OS(O)(=O)=O.[O-]S([O-])(=O)=O.[Mg+2].[F:12][C:13]1[CH:14]=[C:15]([CH:19]=[C:20]([F:22])[CH:21]=1)[C:16]([OH:18])=[O:17].[CH3:23][C:24](O)([CH3:26])[CH3:25]>C1(C)C=CC=CC=1>[F:12][C:13]1[CH:14]=[C:15]([CH:19]=[C:20]([F:22])[CH:21]=1)[C:16]([O:18][C:24]([CH3:26])([CH3:25])[CH3:23])=[O:17]Concentrated H2SO4 (1.74 mL, 31.6 mmol) was added to a vigorously stirred suspension of MgSO4 (15.2 g, 126.4 mmol, 4 eq) in toluene (100 mL). The mixture was stirred for 15 minutes, after which 3,5-difluorobenzoic acid (5 g, 31.6 mmol) and t-BuOH (14.9 mL, 158 mmol, 5 eq) were added successively. The mixture was stoppered tightly and stirred at room temperature until the reaction was complete by TLC analysis. The reaction mixture was then quenched with saturated NaHCO3 solution and stirred until all MgSO4 had dissolved. The organic layer was washed with brine, dried over MgSO4 and concentrated to afford the crude product. The residue was purified by chromatography (silica, Hex) to give tert-butyl 3,5-difluorobenzoate (5.3 g, 78%) as a colorless oil.
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