Cis vs. Trans epoxidation of olefins

[gravenewworld]

Why is it that Cis olefins have a much higher ee% (enantiomer excess) than trans olefins using Jacobsen's catalysts for epoxidation? I have searched high and low for the answer but to no avail. Any help would be very much appreciated. Thanks.

[chem_tr]

I don't know the issue very well, but it led me to think about the added steric hindrance in the case of trans-olefin; the catalyst molecule cannot reach as freely as it does in cis-olefin. So the ee% may be greater in cis-isomer. You know, the catalyst (whatever it is, all of them react in a similar way) must react with the double bond to give an oxygen to the double bond.

[movies]

The transition state for the Jacobsen epoxidation is not very well understood. However, the assumption is that the alkene approaches the manganese-oxo species from over the cyclohexane ring. Approach from over the aromatic rings is disfavored due to steric interactions with the t-butyl groups on the aromatic rings; approach from the "front" (180 degrees from the cyclohexane) is hindered by the t-butyl groups on the aromatic ring as well.

The approach would suggest that the hydrogens on the cis alkene are situated directly over the cyclohexane ring in the transition state, with the plane of the alkene perpendicular to the plane of the catalyst. If you had a cis alkene, however, this type of transition state would force one of the substituents into the cyclohexane ring.

The orientation of the approaching alkene is determined by the steric interaction between the substituents on the alkene and the axial hydrogens on the cyclohexane ring. So, in general, the smaller alkene substituent will over the axial hydrogen of the ring and the larger alkene substituent will approach over the carbon with the axial hydrogen on the opposite side of the catalyst plane.

I hope this description is helpful. You might look at the original Jacobsen paper for a nice description. Here is the reference: J. Am. Chem. Soc. 113, 7063 (1991).