Continuous-Flow Hydrogenation of 4-Phenylpyridine to 4-Phenylpiperidine with Integrated Product Isolation Using a CO2 Switchable System.

A process comprising a continuous-flow hydrogenation reaction integrated with selective water-organic solvent biphasic extraction using CO₂ as molecular switch to control partitioning was devised for the synthesis of arylpiperidines from arylpyridines. The selective hydrogenation of 4-phenylpyridine using heterogeneous carbon-supported metal catalysts was chosen as model reaction. A design-of-experiment approach was used for the identification of suitable reaction conditions under continuous-flow operation. A maximum selectivity for 4-phenylpiperidine of 96% was achieved at 87% conversion suppressing the deep hydrogenation to 4-cyclohexylpiperidine almost completely (⩽5%). The higher basicity of piperidines over pyridines was exploited for selective and reversible protonation of the product upon pressurization with CO₂ separating it quantitatively from the remaining starting material in a water-EtOAc biphasic system. This concept enabled a fully integrated and a salt-free synthetic process using a standard Pd/C catalyst for the hydrogenation coupled with the CO₂-triggered isolation of the desired product 4-phenylpiperidine in 81% yield and 98% purity. [ABSTRACT FROM AUTHOR]