Hydroxypropyl‐α‐Cyclodextrin‐Capped Palladium Nanoparticles: Active Scaffolds for Efficient Carbon‐Carbon Bond Forming Cross‐Couplings in Water

Abstract

A new approach for the preparation of palladium nanoparticles in water from a renewable source, 2-hydroxypropyl-a-cyclodextrin (a-HPCD), which acts both as a reductant and capping agent, is presented. The palladium nanoparticles were characterized by using dynamic light scattering (DLS), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), which revealed the formation of spherical particles in the size range of 2-7 nm. Further analysis by Fourier-transform infrared spectroscopy (FT-IR) and 1 H NMR did not show covalent bonds between cyclodextrins and palladium nanoparticles, suggesting that a-HPCD is only physically adsorbed on the nanoparticle surface, presumably through hydrophobic interactions which limit the mutual coalescence of nanoclusters. The catalytic activity was tested in Suzuki, Heck and Sonogashira reactions in neat water, providing good yields and selectivities of coupling products under very low Pd loadings (0.5-0.01 mol%). Remarkably, the nanocatalyst showed significant stability hence the aqueous phase remained active for four subsequent runs. The combination of a binding site for substrates (the HPCD cavity) and a reactive centre (Pd core) provides a potential to explore functional catalysis in aqueous medium.

Using Size-Exclusion Chromatography to Evaluate Changes in the Sizes of Au and Au/Pd Core/Shell Nanoparticles Under Thermal Treatment

“…The mechanism through which this amino-functionalized CD stabilized the Au and Au/Pd core/shell NPs most likely involved strong interactions between the Au [36] and Pd [37] surfaces and the amino group, presumably through interactions that limited the mutual size variation of the Au and Au/Pd core/shell NPs [38]. Therefore, these CD-capped Au and Au/Pd core/shell NPs were stabilized under thermal treatment, suggesting their great potential for application as catalysts.…”

β‐cyclodextrin‐capped palladium nanoparticle‐catalyzed ligand‐free Suzuki and Heck couplings in low‐melting β‐cyclodextrin/NMU mixtures

“…Ligands are used to support the active palladium(0) species, thus preventing it from deactivation. Native CDs prove to be insufficiently reactive to reduce Pd(II) to Pd(0) under mild conditions . It is worth mentioning that previously Yao et al proposed a mechanism in which the acetate anion acted as a ligand in the rate‐determining oxidative addition step .…”

Sonogashira Reactions Using Nanocatalysts

“…Thus, hydroxypropyl-␣-cyclodextrin-capped Pd-NPs have been obtained by addition of ␣-cyclodextrin partially O-hydroxypropylated to an aqueous solution of PdCl 2 . 50 The active Pd(0) generated [Pd(0)/CD] has been used as a catalyst (0.5 mol%) in the coupling of aryl iodides, such as 4-iodoanisole, or bromobenzene with phenylacetylene or 1-octyne, in the presence of triethylamine in water at 60-70 • C to give the corresponding coupled alkynes such as 17 (Scheme 4.6).…”