SolutionsOlefin metathesis
Exercise 1
What product is formed in the ring-closing metathesis of hepta-1,6-dien-4-ol (protected as a silyl ether)? And of octa-1,7-diene? Why is the reaction carried out at low concentration?
Hepta-1,6-dien-4-ol gives (silylated) cyclopent-3-en-1-ol and ethylene; octa-1,7-diene gives cyclohexene and ethylene. Low concentration favours the intramolecular reaction over intermolecular cross metathesis, which would give oligomers and polymers (acyclic diene metathesis, ADMET).
Exercise 2
Explain why the ROMP of norbornene is practically irreversible, whereas that of cyclohexene does not take place.
The driving force of ROMP is the release of ring strain. Bicyclic norbornene has a high strain (about 100 kJ/mol), which more than compensates for the loss of entropy on polymerization. Cyclohexene has hardly any strain, so the equilibrium lies on the side of the monomer.