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


In the epoxidation mechanism, the π-electrons of the double bond attack 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?
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):