One interesting use of Diels–Alder reactions is to trap very reactive alkenes that cannot be isolated and studied directly. One compound used as a diene for this purpose is diphenylisobenzofuran, which reacts as shown in Fig.P15.81a. (Notice that the formation of an aromatic ring in the product helps ensure that the Diels–Alder reaction is driven to completion.) In the reaction given in Fig. P15.81b, use the structure of the Diels–Alder product to deduce the structure of the reactive species formed in the reaction. Show by the curved-arrow notation how the reactive species is formed, and explain what makes it particularly unstable.

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(a) (b) R H H Br Br Figure P15.81 Ph Ph diphenylisobenzofuran reactive diene + Mg diphenyliso- benzofuran ether solvent R H H Ph Ph Ph afo- Ph + MgBr