Palladium nanoparticles stabilized by reagents like polymers, dendrimers, ionic liquids, macrocycles and distinct organic ligands having hetero donor atoms etc., have owned a fair share of the experimental progress in the field of palladium-based catalysis1. In our group solvent mediated ligand free palladium nanoparticles (LF-PdNPs) had been synthesized without using any external stabilizing and reducing agent and the material was utilized in different -C-C- coupling reactions such as Suzuki, Sonogashira and subsequent synthesis of benzofurans and isobenzofurans2. Recently synthesis and catalytic usage of palladium nanoparticles in different organic transformations under aqueous condition was explored3.
In this work, LF-PdNPs were prepared in MeOH/MeCN mixture following already established method of our laboratory. Pentaerythritol (PE) stabilized palladium nanoparticles (PdNPs/PE) were also prepared in water medium by exploiting the reducing and stabilizing capabilities of PE. Mixed reagents containing PE along with one other among tannic acid (TA), sodium dodecyl sulphate (SDS), triton-X and graphene oxide (GO) were employed for the synthesis of PdNPs like (PdNPs/PE/TA), (PdNPs/PE/SDS) and (PdNPs/PE/GO). All of these pentaerythritol stabilized palladium nanoparticles with and without an additional stabilizer were characterized by several physico-chemical techniques like UV-Vis, PXRD, HR-SEM, EDS and XPS.
Keeping in mind the high hydrogen affinity of palladium, the catalytic effectivity of ligand free palladium nanoparticles (LF-PdNPs) was explored in chemoselective hydrogenation of different organic functionalities such as >C=C (in chalcones, α,β-benzylidine-1,3-diketones and unsaturated esters), -C≡C- (in biaryl acetyenes) and –NO2 (in aromatic and aliphatic nitro compounds) using gaseous hydrogen (H2). The respective products were obtained in very good to moderate yields.
LF-PdNPs catalyzed semi-hydrogenation of internal alkynes remained unsatisfactory when attempted using H2 under aqueous condition. Pentaerythritol stabilized palladium nanoparticles (PdNPs/PE) was then utilized under the same reaction condition wherein formic acid and hydrazine hydrate were employed and found to be effective as hydrogenating agents. Reproducibility in terms of ratios of cis-trans isomers of the product was noticed. Later pentaerythritol stabilized palladium nanoparticles with additional stabilizers namely PdNPs/PE/TA and PdNPs/PE/SDS were employed as catalyst in the semi-hydrogen reaction and the above-mentioned reproducibility was somewhat controlled.
References:
1. (a) Balanta, A.; Godard, C.; Claver, C. Chem. Soc. Rev. 2011, 40, 4973-4985; (b) Athilakshmi, J.; Chand, D. K. J. Chem. Sci. 2011, 123, 875-881; (c) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem. Int. Ed. 2005, 44, 7852-7872.
2. (a) Athilakshmi, J.; Ramanathan, S.; Chand, D. K. Tetrahedron Lett. 2008, 49, 5286-5288; (b) Mandali, P. K.; Chand, D. K. Catal. Commun. 2013, 31, 16-20; (c) Mandali, P. K.; Chand, D. K. Catal. Commun. 2014, 47, 40-44; (d) Mandali, P. K.; Chand, D. K. Synthesis 2015, 47, 1661-1668.
3. (a) Dai, B.; Zhou, Y.; Liu, C.; Chen, J.; Shen, Z.; Zhao, Y. J. Chem. Res. 2020, 45, 540-544; (b) Feng, J.; Handa, S.; Gallou, F.; Lipshutz, B. H. Angew. Chem. Int. Ed. 2016, 55, 8979-8983; (c) Ansari, T. N.; Jasinsk, J. B.; Leahy, D. K.; Handa, S. JACS Au 2021, 1, 308-315.