Barrelene to semibullvalene rearrangement

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

Aside from simple processes such as cis-trans isomerization, the first example of a potential energy surface derived for a photochemical rearrangement or reaction was in 1967. This was work by the Zimmerman group 44-47) in which the hypersurface for the Di-u-Methane rearrangement of barrelene to semibullvalene was obtained note Fig. 13. 

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. 

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. 

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. ... 

Studies of di-ir-methane photochemical rearrangements have been one of the main areas of research in organic photochemistry for many years (for reviews, see Refs. 1-4). The first example of a reaction of this type was reported by Zimmerman in 1967 in the sensitized irradiation of barrelene 1 that yields semibullvalene 2 [5] (Scheme 1). The reaction has been extended to a large number of acyclic and cyclic 1,4-dienes that yield the corresponding vinylcyclo-propanes on irradiation, in the di-ir-methane (DPM) version of the rearrangement. This reaction also takes place when a vinyl unit is replaced by an aryl group. A few representative examples of DPM rearrangements are shown in Scheme 1 [6-9]. 

The di-TT-methane rearrangement was discovered and conceptually developed by H. E. Zimmerman (University of Wisconsin), one of the earliest examples being the photoisomerization of barrelene (1) into semibullvalene (2) (equation 2). Through intensive subsequent work, this photoreaction was shown to be quite general for 1,4-dienes. In view of the major contributions by the original author, the rearrangement is also referred to as the Zimmerman rearrangement. 

One of the first examples of the di-7t-methane rearrangement was found in the photochemical conversion of bicyclo[2.2.2]octa-2,5,7-triene (barrelene, 1) to tricyclo[5.1.0.0 ]octa-2,5-diene (semibullvalene, 2). ... 

A study of the di-Ti-methane reactivity of the barrelene derivatives (73) in zeolites has been published. The reaction in a slurry affords a 77 23 mixture of (74) and (75) when the reaction is carried out in a zeolite the cyclooctatetraene product is suppressed and the two products are obtained in a ratio of 1 99. This enhancement of the di-Ti-methane reactivity occurs with Li" - and Na -exchanged zeolites.Liao and co-workers have reported new reactivity of some barrelenes. The reactions encountered are sensitive to substitution pattern. Thus, the irradiation of (76) with electron withdrawing groups follows the di-TT-methane route, to yield (77) and (78) predominantly. The less heavily substituted derivative (79) behaves differently, and irradiation affords (80) and (81) by the aza-di-7r-methane rearrangement, with (82) formed only in small amounts by the alternative di-Ti-methane path. Calculations have been used to examine the mechanism of the barrelene-semibullvalene isomerization. These results indicate that two biradical intermediates are involved in the T state. Other calculations on the di-Ti-methane rearrangement of barrelene substantiate the Zimmerman mechanism for the sensitized rearrangement. 

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