(49f) Efficient Synthesis of N-Vinyl Carbazole from Acetylene By a Continuous High-Pressure Liquid-Phase Reaction Process with Inherent Safety

Acetylene is a classic chemical raw material, which has been widely used in the production of bulk chemicals and fine chemicals since early 20th century. Even today, acetylene-involved organic synthesis remains important for its shorter path and higher atom economy. As most of acetylene-involved reactions are homogeneously catalyzed, the reaction rate is closely correlated to the concentration of acetylene in the liquid phase. However, increasing acetylene solubility in the liquid phase, achieved by elevating gas phase pressure, presents safety challenges due to the explosive nature of acetylene. As a safety regulation, acetylene must be used at low pressure and usually mixed with an inert gas (such as nitrogen), which creates a contradiction between reaction efficiency and safety. In this work, a continuous high-pressure liquid-phase acetylene microreaction process is rationally designed and successfully realized by employing the strategy of decoupling the acetylene dissolution and successive reaction. As the acetylene reaction is operated in a pure liquid phase with high acetylene concentration, the safety and efficiency of the acetylene conversion process are greatly improved. As a result, N-vinylcarbazole (NVC) is continuously synthesized from dissolved acetylene at 165 °C and 4.5 MPa. Compared to the conventional reaction process, the reaction time is reduced to 5-20 minutes from about 6 hours, and the amount of catalyst is reduced to 0.18-0.55wt% from 4wt%. To further increase the yield of NVC, 4-tert-butylcatechol (TBC) is added to the feedstock as a polymerization inhibitor, which improves the yield of NVC to 88%. Several other vinyl compounds, in addition to NVC, are also continuously synthesized from acetylene using this method, which suggests that the process decoupling reaction strategy is widely applicable to acetylene-involved reactions.