Many kinetic and stereochemical data, as well as direct spectroscopic observations, have accumulated to support the nonclassical, bridged nature of secondary2 and some tertiary3 7-substituted-7-norbornenyl and 7-norbornadienyl carbonium ions. However, when a strongly charge stabilizing substituent is placed at C,, stabilization due to the bridged interaction will be decreased and may actually be cancelled. This has been demonstrated for the 7norbornenyl series by solvolytic studiesza and by direct nmr analysis of fully formed ions.3b
In the 7-norbornadienyl series, the substituent effect has been illustrated by considering the barrier to "bridge-flipping" (eq. l), the free energy difference between the unsymmetrical (ground state) and synxsetrical (transition state) forms of the ion.*' For the parent cation,
Unsymmetrical
Bridging Bridging this difference must be 19.6 kcal/mole or more. For the 7-methyl ion, it is 12.4 kcal/mole, and for the 7-phenyl ion it is less than ". 5.2 kcal/mole. 4b This correlation between decreasing barriers to "bridge-flipping" and increasing stabilizing ability occurs because the substituents stabilize selectively the transition state (i.e., the symmetrical form of the ion) in which delocalization of charge to the unsaturated linkages of the ring system is minimized and delocalization into the substituent is maximized. It is conceivable that, in the extreme where the substituent is very highly stabilizing, the symmetrically bridged ion will be more stable than the unsymmetrically bridged form.3,5 This communication presents evidence that methoxyl is such a substituent. Each of the cations L-2 was generated in PSO,H solution by extraction of the corresponding ketal precursor from pentane or CD#l, at -78'. Spectra were recorded at ca. -6O', and the chemical shifts were assigned (relative to internal CH&l, at T 4.70) as described on the appropriate structures. The nonequivalence of the bridgehead protons arises from the fact that, in order for p-p x overlap to occur between C, and oxygen, the methyl function must lie in the C,-C,-C4 plane, and is thus oriented s to one bridgehead proton and anti to the other. Upon warming the solutions, a degenerate D-anti isomerization (indicated for 3 in eq. 2) was observed for all three ions. This phenomenon was manifested as a reversible coalescence of the