Semibullvalenes, rearrangements

With the exception of the a-naphtho position, bridging to aromatic sites is very unfavorable in the barrelene-semibullvalene rearrangements. If we exclude anthraceno-vinyl bridging, there is insufficient energy available (43 kcal/mole) for vinyl-vinyl bridging (58 kcal/mole) in the lowest anthrabarrelene triplet state. Consequently, it is not surprising that the 7 state is unreactive. In contrast, (76 kcal/mole) and T2 (74 kcal/mole) are not subject to this limitation. Evidence in favor of one or the other of these two possible electronic states is not available. 

An isoelectronic analogue of the -component of benzene is the transition state for the degenerate semibullvalene rearrangement in Scheme 41a. The VB correlation diagram,37 in Scheme 41b, for this process is also analogous to that of benzene and involves two curves which correspond to the VB... 

Scheme 41. (a) Transition State for the Degenerate Semibullvalene Rearrangement, (b) Avoided Crossing of the Corresponding Kekule Structures Along the Reaction Coordinate Mode (t>2), and the Generation of the Twin States by Avoided Crossing, (c) Frequencies of the b2 Mode in the Two States (Ref 225). Delocalized Bonds in c Are Removed for Clarity... 

As a proof of principle, the twin states were characterized for the semibullvalene rearrangement and found to possess virtually identical geometries. As shown in Fig. 6.23, the twin-excited state possesses B2 symmetry as the symmetry of the reaction coordinate of the thermal process. The TS mode, b2, which is imaginary for (A, was shown to be real for (B ) (83). These calculations match the intriguing findings (84,85) in the... 

FIGURE 6.23 The twin states along the b2 reaction coordinate for the semibullvalene rearrangement. When the thermal barrier is NOT much higher than the zero-point levels of the two isomers, the transition state ( F ) region becomes available thermally. Absorption in the transition state region is in the visible, leading to thermochromism at elevated temperatures. 

S. Zilberg, Y. Haas, D. Danovich, S. Shaik, Angew. Chem. Int. Ed. Engl. 37, 1394 (1998). The Twin-Excited State as a Probe for the Transition State in Concerted Unimolecular Reactions. The Semibullvalene Rearrangement. 

DETAILED REACTIVITY OF INTERMEDIATE DIRADICALS IN THE BARRELENE TO SEMIBULLVALENE REARRANGEMENT... 

The Twin-Excited State as a Probe for the Transition State in Concerted Unimolecular Reactions. The Semibullvalene Rearrangement. 

Cyclooctatetraene can be obtained on an industrial scale by metal carbonyl catalyzed thermal tetramerization of acetylene. If cyclooctatetraene is UV-irradiated at low temperature in the presence of acetone, it is reversibly rearranged to form semibullvalene (H.E. Zimmerman, 1968, 1970). 

A further reduction in the barrier and increase in rate is seen with semibullvalene i38), in which strain is increased still more. The for this rearrangement is 5.5kcal/molat -143 C. ... 

The di- r-methane rearrangement is a fairly recent reaction. One of the first examples has been reported in 1966 by Zimmerman and Grunewald with the isomerization of barrelene 8 to semibullvalene 9. This rearrangement reaction occurs in the presence of acetone as photosensitizer, and proceeds from the Ti-state. ... 

However, some substrates, generally rigid bicyclic molecules, (e.g., barrelene, which is converted to semibullvalene) give the di-7t-methane rearrangement only from triplet states. 

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. 

The influence of radical stabilization on the outcome of the rearrangement reactions of a variety of dibenzobarrelenes has been evaluated178. A detailed analysis of the acetophenone-sensitized conversion of the cyano-substituted barrelenes into the corresponding semibullvalenes has been presented179. The outcome of the irradiation of the dibenzobarrelene 331 is dependent upon the excited state involved. Thus direct irradiation affords a cyclooctatetraene and sensitized irradiation converts it into the two... 

The numerous transformations of cyclooctatetraene 189 and its derivatives include three types of structural changes, viz. ring inversion, bond shift and valence isomerizations (for reviews, see References 83-85). One of the major transformations is the interconversion of the cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene. However, the rearrangement of cyclooctatetraene into the semibullvalene system is little known. For example, the thermolysis of l,2,3,4-tetra(trifluoromethyl)cyclooctatetraene 221 in pentane solution at 170-180 °C for 6 days gave three isomers which were separated by preparative GLC. They were identified as l,2,7,8-tetrakis(trifluoromethyl)bicyclo[4.2.0]octa-2,4,7-triene 222 and tetrakis(trifluoromethyl)semibullvalenes 223 and 224 (equation 71)86. It was shown that a thermal equilibrium exists between the precursor 221 and its bond-shift isomer 225 which undergoes a rapid cyclization to form the triene 222. The cyclooctatetraenes 221 and 225 are in equilibrium with diene 223, followed by irreversible rearrangement to the most stable isomer 224 (equation 72)86. 

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