Solvent-Free Michaelis-Arbuzov Rearrangement under Flow Conditions

The first solvent- and catalyst-free procedure for the Michaelis-Arbuzov reaction under flow conditions was developed. A variety of alkylphosphonic esters could be obtained using this protocol starting from the corresponding trialkyl phosphites and even catalytic amounts of alkyl halides with very short reaction times (8.33-50 min) and excellent conversions. In general, this protocol works effectively when the alkyl halide is used in catalytic amounts as low as 5-10% only if it concerns the synthesis of homo alkylphosphonates. One equivalent and an excess of alkyl halides should be used in the reaction with alkyl phosphite if the alkyl group of the selected substrates differ. Thus, it provides a sustainable, fast alternative to the existing methods for the preparation of alkylphosphonates. The isolation of the reaction products is straightforward due to the lack of solvents and a high purity of the obtained products (conv ≥ 99%), and notably, in the catalytic procedures there are only traces of alkyl halides formed after the reaction is complete. The reactions conducted using a glass microreactor chip with an internal volume of 250 μL allow the production of 1.6-1.95 g of organophosphorus esters per hour.