral bipyridine ligand
Wo?rsdo?rfer, Udo,Vo?gtle, Fritz,Nieger, Martin,Waletzke, Mirko,Grimme, Stefan,Glorius, Frank,Pfaltz, Andreas
, p. 597 - 602 (2007/10/03)
Combined methods of cyclophane and bipyridine synthesis open the way to the new planar-chiral 2,2'-bipyridine 1, whose CD spectrum is strongly dependent on metal salts. The absolute configuration of the chiral precursor 2 could be assigned by comparison of experimental and theoretical CD spectra. The usefulness of planar chirality in heterocyclic transition metal ligands is revealed in experiments towards stereoselective catalysis using 1.
