title: Selected substituent effects on the rate and efficiency of formation of an eight-membered ring by RCM.
abstract: Studies of a range of reactions forming cyclooctenones highlight a discrepancy between cyclization rate and cyclization efficiency. Cyclization rates change modestly as the oxygen function at the allylic position is varied, and increase upon gem-dimethylation. Cyclization efficiency has also been quantified for four substrates, revealing a range of effective molarities (EMs) of 2 orders of magnitude that are substituent dependent. The most efficient cyclization appears to result from suppression of the cross-metathesis pathway through which oligomerization begins, rather than from a particularly rapid cyclization reaction. In the presence of a Ti(IV) cocatalyst, diene monomers transform smoothly to eight-membered-ring products without the intermediacy of dimers or other oligomers, indicating that the cyclizations are kinetically and not thermodynamically controlled. The gem-dialkyl effect is also shown to be kinetic.
authors.affiliation: Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom.
authors.forename: Lisa
authors.initials: L
authors.lastname: Mitchell
authors.forename: John A
authors.initials: JA
authors.lastname: Parkinson
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Percy
authors.forename: Kuldip
authors.initials: K
authors.lastname: Singh