Allenes, cycloadducts general reactions

Cyclonona-1,2-diene (1), with its allenic chromophore bent ca. 10 ° from linearity, is the smallest unsubstituted cyclic allene which is kinetically stable at ambient temperature. " Benzene sensitized irradiation of this cyclic allene yielded tricyclo[4.3.3.0 ]nonane (3) (cj) = 0.17 in the vapor phase), while direct irradiation afforded four isomers, one of which was believed to be the tricyclononane 3. " The remaining products were not identified. In what appeared to be a general reaction of both linear and cyclic allenes, a cyclopropylidene intermediate 2 was proposed. Subsequently, the formation of cycloadducts 4 and 5 from the irradiation of 1 in benzene solution was reported. " ... 

Concerning the structure, the cyclopropane derivatives 524—526 deviate from the generally observed cycloadducts of cyclic allenes with monoalkenes (see Scheme 6.97 and many examples in Section 6.3). The difference is caused by the different properties of the diradical intermediates that are most likely to result in the first reaction step. In most cases, the allene subunit is converted in that step into an allyl radical moiety that can cyclize only to give a methylenecyclobutane derivative. However, 5 is converted to a tropenyl-radical entity, which can collapse with the radical center of the side-chain to give a methylenecyclobutane or a cyclopropane derivative. Of these alternatives, the formation of the three-membered ring is kinetically favored and hence 524—526 are the products. The structural relationship between both possible product types is made clear in Scheme 6.107 by the example of the reaction between 5 and styrene. 

The presence of the triphenylphosphonium substituent makes 351 (Scheme 2.122) an active dienophile. Resulting cycloadducts such as 352 are immediately recognized as precursors for the formation of ylides. Products with an exocyclic double bond such as 353 are easily synthesized via a tandem sequence of Diels-Alder and Wittig reactions. The formation of adduct 353 could have been achieved via the Diels-Alder reaction with the allene CH2 = C = CHR, but this direct route is generally unapplicable owing to the low activity of allenes as dienophiles. 

Like ketenes, allenes generally undergo [2 + 2] cycloadditions with alkenes affording methylene cyclobutanes . In reactions with 1,3-butadienes, both Diels-Alda- adducts and [2 -F 2] cycloadducts are formed. Cyclopentadiene, however, has been reported to react with several allenes to give exclusively Diels-Alder adducts. From the sevraal possible mechanisms by which [2 -F 2] cycloaddition reactions of allenes could occur,... 

Allenes and cumulenes in general are more reactive than alkenes in undergoing cycloaddition reactions with isolated non-activated double bonds. The stereochemistry of the major cycloadducts can be predicted from the most stable diradicals formed upon initial bonding of the sp carbon to the allene. The [2-1-2] cycloaddition reaction of allenes to olefins proceeds thermally, photochemically and at room temperature, using a Lewis acid catalyst. The mechanism of these reactions are considered to be stepwise processes, with radical or ionic intermediates. 

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