Arenes intramolecular cycloaddition with

Most compounds studied have a carbonyl group conjugated with the benzene ring and nearly all reactions are intramolecular cycloadditions. In most cases, the alkenyl moiety is connected to the arene ring via an oxygen atom, ortho or para with respect to the carbonyl group. 

Cyclization of dienynes.1 This Ni(0) catalyst in combination with a triarylphos-phite, particularly tri-o-biphenyl phosphite, permits intramolecular cycloaddition at 25° of [4+2] dienynes, in which the diene and the alkyne are separated by 3- and 4-atom units. This reaction is a useful route to products containing a cyclohexadiene group, which are oxidized to an arene by DDQ. 

The next selectivity issue, exo/endo preferences, can be predicted for both the ortho and meta modes of cycloaddition on the basis of secondly orbital interactions (FMO treatment) and by electrostatic considerations involving polarized species (54) and (27). In general, intermolecular reactions with simple al-kenes proceed with endo selectivity. Heteroatom-substituted or polarized alkenes (equation 11) give exolendo mixtures, whose composition can be explained by electrostatic considerations. Intramolecular cycloadditions of simple alkenes and arenes joined by a three-atom tether generally proceed with high exo selectivity due in part to orbital alignment effects. In all cases, alkene geometry is preserved, except for sterically encumbered alkenes, in which case excitation transfer from the arene to the alkene can occur. 

An intramolecular cycloaddition of aza-diene 41 with arene chromium complex in the presence of Lewis acid afforded the chromium-complexed tetrahyd-roquinoline derivatives in good yields with high diastereoselectivity (Eq. 29)... 

The Lewis acid-mediated [4+2] cycloaddition of conjugated enynes with alkynes affords arenes however, it is limited to the intramolecular cycloaddition [44]. The use of transition metal complexes realizes the catalytic intermolecular variants. The first example was reported in the paUadium(O)-catalyzed homo-[4+2] cycloaddition of monosubstituted conjugated enynes giving substituted styrenes (Scheme 21.38) [45]. Importantly, this reaction proceeded with definite regioselectivity. 

In 2006, Liu and coworkers reported a gold-catalyzed intramolecular cycloaddition of diynes with tethered arenes 118 to synthesize l,3-dihydroindeno[2,l-c]pyran or 2,3-dihydro-l//-indeno[2,l-c]pyridinederivatives 120 (Scheme 12.52) [56]. On the basis of deuterium labeling, they believed that the first reaction step is the intramolecular arylation of one alkyne to form vinylgold(I) intermediate 119,... 

The irradiation of benzene solutions of CA in the presence of the 1,1-diarylethenes 46 yields the [27H-27i]-cyclobutane adducts 47 and the substituted quinones 48, both of which undergo further photochemistry to give the products 49 and 50 of intramolecular arene-ethene cycloaddition and 6Jt-elec-trocychzation, respectively (Figure 87.10). It is proposed that products 48 arise from a single electron-transfer process from the donor ethene to the Tj chloranil, and that the formation of the cyclobutane adducts are formed from the 1,4-biradicals without the involvement of an electron-transfer process. This conclusion is supported by the observation that the ratio of 47 48 increases with an increase in both the oxidation potential of the diarylethenes and in the positive AG value for electron transfer between the addends furthermore, the formation of 48 is greatly favored by an increase in solvent polarity. 

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. 

A major initial limitation of the benzocyclobutene approach to o-quinodimethanes was the lack of efficient, large-scale syntheses for many specifically substituted derivatives. Fortunately, recent developments have lemov much of this impediment. Q>nceptually, the synthesis of benzocyclobutenes from aromatic precursors can be envisaged in only a limited number of ways. These include [2 -i- 2] cycloadditions involving benzynes and alkenes, intramolecular cyclization on to a benzyne, cyclizations involving arene anions, and electrocyclic closure of o-quinodimethanes. Benzocyclobutene derivatives can also be prepared by aromatization of bicyclo[4.2.0]octanes. Detailed discussion of variations to these approaches can be found in the cited reviews. The cobalt catalyzed co-oligomerization of 1,5-hexadiynes with al-kynes, especially bis(trimethylsilyl)acetylene, has also been employed for the preparation of specifically substituted benzocyclobutenes. In the latter case the cyclobutenes are often not isolated but converted directly to o-quinodimethanes and subsequent products. ... 

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