Hi I am trying to figure out the mechanism of a reaction containing a lithium carbanion and 1,2-dibromoethane in THF. The lithium carbanion is generated by deprotonating using LDA. Then 1,2-dibromoethane is added to get the desired product (see image) enter image description here

I think that the carbanion is brominated, otherwise this reaction does not make sense, but I really don't understand the mechanism. 1,2-dibromoethane can undergo a reductive elimination of the bromide-ions to form bromine and ethene. But in this specific reaction, I have no reductive agent... So how does the carbanion get brominated? Is it simply a fragmentation reaction, where the carbanion attacks the Br of 1,2-dibromoethane?

These are my thoughts on the intermediates. enter image description here enter image description here Thank you!

  • $\begingroup$ Important thing to consider here is that organolithium there doesn't have carboanion - for an accurate mechanism Li has to be coordinated all the time. $\endgroup$
    – Mithoron
    Nov 17 '19 at 1:05
  • $\begingroup$ related chemistry.stackexchange.com/questions/31094/… $\endgroup$
    – Mithoron
    Nov 17 '19 at 1:17
  • $\begingroup$ The gamma picoline is the reducing agent and ethylene dibromide is the oxidizing agent. Gamma picoline is oxidized; ethylene dibromide is reduced. $\endgroup$
    – user55119
    Nov 17 '19 at 1:22
  • $\begingroup$ @Mithoron I have corrected the mechanism. Does this seem correct now? The negative charge should be on the nitrogen after the picoline is deprotonated as it can accept the electrons. $\endgroup$ Nov 17 '19 at 11:30
  • $\begingroup$ Ah, you wrote just what I thought, but unfortunately a moment later I though that it can't work like that :( It's not a substitution, but transmetalation. Bromide would also be coordinated. Some digging for papers would be needed to answer this... $\endgroup$
    – Mithoron
    Nov 17 '19 at 17:17

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