Method for preparing boric acid ester using uncatalyzed hydroboration of carboxylic acid

#The invention relates to the application field of green chemistry, particularly relates to a method for preparing the borate ester by non-catalytic hydroboration with carboxylic acid.

#Organic boronic acid esters can be regarded as derivatives in which the hydrogen in orthoboric acid B(OH)3 is replaced by organic groups, in addition to metaborate (ROBO)3. Due to its stability and low toxicity, borate is widely used in various fields. It is a main raw material for the synthesis of boron-containing compounds. Borate compounds can be used not only as rust inhibitors, preservatives, polymer additives, anti-wear additives, automobile brake fluids, gasoline additives, flame retardants, but also as lubricant additives.

#The reduction catalyst system reported in the literature for the synthesis of borate esters is mainly the carboxylic acid hydroboration catalyzed by LiAlH4 and NaBH4, SmI2—H2O-Et3N and various transition metal complex catalysts. However, these methods have obvious shortcomings:LiAlH4 and NaBH4 systems have great safety risks, the SmI2—H2O-Et3N system requires a lot of excess reagents, and the Ru, Rh, Ir, Co transition metal complex systems require high temperature and high pressure. The difference between the nucleophilic addition reaction activity of carboxylic acid and aldehyde and ketone: (1) The active hydrogen of carboxylic acid is easy to leave, so its two O's are actually equivalent, so from the perspective of steric structure, the steric hindrance of carboxyl group is more Large, and the existence of carboxyl hydrogen bonds makes the electron cloud density of the entire carbonyl group larger, and it is more difficult for nucleophiles to attack the active center; (2) Nucleophiles attack the carbonyl carbon first, which is related to the electron cloud density on the carbon. Small advanced attacks, such as aldehydes and ketones have a smaller electron cloud density than esters and amides, so they are highly active and will react preferentially. Those with large steric effects are not easy to react; (3) When forming a transition state, it depends on the leaving group, aldehyde The leaving groups of ketones are alkyl and hydrogen, both of which are not easy to leave. Therefore, aldehydes and ketones only undergo addition without elimination, which is different from carboxylic acids and their derivatives. On the one hand, the existing methods use catalysts that are difficult to synthesize and cost high; on the other hand, the catalytic reaction requires a reaction temperature of 60° C. and a reaction time of 24 hours.

#FIG. 1 is the schematic diagram of the reaction in Example 1;

#FIG. 2 is the schematic diagram of the reaction in Example 18.

#The following further describes the present invention with reference to the embodiments:

#Under the protection of N2, loading the benzoic acid (61.1 mg, 0.5 mmol) to the reaction bottle, which was treated by dehydration and deoxidation, then adding a pinacolborane (218 μL, 1.5 mmol) with pipette gun, after reacting for 12 h at room temperature, the reaction was removed from the glove box, the reaction was terminated, and the borate was obtained. The mes-trimethoxybenzene (84.15 mg, 0.5 mmol) was used as the internal standard. The stock solution was dissolved in CDCl3 stirring for 10 minutes, sampled, and equipped with nuclear magnetism. After calculation, conversion rate of 1H is 99%, and the reaction diagram is shown in FIG. 1. 1H NMR analysis of the product is: 1H NMR (400 MHz, CDCl3): δ 7.22-7.32 (m, 5H, ArH), 4.92 (s, 2H, CH2), 1.26 (s, 36H, CH3). The boric acid ester was further hydrolyzed into alcohol, adding 1 g silica gel with 3 mL methanol as the solvent at 50° C. for 3 h. After the reaction, the product pure primary alcohol was extracted three times with ethyl acetate, and the organic layers were combined, dried with anhydrous sodium sulfate, the solvent was removed under reduced pressure, and purified by silica gel (100-200 mesh) column chromatography, washed with ethyl acetate/hexane (1:5 volume ratio) mixture, with a separation yield of 92%. For the reaction diagram, see the FIG. 1. 1H NMR analysis of the product is: 1H NMR (400 MHz, CDCl3): δ 7.22-7.29 (m, 5H, ArH), 4.61 (s, 2H, CH2), 1.87 (br s, 1H, OH).

#Under the protection of N2, loading the benzoic acid (60.3 mg, 0.5 mmol) to the reaction bottle, which was treated by dehydration and deoxidation, then adding a pinacolborane (289 μL, 2 mmol) with pipette gun, after reacting for 6 h at room temperature, the reaction was removed from the glove box. The mes-trimethoxybenzene (83.05 mg, 0.5 mmol) was used as the internal standard. The stock solution was dissolved in CDCl3 stirring for 10 minutes, sampled, and equipped with nuclear magnetism. After calculation, conversion rate of 1H was 93%. Continuing hydrolyzing to obtain pure primary alcohol with an isolated yield of 85%.