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15The starting material for our epoxide is citronellyl pivalate, a tri-substitutedalkene. Based upon the previously described mechanism, one would think thatthis compound would be a poor epoxidation substrate for Jacobsen’s catalyst,however Jacobsen has described this very approach to the reaction. 18 According tothisR1 R2R1OO R2Mncis alkene-epoxide transition stateMntrans alkene-epoxide transition stateFigure 12. Trans-Alkene approach produces steric repulsion 13-15paper, tri-substituted alkenes are indeed excellent substrates for a Jacobsenepoxidation and many such reactions have been carried out with high reactionrates and enantioselectivity. Figure 13 includes the six tri-substituted alkeneswhich showed promising yields and enantiomeric excesses. Also included inFigure 13 is the one unconjugated tri-substituted alkene that underwent Jacobsenepoxidation.In order to explain these seemingly contradictory results, Jacobsen had tore-evaluate his proposed mechanism for epoxidation. 18 This new theory involveda “side-on” skewed attack of the alkene upon the manganese-oxo complex thatinvolved the formation of two possible intermediates. With a cis-alkene, one lowenergy activated complex and one high energy activated complex are possible.With trans-alkenes, both possible activated complexes are high energy speciesbecause either approach of the alkene would produce the same steric repulsion.Therefore a trans-alkene would not proceed with appreciable yield and