Catalysis carbene

Keywords Carbene Catalysis Complex Iron Nitrene... 

C-C-Bond Formations Employing N-Heterocyclic Carbene Catalysis... 

Enders D, Niemeier O, Balensiefer T (2006c) Asymmetric intramolecular cros-sed-benzoin reactions by N-heterocyclic carbene catalysis. Angew Chem Int Ed Engl 45 1463... 

Enders D, Kallfass U (2002) An efficient nucleophilic carbene catalyst for the asymmetric benzoin condensation. Angew Chem Int Ed 41 1743-1745 Enders D, Niemeier O (2004) Thiazol-2-ylidene catalysis in intramolecular crossed aldehyde-ketone benzoin reactions. Synlett 2004 2111-2114 Enders D, Niemeier O, Balensiefer T (2006) Asymmetric intramolecular crossed-benzoin reactions by N-heterocyclic carbene catalysis. Angew Chem Int Ed 45 1463-1467... 

With respect to the application of asymmetric carbene catalysis as a tool for enantioselective synthesis, the last decade s major success is based on substantial improvements in catalyst development. Early reports dealt with implementing chirality in thiazolium scaffolds (Sheehan and Hunneman 1966 Sheehan and Hara 1974 Dvorak and Rawal 1998), but their catalytic performance suffered from either low yields or low ee-values. In this regard, the investigation of triazole heterocycles as an alternative core structure (Enders et al. 1995) has played a crucial role to provide heterazolium precatalysts improving both asymmetric benzoin and Stetter reactions. An intramolecular Stetter reaction yielding chromanones upon cyclization of salicylaldehyde-derived substrates is commonly used as a benchmark reaction to compare catalyst efficiency (Scheme 1 Ciganek 1995 Enders et al. 1996 Kerr et al. 2002 Kerr and Rovis 2004). 

Within the field of nucleophilic carbene catalysis, with the exception of umpolung-directed transformations (see below), one of the most important classes in terms of applications already realized are transesterifications (Scheme 5 Grasa et al. 2003 Nyce et al. 2002 Singh et al. 2004) and related reactions (Dove et al. 2006). 

The second major class of non-umpolung nucleophilic carbene catalysis comprises reactions by initial NHC-activation of various silicon compounds. Their proposed common pathway is thought to lead to a hypervalent silicon complex4 and thus provide carbene-catalyzed activation of the corresponding nucleophiles such as TMSCN, TMSCF3 etc. (Kano et al. 2006 Song et al. 2005 2006). It is not only certain carbon-silicon bonds that can be effectively activated, but a comparable activation of Si-O bonds, e.g. of trimethylsily enol ethers etc., allows for mild, NHC-promoted Mukaiyama aldol reactions (Scheme 6 Song et al. 2007). 

Zeitler K (2006) Stereoselective synthesis of (E)-afi—unsaturated esters via carbene-catalyzed redox esterification. Qrg Lett 8 637-640 Zeitler K, Mager I (2007) An efficient and versatile approach for the immobilization of carbene precursors via copper-catalyzed [3+2]-cycloaddition and their catalytic apphcation. Adv Synth Cat 349 1851-1857 Zhao GL, Cordova A (2007) A one-pot combination of amine and heterocyclic carbene catalysis direct asymmetric synthesis of fi-hydroxy and fS-malonale esters from a,fS-unsaturated aldehydes. Tetrahedron Lett 48 5976-5980 Zhou ZZ, Ji FQ, Cao M, Yang GF (2006) An efficient intramolecular Stetter reaction in room temperature ionic liquids promoted by microwave irradiation. Adv Synth Cat 348 1826-1830... 

For comprehensive reviews that provide numerous examples of cyclopropanation based on Rh- and Cu-carbene catalysis, see M. P. Doyle, M. A. McKervey, and T. Ye, Modem Catalytic Methods for Organic Synthesis with Diazo Compounds, Wiley New York, 1998 M. P. Doyle and D. C. Forbes, Chem. Rev., 1998, 98, 911 and M. P. Doyle, J. Org. Chem., 2006, 71, 9254. 

Total Synthesis of (—)-7-Deoxyloganin Exploiting N-Heterocyclic Carbene Catalysis with a,p-Unsaturated Enol Esters... 

Although NHC (N-heterocyclic carbene) catalysis is still underdeveloped compared to other areas of organocatalysis, the number of enantioselective methodologies regardingthe direct C—H bond functionalization of aldehydes... 

Keywords Carbene Catalysis Cytochrome P450 Iron porphyrin Ruthenium... 

Application of 1,2,3-triazolyHdenes as versatile NHC-type ligands Synthesis, properties, and application in catalysis and beyond 13CC1145. Applications of N-heterocycHc carbene catalysis in total synthesis 12AG(E)11686. 

SCHEME 13.12. Enantiopure a-fluoro carboxylic acids viaiV-heterocyclic carbene catalysis. 

SCHEME 13.14. Enantiopure a-chloroesteis via IV-heterocyclic carbene catalysis. 

Chiral base catalysis is one of the most versatile and broadly applicable types of catalysis. In particular, the potential of tertiary amines to act both as a base and as a nucleophilic catalyst makes chiral tertiary amines like Cinchona alkaloids a privileged catalyst structure in modem synthesis chemistry. In addition, the field of achiral phosphine and carbene catalysis has proven its potential in numerous applications in the past and it is probably only a matter of time until chiral phosphines and carbenes will also be used routinely for other presently demanding natural product total synthesis (Table 7). 

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