Cycloalkenes cyclopentadiene

The mixtures are particularly dangerous with alkenes, cycloalkenes and dienes. Accidents have been reported with propene, butenes, isobutylenes, 1-hexene, butadiene and cyclopentadiene. However, the reaction below is not thought to be dangerous if one operates at a temperature of 30°C and under two bar. 

Oxidation with lead tetraacetate is a far less selective process.490,491 Studied mainly in the oxidation of cycloalkenes, it gives stereoisomeric 1,2-diol diacetates, but side reactions (allylic acetoxylation, skeletal rearrangement) often occur. A change in reaction conditions in the oxidation of cyclopentadiene allows the synthesis of different isomeric mono- and diesters.492... 

Another common strategy for construction of fused cyclooctanones is to first build a fused cyclobutanone by [2 + 2] cycloaddition of a vinylketene to a cycloalkene. Equation (58) illustrates this approach with Paquette s synthesis of the tricyclic skeleton of the ophiobolins. Cyclobutanone (108) is assembled by addition of a vinylketene to cyclopentadiene. Cyclopentenyllithium then adds to the less-hindered face of (108), and the lithium alkoxide undergoes a spontaneous anionic oxy-Cope rearrangement to afford the central cyclooctane ring. 

Carbocations can be generated by the protonation of unsaturated hydrocarbons such as alkenes and cycloalkenes [49,52], cyclopentadienes [57], benzenes and naphthalenes (Eq. 24) [58], pyrenes and cyclophanes [59], unsaturated heterocycles [60], and their derivatives with carbon-heteroatom multiple bonds [2], including carbonyl and nitrile compounds and diazoalkanes [61]. 

Cyclic allyl residues enable the synthesis of cycloalkenes having a 1-hydroxyalkyl side chain. Cyclic Ti complexes, like (118) and (119) are readily obtained (Scheme 12) by treatment of (n -C5H5)2TiCl (117) with i-butylmagnesium chloride and cyclohexadiene or cyclopentadiene respectively, in TOF at -40 C for 10 min. Such complexes are quite unstable and can be prepared only in situ. Propanal... 

The geometry of a carbon-carbon double bond can also be distorted in monocyclic compounds. Cycloalkenes with a trans double bond show significant deviations from normal bond angles when the rings are small. Nevertheless, frans-cyclooctene (33) has been isolated, and trflws-cyclohep-tene and fra s-cyclohexene have been implicated as reaction intermedi-ates. ° ° Even cis double bonds may be highly strained in very small rings. Billups and Haley obtained evidence for the existence of spirocyclopenta-diene (34) as a transient species whose existence was confirmed by trapping with cyclopentadiene. °... 

Diels-Alder reactions continue to be of value for the interception of highly reactive dienes or dienophiles. Thus, irradiation of 2-cyclo-octen-l-one or of 2-cyclohepten-l-one in the presence of excess cyclopentadiene affords the norbornene derivatives (173 n = 5 or 4 respectively), which undergo retro-[4 -I- 2] addition on distillation. Presumably the trans-2-cycloalken-l-ones are reactive intermediates in these processes. Elimination of acetic acid from bis(trifluoromethyl)cyclopentadienylcarbinyl acetate yields the reactive 6,6-bis(trifluoromethyl)fulvene, which rapidly dimerizes by [4 + 2] cycloaddition the fulvene intermediate can be intercepted by similar addition to cyclopentadiene. The adduct (174) is formed on reaction of cyclopentadiene with the imine produced by iodine-catalysed rearrangement of NN-... 

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