Synthesis
1. Synthesis of (trimethylsilyl)methylboronic acid: Under nitrogen protection, magnesium metal (0.33 mol) and a few small grains of iodine were added to a reaction flask equipped with a condenser and a constant pressure feeding funnel. While stirring, 30 mL of a mixed solution of trimethylsilyl bromide (0.3 mol) dissolved in 250 mL of 2-methyltetrahydrofuran was added dropwise. The mixture was heated to above 40°C, and after the reaction was initiated, the remaining solution was slowly added dropwise. The mixture was refluxed for 3 hours and then cooled to room temperature.
In another reaction flask, under nitrogen protection, add 0.35 mol of trimethyl borate and 80 mL of 2-methyltetrahydrofuran. After stirring evenly, cool to -20°C and slowly add the prepared Grignard reagent over 2-3 hours. Maintain the temperature and continue the reaction for another 2-2.5 hours. When the reaction is confirmed to be no longer changing, raise the temperature to 0°C and add 10% hydrochloric acid to adjust the pH to 2. Extract the aqueous layer once more with 2-methyltetrahydrofuran. Combine the organic layers and use them directly in the next synthesis step.
2) Synthesis of methylboronic acid trimer: Add the organic layer obtained above to a reaction flask equipped with a reflux separator and 0.4 mol of tetrabutylammonium fluoride hydrate. The system quickly dissolves under stirring at room temperature. After the reaction is complete, evaporate the solvent to dryness under normal pressure. Add 120 mL of toluene to the mixture and begin reflux separation. When more than the calculated amount of water has been separated and no more water is separated, connect a distillation apparatus to distill off the methylboronic acid trimer generated after the reaction.
3) Synthesis of methylboric acid: The methylboric acid trimer obtained after the above distillation was added to 4.5 g of water and stirred at room temperature for 30 minutes, resulting in the precipitation of a solid. After adding n-heptane and stirring, the mixture was filtered to obtain 11.2 g of pure white solid methylboric acid, with a yield of 62%. The product had a GC content of 98.2%, a melting point of 88-90℃, and a NMR structure that was consistent with the given information.
