Cycloadditions of benzene

Furan was used as building block in the total synthesis of ( )-palasonin <96JOC4816, 96TL1129>. A synthetic protocol to assemble a benzene ring mimicking the [4+2] cycloaddition of benzene and fiiran (or cyclopentadiene) was reported (Scheme 1, <96T14247>). 

The [2 + 2] cycloaddition of benzene derivatives with alkenes was also carried out using photosensitization. Maleimide 36 was added to benzene in high yields (Scheme 5.8, reaction 16) [42]. In this case, the sensitizer acetophenone 37 transfers its triplet energy to 36, after which the cycloadduct VII reacts immediately with an... 

De Keukeleire, D., Bako, P., and Van der Eycken, E. (2000) Intramolecular ortho and meta photocycloadditions of 4-phenoxybut-l-enes substituted in the arene residue with carbomethoxy, carbomethoxymethyl, and 2-carbo-methoxyethyl groups. Journal of Photochemistry and Photobiology A Chemistry, 133, 135-146 (c) Wender, P.A. and Dore, T.M. (1995) Intra and inter-molecular cycloadditions of benzene derivatives, in CRC Handbook of Organic Photochemistry and Photobiology (eds W.M. Horspool and P-.S. Song), CRC Press, Boca Raton, pp. 280-290. 

Gilbert, A. (2004) Intra- and intermolecular cycloadditions of benzene derivatives, in CRC Handbook of Organic Photochemistry and Photobiology,... 

In 1987, Comelisse and coworkers reported a semi-empirical calculation for the meta cycloaddition of benzene with ethylene, which impressively consolidates many experimental aspects of this reaction and flts well with the Mattay analysis. This calculation, performed at the CNDO/S and MNDO levels, supports an exciplex mechanism with charge polarization, in accord with Mattay s treatment and a related mechanism developed earlier in the Comelisse laboratories. According to this mechanism (equa-... 

Intramolecular analogs from the meta cycloaddition of benzene with alkenes are known. Thus, irradiation of 6-phenylhex-2-ene yielded two adducts 4 and 5 in equal amounts from the (Z)-... 

Intramolecular ortho-cycloaddition of benzene-ethylene systems... 

There are large number of cycloaddition reactions of benzene and its derivatives. Correlation diagram can be constructed to predict them which can be represented by examples of ortho-, meta- and para-cycloadditions of benzene and ethylene or benzene and butadiene. Ortho-, meta- and para-additions give different products. 

Fig. 5.11. Correlation diagram for pora-cycloaddition of benzene and ethene.

Dimeric product is supposed to be formed by thermally allowed n +ita cycloaddition of initial products [A] and (B). Correlation diagram for n +n cycloaddition of benzene and trans-1, 3-butadiene and benzene with cis-1, 3-buta- diene are shown in Fig. 5.12 and 5.13, respectively. 

Product from cycloaddition of benzene and trans-butadiene is predominates. To benzene trans-butadiene adds more readily, it +n cycloaddition is photochemically allowed. Formation of charge transfer complex between two ground state molecules or an exciplex between excited state of one molecule and ground state of other may lower the energy barrier to this pericyclic reaction, it +7c cycloaddition may occur is case of two aromatic substrates also. For examples, dimerization of naphthalene and anthracene is very common (shown on next page). 

Homocyclic Systems.— The photo-initiated cycloaddition of benzenes and alk-ynes continues to be a route of choice for cyclo-octatetraenes (cot s). Thus while hexafluorobenzene and various phenylacetylenes photo-add, giving the cot s (3) and (4) by thermal reorganisation of the isolable bicyclic intermediate (2), ... 

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