N-杂环卡宾

N-Heterocyclic Carbene Ligands
Rapid progress in cross-coupling reactions of unactivated substrates catalyzed by metal compl transformed the chemical marketplace through introduction of an extensive library of achiral an phosphine ligands. The positive effects of phosphine ligands are exemplified in the numerous r highly efficient systems in terms of turnovers and selectivities applied to traditional industrial pro as hydrogenation; however, the disadvantages in the high cost of producing tertiary (especially phosphines and their degradative tendency in converting to phosphine oxides have only now b addressed by the rich field of NHC ligands utilized in homogeneous catalysis.1
The advantages of N-heter ocyclic carbene’s as ancillary ligands are: 1) they are stronger s-don phosphines enabling favorable rates of palladium catalyzed oxidative addition of aryl chlorides, metal-carbenic bond of the NHC complex favors tight binding kinetics, therefore lessening ligan dissociation, 3) the presence of sterically encumbering groups bound to the N-atoms facilitate r elimination of the product from palladium, and 4) the activity of NHC ligands can be modified by introduction of electronic directing substituents remotely, as witnessed in the synthesis of benzim that contain electronically dissimilar groups on the aromatic backbone.2
A wide range of NHC ligands are now commercially available which exhibit high activities in va important organic transformations when combined with metal pre-catalysts. NHC imidazolidine sterically encumbering groups such as mesityl, isopropyl, and adamantyl have been used in the Pd-catalyzed cyclization of anilides,3 amination of aryl chlorides,4arylation with ester enolates to esters,5 Sonogashira reactions of unactivated alkyl bromides,6 and the ruthenium-catalyzed RC (Scheme 1-5).7
This last example shows the power of NHC ligands to stabilize the metal active species in solut imparts the ability of the catalyst to effect RCM on highly substituted and electron-poor olefinic yield tetrasubstituted products. Such pronounced reactivity was the exclusive domain of the Sh alkylidene catalysts.8 However, use of these imidazolidines leads to a vast improvement in perfo early ruthenium metathesis catalysts and provides additional stability features versus the Shroc Our goal at Sigma-Aldrich is to accelerate cutting-edge research projects in NHC ligand mediat To this end we are the first to offer a ligand kit composed of a diverse set of sterically demandin ligands.
Product Name Pro
6 NHC Ligand Kit I
6 1,3-Diisopropylimidazolium tetrafluoroborate 96%
6 1,3-Bis(1-adamantanyl)imidazolium tetrafluoroborate
6 1,3-Bis-(tert-butyl)-4,5-dihydro-1H-imidazolium tetrafluoroborate
6 N,N'-(2,4,6-Trimethyl)dihydroimidazolium chloride
6 N,N'-(2,6-Diisopropylphenyl)dihydroimidazolium chloride
References
1. Herrmann, W. et al. Angew. Chem. Int. Ed.2002, 41, 1290.
2. Organ, M. G. et al. Org. Lett.2005, 7, 1991.
3. Hartwig, J. F. et al. J. Org. Chem.2001, 66, 3402.
4. Hartwig, J. F. et al. Org. Lett.2000, 2, 1423.
5. Hartwig, J. F. et al. J. Am. Chem. Soc.2002, 124, 12557.
6. Fu, G. C. et al. J. Am. Chem. Soc.2003, 125, 13642.
7. Nolan, S. P.; Furstner, A. et al. J. Org. Chem.2000, 65, 2204.
8. (a) Schrock, R. R. et al. J. Am. Chem. Soc.1990, 112, 3875;
(b) Schrock R. R. Top. Organomet. Chem.1998, 1, 1.。