Synthesis of gem‐Dichlorocyclopropanes Using Liquid–Liquid Slug Flow

By analyzing the causes of precipitation and clogging in continuous flow dichlorocyclopropanation, we established that the flow reaction can be performed only with T‐junctions and microtubes. Unlike tertiary amines, when quaternary ammonium salts were used as PTCs, precipitation and clogging in microchannels did not occur, and an excellent yield of up to 99% was obtained after optimizing the reaction temperature, reactant concentration, and retention time. In the scale‐up experiment, the reaction yields slightly decreased as the inner diameter (ID) of the microchannel was increased, but the calculated throughput increased from 2.41 g/h (ID: 1.5 mm) to 3.46 g/h (ID: 1.9 mm) and 5.13 g/h (ID: 2.4 mm) We established that the continuous flow dichlorocyclopropanation can be performed only with T‐junctions and microtubes. When quaternary ammonium salts were used as phase transfer catalysts, precipitation and clogging in microchannels did not occur, and an excellent yield of up to 99% was obtained in 20 min.