(191e) Pd/TOMPP-Catalyzed Telomerization of 1,3-Butadiene with Polyols and Carbohydrates: New Opportunities for Catalytic Biomass Valorization

The sustainable production of valuable bulk chemicals from biomass as an alternative for petrochemicals is an important and pressing scientific challenge. We therefore aim to develop clean and efficient catalytic processes for the conversion of renewable feedstocks. A particularly interesting technology is the conversion of various biomass-derived polyhydric compounds by palladium-catalyzed telomerization of 1,3-butadiene [1]. This 100% atom-efficient reaction converts alcohols in one step to the corresponding C8-ethers which, after hydrogenation, have many potential applications as surfactants and cosmetics, but also as chemical building blocks for the pharmaceutical industry.

Since its initial discovery the Pd-catalyzed telomerization reaction has matured and is currently commercially applied for the production of 1-octanol. Phosphine-based ligand systems are most commonly used, e.g. the water-soluble tris(m-sulfonato-phenyl)phosphine ligand (TPPTS), but more recently also imidazolium-based carbenes have been reported [1]. In this context, we wish to report here the results of broad substrate screening obtained using our newly developed homogeneous catalyst system: Pd/TOMPP (tris(o-methoxyphenyl)phosphine) [2-5]. We initially communicated that Pd/TOMPP is a highly active and selective catalyst for the conversion of glycerol, an important biomass-derived platform molecule [2,3]. It is important to note here that these telomerization reactions were achieved without the use of solvent and without the need for an added base. Encouraged by these results, we have now greatly expanded the scope of the Pd/TOMPP catalyst system to include many of the major renewable oxygenates, such as various polyols, sugar alcohols and carbohydrates [4,5]. Catalytic results show that the Pd/TOMPP combination rivals the activity and selectivity of any of the known catalysts over the whole range of substrates.

The telomerization activity of the Pd/TOMPP catalyst was screened using around 20 different substrates which include various (sugar) alcohols, polyols and the representative carbohydrate classes. In case of liquid substrates, reactions were carried out both solvent- and base-free. For solid substrates the use of a solvent was necessary to allow for their participation in the telomerization reaction. After optimization of reaction conditions, high conversions were obtained in very short reaction times; depending on the substrate 12-70 min were needed for near full conversion of the alcohol substrate. In addition, the reactions could be performed with low butadiene-to-substrate ratios. High selectivities towards the desired monotelomer (well over 80% for most substrates) were obtained for the polyol substrates and relatively low degrees of substitution were observed for the carbohydrates and sugar alcohols. For example, selectivities for the mono-telomer for 1,2-butanediol and 1,2-propanediol were close to 90 % with TONs of 7200 and 7800, resp. Also, exceptionally high TOFs of up to 350000 h-1 were obtained, illustrating the high activity of Pd/TOMPP system. Catalyst activity was found to depend on the type and properties of the alcohol (e.g. coordination properties, acidity, hydrophobicity) and activity followed the order 1,2-butanediol > 1,4-butanediol > 1,2-propanediol > 1,3-propanediol > ethanediol [4]. A structure/activity relationship could also be established for the telomerization activity with the different classes of carbohydrate substrates. In addition, a strong dependence of the reaction rate on the length of the substrate chain was observed for the sugar alcohols [5].

In conclusion, we have successfully applied the Pd/TOMPP system in the telomerization of 1,3-butadiene with various biomass-based oxygenates. Low catalyst loadings were used and high TONs and TOFs were obtained together with high selectivities to the desired products. A relation between the structure and physicochemical properties of the substrate and catalytic activity was identified.

We would like to thank the ASPECT-ACTS Program for financial support.

[1] Behr, A.; Becker, M.; Beckmann, T.; Johnen, L; Leschinski, J.; Reyer, S. Angew. Chem. Int. Ed. 2009, 48, 3598-3614.

[2] Palkovits, R.; Nieddu, I.; Klein Gebbink, R. J. M.; Weckhuysen, B. M. ChemSusChem 2008, 1, 193-196.

[3] Palkovits, R.; Nieddu, I.; Kruithof, C.A.; Klein Gebbink, R. J. M.; Weckhuysen, B. M. Chem. Eur. J. 2008, 14, 8995-9005.

[4] Palkovits, R.; Parvulescu, A. N.; Hausoul, P. J. C.; Kruithof, C.A.; Klein Gebbink, R. J. M.; Weckhuysen, B. M. Green Chem. 2009, in press

[5] Hausoul, P. J. C.; Bruijnincx, P. C. A.; Klein Gebbink, R. J. M.; Weckhuysen, B. M., to be submitted