Bicyclo hexenyl cation

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

It has been found that cyclohexa-2,4- and -2,5-dienones undergo a light-induced valence isomerization reaction in strong acid analogous to the alkylbenzenes, to yield 2-hydroxy-bicyclo[3,l,0]hexenyl cations. The hydroxybenzenium ion (34), for example, underwent a clean photoisomerization to 35 at temperatures below — 60° (Paxrington and Childs, 1970). Cation 35 was also produced upon similar irradiation of 36. 

FIGURE 10. PMO interaction diagrams for the frontier orbitals of bicyclo[2.1.0]pentene (left) and bicyclo[3.1.0]hexenyl cation (right) according to Jorgensen73. The Walsh orbitals of the cyclopropyl ring are denoted by ws and wa, electrons by dots. Compare with Figure 11... 

In the 4q case, 6 is not an important contributor to the ground state description of the properties of either 4 or 5. However, with 4 there are alternative modes of homoconjugation possible that involve the external cyclopropane bonds30,32,37 38. This is shown in Scheme 2 for the bicyclo[3.1.0]hexenyl cation. This alternative mode of conjugation of a cyclopropane in a 4q situation, an option not available to the parent 4q antiaromatic unsaturated ring systems39, leads to a fundamentally different set of properties and reactions of these systems as compared to the potentially homoaromatic 4q + 2 cases. 

SCHEME 2. Delocalization in the bicyclo[3.1.0]hexenyl cation C. Organization of the Chapter... 

While considerable work has been reported on the bicyclo[3.1.0]hexenyl cation and its derivatives, the results of these studies have not been reviewed as extensively as those of the corresponding homoaromatic systems. The most detailed accounts of these systems are those of Koptyug132 and Barkhash133. Numerous reviews on the cyclohexadienyl cations have appeared132 134. 

No other experimental structure determinations of bicyclo[3.l.0]hexenyl cations have been reported. Theoretical calculations have been undertaken by Hehre105 151 and Cremer and colleagues30. While in each of these instances the calculations were at a low level, the results are consistent with the presence of a short C(l)—C(5) and long C( 1)—C(6)/C(5)—C(6) internuclear distances for the parent cation. 

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. 

In summary, the bicyclo[3.1.0]hexenyl cations clearly show that homoconjugation is an important factor in determining the chemistry and properties of these cationic systems. The properties of the bicyclo[3.1. Ojhexenyl cations are sharply different from those of the homotropenylium and cyclopropenium ions, reinforcing the designation of the latter two cations as being examples of homoaromatic systems. 

Bicyclo[2.1.0]pentene, 116, can be considered to be the prototype of a neutral homoantiaromatic molecule. The types of structural and bonding effects found for this molecule parallel in many respects those found for the bicyclo[3.1.0]hexenyl cation reported above. Further studies on both of these 4q systems will likely be rewarding in terms of fully understanding the nature of cyclopropyl homoconjugation and homoantiaromaticity. 

Several substituted bicyclo[3.1.0]hexenyl cations 42 and 43 were prepared by Olah and coworkers73, and they all showed extensive charge delocalization into the cyclopropyl group as shown by the significantly deshielded l3C absorptions for the methylene carbons (89-124 ppm). 

Berson and coworkers95 observed that the parent bicyclo[3.1.0]hexenyl cation 86 undergoes circumambulatory migration with an activation barrier of 15.1 kcalmol" (equation 55). 

Examples Cyclobutenyl Bicycio [2.1.0] Bicyclo [3.1.0] cation pentene hexenyl cation... 

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. 

The structure of the l,2,3,4,5-pentamethylbicyclo[3.1.0]-hexenyl cation [170] was established by Childs et al. (1968) by H-nmr spectroscopy and a considerable delocalization of positive charge into the cyclopropane ring (112) was proposed. The structures have been extensively discussed by Childs et al. (1974 and references therein) for other bicyclo[3.1.0]hexenyl cations. 

If C(6) is substituted with very good carbonium ion stabilizers the transition-state structure [173] becomes more stable than the bicyclo[3.1.0]-hexenyl cation structure. Thus treatment of a series of 5-acyl-1,2,3,4,5-pentamethylcyclopenta-1,3-dienes with AlgCle produced cleanly the corres-... 

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