Carbenes clean up

Laser light avoids side-product formation in ligand synthesis.

Research at the University of York, UK, has uncovered a new, one step synthesis for making highly reactive rhodium-carbene complexes.

Interest in carbenes as ligands in organometallic chemistry has grown due to their high reactivity and subsequent applications in organic synthesis and catalysis. However, indirect, multiple-step syntheses are still needed to form transition metal carbene complexes.

Now, Robin Perutz and his team have devised a direct procedure, by using photolysis to generate the carbene ligand in situ.

Free carbene formation often suffers because undesirable side-products are produced when the carbene attacks solvent or reacts with the diazo starting material. But by using a mercury arc fitted with an appropriate wavelength filter and deuterated benzene at room temperature, the team saw that azine, R ₂CNNCR ₂, a dimer of the rhodium complex starting material, as well as the desired product were all formed. However, by using a laser tuned to the absorption spectrum of diphenyldiazomethane, specific irradiation was achieved without degrading the rhodium starting material.

The new technique is carried out at low temperature, using laser irradiation and with deuterated chlorobenzene as the solvent. When the desired carbene complex was made in this way, its formation rate was significantly faster than the rate of both solvent attack and azine side-product formation. Not only that, but the carbene was the only product formed.

’This route may prove important when other synthetic approaches are unavailable,’ explains Perutz, who continues by saying: ’one challenge is now to scale up these reactions and isolate the products. Another is to extend the principle to even more reactive carbenes’.

Andrew West