CYCLOPROPANES . AN ANTI-MARKOVNIKOV CLEAVAGE AND EVIDENCE FOR A SIGMA-COMPLEX PROTONIUM ION

The first example of anti-Markovnikov cleavage of a cyclopropane ring has been discovered in the acid-catalyzed reactions of the bicyclo[2.2.2]octane adduct (1). Cleavage of the cyclopropane ring by hydrobromic or acetic-sulfuric acids was shown to proceed in high yields to addition products without methyl groups, the reverse of electronic prediction. Use of deuterio acid allowed determination of the stereochemistry of proton attachment in the cleavage, which was shown to proceed exclusively via initial formation of a three-center protonium ion (σ complex). Steric inhibition to normal collapse of this ion by nucleophile attack is suggested as the cause of anti-Markovnikov cleavage in this instance. © 1969, American Chemical Society. All rights reserved.