Dewar benzene rearrangement

The analogous adducts of artemether Ic and fenozan 4 were also obtained and characterized in each case porphyrin adducts were isolated in 20-30% yield. When artemisinin was treated with manganese(II) TPP in the presence of cyclohexene, Robert and Meu-nier also failed to observe any of the corresponding epoxide. The authors suggest that the Dewar benzene rearrangement and the oxidation reactions observed by Posner... 

The skeletal rearrangement of various strained cyclic compounds is carried out with a catalytic amount of soluble complexes of PdCl2. Namely, the rearrangements of bulvalene (67) to bicyclo[4.2.2]deca-2,4,7,9-tetraene (68)[54], cubane (69) to cuneane (70)[55], hexamethyl Dewar benzene (71) to hexa-methylbenzene (72)[56], and 3-oxaquadricyclanes[57] and quadricyclane (73) to norbornadiene[58-60] take place mostly at room temperature. Reaction of iodocubane (74) with a terminal alkyne catalyzed by Pd(0) and CuBr unexpectedly affords an alkynylcyclooctatetraene 75, without giving the desired cubylalkyne 76. Probably the rearrangement is a Pd-catalyzed reaction[61]. 

In 1966, a photochemical rearrangement by ultraviolet (UV) irradiation of hexafluorobenzene to hexafluorobicyclo[2 2 0]hexa-2,5-diene was achieved Since then, many reactions analogous to the valence tautomerism of benzene and bicy clo[2 2 0]hexadiene (1 e, Dewar benzene), as well as of fluonnated benzvalene and... 

When uradiated, fluonnated isomers of Dewar benzene yield pnsmane derivatives that rearrange thermally to benzene Photolysis of hexakis(mfluororaethyl)benzvalene ozonide gives tetrakis(tnfluoromethyl)cyclobutadiene and its dimer [J47]... 

Figure 15.29 The Cope rearrangement and Dewar benzene ring-opening reaction...

Bicyclohexadiene, also known as Dewar benzene, is extremely stable despite the fad that its rearrangement to benzene is energetically favored. Explain why the rearrangement is so slow. 

Thermolysis of the hexamethyl-substituted Dewar benzene-phenylazide adduct 1 in refluxing Decalin affords the 1.2,3-triazonine derivative 2 as the major product in form of a yellow-orange powder.30 A bicyelic isomer is also isolated which arises from rearrangement of 2 by purification on alumina and is readily converted back to 2 on heating to 80 C. 

In the abovementioned cases preparation of the propellanes was direct. A very nice instance exists, however, of rearrangement of a dispiran, 26, to Dewar benzenes 27 which happen to be [n.2.2]propelladienes. Silver ion (silver perchlorate at —20 °C) promotes the isomerization, as shown 10). 

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... 

Figure B14.1. Orbital correlation diagram for rearrangement of Dewar benzene to benzene. Two vertical planes of symmetry are preserved.

As will be seen in a later section, substituted benzenes rearrange photochemically. Thus o-xylene isomerizes to m-xylene and 1,3,5-tri-isobutyl benzene isomerizes to the 1,2,4- and the 1,2,3-triisobutyl benzenes.410 Such isomerizations conceivably could proceed through free-radical intermediates but Wilzbach and Kaplan and their coworkers have shown that the ring carbon to which the moving substituent is attached also changes position with the substituent. These authors offer the very reasonable explanation that the formation of benzvalene followed by rupture of bonds other than the new ones just formed could lead to rearrangements of the type in question. It should be noted that both benzvalene and prismane could serve as intermediates in this way but that Dewar benzene could not. 

Density functional theory, 21, 31, 245-246 B3LYP functional, 246 Hartree-Fock-Slater exchange, 246 Kohn-Sham equations, 245 local density approximation, 246 nonlocal corrections, 246 Density matrix, 232 Determinantal wave function, 23 Dewar benzene, 290 from acetylene + cyclobutadiene, 290 interaction diagram, 297 rearrangement to benzene, 290, 296-297 DFT, see Density functional theory... 

Shifts of atoms or groups in fluorine-containing aromatic or heteroaromatic compounds are covered in this section. Shifts due to valence isomerization (benzene-Dewar benzene-prismane-benzvalene), however, are covered in Section 5.3.4.3. A major review1 on polyfluoroaromatic and heteroaromatic compounds has been written by G.M. Brooke, one of the principal contributors in this area, covering the literature until the end of 1995 (see pp 56-59 of ref 1 for the section on rearrangement reactions). 

The bicyclic pentafluoro oxo derivative 13 of Dewar benzene (see Section 5.3.4.3.) rearranges to pcntafluorophcnol when photolyzed in solution, in contrast to the desired decarbonylation to form the bicyclo[2.1.0]pentene systems.16... 

Perfluoro(hexamethylbenzene) (20) rearranges to the three valence isomers, benzvalene 21, Dewar benzene 22, and prismane 23, when photolyzed in perfluoropentane23,24 or in the vapor... 

Perfluoro(3,5-dialkvlpyridazines) 35 rearrange photolytieally to the 2,6-disubstituted pyra/ines 36.40 The mechanism that best explains these results involves rearrangement of an intermediate Dewar benzene structure 37 to 38, followed by rearrangement to the pyrazine structure (Scheme 12).40... 

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