Question

A graduate student tried to make o-fluorophenylmagnesium bromide by adding magnesium to an ether solution of o-fluorobromobenzene. After obtaining puzzling results with this reaction, she repeated the reaction by using as solvent some tetrahydrofuran that contained a small amount of furan. From this reaction, she isolated a fair yield of the compound that follows. Propose a mechanism for its formation

Short answer

When O-fluoro bromobenzene is added to magnesium in tetra hydra furan solution it forms O-fluoro phenyl magnesium bromide. But this compound forms the benzyne intermediate.

Step-by-step solution

Formation of benzyne intermediate

When O-fluoro bromobenzene is added to magnesium in tetra hydra furan solution it forms O-fluoro phenyl magnesium bromide. But this compound forms the benzyne intermediate.

Formation of Diels alder adduct

As the solvent contains a small amount of furan, the benzyne undergoes a Diels alder reaction to form the adduct. The reaction mechanism is represented as follows: