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What is the decarboxylation product of the following species: enter image description here

It may seem straightforward but the product enolate involves a double bond with one end at the bridgehead carbon which is impossible/very unstable. What is the correct product and how is the product reached mechanistically.

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  • $\begingroup$ Could the resulting enol tautomerize? $\endgroup$ – Lighthart Apr 8 '15 at 17:26
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    $\begingroup$ Yes it could. But the enolate must surely form initially? $\endgroup$ – RobChem Apr 8 '15 at 17:27
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    $\begingroup$ Enolization is a blindingly fast reaction. Even though the intermediate is unstable, the easy pathway forward might allow formation of the resulting ketone, despite the bridgehead. $\endgroup$ – Lighthart Apr 8 '15 at 17:38
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${\beta}$-keto acids typically decarboxylate very smoothly at temperatures between 100-200°C. The mechanism involves a cyclic, 6-membered transition state that first produces an enol intermediate (see figure).

enter image description here

However in the case of your bicyclo[2.2.1] ${\beta}$-keto acid, a bridgehead double bond would result. As you correctly point out, such a structure would be very high energy. Therefore this compound does not decarboxylate at normal temperatures (reference, see p. 218, scheme 6.53).

I'm sure if you heated it hot enough something would happen. It may decarboxylate via a free radical mechanism. The resultant bridgehead free radical could fragment or dimerize. Alternately, and my preferred guess, would have the starting compound, in equilibrium with its non-bridgehead enol tautomer, undergo a reverse Diels-Alder reaction as pictured above.

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    $\begingroup$ @RobChem Just curious (I like to learn too), did your instructor present a different answer\explanation? $\endgroup$ – ron Apr 12 '15 at 17:36
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    $\begingroup$ My term hasn't started again yet. I'll let you know $\endgroup$ – RobChem Apr 21 '15 at 12:22
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    $\begingroup$ she said that it does not decarboxylate $\endgroup$ – RobChem May 14 '15 at 19:15
  • $\begingroup$ Thanks for getting back on this. So is my answer acceptable? $\endgroup$ – ron May 14 '15 at 19:23
  • $\begingroup$ Well, she just said it wouldn't decarboxylate. I didn't offer her your response as a possibility I'm afraid. Although, she seemed to suggest heating it wouldn't decarboxylate it. She didn't suggest anything else so I guess nothing happens until it combusts. $\endgroup$ – RobChem May 14 '15 at 19:32

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