Reaction of Phenylmagnesium Bromide with Boronic Acid Trimethyl Ester

3 Reaction of Phenylmagnesium Bromide with Boronic Acid Trimethyl Ester 

Aryl and alkyl boron compounds have received significant research attention as building blocks in organic synthesis. One of the most straightforward methods for making such compounds is the reaction of a boronic acid ester with a Grignard reagent. However, the reaction 

N-Acyliminium ions are versatile intermediates in organic synthesis they not only react with various nucleophiles such as electron-rich aromatic and heteroaromatic compounds but also undergo cycloaddition reactions with unsaturated compounds.It is especially noteworthy that N-acyliminium ions serve as electron-deficient 4t7 components in [4 + 2] cycloaddition with alkenes and alkynes. This reaction serves as a useful method for the construction of heterocyclic rings containing a nitrogen atom. Acyclic structures containing amino and hydroxyl groups can also be synthesized from the initially formed cyclic compounds. 

However, the reaction with styrene gives rise to the formation of a significant amount of polymeric by-products. The cationic compound seems to react with another molecule of styrene, and further reactions eventually give rise to the formation of the polymer. This is also assigned to a problem of competitive consecutive fast reactions. In fact, the yield of the cycloadduct strongly depends on the method of mixing, as shown in Table 8.6. Although the addition of styrene to a solution of the 

N-acyliminium ion (method A) gives the cycloadduct in 57% yield, the addition of a solution of the N-acyliminium ion to styrene (method B) gives the cycloadduct in only 20% yield and a significant amount of the polymeric product (ca. 80% based on styrene) is formed, presumably because of the higher initial concentration of styrene. The simultaneous addition of two reaction components (method C) gives essentially the same results as method A. 

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Table 8.5 Product selectivity of the reaction of phenylmagnesium bromide with boronic acid trimethyl ester...

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