Chemoselectivity of Reactions with Syngas Indicating Water in the Coordination Sphere of Rhodium

摘要

Hydroformylation of propylene belongs to the industrial processes in which the principle of biphasic aqueous catalysis (1) is applied. In this type of catalysis, the catalysts are soluble in water and have only negligible solubility in the organic phase containing the educts and products. This is achieved by modification of the ligands with hydrophilic groups such as carboxylate or sulfonate groups. The best known example for a water-soluble phosphine is TPPTS (Pfm-C_6H_4-SO_3Na]_3). Biphasic catalysis allows the catalyst to be separated from the organic phase by simple phase separation. Although this technology has major advantages, it has not been industrially applied for the conversion of long-chained olefines with syngas to form aldehydes for the production of plasticizers. To overcome the limitations in activity caused by decreasing water-solubility of the olefinic substrates with growing carbon chain length, the addition of surfactants has been suggested. This in turn can disturb the phase separation by formation of stable microemulsions. A catalyst that is likely to form aggregated structures but still allows simple catalyst separation has recently been reported by Hanson et al. (2). This aqueous Rh complex catalyst is much more active than Rh/TPPTS systems in the hydroformylation of oct-1-ene. Catalysts embedded in micelles, vesicles or other aggregated structures are expected to gain importance in biphasic catalysis with substrates having low solubility in water. In principle, the catalysis could happen in the aqueous bulk phase, in the hydrophobic regions inside of micelles or related structures, or in the interface layers between hydrophilic and hydrophobic micro phases. For an optimal design of biphasic aqueous catalysts, it is important to know where the catalyst works. Cornils et al. (3) used a productivity function of interfacial area for the refinement of a kinetic model. The refined model assumed that the hydroformylation of propylene occurs in the interphase region between water and the organic phase. It described the experimental results more accurately than a model for the reaction taking place in the aqueous bulk phase. Further indicators, such as a chemical probe giving different results in aqueous and organic environments would be desirable to shed light on the problem 'where the reaction takes place' (3). We found the conversion of 2,5-dimethoxy-2,5-dihydro-furan (1) with syngas to be such a probe reaction.