Pd-nanoparticles on end-functionalized poly(lactic acid)-based stereocomplexes: Selectivity in C=C bond hydrogenation reactions

The selective hydrogenation of the C=C double bond in ?,?-unsaturated carbonyl compounds is an important organic transformation because it leads to the saturated counterparts which are widely applied in synthetic and pharmaceutical chemistry. [1] The classical heterogeneous catalyst for the latter catalytic transformation is Pd on carbon (Pd@C), [2] which lacks of chemoselectivity due to charge transfer from the graphite structure of the carbon to the surface metal atoms. As a consequence, the binding energy for the C=C double bond decreases, fostering C=O bond hydrogenation. [3] Alternatively to carbon supports, functional organic polymers, [4] such as polyamines [5] and polyketones [6] have shown to be promising supports for Pd-nanoparticles (NPs) which catalyze the hydrogenation of cinnamaldehyde to 3-phenylpropanal showing chemoselctivities between 84 and 88%. In this presentation we focus on Pd-NPs, synthesized by metal vapor synthesis (MVS) technique [7] on differently end-functionalized poly(lactic acid) (PLA)-based stereocomplexes. [8] The chemical nature of the end-group notably influenced the dispersion (i.e. Pd-NPs' size) and stabilization of Pd-NPs against sintering in the course of the catalytic hydrogenation of ?,?-unsaturated carbonyl compounds in THF solution and under solventless experimental conditions. As a result, Pd-NPs, stabilized by a 2,2'-bipyridine-functionalized PLA stereocomplex (Pd@LBiPy), exhibited the best performance in terms of catalytic activity, chemoselctivity for the C=C bond hydrogenation of cinnamaldehyde (i.e. Pd@LBiPy (99%) vs Pd@LH (60%)) and recyclability, outperforming Pd@C (55%). [1] H.U. Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner, M. Studer, Adv. Synth. Catal. 345 (2003) 103. [2] X. Ji, X. Niu, B. Li, Q. Han, F. Yuan, F. Zaera, Y. Zhu, H. Fu, ChemCatChem 6 (2014) 3246. [3] F. Delbecq, P. Sautet, J. Catal. 152 (1995) 217. [4] P.Kaur, J.T. Hupp, S.T. Nguyen, ACS Catal. 1 (2011) 819. [5] Y. Gao, C.-A. Chen, H.-M. Gau, J.A. Bailey, E. Akhadov, D. Williams, H.-L. Wang, Chem. Mater. 20 (2008) 2839. [6] A.M. Raspolli Galletti, L. Toniolo, C. Antonetti, C. Evangelisti, C. Forte, Appl. Catal. A: Gen. 447-448 (2012) 49. [7] G. Vitulli, C. Evangelisti, A.M. Caporusso, P. Pertici, N. Panziera, S. Bertozzi, P. Salvadori in: B. Corain, G. Schmid, N. Toshima (Eds.), Metal Nanoclusters in Catalysis and Materials Science: The Issue of Size Control, Elsevier, Amsterdam 2008, chapter 32. [8] G. Petrucci, W. Oberhauser, M. Bartoli, G. Giachi, M. Frediani, E. Passaglia, L. Capozzoli, L. Rosi, Appl. Catal. A: Gen. 469 (2014) 132.