Semibullvalenes structure

The highly reactive semibullvalene structure undergoes facile acid-catalyzed 1,4-addition of protic solvents to one of the vinylcyclopropane units.201 The reaction... 

Acetone sensitization provided semibullvalene (38) while direct photolysis gave cyclooctatetraene (39). Several structural representations of semibullvalene are shown below ... 

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 ... 

Table 3 lists all polyenes whose radical cations have been investigated by one or other of the above-described techniques and some of the structures listed are shown below the table. Note that some nonconjugated dienes do not retain their structure upon ionization [e.g. semibullvalene 104 (equation 61) or the cyclopentadiene dimers 126 and 294 (equation 62)] but break a bond to form a bisallylic radical cation, a rather common tendency of radical cations that have this possibility. 

An electron-diffraction study of semibullvalene [83] revealed the anticipated Cs symmetry with reasonable correspondence between calculated and experimental structural parameters (Wang and Bauer, 1972). 

The primary piece of information obtained from most theoretical calculations is the molecular structure. If homoaromatic interactions are important in a molecule, they may cause the molecule to adopt an unusual geometry. In suitable radicals, ESR evidence has been taken to indicate systems of high symmetry which in turn has been interpreted in terms of homoaromatic interactions (Dai et al., 1990). A computational example of this effect is shown in the semiempirical calculations of Williams and Kurtz (1988) on the bisannelated semibullvalene [108]. Here simple configuration interaction... 

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. 

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. 

Radical cations (21 +) derived from semibullvalene + or barbaralane belong to a different structure type. The ESR spectrum of 21 + (a = 3.62 mT, 2H a = 0.77 mT, and strong CIDNP effects support a structure in which... 

Interestingly, the original referees did not like the terms barrelene and semibullvalene with the comment that these were unesthetic and that it was unlikely that any further compounds of these structures would be encountered. 

SCHEME 41. Semibullvalenes calculated to possess symmetrical ground state structures... 

The structures of semibullvalene derivatives have been determined. It was initially suggested that there was a systematic variation in the C(2)-C(8) and C(4)-C(6) distances as the barrier to interconversion of the two valence tautomers varied. Such a variation would be good evidence for cyclic delocalization. However, recent work of Jackman, Quast and coworkers using CP-MAS 13C NMR has shown that the earlier work did not take into account the presence of two valence tautomers263. 

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... 

Another structural possibility for a hexadiene radical cation arises, when the two allylic moieties are linked in pairwise fashion to two- or three-carbon spacers. This structure type can be approached by oxidation of molecules such as semibullvalene [391-393] or barbaralane [394]. In the resulting radical cations, the two allylic moieties are held in close proximity model considerations suggest a non-bonding C—C distance of 2.2-2.3 A, considerably closer than for the previously discussed structure type. At this distance, a moderately strong interaction of the twin moieties cannot be excluded. Accordingly, we assigned cyclic conjugated structures to radical cations derived from semibullvalene (-> 138 cf. 

In contrast, CIDNP results indicate that the radical cations of barbaralane (157, X = C = 0) and semibullvalene (157, X = —) correspond to the elusive structure type with a single minimum [391, 424]. The spin density resides primarily on the termini (C-2,4,6,8) of the twin allyl moieties, whereas the remaining (internal) carbons of the 5 jr-electron perimeter have negative spin density. This spin density distribution reflects the coefficients of orbital 158, the HOMO of a bis-homoaromatic structure (Fig. 32) [424], More recently, ESR results have confirmed this assignment [392, 393],... 

Suitable substitution can also decrease the isomerization barrier. As predicted using essentially qualitative theory the semibullvalene system is markedly susceptible to such modulation. For example 3,7-dicyano substitution increases the barrier in 1,5-dimethylsemibullvalene from 6 to 10 kcalmol" while 2,6-dicyano substitution decreases the barrier to ca. 4 kcal mol" L (Strictly speaking, the comparison is being made between data on the isomeric dimethyldicyano semibullvalenes and that of the parent hydrocarbon. ) Recently, it has been shown that l,5-dimethyl-2,6-dicyanosemibullvalene has a thermally accessible isomer that corresponds to a delocalized, homoaromatic structure (70), i.e. the cyclopropane has disappeared It is perhaps not so obvious when a cyclopropane is a cyclopropane. 

In fact, barrelene rearranges by the di-7t-methane pathway to semibullvalene only when acetone sensitized direct irradiation produces cyclooctatetraene as the major photoproduct. A number of similar examples exist, each demonstrating the general phenomenon of preferred triplet multiplicity for di-7r-methane rearrangements in rigidly constrained systems, i.e. structures which prohibit free rotation about the Ti-bonds. ... 

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