Aryl cyclodimerization

In the course of research on catalysis by synthetic metalloporphyrins, Tagliatesta and coworkers reported a unique cyclodimerization reaction of aryl acetylenes to give 2-aryl naphthalenes (6, Scheme 9.2) [5]. Ru(CO) and RhCl complexes effectively promote this reaction, with the latter catalyst (8) providing generally superior yields and selectivities for a small range of substrates (Table 9.1). As a synthetic method, Tagliatesta s cyclodimerization reaction is most remarkable for its efficiency. Yields of up to 78% were observed at a substrate/catalyst ratio of 5700 1. Successful recycling of recovered catalyst was also demonstrated. 

Reaction of epichlorohydrine with aryl sulfonamides afforded a mixture of the cyclodimerization products, as and trans l,5-bis(arylsulfonyl)-3,7-dihydroxyoctahydro-l,5-diazocines 90a,c <1995JOC1959, 1998JOC1566, 2006LJSP7145003>. 

In the presence of Ni(0) catalysts, methylenecyclopropanes cyclodimerize at temperature as low as —15 °C. The chemoselectivity of these reactions strongly depends on the substitution pattern of the substrates. [2+2]-Cyclodimerizations are restricted to methylenecyclopropanes bearing no further substituents at the exo-cyclic double bond. The substitution pattern at the threemembered ring (R = alkyl, aryl) determines whether four-membered rings, five-membered ring or open-chain products (or a mixture of all those) are obtained. 

Upon photosensitization with (- )-menthyl salicylate 48a (X = 2-OH), pyromel-litate 49a, 1- and 2-naphthalenecarboxates 41a and 42a, 2,3- and 2,6-naphthalene-dicarboxylates 43a and 91a, and/or 9-anthracenecarboxylate 73a in acetonitrile at 25 or -40°C, aryl vinyl ethers 87 (X = H or Cl) and 4-methoxystyrene 89 cyclodimerize to the corresponding cyclobutane derivatives 88 and 90 (only the trans isomers are chiral) in low to good yields (Scheme 15) [64]. However, the... 

Cycloadditions involving ketene derivatives as one or both reaction partners are assumed to be rare examples of concerted [tTj - - n cycloadditions [81]. The activation volumes determined for the [2 + 2] cyclodimerization of diphenylketene [82] and the [2 + 2] cycloadditions of diphenylketene to various enolethers [83] gave values of —30 cm mol and —22 to -52 cm mol , respectively, and were highly negative. Thus, the effect of pressure leads to a powerful acceleration of these [2 + 2] cycloadditions comparable to that with Diels-Alder reactions, a characteristic which is usefiil for synthetic purposes. For example, various /1-lactams can be easily synthesized by pressure-induced [2 + 2] cycloaddition of alkyl and aryl isocyanates and enolethers [84]. 

Cyclodimerization of fluorinated alkenes was observed as early as 1947 during the pyrolysis study of polytetrafluoroethylene (PTFE) [54]. The ability of fluorinated alkenes to dimerize with themselves has been attributed to the energy relief of fluorinated double bond strain [4]. In the same fashion, the aryl trifluorovinyl ether groups (Ar—O—CF=Cp2) underwent thermally activated [2-1-2] cycloaddition to form bisaryloxy-substituted PFCB rings. Scheme 14.9 depicts the model reaction of cycloaddition of aryl trifluorovinyl ethers. The thermal cyclodimerization of methyl... 

Aryl(quinoxalinyl)ketenes 53, generated in the thermolysis of the corresponding hydro-2,3-furandiones 52, undergo [4+2] cyclodimerizations across the a-imidoylketene configuration with the ketene C=C bond as the dienophile to give the cyclodimers 54, in 74-80 % yield... 

Antimony pentachloride Cyclodimerization of aryl isothiocyanates N-(2-Benzothiazolyl)thiocarbamyl chlorides... 

By a related carbenoid approach, tetrakis(aryl, trifluoromethyl)[4]radialenes 62 were obtained as mixtures of four diastereoisomers besides the [3]cumulenes 85 (Scheme 4.17) [58, 77]. The thermal cyclodimerization of 85 to form diastere-omeric mixtures of [4]radialenes 62 [58, 77] has already been mentioned. 

Novel polymers containing alternating perfluorocyclobutane and aromatic ether subunits have been prepared by polymerization of aryl trifiuorovinyl ether monomers via the thermal [2tc -h 2n cyclodimerization of the trifiuorovinyl ether functionality (Scheme XII) (5(5). The dimerization of the trifiuorovinyl ether moiety is not a concerted pericyclic reaction, rather proceeds through a diradical intermediate. Dimerization affords predominately 1,2-substituted perfluorocyclobutane rings with a mixture of cis and trans isomers. Both linear and crosslinked polymer systems were synthesized using bis- and tris(trifluorovinyoxy) monomers, respectively. These polymers display both a low dielectric constant (2.5) and very low moisture absorption. 

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