“An efficient didehydroxylation method for the biomass-derived polyols glycerol and erythritol. Mechanistic studies of a formic acid-mediated deoxygenation” Arceo, E.; Marsden, P.; Bergman, R. G.; Ellman, J. A. Chemical Communications, 2009, 23, 3357. 10.1039/b907746d
A major focus in the area of “green” methods development is the mild and selective removal of functionality from readily available bio-derived feed stocks. Unlike petrochemical derived starting materials, the majority of biomass is highly oxygenated (think carbohydrates or lignin), and before the carbon embedded within this framework can be used for all-purpose chemical manufacturing the oxygen must be removed.
Recently, a collaborative effort from the Bergman and Ellman groups at the University of California, Berkeley described such a procedure for the di-deoxygenation of glycerol to form allyl alcohol. It is important to note that the conversion of glycerol into value added chemicals has received a great deal of interest since it is a byproduct of biodiesel manufacturing. [This review by Zhou et al. DOI: 10.1039/b707343g should pique the interests of those interested in glycerol conversion.] With the recent expansion of biofuels, the authors recognize that glycerol is a very attractive starting material for the synthesis of more valuable chemicals. (See Scheme 1) In this particular case, the authors are targeting allyl alcohol, which addition, it’s important to point out that allyl alcohol (an important industrial chemical in its own right) is currently produced by oxidation of propene, which is ultimately derived from petrochemicals.
So, let’s get on to the chemistry. In the current process, glycerol is converted to allyl alcohol at elevated temperatures (230-240 oC- a very reasonable temperature for manufacturing purposes) in the presence of formic acid (itself a high-volume commodity chemical) to provide the product in 80% isolated yield. Importantly, the authors found that passing a steady stream of N2 through the reaction medium was required to achieve good isolated yields of allyl alcohol, as they believe the carrying gas helps to whisk away the sensitive product which otherwise suffers severe decomposition.
In typical Bergman fashion, a series of mechanistic experiments provide compelling evidence for their proposed mechanism. First off, deuterium labeling experiments (Scheme 3, Equations 1 and 2) suggest that formic acid is not serving as a hydride donor. In addition, the reaction is stereospecific (Scheme 3, Equations 3 and 4), as syn and anti diols provide cis and trans olefins respectively.
The authors also demonstrate that this method can be applied to a variety of simple cyclic and acylic diols, but the substrate scope doesn’t really delve into functional group compatibility, so its utility on more complicated substrates remains questionable (It’s probably worth noting that neat formic acid at temperatures >200 oC isn’t what would be considered “mild”).
There is one more important substrate that the team looked at, and that’s the di-deoxygenation of erythritol. Erythritol is a four-carbon sugar-alcohol, accessible via yeast or fungal fermentation of glucose from corn starch (see Scheme 1 to see how this fits into biomass conversion). Under these reaction conditions, erythritol is converted to 2,5-dihydrofuran, albeit in modest yield (Scheme 4). Now, this reaction might need some further optimization before it’s operational on industrial scale, but the concept is a great one. Current methods for the production of 2,5-dihydrofuran start from petrochemical feedstock and require multistep processes involving either ring-closing-metathesis or rearrangement of vinyl oxirane. The authors do provide some mechanistic insight, suggesting that erythritol is initially converted to cyclic diol 6 (They prepare 6 via a known cyclo-dehydration procedure, reported here DOI : 10.1021/op0601058). Intermediate 6 then undergoes the formic acid mediated di-deoxygenation to provide the dihydrofuran product.
In the end, the authors have provided a simple procedure for the preparation of two important commodity chemicals starting from biomass. It will be interesting to see where this chemistry leads, and to what extent synthetic organic chemists will pick it up as a useful alternative to the synthesis of olefins.