“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.
The reaction was successful for a range of aliphatic (e.g., methanol and ethanol) and benzylic primary alcohols and tolerated the presence of aryl flouro and ether groups as well as silyl groups. The authors note that benzylic alcohols are a central component of lignin. The reaction did not tolerate aryl nitro, chloro or ester groups.
The authors then explored the mechanistic details of the reaction, noting that small amounts of the aldehyde were detected during the reaction and that using the aldehydes directly provided the hydrocarbon under the catalytic conditions using either 1 or 2. Furthermore, the presence of CO did not inhibit the reaction at 1 atm, but did inhibit the reaction at 5 atm. No aldehyde or hydrocarbon was detected in the absence of light.
The observation that catalysis with 1 and 2 proceeds at different rates suggests distinct intermediates, although no catalytic intermediates were detected in the authors’ present study. The catalytically active intermediate from 1 is likely ToMRh(CO) (3), provided by photolytic dissociation of CO. In contrast, photolytic CO dissociation from 2 would afford the dihydride ToMRhH2 (4).
The hypothesis that CO dissociation is required for catalysis is supported by two observations: (i) the lack of thermal reactivity of 1 with alcohols and (ii) the reaction of ToMRh(H)2(NCCH3) with cyclohexanemethanol, which provides cyclohexane as well as 2 (likely via acetonitrile loss to give the proposed dihydride 4).
With this information, the authors propose the following, simultaneously-operative catalytic cycles. It is interesting to me that catalysis from 1 requires two photons per cycle, while just a single photon is required for 2. I’m not sure how you could experimentally confirm this, though.
In summary, the Sadow group describes a new application for their rhodium tris(oxazolinyl)borate complex as a photocatalyst for the decarbonylation of alcohols. Although I didn’t cover it, they also apply this catalyst to the photocatalytic conversion of amines to imines, a reaction which follows a different route than alcohol decarbonylation – namely, dehydrogenation of the primary amine to form an aldimine followed by reaction with a second equivalent of amine to form the observed imine product with the presumed loss of ammonia. It is thus noteworthy that their catalyst can handle both oxygen and nitrogen functionality. Going forward, I hope to see this work extended to diols and polyols.