Cope rearrangement of semibullvalene

Andrae D, Barth I, Bredtmann T, Hege H-C, Manz J, Marquardt F, Paulus B (2011) Electronic quantum fluxes during pericyclic reactions exemplifled for the cope rearrangement of semibullvalene. J Phys Chem B 115 5476... 

GonzAlez-Navarrete P, Andres J, Berski S (2012) How a quantum chemical topology analysis enables prediction of electron density transfers in ehemieal reactions. The degenerated cope rearrangement of semibullvalene. J Phys Chem Lett 3 2500-2505... 

Jackman LM, Fernandes E, Heubes M et al (1998) The effects of substituents on the degenerate cope rearrangement of semibullvalenes and barbaralanes. Eur J Org Chem 10 2209-2227... 

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-... 

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). 

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.

The (BLW)PBE/IGLOO-III level Vc2C8 couplings calculated by Jana et al for semibullvalenes have been found to correlate with the (BLW)-B3LYP/6-311 + G(d,p) level calculated reaction barrier heights for the Cope rearrangements of these compounds. 

Reviews of the Cope reaction, electronic quantum fluxes during the rearrangement of semibullvalene," the chemistry of hetero-substituted isocyanates and isothiocyanates RX-NCY (X = R2N, RO, RS Y = 0, S) and the isomeric cyanates RX-OCN, thiocyanates RX-SCN, nitrile oxides RX-CNO, and nitrile sulfides RX-CNS have appeared. ... 

Jiao H, Schleyer P v R (1993) Elimination of the barrier to cope rearrangement in semibullvalene by Li complexatton. Angew Chem Int Ed Engl 32 1760-1763... 

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.

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... 

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. 

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... 

---