Bicyclo hexenyl cations electron delocalization

Berson and Jenkins have looked for a comparable circumambulation in the parent homotropenylium ion 11 using the 4-deuterium labelled ion (Scheme 23)157. They were unable to detect the occurrence of any circumambulation prior to decomposition of the ion and, as a result, it was only possible to obtain a lower limit of 27 kcal moT1 for the barrier for circumambulation. Hehre calculated (HF/STO-3G) the barrier to thermally induced circumambulation in 11 as being43 kcal mol1105151. Once more it is clear that there is a fundamental difference in the properties of the bicyclo[3.1.0]hexenyl cations and the homotropenylium ions which can be attributed to the difference in electron delocalization of the two systems. 

FIGURE 14. CC bond orders n and bond ellipticities a of cyclopropyl homoconjugated molecules (a) norcaradiene, (b) bicyclo[2.1.0]pentene, (c) bicyclo[3.1.0]hexenyl cation. On the right, the preferred mode of electron delocalization is indicated by dashed lines. Also given is the number of delocalized electrons as calculated from topological bond orders. See text... 

Information on electron delocalization in the bicyclo[3.1.0]hexenyl cations is available from their reported NMR spectra Data obtained with a variety of systems point to a completely different charge delocalization pattern to that found with the homotropenylium ions. For example, Olah and colleagues have obtained the NMR spectrum of the parent ion"", 61, and compared this with those of 42 and 11. As can be seen from the data summarized in Scheme 18, the chemical shifts of the five-membered ring carbons of 61 resemble those of the cyclopentenyl cation. There is a considerable difference in chemical shifts, and hence charge distribution, at C(2), C(4) and C(3) of 61. There is no evidence for the fairly even charge distribution as is found for the homotropenylium and homocyclopropenium ions (see previous Sections III. A and III. B). It was also noted by Olah that the chemical shift of C(6) is consistent with large delocalization to this position, i.e. to conjugation of the allyl system of 61 with the external cyclopropyl bonds. 

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