Metal Nanoparticles on Halloysite Applied in Stereoselective Hydrogenation of Tetrasubstituted Alkenes.

The ease of hydrogen-activation by platinum-group metals in combination with metal nanoparticle-halloysite support interactions impacts the catalytic hydrogenation of tetrasubstituted alkenes. Thus, Pd, Pt, and Ru nanoparticles were synthesized in a one-pot process using quinidine as a stabilizer, immobilized on halloysite nanotubes at low and high metal loadings. Structural analyses revealed that low metal loading favors small, well-dispersed nanoparticles, whereas higher loading induces aggregation except for Ru. Among all materials, Pd-A (4.1 wt% Pd) displayed the best balance between particle size and homogeneous distribution on the support, resulting in the highest catalytic activity and chemoselectivity. Moreover, Pd-A enables the hydrogenation of sterically demanding endocyclic and exocyclic oxazolidinones under mild conditions (3 bar H₂, 80 °C-100 °C), affording the corresponding saturated products with complete syn-diastereoselectivity. Deuteration studies using multi-nuclear NMR demonstrate that hydrogenation of the endocyclic oxazolidinone proceeds via direct addition to the tetrasubstituted C═C bond, accompanied by benzylic H/D exchange arising from C-H activation. Overall, the study of the catalytic activity of the as-prepared metal nanocomposites in terms of reactivity and selectivity permitted to conclude that Pd-A, a zero-valent Pd-halloysite nanocomposite, is the best heterogeneous catalyst of the series in terms of activity and selectivity for the hydrogenation of sterically hindered alkenes.