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Instead of a thermodynamic preference for the intermediate leading to the exo diastereomer (with equilibration to starting material), de Ler(ÈËÃû) a proposes a higher energy transition state in the rate-limiting step for formation of the endo diastereomer. The endo isomer arises from activation of the indole 2,3-¦Ð bond by the electrophile to form diastereomeric benzylic carbocations (exo diastereomer, 74, Figure 8B), then formation of an azetidine, 75, and finally concerted rearrangement to the pyrroloindoline product, 76. The computed energies for the diastereomeric transition states predicts that the exo product should predominate for both bromination and selenation. |
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