FAST REARRANGEMENT OF exo-NORBORNANOL IN STRONG ACID SOLUTIONS

Abstract

A B S T R A C T Nuclear magnetic resonance spectra have been obtained a t several temperatures for solutions o f 2-exo-norbornanol in strong acids. T h e spectra change with temperature reversibly. I t isconcluded that the 2-era-norbornanol undergoes very rapid rearrangement, the fast steps being 2,6 and 3,2 hydride shifts, respectively. I N T R O D U C T I O NThere is a vast literature which concerns carbonium ion rearrangements of bicyclic compounds (1-3). Of the inany systems studied none has been the subject of such intensive investigation as the reactions of 2-substituted norbornanes. These detailed kinetic (4) and isotopic (5) studies of solvolysis and deamination reactions reveal the operation of 3,2 and 6,2 hydride shifts, respectively, and Wagner-iueerwein rearrangements and imply the intermediacy of several different carbonium ions. Hitherto these rearrangements have been observed in irreversible systems. In this work it will be shown by the methods of nuclear magnetic resonance (n.1n.r.) spectroscopy that 2-exo-norbornanol is stable in strong acid solutions and a t the same time undergoes fast skeletal rearrangements. R E S U L T S A N D DISCUSSIONThe n.m.r. spectruln of 2-exo-norbornanol in Inethylene chloride is shown in Fig. I. The absorptions a t 6.19 tau and 7.87 tau arise froin CHOH and the bridgehead hydrogens respectively. The remaining hydrogens on C3, C3, C6, and C7 give the multiplets in the region of 8.8 tau; the line for OH appears a t 8.21 tau. The same spectrum is obtained using dimethyl sulfate and methanol as solvents with the exception that the chemical shift of the OH proton changes with solvents due to differences in hydrogen bonding. The chemical shifts of the solvent protons are as listed in the literature (6). In all these solutions the n.m.r. absorption for the OH hydrogen consisted of a single line. Thus 0-H exchange was too fast to permit observation of CzH, OH coupling. However, using trifluoroacetic acid as solvent, the line for C2H appears a t 5.40 tau and the rest of the spectrum ( Fig. 2A) has a slightly different appearance froin those obtained with the other solvents. Furthermore, due to fast proton exchange, the OH resonances in the acid and alcohol are merged. Since 2-exo-norbornanol was quantitatively recovered from trifluoroacetic solution, these changes must be due to a specific solvent effect.When 2-exo-norbornanol is dissolved in 97% sulfuric acid or mixtures of trifluoroacetic acid and sulfuric acid the peak due to CHOH disappears and the remaining spectrum consists of two broad lines a t 7.8 tau and 8.8 tau (Fig. 2E). The starting material was quantitatively recovered from these solutions also. Hence the effects found here do not arise from unusual chemical reactions. On heating the solutions of 2-exo-norbornanol in H2SO4-CF3C02H between 0 and 60" the resonance due to CHOH broadens and disappears

Some observations on isotopic scramblings accompanying acetolysis in the 2-norbornyl system

“…Thus it appears that there could be some similarities in the processes of scrambling in the reaction of exo-1-OAc in HOAc-NaOAc and in the treatment of exo-l-OH in F3CCOOH-H,S04. In sharp contrast to the findings of Fraenkel and coworkers (17), the norbornyl cation generated in "super acid" by Schleyer and coworkers (18,19) behaved quite differently. At temperatures between -5 and +37", a one-peak n.m.r.…”

“…From the examination of the n.m.r. spectra of exo-1-OH in F3CCOOH-H,S04 under various conditions, with the amount of H,S04 ranging from 5-60% and the temperature ranging from 0-40°, Fraenkel and coworkers (17) have concluded that under these conditions, 3,2-and 6,2hydride shifts take place faster than Wagner-Meerwein rearrangements. Thus it appears that there could be some similarities in the processes of scrambling in the reaction of exo-1-OAc in HOAc-NaOAc and in the treatment of exo-l-OH in F3CCOOH-H,S04.…”

The synthesis of dehydro‐2‐norbornyl and 2‐norbornyl derivatives labeled at C‐3 with 14C

“…In the reaction vessel were placed 15.0 g (0.21 mole) of l-14C-2-cyanoethanol (V1, prepared in 80 % yield from V + NaCN (13)), 6.0 g of fused KOAc, 0.2 g of hydroquinone and 15 ml of diethylene glycol. The reaction mixture was gradually heated in a Wood's metal bath until the bath temperature was maintained at 2300 C. With the steam-heated reflux condenser preventing the distillation of the solvent, the distillate containing water and VII began to appear when the bath temperature reached 1800 C and the heating was stopped when no more distillate appeared at 230° C. The organic layer of the distillate was separated, dried over CaCl, and fractionated to give 9.2 g (82 %) of VII, b.p.…”