We report a unified continuous flow strategy for the synthesis of three commercially important neonicotinoid insecticides: Thiamethoxam, Clothianidin, and Imidaclothiz, directly from inexpensive propionaldehyde. Central to this strategy is the efficient preparation of the key intermediate 2-chloro-5-bromomethylthiazole (CBT) via a telescoped flow sequence integrating α-bromination/cyclization with thiourea, a Sandmeyer chlorination using NaNO2/HCl in a 3D microreactor, and a photoinduced benzylic bromination. Subsequent transformation of CBT using a K2CO3-packed-bed flow reactor enabled the synthesis of Thiamethoxam and Imidaclothiz, while Clothianidin was accessed through acid-mediated hydrolysis of Thiamethoxam. This continuous flow approach ensures the safe handling of hazardous reagents and unstable intermediates. Compared to the corresponding batch route (7.5 h, 16.5% overall yield), the flow process shortens the overall reaction time to 1 h, and increases the overall yield to 32%. This work showcases the potential of continuous flow technology for developing efficient, scalable, and sustainable synthetic routes to complex agrochemicals from simple feedstocks.
Lu et al. (Mon,) studied this question.