Nature of the 5‐norbornen‐2‐yl Cation Intermediate in the acetolysis of 2‐deuterio‐endo‐5‐norbornen‐2‐yl brosylate

Abstract

Summwy. Mass spectrometrical and 1H-NMIL-analyses of the exo-5-norbomen-2-yl acetate, formed by acctolysis of endo-5-norbonen-2-yl-2-exo-d brusylatc, demonstrate that thc deuterium initially on C(2) migratcs partially (30%) onto C(l) (mechanism l a or Ib). No dcuterium could bc detccted on the other positions, which shows that C(1-7) migration is insignificant. W-NMR.-analysia of th? deutcriatcd nortricyclyl acetate obtained as main product shows that the deuterium is equally and uniquely distributed between positions C(l) ancl C(G). This indicates that the nortricyclyl derivatives do not arise frotri nucleopliilic attack on C(5) of asymmetrical norbornenyl interrnediatcs, but from the reaction of a symmctrical nortricyclyl cation interrnediatc with solvent (mechanism Tb).Since the pionecring work of Roberts [11 and Winskin 121 on thc solvolysis of a m -and endo-5-norborncn-2-yl derivatives 1-X and 2-X many papers have dealt with the cationic intermediate, thc nature of which ha5 still not been cstablished satisfactorily [3J. Wc discuss briefly the main fcatuws of this homoallylic system and preacnt cxperirnental results that allow, for the first time, it clear distinction bctween five possible mechanisms Is, Ib, 11, 111 and I V of the dcgencrate rcarrangemcnt of the cationic intermcdiatc formed in the acetolysis of the endo-5-norbornen-2-yI brosylate.Introduction. -The participation of the C(5-6) double bond in the solvolysis of em-S-norbornen-2-yl derivatives 1-X was indicated by the relative rate of acetolysis ef exo-5-norbornen-Z-yl 9-bromobenzenesulfunate (1-OBs), which was found to be of the norpinenyl cation 7 in the interconvertion of dehydronorbonyl cations cannot therefore be excluded a prioris).In the acetolysis and formolysis of 1-and 2-OBs or deamination of 2-NH,, Rob& [12] observed that 30 to 48% of the initial 14C label at C(2,3) had migrated into the other psiitions C(1,4,5,6,7) of the exo-5-norbornen-2-yl products. These results suggested the occurance of an equilibrium betwcen two asymmetrical cationic intermediates 4 arid 4' via a symmetrical intermediate or transition state of type 10, i. e. a Wagaer-Meerweirt rearrangement involving the migration of the C( 1-6) bond.Depending upon the reaction conditions and particularly upon the reactivity of the nucleophile, the label scrambling was partial or complete. This interpretation is supported by the solvolysis' of 2-dcuterio-1-X derivatives [13] and by DC1 additions to norbornadiene and quadricyclane [14].In the acetolysis of em-3-deuterio-1-OBs CristoE 1151 observed that the deuterium is distributed equally between the e x 0 3 and syw7 positions of the 1-OAc isolated.It was further shown that the deuterium migration in the starting brosylatc was faster than nucleophilic attack by solvent yielding I-OAc and 3-OAc. In contrast to Robert's conclusion it was suggested that a symmetrical cation intermediate of type 10 is primarily formed, thus removing the necessity of an asymmetrical norbornenyl cation intermcdiateY). The participation and migration...

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

“…13C-NMR. spectroscopy established that the deuterium substituted exclusively at C (2) (almost complete disappearance of the signal of the carbon atom bearing the H20+ group was observed at < -50"; the C(2) was saturated by relatively fast proton noise decoupled pulses [18] (see Fig. 1D).…”

The use of lanthanides for solution structure determination of biomolecules by NMR. Evaluation of the methodology with EDTA derivatives as model systems

“…This question is addressed below with special reference to the reliability of using the temperature dependence of the paramagnetically shifted resonances for making such assignments. Given the avidity with which lanthanide ions are bound by some proteins, it is inevitable that slow exchange obtains in many protein-lanthanide complexes, thereby complicating the ultimate interpretation of the NMR (8) Solomon, I. Phys. Rev.…”