Fulvenes rearrangement

SKATTEB0L DihalocydapropaneRearrangemant Rearrangement of gem-ditialocyclopropanes to ailenes or of vinyt dihalocyclopropanes to eyciopentadienes and fulvenes by MeU... 

Cyclopentadienyl radical can combine with methyl radicals to form fulvene, along with a subsequent loss of H atom. Fulvene may also be formed by reactions of the iso isomer of C4H5 with aeetylene. Once fulvene adds an H atom, subsequent rearrangement to benzene has been shown to be thermodynamically favorable using quantum chemieal methods.Cyelopentadienyl radicals may also combine with eaeh other and then rearrange to form naphthalene. 

Dimethyl anthracene and diphenyl isobenzofuran form remarkably stable233 cyclopropanone derivatives (353/354), whilst with other diene components (butadiene, tetracyclone, and fulvene) the primarily formed Diels-Alder adducts either suffer ketalizing attack of the solvent (356 - 357, 359 - 358/360) or undergo irreversible changes such as decarbonylation to 362 or rearrangement to 355. 

When the cyclic acetylene 260 is generated from a suitable precursor, it undergoes an isomerization reaction spontaneously generating naphthalene (263) and benzo-fulvene (264) as the finally isolable products. Very likely the process begins with a retro-Diels-Alder reaction to the [3]cumulene 261, which in a cascade reaction via the semicydic allene 262 rearranges to 263 and 264 [111]. 

The tetraphenylcyclopentadiene 160 affords the peroxide 161, which rearranges on heating to the bis-epoxide 162 (equation 89)81. In the case of the photooxygenation of the fulvene 163, only the rearrangement product 164 could be isolated (equation 90)81. 

Arylsubstituted cyclopentadienes such as 1,2,3,4-tetraphenylcyclo-pentadiene (369) give rise to isolable endoperoxides249 which can be thermally rearranged to m-diepoxides. From the corresponding fulvene derivative (372) only the diepoxide 374 was isolated which probably originates from a thermally unstable endoperoxide 37i.249,250... 

Oxaquadricyclanes with a-H-containing substituents on C2 and C4 may undergo a competing thermal or acid-catalyzed rearrangement to afford enol ethers.46 Acid or Lewis acid catalysis may also lead to fulvene derivatives.46... 

A less usual method for the generation of starting dienones is thermal rearrangement of fulvene-l,2-dioxetanes 139a,b, leading to 2//-pyrans (140a,b).182... 

The rearrangement of fulvene (64) to benzene has been studied by dieoretical methods.74 The favoured pathway involved rearrangement to iso fulvene (65) (barrier 41.6 kcal mol-1), ring opening to cyclohexadienecarbene (66) (barrier 74.3 kcal mol-1), and 1,2-Hf shift to form benzene (barrier 59 kcal mol-1). The discrepancy between die calculated activation energy and the value determined by very low-pressure pyrolysis is suggested to be due to surface effects. 

In connection with the above discussion, formation of 3,3-disubsti-tuted 2 (3 H)-oxepinones (73) in the dye-sensitized photooxygenation of 6,6-disubstituted fulvenes is of special interest. 57>58> The reaction may be pictured in terms of an allene oxide intermediate which, as shown in Scheme 11, isomerizes to a cyclopropanone, followed by intramolecular rearrangement. 

The radical cations of fulvene systems are of interest, because steric and electronic factors might favor a perpendicular structure and because the energy difference between the respective cis and trans isomers are expected to be small. However, the chloranil photosensitized reaction resulted in CIDNP effects, indicating planar or slightly twisted structures. The Z- and E-2-tert-butyl-6-(dimethylamino)fulvene [20, R = — N(CH3)2] radical cations rearrange readily whereas di-/er/-butylfulvene [20, R = — C(CH3)3] showed no interconversion under comparable experimental conditions [160]. 

The dienylic cations 44 with cyclopropyl and phenyl groups were also prepared and characterized by the protonation of respective fulvenes. Other cyclopropyl substituted allyl cations include acyclic 1,3- and 1,4-disubstituted allyl cations 45 and 46 The charge in these cations is localized mainly on the carbon adjacent to the cyclopropyl group. The rearrangement of these cations at higher temperatures was also studied. ... 

Interestingly, the reaction of dimethyl fulvene (R = Me) with 2//-azirine 819 in an ultrasonic bath (neat) yielded the alkylation product 825. The stmcture of 825 was unambiguously assigned by single crystal X-ray structure. A possible mechanism to account for the formation of 825 involves an initial [4-1-2] cycloaddition followed by a subsequent rearrangement of the initially formed Diels-Alder cycloadduct 824 (Scheme 204). 

An unusual example of the allene oxide-cyclopropanone isomerization is found in the results of singlet oxygen oxygenation of 6,6-disubstituted fulvenes (18) The production of 3,3-disubstituted 2(3//)-oxepinones (20) in this process can be rationalized by the sequence of transformations outlined in equation 8. The initially formed endoperoxide (19) undergoes a well-precedented rearrangement to the epoxy ketone (19a) which contains the allene oxide residue. Rearrangement to the cyclopropanone form is... 

---