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Rh-catalyzed Alcohol Deoxygenation

“Acceptorless Photocatalytic Dehydrogenation for Alcohol Decarbonylation and Imine Synthesis.” Ho, H-A.; Manna, K.; Sadow, A. D. Angew. Chem. Int. Ed. 2012, 51, 8607-8610. DOI: 10.1002/anie.201203556

The use of biorenewables as feedstock chemicals for commodity chemicals as well as fuels requires mild, selective removal of oxygen-containing functional groups. This is in direct contrast to the production of these chemicals from petroleum products, which, at least for highly functionalized target molecules, necessarily involves oxygenation of hydrocarbons.

There are a large amount of methods development currently underway and I highlight the recent report from the Sadow group on the decarbonylation of alcohols under Rh catalysis. I think the described reaction is a good example of green chemistry, as the reaction is high-yielding, selective, and performed at room temperature under photocatalytic conditions. One serious drawback is the use of benzene as the solvent, although toluene works as a solvent in at least some cases.

Reasoning that photolysis would prevent catalyst inhibition by CO binding, the researchers first screened Rh(I) catalysts under photocatalytic conditions with the test substrate cyclohexanemethanol. Unfortunately, no cyclohexane was observed under these reaction conditions. The group then tested Rh and Ir compounds known for C-H activation, such as Cp*Ir(CO)2 and Tp*Rh(CO)2, and did observe cyclohexane for one of the tested catalysts, albeit in low yield (36 % NMR yield with Tp*Rh(CO)2). CO and H2 were also observed, consistent with the targeted alcohol decarbonylation reaction. Interestingly, using their previously reported rhodium tris(oxazolinyl)borate complex ToMRh(CO)2 (1) improved the yield to > 95%. Furthermore, the related dihydride, ToMRh(H)2CO (2) was roughly three times slower and the Ir complex ToMIr(CO)2 was inactive for this reaction.