Arylboron Formation Investigated in Micro Reactors

3 Arylboron Formation Investigated in Micro Reactors Organic synthesis 40 [OS 40] Formation of phenyl esters of boronic add 

The formation of phenyl esters from boronic acid and phenyhnagnesium bromide in THF is a fast liquid reaction involving contacting of two reactants, Rj (boronic acid) and R2 (phenyhnagnesium bromide), dissolved in the same solvent to yield a liquid mixture [48,108], This mixture is post-processed by a fast hydrolysis. This step was performed conventionally in a batch mode. 

The two reactants Rj and R2 form an intermediate Ij which reacts to a second intermediate I2 via two reaction pathways, one needing the presence of R2 [48, 108]. A number of additional reactions decrease the selectivity and yield. First, the reactants Rj and R2 are labile to moisture. Hydrolysis results in the formation of the side product reactants S j and 82- By more complex pathways, three other known 

The intermediates Ij and I3, however, precipitate in the course of the reaction [48, 108]. They are dissolved again by post-processing to the product Pj and the consecutive product Cj. 

Compared with the simple and flexible laboratory set-up, the scale-up set-up is said to be more robust and user-friendly [48, 108]. By selecting the proper delay tube via the 5/2-way valve, the residence time was varied 5, 10, 26 and 120 s at 10 and 20 °C, and 1, 5, 10 and 26 s at 30 and 40 °C. The stainless-steel tubes used had inner diameters of 0.7, 3.8, 4.8, 9.3 and 21.2 mm and a length of 1000 mm (four tubes were permanently moimted and one was exchanged, if needed). 

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