Home MetathesisN-Heterocyclic Carbene (NHC) Compounds

N-Heterocyclic Carbene (NHC) Compounds

Rovis Catalysts

Rovis has demonstrated that triazolium salt (667080) in the presence of a base can act as an N-heterocyclic carbene organocatalyst¹ in highly enantioselective intramolecular Stetter reactions.² The Stetter reaction (conjugate addition of an aldehyde to an α,β-unsaturated compound) is a superb method for the construction of 1,4-dicarbonyl compounds bearing quaternary stereocenters (Scheme 1).

Scheme 1.

Bode Catalysts

Beginning in 2004, Bode and co-workers extended the use of NHCs as organocatalysts by developing new approaches to the catalytic generation of reactive species including activated carboxylates, homoenolates, and enolates. Critical to the success of these processes is an N-mesityl substituent on an imidazolium or triazolium NHC precursor. These novel catalysts and reactions make possible a new generation of highly enantioselective annulations from simple starting materials under mild reaction conditions, usually at room temperature and without added reagents. For example, chiral (683973) catalyzes an inverseelectron demand Diels–Alder reaction of activated enals and α,β-unsaturated imines to afford cis-dihydropyridinones with outstanding selectivity (Scheme 2).³ The use of α-chloro aldehydes as starting materials is viable, and also makes possible enantioselective annulations with enones using less than 1 mol % of the chiral NHC catalysts (Scheme 3).⁴ These catalysts promote highly enantioselective cyclopentene-forming annulations of simple enals and activated enones (Scheme 4).⁵ Achiral catalyst (688487) is also useful for redox esterifications and amidations of α-functionalized aldehydes.⁶

Scheme 2.

Scheme 3.

Scheme 4.

Materials

References

Marion N, Díez-González S, Nolan S. 2007. N-Heterocyclic Carbenes as Organocatalysts. Angew. Chem. Int. Ed.. 46(17):2988-3000. https://doi.org/10.1002/anie.200603380

Kerr MS, Rovis T. 2004. Enantioselective Synthesis of Quaternary Stereocenters via a Catalytic Asymmetric Stetter Reaction. J. Am. Chem. Soc.. 126(29):8876-8877. https://doi.org/10.1021/ja047644h

He M, Struble JR, Bode JW. 2006. Highly Enantioselective Azadiene Diels?Alder Reactions Catalyzed by Chiral N-Heterocyclic Carbenes. J. Am. Chem. Soc.. 128(26):8418-8420. https://doi.org/10.1021/ja062707c

He M, Uc GJ, Bode JW. 2006. Chiral N-Heterocyclic Carbene Catalyzed, Enantioselective Oxodiene Diels?Alder Reactions with Low Catalyst Loadings. J. Am. Chem. Soc.. 128(47):15088-15089. https://doi.org/10.1021/ja066380r

Chiang P, Kaeobamrung J, Bode JW. 2007. Enantioselective, Cyclopentene-Forming Annulations via NHC-Catalyzed Benzoin?Oxy-Cope Reactions. J. Am. Chem. Soc.. 129(12):3520-3521. https://doi.org/10.1021/ja0705543

Sohn SS, Bode JW. 2006. N-Heterocyclic Carbene Catalyzed C?C Bond Cleavage in Redox Esterifications of Chiral Formylcyclopropanes. Angew. Chem. Int. Ed.. 45(36):6021-6024. https://doi.org/10.1002/anie.200601919

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