Selected Substituent Effects on the Rate and Efficiency of Formation of an Eight-Membered Ring by RCM

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.