Lithium-halogen exchange 3-bromo thiophene

If 3-bromothiophene is added to butyllithium at a sufficiently low rate, no 2-lithio-thiophene is formed, because the halogen-lithium exchange is much faster than the vicinal metallation of the bromo compound. Support for the mechanism of trans-metallation was provided by adding an equivalent amount of thiophene to a solution of 3-lithiothiophene in THF, cooled at —30 °C after 1.5 h only the 2-lithio derivative was present in the solution [9]. In Et20 this trans-metallation was very slow. 

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