Pd-EPS: a new biogenerated catalyst for aqueous biphasic carbonylations

Catalytic biphasic reactions are widely used because of their advantages over conventional homogeneous catalytic syntheses as the easy separation and reuse of the expensive catalyst and the environmental aspects of the chemical process. In particular, the use of both water as co-solvent for biphasic reactions and easily recyclable catalysts are highly desirable for the realization of green processes [1-3]. Bio-generated metal-binding polysaccharides may be novel sustainable materials with interesting catalytic properties for many synthetic applications; in this context we prepared a new Pd species bound to an exopolysaccharide (EPS) directly produced by alive bacterial cells of Klebsiella oxytoca BAS-10 grown in static mode in the presence of Pd(NO3)2 [4]. Pd-EPS, was treated with 1 MPa of H2 at 30°C for 21h, so furnishing a microstructured catalyst (called activated Pd-EPS) constituted by a mixture of Pd(II)- and Pd(0)-EPS in the ratio 1.9/1. Activated Pd-EPS was used as catalyst in the aqueous biphasic carbonylation of 2-iodo-5-alkylthiophenes (I), to afford the corresponding 2-formyl-5-alkylthiophenes (II), important flavours and fragrances. The reaction was carried out at 60-80°C for 24h at 5 MPa of CO in the presence of TES or PMHS as hydrogen donor and of different bases as Na2CO3, quinine, TMEDA, etc.. Almost quantitative substrate conversions were obtained and aldehydes II were formed with very high selectivity (89-93%).