One Carbene Helps Another

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Science  03 Sep 2004:
Vol. 305, Issue 5689, pp. 1371
DOI: 10.1126/science.305.5689.1371c

Homogeneous copper catalysts are widely used to add electrophilic carbenes to organic substrates. In a typical variant of the reaction, the Cu center stabilizes a carbene formed by N2 loss from an ethyl diazoacetate (EDA) precursor; next, the carbene can transfer from Cu to an olefin to form the desired cyclopropane derivative. Unfortunately, the Cu- carbene complex also tends to react with another EDA molecule, giving undesired carbene dimers.

Fructos et al. have prepared a Cu(I) chloride catalyst that effectively eliminates the EDA dimerization pathway, while transferring a carbene to olefins, alcohols, and amines at high rates and efficiencies. It turns out that the key to this catalyst is another carbene, bound to Cu as a ligand. Unlike the electrophilic reagent derived from EDA, the ligand is an electron-rich substituted N-heterocyclic carbene, a class of molecule increasingly used as an alternative to phosphines and amines in coordination compounds. How EDA dimerization is avoided is not yet clear, but the authors speculate that the order of steps may be reversed, with olefin (or alcohol or amine) coordination to the Cu complex preceding reaction with EDA. — JSY

J. Am. Chem. Soc. 10.1021/ja047284y (2004).

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