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Ring closing metathesis (RCM) reactions using Grubb's 1st generation catalyst form rings with an alkene. If the ring size permits, then a mixture of E and Z products can result but the dominant isomer cannot be predicted. By synthesizing a series of substituted 10-membered lactones and measuring the resulting E/Z ratios after RCM we are examining how steric effects control the stereoselectivity of the reaction. differences in the relative stabilities of E and Z RCM intermediates have been explored computationally at the DFT-B3LYP level of theory for series of methyl substituted and t-butyl substituted 10-membered lactones. For most substrates, the difference in energy between the two intermediates is negligible. We have synthesized five methyl substituted RCM substrates and are currently determining experimental E/Z rations for this series. We have so far observed ~1:1 E/Z ratios for substrates with a methyl group distant from the forming alkene.