Methyl chloroformate → Methyl 4-bromothiophene-3-carboxylate

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A solution of isopropylmagnesium chloride (1.0 g, 9.7 mmol, 1.2 eq.) in THF (5 mL) was added to a solution of 3,4-dibromo-thiophene (2 g, 8.3 mmol, 1.0 eq.) in THF (15 mL), and stirred for 3 hours while the temperature was maintained at 0-5° C. in an ice/salt bath. Methyl chloroformate (1.6 g, 16.9 mmol, 2 eq.) was added dropwise with stirring. The resulting solution was stirred for an additional 16 hours at room temperature. The reaction was then quenched with water (1 mL). The resulting mixture was concentrated under vacuum, then diluted with EtOAc (50 mL). The resulting mixture was washed 1N HCl (1×100 mL) and saturated aqueous NaCl (2×100). The organic layer was concentrated to provide a crude residue, which was applied onto a silica gel column and eluted with EtOAc/PE (1:50) to yield 4-bromothiophene-3-carboxylic acid methyl ester (1.0 g) as a light yellow solid.

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