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<p>The synthesis, characterization,
and activity of Pd-doped layered double hydroxides (Pd-LDHs) for for
acceptorless amine dehydrogenation is reported. These multifunctional catalysts
comprise Brønsted basic and Lewis
acidic surface sites that stabilize Pd species in 0, 2+, and 4+ oxidation
states. Pd speciation and corresponding cataytic performance is a strong
function of metal loading. Excellent activity is observed for the oxidative
transamination of primary amines and acceptorless dehydrogenation of secondary
amines to secondary imines using a low Pd loading (0.5 mol%), without the need
for oxidants. N-heterocycles, such as indoline, 1,2,3,4-tetrahydroquinoline,
and piperidine, are dehydrogenated to the corresponding aromatics with high
yields. The relative yields of secondary imines are proportional to the calculated
free energy of reaction, while yields for oxidative amination correlate with the
electrophilicity of primary imine intermediates. Reversible amine
dehydrogenation and imine hydrogenation determine the relative imine:amine
selectivity. Poisoning tests evidence that Pd-LDHs operate heterogeneously,
with negligible metal leaching; catalysts can be regenerated by acid dissolution
and re-precipitation.</p>
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Reply to the ‘Comment on “In Defence of Oxidation States”’ by J. C. Green, Dalton Transactions, 2022, 51, DOI: 10.1039/D1DT02930D
