Secondary 2‐Norbornyl Cation Intermediates Substituted at C(5) and C(7) by Electron‐withdrawing Groups. Addition of fluorosulfuric acid to unsaturated norbornane derivatives

Low temperature (−130° to −110°) addition of exo ‐norborn‐5‐en‐2‐ol ( 7 ) to excess HSO 3 F in SO 2 CIF yielded a mixture of exo ‐5‐(fluorosulfonyloxy)‐ exo ‐2‐ and endo ‐2‐norbornylhydroxonium ions ( 9+10 ) under kinetic control that was different from the mixture of 9+10 obtained by addition of endo ‐norborn‐5‐en‐2‐ol ( 8 ) to HSO 3 F under kinetic control. These mixtures differed from the mixture of 9+10 observed at higher temperature (−80° to −60°) (thermodynamic control). Addition of 3‐nortricyclanol ( 23 ) or exo ‐2, 3‐epoxynorbornane ( 24 ) to HSO 3 F at ‐−120° ± 10° yielded a mixture containing the exo ‐2‐(fluorosulfonyloxy)‐ anti ‐7‐ and syn ‐7‐norbornylhydroxonium ions ( 26+27 ) as major adducts. Qualitative rates of the isomerization of 26+27 to the more stable ions 9+10 and of the isomerization 9 ⇄ 10 were evaluated. The solvolysis of 9+10 in HSO 3 F yielded the exo ‐2, exo ‐5‐ and exo ‐2, endo ‐5‐norbornanediyl bis (fluorosulfates) ( 21+22 ). Norbornadiene and quadricyclane added 2 equivalents of HSO 3 F and furnished kinetically a mixture of exo ‐2, anti ‐7‐ and exo ‐2, syn ‐7‐norbornanediyl bis (fluorosulfates) ( 36+37 ) as major adducts. The latter 36+37 were isomerized into a kinetic mixture of the more stable isomers 21+22 . The rates of these isomerizations were compared. The use of DSO 3 F and ( exo ‐2‐D)‐norborn‐5‐en‐2‐ol ( 15 ) confirmed that heterolyses of the fluorosulfates were responsible for the observed isomerization; elimination‐addition processes occurred but much more slowly. The results are interpreted in terms of substituted classical and σ‐bridged secondary 2‐norbornyl cation intermediates. It appears that the electron withdrawing substituents FSO 3 and H 2 O + (HO) destabilize the σ‐bridged 2‐norbornyl cation more at C(5) than C(7). If the σ‐bridged ions 5‐Z substituted at C(5) by Z = FSO 3 or H 2 O + (HO) are transition states in the isomerization of the corresponding classical ions 3‐Z, 4‐Z , the free enthalpy difference between the ‘non‐classical’ σ‐bridged ion and the classical ions is not higher than the energy barrier to the quenching of the latter intermediates by FSO in HSO 3 F/SO 2 CIF.