Final answer:
The treatment of 1,8-nonadiene with Grubbs catalyst results in a cyclic product through a ring-closing metathesis reaction, likely forming a nine-membered ring with a double bond.
Step-by-step explanation:
When 1,8-nonadiene is treated with Grubbs catalyst, a ring-closing metathesis (RCM) reaction occurs, producing cyclic products.
During RCM, the Grubbs catalyst, which is a ruthenium-based metathesis catalyst, facilitates the formation of carbon-carbon double bonds, resulting in the formation of cyclic olefins. For 1,8-nonadiene, a nine-membered ring with a single double bond is most likely formed, since the two double bonds in the substrate are in the correct positions to be closed by the catalyst's action.
While this question mentioned a Diels-Alder reaction, that information is not directly relevant since Grubbs catalyst is used for metathesis reactions rather than Diels-Alder reactions.