A graduate student attempted to form the iodohydrin of the alkene shown below. Her analysis of the products showed a good yield of an unexpected product. Propose a mechanism to explain the formation of this product.

When iodine molecule reacts with the given alkene, the formation of iodonium ion takes place. Iodonium ion is a three-membered cyclic intermediate containing iodine that is reactive due to increased angle strain in the ring and also there is a positive charge on the iodine atom which is not stable. Thus, the cyclic ring opens up in the next step.

Formation of iodonium ion

Cyclic iodonium ion opens up to release strain and neutralize positive charge on iodine atom, and there occurs intramolecular attack of carbonyl oxygen as nucleophile on the carbocation formed. The carbonyl oxygen is more nucleophilic than hydroxyl oxygen due to resonance stabilization of the intermediate. Then, after proton removal by water, we get our required product.

Intramolecular attack of carbonyl oxygen as nucleophile