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A problem in a popular organic chemistry text (1) requests the mechanism for the isomerization of 2,7-dimethylocta-2,6-diene 1 with phosphoric acid to 1,1-dimethyl-2-isopropenylcyclopentane 4. The expected answer involves protonation to the more stable 3deg carbocation 2 which adds to the remaining double bond to give the more stable 3deg carbocation 3 that loses a proton to form 4. not the more substituted 2-isopropylidene-1,1-dimethylcyclop entane 5. This apparent failure of a logical extension of Saytzeff's rule led to the following discussion, calculations, and literature search.
Because of the alkenelike transition state of the product-determining step of an E1 reaction, the most stable alkene is formed preferentially. According to Saytzeff's rule this should be the most substituted alkene 5. However, if 5 were the less stable isomer, it would not be the preferred product. Such exceptions to Saytzeff's rule are known (2). Possible reasons for an increased energy content of 5 might be steric hindrance between the gem dimethyl groups and one of the allylic methyl groups and/or the bond angle distortion necessary to place the double bond exo to the five-membered ring. This hypothesis was tested and discarded on the basis of the calculated heats of formation [H sub f ] of the energy minimized conformations of 4 and 5 that show the latter to be more stable by 3.5 kcal/mol (see the figure.)(1) (Figure omitted)
Might the reaction of 1...