Stereochemistry of acid-catalyzed cleavage of 3-chloro- and 3-bromonortricyclene in deuterated medium. Evidence for edge protonation of nortricyclenes

Abstract

Cleavage of 3-chloronortricyclene (4c) and 3-bromonortricyclene (4d) occurs faster than solvolysis in D2SO4–DOAc at 70–75 °C and yields syn-7-, anti-7-, exo-5-, and endo-5-halo-exo-2-norbornyl acetates as products. That the 7-chloroacetates comprise 70% of the reaction mixture, that 30% of the 5-chloro-exo-2-norbornyl acetates which comprised 28% of the reaction mixture are derived via rearrangement of the syn-7- and anti-7-chloronorbornyl cations, and that the deuterium at C-6 is at least 90% stereochemically pure endo is evidence that cleavage of the bond farthest removed from the electron withdrawing halogen is preferred 5:1 over the other bonds of the three-membered ring. The Stereochemistry of the ring-opening is rationalized on the basis of cleavage via edge-deuteronation with formation of classical halocations as intermediates. The location of deuterium in the 5-halo-exo-norbornyl acetates is used to define the pathways whereby the 5-haloacetates are formed. This study also establishes that C-6 endo deuterated 7-chloro- and 7-bromo-exo-2-norbornyl brosylates can be synthesized from the corresponding 3-halonortricyclenes.