Cope rearrangements semibullvalenes

Based on the structure drawn for semibullvalene (38), it appears that there should be five different types of protons. However, because of the following Cope rearrangement only three types are observed (—110 to +117°) by NMR ... 

Fig. 19. Two examples of degenerate Cope rearrangement, a) 1,5-hexadiene b) semibullvalene.

Given that the boat transition state 8 is unfavourable, it is at first sight surprising that the Cope rearrangements of bullvalene (14), barbaralane (15), and semibullvalene (16) should take place so readily given that the transition states (17) of these reactions are derivatives of 8. We therefore decided 3S-) to calcu-... 

This is perhaps the area where there is the most optimism of attaining the elusive goal of neutral homoaromaticity. It has been suggested that semibullvalene [83], which undergoes degenerate Cope rearrangements through a homoaromatic transition state [83b] (2) with extremely low... 

Shortly after this prediction, Schroder (1963) isolated bullvalene. Numerous studies amply demonstrated the facile Cope rearrangement of [84] and its derivatives (see for example Schroder and Oth, 1967 Doering et al., 1967). Theory and experiment agree that, by pinching the methano bridges closer together, the rate of the Cope process increases in the sequence semibullvalene [83] > barbaralane [85] > bullvalene [84] (Dewar and Schoeller, 1971 Anastassiou et al., 1975). 

Following the suggestion that donor-acceptor (Dewar-Hoffmann) semi-bullvalenes [83a] would have a lower activation barrier for the Cope rearrangement, or even a homoaromatic ground state (Hoffmann and Stohrer, 1971 Dewar and Lo, 1971), numerous syntheses and studies of appropriately substituted semibullvalenes have been reported. In fact, this aspect of the search for homoaromatic semibullvalenes has been the most extensively investigated (for a partial summary of this work see Quast et al., 1985 Gompper et al., 1988, and references cited therein). 

Following the calculations (Williams and Kurtz, 1988) which led to the prediction that the bisannelated semibullvalene [108] would be homoaromatic, the Mullen group prepared the first (and, to date, only) example of a bisannelated semibullvalene ([109]) (Kohnz et al., 1989). The Cope rearrangement in [109] is extremely facile the estimated upper limit for the free energy of activation is 3.6 kcal mol-1. 

In principle, the divinylcyclopropane structure discussed here is incorporated into very well known systems such as bullvalene 547, barbaralane 548 and semibullvalene 549, which very easily undergo a Cope rearrangement. 

A number of semibullvalenes and some barbaralanes, which undergo a facile Cope rearrangement via bicyclooctadienyl diradicals, produce various cyclic peroxides on exposure to triplet oxygen ". Thus, cyclopentano semibullvalene 82a is peroxidated with air to give a mixture of 5-membered 83a and 6-membered endoperoxide 84a in a 1 2 ratio, whereas the cyclohexano derivative 82b yields only the 6-membered cyclic peroxide 84b (Scheme 15). The energy in the highly strained mono(Dewar benzene) isomer of... 

Figure 6. Series of IR femtosecond/picosecond laser pulses for the sequence of vibrational transitions SBV(u = 0) - SBV(u = 6) - SBV(t> = 1) for laser control of the Cope rearrangement of the model substituted semibullvalene (SBV) shown in Fig. 2 (adapted from Ref. 26). The notations are as in Fig. 3. The electric field is scaled by the scaling factor / of the effective charge associated with the dipole function jt =/ e q.

Cope rearrangement is known to take place in semibullvalene (S4)368 and bull-valene ( 5)369. This process is quite slow compared to the electron-diffraction process, and the bond distances of (84) and (85) are therefore found to be similar to normal single and double bonds. 

H. Quast, M. Seefelder, Angew. Chem. Int. Ed. Engl. 38, 1064 (1999). The Equilibrium between Localized and Delocalized States of Thermochromic Semibullvalenes and Barbaralanes—Direct Observation of Transition States of Degenerate Cope Rearrangements. 

A very rapid degenerate Cope rearrangement was observed for tricyclo[3.3.0.0 ]octa-3,6-diene(semibullvalene), 301. The H-NMR spectrum showed type A protons at 5.08 ppm, type B at 4.17 ppm and type C at 2.97 ppm and was unchanged between — 110° C and -I-117° C three absorptions only were shown in the C-NMR spectrum. ... 

Anet, Meinwald and their coworkers froze out the degenerate Cope rearrangement in semibullvalene at — 160°C and reported five and C absorptions and these are shown in Table 18 . 

Williams, R. V., Kurtz, H. A. A theoretical investigation of through-space interactions. Part 3. A semiempirical study of the Cope rearrangement in singly annellated semibullvalenes. J. Chem. Soc., Perkin Trans. 21994,147-150. 

The ability to measure low temperatures using C is also important, and one very sensitive system was based on the temperature dependent C chemical shifts for certain substituted semibullvalenes that exist as a pair of different valence tautomers. Equilibration between tautomers occurs rapidly on the NMR timescale via non-degenerate Cope rearrangement, and this system is suitable for measurements from 100 to 300 K, using chlorodifluoromethane and perdeuterated dimethyl ether (3 1 or 4 1 v/v) as solvents. The two carbons 6 and 2, shown in Fig. 8, have very different chemical shifts, with the difference, in terms of an equilibrium constant K, given by ... 

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