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What is the product formed when 1,2-dimethylcyclohexa-1,4-diene undergoes epoxidation with one equivalent of m-CPBA (meta-chloroperbenzoic acid)?

Epoxidation of 1,2-dimethylcyclohexa-1,4-diene with m-CPBA

In the epoxidation mechanism, the π-electrons of the double bond attack m-CPBA:

Mechanism of epoxidation with m-CPBA

So, methyl groups attached to the double bond should increase its nucleophilicity. On the other hand, since it is a nucleophilic attack, the methyl groups should also cause considerable steric hindrance. So what should the answer be?

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The interaction of the alkene with m-CPBA is primarily nucleophilic/electrophilic in nature, i.e. the π MO of the alkene attacks the O–O σ* orbital. Therefore, the more electron-rich alkene (i.e. more highly substituted alkene) preferentially reacts.

With methyl groups there probably isn't too much steric hindrance. m-CPBA approaches above/below the alkene and doesn't directly attack over the methyl groups.

Indeed, the monoepoxidation of 1,2-dimethylcyclohexa-1,4-diene is reported to proceed selectively at the more electron-rich tetrasubstituted double bond (J. Org. Chem. 2006, 71 (4), 1653–1657):

Epoxidation to form 1,6-dimethyl-7-oxabicyclo[4.1.0]hept-3-ene

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    $\begingroup$ How tolerant is this against aromatisation? You have a 1,4-cyclohexadiene, you look at it in the wrong way and it gives you a benzene ring. $\endgroup$ – Martin - マーチン May 6 '17 at 14:28
  • $\begingroup$ @Martin-マーチン there's one record of this substrate being oxidised, with DDQ instead of mCPBA. I suppose it may form 1,2-dimethylbenzene if the epoxide is opened - form the diol then dehydrate $\endgroup$ – orthocresol May 6 '17 at 14:54

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