GENERAL METHODS: Tert-butyl nitrite (155 mg, 1.1 mmol) was added dropwise to a mixture containing bis(pinacolato)diboron (127 mg, 0.5 mmol), p-aminobenzoic acid (61 mg, 0.5 mmol) and eosin Y (0.01 mmol). The reaction mixture was stirred in acetonitrile (3 mL) under blue LED irradiation for 2 h at room temperature (the progress of the reaction was monitored by TLC). Upon completion of the reaction, the mixture was diluted with ethyl acetate (5 mL) and then filtered through diatomaceous earth. The filtrate was extracted with ethyl acetate (3 x 10 mL) and the combined organic phases were washed with saturated saline and dried over anhydrous Na2SO4. Concentration under reduced pressure afforded the crude product, which was purified by silica gel column chromatography using hexane-ethyl acetate (98:2) as eluent to afford pure 4-carboxyphenylboronic acid pinacol ester as a pale yellow viscous liquid (208 mg, 88% yield).IR (pure samples)2978, 2933, 2839, 2526, 2050, 1950, 1911, 1724, 1605, 1570 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.33 (s, 12H), 7.82 (s, 3H), 6.89 (d, J = 8.0 Hz, 2H), 7.75 (d, J = 8.0 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 24.9 (4C), 55.2, 83.6 (2C), 113.4 (2C), 136.6 (2C), 162.3.The spectral data of the obtained compounds are in agreement with the 4-carboxyphenylboronic acid pinacol ester reported in the literature.
