Flash Flow Pyrolysis: Mimicking Flash Vacuum Pyrolysis in a High-Temperature/High-Pressure Liquid-Phase Microreactor Environment

Flash vacuum pyrolysis (FVP) is a gas-phase continuous-flow technique where a substrate is sublimed through a hot quartz tube under high vacuum at temperatures of 400–1100 °C. Thermal activation occurs mainly by molecule–wall collisions with contact times in the region of milliseconds. As a preparative method, FVP is used mainly to induce intramolecular high-temperature transformations leading to products that cannot easily be obtained by other methods. It is demonstrated herein that liquid-phase high-temperature/high-pressure (high-T/p) microreactor conditions (160–350 °C, 90–180 bar) employing near- or supercritical fluids as reaction media can mimic the results obtained using preparative gas-phase FVP protocols. The high-T/p liquid-phase “flash flow pyrolysis” (FFP) technique was applied to the thermolysis of Meldrum’s acid derivatives, pyrrole-2,3-diones, and pyrrole-2-carboxylic esters, producing the expected target heterocycles in high yields with residence times between 10 s and 10 min. The exact control over flow rate (and thus residence time) using the liquid-phase FFP method allows a tuning of reaction selectivities not easily achievable using FVP. Since the solution-phase FFP method does not require the substrate to be volatile any more —a major limitation in classical FVP—the transformations become readily scalable, allowing higher productivities and space–time yields compared with gas-phase protocols. Differential scanning calorimetry measurements and extensive DFT calculations provided essential information on pyrolysis energy barriers and the involved reaction mechanisms. A correlation between computed activation energies and experimental gas-phase FVP (molecule–wall collisions) and liquid-phase FFP (molecule–molecule collisions) pyrolysis temperatures was derived.