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What happens when this bicyclic compound is treated with KH in THF?

Reaction scheme

Does $\ce{KH}$ simply act as a base here? $\ce{H-}$ from $\ce{KH}$ could potentially act as a nucleophile, but I don't find any electrophilic centre in the compound. What happens?

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The reaction in question is one of the early examples of an oxy-Cope rearrangement [1] in which a 3-hydroxy-hexadiene undergoes a [3,3]-sigmatropic rearrangement to afford a ketone.

oxy-Cope rearrangement

The general mechanism is shown below:

enter image description here

Source: Organic Chemistry, Oxford University Press, 2 ed. Clayden, Warren and Greeves

In a general sense, the reaction is a [3,3]-sigmatropic rearrangement (if you look at the curly arrows, you'll see that we're essentially transposing a sigma bond from one end of the pi system to another).

The presence of the oxygen provides a driving force for the reaction, since the enol(ate) formed during the rearrangement is able to tautomerise, yielding the ketone product.

In the specific case you've given, the presence of a base means that during the sigmatropic rearrangement, the oxygen is deprotonated and therefore anionic. This specific case, using an alkoxide, is a specific kind of reaction known as an anionic oxy-Cope. The anion is thought to help speed up the reaction, to the extent that minimal heat is usually required.

enter image description here


References:

[1] J. Am. Chem. Soc., 1964, 86,5019. DOI: 10.1021/ja01076a067

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  • $\begingroup$ If you want to challenge yourself further with this reaction, consider what the product would be if we substituted one of the hydrogen atoms on the vinyl group for a deuterium atom. $\endgroup$
    – Zhe
    Commented May 12, 2017 at 21:58

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