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2-Methylcyclohexanone is treated with meta-chloroperoxybenzoic acid (mCPBA). What is the major product of this Baeyer-Villiger oxidation?

1. A ring-expanded 7-membered lactone with a methyl substituent at the carbon adjacent to oxygen (methyl migrates preferentially)
2. A 7-membered lactone with the oxygen inserted between the carbonyl and the less-substituted carbon (methyl group retained on ring carbon)

Correct answer: A ring-expanded 7-membered lactone with a methyl substituent at the carbon adjacent to oxygen (methyl migrates preferentially)

Solution

mCPBA oxidises ketones to esters/lactones via the Baeyer-Villiger mechanism. In 2-methylcyclohexanone, the carbon bearing the methyl group is secondary (more substituted) and migrates preferentially, inserting oxygen on the other side, giving a 7-membered lactone (caprolactone derivative) with the methyl group on the carbon adjacent to the ester oxygen.

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