Barrelene

A classic example of the di-rr-methane photorearrangement is afi orded by the reaction of barrelene (37) 

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

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

The direct photolysis of barrelene to cyclooctatetraene can be pictured as foUows  

A precedent for this type of reaction can be found in the formation of benz[/]oxepin (42) from the epoxide (41)  

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Barrelene was obtained via a double Diels-Alder reaction from a-pyrone with methyl acrylate (H.E. Zimmerman, I969A). The primarily forming bicyclic lactone decarboxylates in the heat, and the resulting cyclohexadiene rapidly undergoes another Diels-Alder cyclization. Standard reactions have then been used to eliminate the methoxycarbonyl groups and to introduce C—C double bonds. Irradiation of barrelene produces semibullvalene and cyclooctatetraene (H.E. Zimmerman. 1969B). 

Strong acids or superacid systems generate stable fluorinated carbocations [40, 42] Treatment of tetrafluorobenzbarrelene with arenesulfonyl chlorides in nitro-methane-lithium perchlorate yields a crystalline salt with a rearranged benzo barrelene skeleton [43] Ionization of polycyclic adducts of difluorocarbene and derivatives of bornadiene with antimony pentafluonde in fluorosulfonyl chloride yields stable cations [44, 45]... 

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. ... 

We reacted 2 first with bromine in chloroform at 10 C. iH NMR studies have revealed that the reaction mixture was very complex and consisted of six products. This mixture was submitted to silica gel column chromatography. Careful repeated chromatography followed by fractional crystallization allowed us to isolate ten products (Scheme 3). IR analysis indicated that a hydroxyl group was incorporated in compounds lfi-19. Therefore, we assume that these products have been formed by partial hydrolysis of compounds lfl-14. Structural determination of compounds lfl-19 revealed that the barrelene skeleton was rearranged completely. 

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. 

The spectral properties of benzobarrelene are as follows infrared (potassium iodide) cm. strong peaks 1460, 1325, 790, 750, 690, 660 proton magnetic resonance (chloroform-d) S (multiplicity, number of protons, assignment) 4.9 (multiple , 2, bridgehead H), 6.8-7.3 (multiple , 8, aryl and vinyl H). The reported melting point is 65.5-66°.From 20 g. of tetrachlorobenzo-barrelene the submitters obtained 8.3-8.8 g. (79-83%) of benzobarrelene, m.p. 64-65°. 

Oxepin is in equilibrium with benzene oxide by a [3,3]-sigmatropic shift. Advantage has been taken of this equilibrium to develop a short synthesis of barrelene. Outline a way that this could be done. 

In mechanism (8.43) the bridgehead hydrogens of barrelene should be found at the a positions of semibullvalene (2a, 0/3,0y). Mechanism (8.44) can give three different hydrogen-label distributions. If the final bond formation is concerted with bond fission, and bond fission and formation take place at the same carbon atom [mechanism (8.44A)], the label distribution should be (la, 0/3, ly). If bond formation is concerted with bond fission but with a preference for bond formation at the carbon allylic to bond fission [mechanism (8.44B)], the label distribution should be (2a, 0/3, Oy). If there is a symmetric allylic biradical which has a finite existence [mechanism (8.44AB)], then the hydrogen-label distribution should be (1.5a, 0/3,0.5y). 

Benzobarrelene is similar to barrelene in that rearrangement to benzo-cyclooctatetraene takes place from the singlet state while benzosemibullvalene is formed from the triplet state<44> ... 

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. 

Can a fortuitous combination of mechanisms (8.43) and (8.44A) account for the observed hydrogen-label distribution in the formation of semi-bullvalene from barrelene If so, how can you eliminate this possibility ... 

Benzyne reacts with benzene to give a mixture of products in low yield. The original experiments 38> showed that the 1,4-cyclo-adduct (benzo-barrelene) (19), the valence-bond isomerised 1,2-cyclo-adduct (benzo-cyclo-octatetraene) (20), and the product of insertion into a carbon-hydrogen bond (biphenyl) (21), were obtained in 2,8, and 6% yields respectively. 

Unfortunately the structural formula assigned was incorrect. A reinterpretation of the spectral data 44>, showed that the compound was 5,6,7,8-tetrafluoro-1,4-dihydro- 1,4-ethenonaphthalcne (tetrafluorobenzo-barrelene) (24). 

Synthetic applications of other decarbonylation reactions are found in the conversion of cyclooctatetraene to barrelene 250), with the photodecarbonyla-tion of a Diels-Alder adduct as key step (2.31) and the preparation of tetrathioesters from 1,3-dithioles (2.32) 251). The most remarcable application of such a reaction up to date is the synthesis of tetra t.butyltetrahedrane from a tricyclic ketone precursor (2.33) 252). 

Early applications of this reaction are found in the conversion of barrelene to semibullvalene 324) (3.15) or in the preparation of an azabullvalene (3.16) 325). In a similar reaction azabarbaralene has been prepared from aza-bicyclononatriene 326). 

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 asymmetric Diels-Alder cycloadditions of enantiopure (5)-5-(/ -tolylsulfinyl)-1,4-benzoquinones with Dane s diene under thermal and Lewis acid conditions produce tetracyclic quinones after spontaneous elimination of the sulfinyl group.The Diels-Alder reaction of barrelene with o-benzoquinone produces tetracyclo[6.2.2.2 .0 ]tetradeca-9,ll,13-triene-4,5-dione. Under kinetic control, the Diels-Alder cycloaddition of 2,3-dicyano-p-benzoquinone (98) with cyclopentadiene in MeOH produces the single cycloadduct (99) (Scheme 38). ... 

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]. 

Norbornadiene adds typical dienophiles in a homo-Diels-Alder reaction whereby, for example, the adduct 86 is obtained with tetra-cyanoethylene. Dimethyl acetylenedicarboxylate and dicyanoacetyl-ene add to barrelene (bicyclo[2.2.2]octa-2,5,7-triene) in a similar way. °... 

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