A facile chromatographic method for purification of pinacol boronic esters has been developed. Impregnation of silica gel with boric acid was effective both for thin layer chromatography (TLC) and for flash column chromatography. Purification of a series of pinacol boronic esters was successful by suppressing loss of the compounds due to over-adsorption.Separation and isolation of organic compounds is seemingly a routine technique in organic synthesis. Among various separation methods, chromatographic separation methods using silica gel hold a prominent position, which ideally enables the separation of any compounds in a rapid and facile manner. However, especially for a new class of compounds, the chromatographic separation sometimes becomes a formidable obstacle and requires an innovative development of a reproducible and reliable method. 25 We recently faced such a problem during chromatographic isolation of pinacol arylboronic esters 6 and realized that a similar issue often accompanies boroncontaining compounds which are important and ubiquitous substrates for organic synthesis mainly owing to the everincreasing importance of SuzukiMiyaura coupling reactions. 7 We herein report a facile chromatographic method for purification of pinacol boronic ester derivatives. We found that impregnation of silica gel with boric acid is effective to suppress the undesired over-adsorption of the compounds and hence reduces the loss of materials upon chromatographic separation.We found that the shortage in silica gel chromatography for pinacol arylboronic esters originated from an over-adsorption of the substrates. One of the most frequent issues associated with the separation of arylboronic esters is the low recovery efficiency through chromatography, which can also be found as an immobile or tailing spot on TLC plates. As shown in Figure 1a, when we developed 2,7-diborylnaphthalene 1 8 as a model substrate on a TLC plate (Merck, silica gel 60F 254 ; eluent: 10% AcOEt/hexane), two spots were observed: one at the eluted position of R f = 0.4 and the other near the spotted origin (R f µ 0) through the visualization by UV irradiation and staining with p-anisaldehydeH 2 SO 4 reagent. 9 The analysis of both spots by TLC DART MS (direct analysis in real time mass spectrometry) before the staining showed the presence of 1 in both spots. 10Thus, we detected identical ionized species of 1 (m/z 381 and 398) from both spots, and the ions were assigned as a protonadduct and an ammonium-adduct, respectively (Figures 1b and S1 18 ). The results showed that the arylboronic ester 1 remained partly at the spotted origin without any apparent degradation and hence indicates that the insufficient recovery of arylboronic esters from the chromatography does not originate from decomposition but from uneven over-adsorption that results in residual bands or spots upon chromatography. 11We hypothesized that the undesired over-adsorption around the spotted origin may be due to interactions of the vacant orbital on boron with nucleophilic moieties ...

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