The 7-methoxynorbornadienyl cation: evidence for a symmetrically bridged structure

Cope interconversions of the 9-methoxy-endo-tricyclo[4.2.1.0]nona-3,7-dienyl cation and its [4.3.0.0]tautomer. Evidence of enhanced ionic stability by delocalization of neighbouring ? bonds

“…Such shifts are typical for methoxy-groups attached to cationic centres. The small separation (0. spectrum of both isomers of (11) shows comparable resonances a t 6 9.10, 7.57, and 8.87, 7.30 which are consistent with methoxy-stabilized allylic systems. The mutual coupling of 5 Hz indicates that the allylic system forms part of a five-membered ring.1° Ion (12), which occurs similarly in two isomeric forms (12a and b) (1 : Z), shows analogous spectral data (Table 2).…”

“…The The high degree of ionization of compounds (7b,c) in liquid SO, is attended by their skeletal rearrangement, resulting in the formation of the stable ion (ll). * Factors which should be considered with regard to the stability of (11) relative to (7+) are the resonance energy of its allylic system and the increased ring strain of its ring-contracted structure.…”

“…The difference in strain energy is of the order of 14 kcal mol-l.12 This figure is the difference in standard free energy of bicyclo[2.2. llheptane and bicyclo[3.2.0]heptane systems. However, the estimated resonance energy of an allylic cation is 20 kcal m01-l.l~ Therefore, ion (11) will be stabilized substantially with respect to (77. In contrast, the covalent derivative ( 17) of ( 11) will be 14 kcal mol-l higher in energy than (7b,c).…”

“…spectrum (Figure), which shows an increasing amount of (7b,c) a t higher temperatures. The decrease in dipole moment of liquid SO, a t higher temperatures may account for this phen0menon.O When FS0,H was added to the solution of (7b,c) in liquid SO,, (11) only was observed, even a t temperatures up to -10 "C. However, in this medium two isomers of (11) were present [( 1 la,b)], which apparently interchange in pure liquid SO, in which only one structure could be observed. * The assignment of rearranged structure (11) is based on its lH and 13C n.m.r.…”

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1–SN2 Interface

“…Such measurements were made by the Winstein group who determined a barrier of 18.4 kcal.mol −1 for the 7-methoxy-7-norbornyl cation by VT NMR spectroscopy in fluorosulfonic acid (Scheme 3). 35 The barrier for syn-anti equilibration of the 2-methoxy-2-norbornyl cation was higher and estimated to be at least 19.5 kcal.mol −1 35 . While there is considerable variation between the stabilization of ~30 kcal.mol −1 calculated from the hydride ion affinities and the barriers to rotation, on the whole they are in reasonable agreement with estimates of there being approximately 60% π-character in oxocarbenium ions derived by alkylation or protonation of aldehydes and ketones as judged by Olah and others on the basis of 17 O, 1 H, and 13 C chemical shift measurements.…”