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

Abstract

AbstractLow temperature (−130° to −110°) addition of exo‐norborn‐5‐en‐2‐ol (7) to excess HSO3F in SO2CIF 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 HSO3F 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 HSO3F 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 HSO3F yielded the exo‐2, exo‐5‐ and exo‐2, endo‐5‐norbornanediyl bis (fluorosulfates) (21+22). Norbornadiene and quadricyclane added 2 equivalents of HSO3F 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 DSO3F 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 FSO3 and H2O+ (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 = FSO3 or H2O+ (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 HSO3F/SO2CIF.