Researchers in the US have developed an iridium catalyst that promotes carbon-carbon bond formation between methanol and allenes. The process enables direct conversion of methanol to higher alcohols, with no byproducts.
With a global annual production exceeding 35 million tonnes, methanol has the potential to be an attractive carbon feedstock for chemical production. At present, methanol is generated from methane via synthesis gas, a mixture of carbon monoxide and hydrogen. It can also be produced renewably from the gasification of cellulose.
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Methanol's ability to react at the oxygen atom is well known, but its ability to react at carbon is not. The only examples of C-C bond formation using methanol are the Monsanto and Cativa processes, in which rhodium or iridium catalysts convert methanol and carbon monoxide to acetic acid.
Now Michael Krische and colleagues at the University of Texas at Austin in the US, have developed a new iridium complex, which can catalyse the C-C coupling of methanol with a range of allenes. The overall reaction - a hydrohydroxymethylation - adds the C-H bond of the methanol molecule across the pi bond of the allene molecule. Crucially, as all the atoms in the reactants appear in the product, no stoichiometric byproducts are generated.
'When one reacts methanol with olefins, alkynes or allenes, one typically observes C-O bond formation,' says Krische. 'The iridium complex we have developed promotes hydrogen transfer between alcohols and pi-unsaturated reactants, triggering generations of formaldehyde allyliridium pairs, which combine to form higher alcohols.'
Alan Goldman, who researches organometallic catalysis at Rutgers University in the US, says the team's process has potential: 'There's certainly a lot of interest in forming C-C bonds in general and [this group has] formed them from carbon hydrogen bonds, which are not considered to be functional groups.'
The reaction is regioselective, producing higher alcohols that possess quaternary carbon centres, which are typically challenging to prepare. 'What would make it very exciting is if they can do this enantioselectively or diastereoselectively,' says Goldman. Krische and his colleagues are already looking at new catalysts which might achieve this goal. In the long term, Krische also hopes to develop cheaper iron catalysts for the process.
A further limitation is that although the methanol used is highly abundant the allenes are not. 'It would be nice to couple methanol to alpha-olefins, which are direct products of petroleum cracking,' says Krische.
Manisha Lalloo
RSC
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