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Here is the reaction that I didn't find the mechanism of; maybe you can help:
The conditions is zinc dust, when the solvent is acetic acid at 110°C 1h. the reagents are Sodium acetate, 2,3,4,5-tetraacetoxy-1,6-bromohexane and the product is 3-4-diacetoxy-1,5-hexadiene. There is also some evolution of gas (I don't know what it is, $\ce{HBr}$ or $\ce{CO2}$).
What reaction can lead to this elimination when it seems that no proton is leaving the molecule (instead of bromide) and acetate is leaving?

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  • $\begingroup$ Welcome to chemistry.SE! You might not see it a bad idea if you take some of your time to take a tour or visit the help center for common questions about our community. I suggest you see this link and this link in order to learn how to make your posts better. $\endgroup$ – It's Over Dec 27 '14 at 13:53
  • $\begingroup$ Pardon me? I edited your Q but it seems that there are little typos you got there. Did you really use Actinium Hydroxide as your solvent?! Even if you did so, formula shouldn't be AcOH; am I wrong? $\endgroup$ – It's Over Dec 27 '14 at 13:55
  • $\begingroup$ i meant an acetic acid sorry $\endgroup$ – k.s Dec 27 '14 at 14:59
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What reaction can lead to this elimination when it seems that no proton is leaving the molecule (instead of bromide) and acetate is leaving?

This is a reductive elimination reaction. The classic textbook example is the reductive elimination of trans-1,2-dibromocyclohexane to cyclohexene in the presence of zinc dust and acetic acid.

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It is basically a redox reaction where the zinc metal is oxidized and the dibromocyclohexane is reduced. As with many reactions involving electron transfer from metal surfaces, the mechanism of this reaction is not fully understood. Perhaps the zinc inserts itself into the carbon-bromine bond as a first step, but in any case, electron flow proceeds in the direction shown above.

The reaction has found many uses and has been widely modified to include cases such as yours where a bromine and another group are eliminated to form a double bond. Here is a link to a paper where one of the bromines is replaced by an alkoxy group and the same zinc-mediated reductive elimination occurs. Here is a link to another paper where zinc is used to perform a reductive elimination in a series of compounds where both bromines have been replaced, one by acetate and the other by hydroxyl.

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