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Platinum Metals Rev., 2003, 47, (2), 73

Rhodium-Based Production of Aromatics

Ongoing Work In The Laboratory Of Our Rhodium Bicentenary Competition Winner

  • R. H. Crabtree
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We have been working towards creating newroutes to arene compounds by looking at CH activation chemistry. An initial step towards this goalhas now been published as a Journal of the American Chemical Society communication (1). The results have provided us with material for a full paper onthis topic and a related reaction now published in Chemical Communications (2).

In initial work we looked for the cyclometallation-induced formation of M–C bonds to sp2carbons of aryl and vinyl groups. These were successfully achieved for a whole range of substrates.However, the reaction was only successful with iridium, not rhodium. Nevertheless rhodium is tried each time and it is likely that we will get a similar rhodium-induced reaction in the future.

The reaction shown below produces cyclomet-allated species for a series of substrates such as enones and aryl ketones. Attempts to insert CH2 = CHR species (such as the usually particularlyreactive CH2 = CHSi(OEt)3) were unsuccessful, so alkynes were tried. The reaction with alkynes was remarkable, giving a double alkyne to vinylidene rearrangement, followed by an insertion into M–H to give a vinyl, followed by a second alkyne to vinylidene rearrangement and a second insertion. The resulting butadienyl proved to have a ‘π-agostic’ structure with M...C bond lengths much longerthan those of single bonds (2.5 Å was typical). No subsequent treatment, however, has so far given the required coupling.

Deuterium labelling showed that the RC≡CD deuterium 1,2-migrates selectively to provide RDC≡C in the vinylidene. This occurs without oxidative addition because that would lead to a fluxional 7-coordinate M(H)(D) species or more likely to a 6-coordinate M(H–D) species. Both processes are excluded because they would scramble the isotope. The 1,2-migration reaction isstrictly intramolecular within each RC≡CD group,because with RC≡CD and R′C≡CH both presenting the same reaction mixture (crossover reactionconditions), RDC=C and R′HC=C are the only resulting vinylidenes.

In future work, we hope to persuade rhodiumto carry out related coupling reactions, perhaps by moving to the N -heterocyclic carbene ligand system (L′) that has been so successful in alkene metathesis and palladium coupling. Indeed we already have the required [(cod)RhL′CI] species in hand for trials.

Other work (3) includes the synthesis of rhodium and iridium(I) complexes of N -heterocyclic carbenes by transmetallation from the corresponding silver(I) complexes; and the involvement of rhodium in an unusual norbomadiene to nortricyclyl rearrangement.

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Reference

  1. 1
    X. li,, C. D. Incarvito, and R. H. Crabtree, J. Am.Chem. Soc., 2003, DOI: 10.1021/ja0297281
  2. 2
    J. A. Mata,, E. Peris,, C. Incarvito and R. H. Crabtree, Chem. Commun., 2003, ( 2 ), 184
  3. 3
    A. R. Chianese,, X. Li,, M. C. Janzen,, J. W. Faller and R. H. Crabtree Organometallics, 2003, DOI:10.1021/om021029+

The Author

Professor Robert H. Crabtree of Yale University, U.S.A., won theJohnson Matthey Rhodium Bicentenary Competition in 2002. The competition celebrates the 200th anniversary of the discovery ofrhodium in 1804 by William Hyde Wollaston.

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