Friday, January 25, 2019

organic chemistry - 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)?


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?



Answer



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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