Designing Carbenes

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Science  19 Aug 2005:
Vol. 309, Issue 5738, pp. 1155
DOI: 10.1126/science.309.5738.1155b

In the past 10 years, N-heterocyclic carbenes (NHCs) have grown from being regarded as chemical curiosities to become versatile ligands for a wide range of homogeneously catalyzed reactions. Most carbenes (molecules bearing a two-coordinate divalent carbon) are unstable at room temperature, but in an NHC, the unsaturated carbon is stabilized by electron-donating nitrogen atoms on either side.

Lavallo et al. have designed a different type of carbene, termed a cyclic alkyl(amino) carbene (CAAC), in which one of the nitrogens is replaced by a quaternary alkyl center. By appending bulky groups such as cyclohexane to this center, the authors can prepare compounds with stabilities comparable to those of NHCs, but sporting distinct steric and electronic properties. As a ligand, the CAAC is a strong σ-electron donor, and crystallography of a CAAC-coordinated palladium complex reveals steric crowding, particularly close to the metal center. One practical result of these properties is the efficient catalysis by this Pd complex of unactivated aryl chloride couplings to the α-position of aldehydes and ketones. — JSY

Angew. Chem. Int. Ed. 10.1002/anie.200501841 (2005).

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