Experimental Observations A Focus on Stereochemistry

As we have al ready mentioned, two of the important factors in sigmatropic shifts are the stereochemistry of the migrating group and whether migration occurs to the same or opposite faces of the ir system. Here, we look at some examples that verify the predictions given above. 

One of the most common shifts is the [1,5] hydrogen shift. The reaction typically shows a large kinetic isotope effect of 5 at 200 C for the migrating hydrogen, indicating consider- 

Demonstration of the [ 2., -I- 2J reaction. B. Differences when methyl groups are used. 

Another very common sigmatropic shift is the [1,2] hydride shift associated with car-bocations (Eq. 15.30). Although not usually analyzed this way, these migrations involve a cyclic, two-electron system and are Hiickel aromatic (see margin). Viewing these reactions this way nicely rationalizes why comparable [1,2] shifts are not seen in carbanions that is, they would involve a four-electron Hiickel antiaromatic transition state. 

Another interesting and useful reaction is the [2,3] sigmatropic shift (Eq. 15.34). It is a six-electron process, where two of the electrons are contributed by a lone pair, and thus only five atoms are involved. Sulfoxides and various types of allylic ylides are common sub- 

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