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slide notes I was reviewing Dave Evans' notes for stereochemistry of C=X electrophile additions, and found this slide detailing differences in additions to cyclic oxo-carbenium electrophiles, but I don't understand why this selectivity is seen. With the OBn group, I can see that adding on the same face as the OBn is favored, compared with the methyl substituent where adding trans is favored, but I cannot figure out why.

In looking at the mechanism for allylsilane addition I can see that a positive charge builds up on the nucleophile's central carbon. Is this what is stabilized by the neighboring OBn group that effects selectivity?

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  • $\begingroup$ The direction of attack of the nucleophile is governed by the geometry of the oxocarbenium ion, not directly by the substitution. The nucleophile comes in from the direction that will cause it to become axial in the product because the transition state is chair-like and lower in energy than whatever happens attacking the other side. $\endgroup$ – Zhe May 1 at 20:10
  • $\begingroup$ After 15 years, I can't quite remember how the electrostatic effects come into play here. Is it to control the reactive conformation (since attack is axial) or is it to help direct the nucleophile? Maybe look up the Woerpel articles... $\endgroup$ – Zhe May 1 at 20:11

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