Thermal cyclodimerization

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

The thermal cyclodimerization of fluoroalkenes represents a route to fluorinated cyclobutanes. In most cases the neat alkene is heated in a sealed tube for prolonged periods of time. For symmetrical fluoroethenes head-to-head dimerization predominates. For instance, 1.1-dichloro-2,2-difluoroethene (1) undergoes thermal dimerization (180°C, 14 days) to give the hcad-to-hcad dimer 2 although no yield was reported.1... 

An earlier paper on the thermal cyclodimerization of 3 (R1 = F R2 = Ph) also reported the formation of a cis/trans mixture which was separated by gas chromatography.3 This structure contradicted an earlier assignment of the head-to-tail structure for this dimer.4... 

Strained alkenes undergo thermal cyclodimerization often spontaneously. For instance, the fl t/-Bredt bicycloalkenes 9 and 10 formed as transients give the cyclodimers 11 and 12, respectively, along with oligomeric substances.7... 

The strain associated with small-ring compounds often engenders chemical properties not encountered in larger ring systems. Methylenecyclopropanes and cyclobuta-1,3-dienes are known to undergo thermal cyclodimerization. 

One of the problems associated with thermal cyclodimerization of alkenes is the elevated temperatures required which often cause the strained cyclobutane derivatives formed to undergo ring opening, resulting in the formation of secondary thermolysis products. This deficiency can be overcome by the use of catalysts (metals Lewis or Bronsted acids) which convert less reactive alkenes to reactive intermediates (metalated alkenes, cations, radical cations) which undergo cycloaddilion more efficiently. Nevertheless, a number of these catalysts can also cause the decomposition of the cyclobutanes formed in the initial reaction. Such catalyzed alkene cycloadditions are limited specifically to allyl cations, strained alkenes such as methylenccyclo-propane and donor-acceptor-substituted alkenes. The milder reaction conditions of the catalyzed process permit the extension of the scope of [2 + 2] cycloadditions to include alkene combinations which would not otherwise react. 

Dispiro[2.1.2.1]octane is obtained as the only product in the presence of Ni(0), in contrast to the result of the thermal cyclodimerization 143). According to the polarization of the 7i -orbital of methylenecyclopropane one would expect the symmetrical nickelacyclopentane VII as an intermediate 168), leading to dispiro[2.0.2.2]octane (Eq. 70). Obviously, steric constraints in the nickelorganic intermediate 148b) are responsible for the exclusive formation of dispiro[2.1.2.1]octane. 

Dispiro[2.1.2.1]octane is obtained as the only product in the presence of Ni(0), in contrast to the result of the thermal cyclodimerization According to the polariza-... 

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... 

Our ongoing strategy to develop new fluorosilicone materials has been to combine the well known preparative chemistry of silicon containing compounds with the relatively new development of perfluorocyclobutane (PFCB) aromatic ether polymers. PFCB materials are prepared by the thermal cyclodimerization of trifluorovinyl ether (TFVE) monomers." " Recently we developed a synthetic... 

The thermal cyclodimerization of allenes proceeds via a stepwise [2-1-2] cycloaddition reaction, the intermediate being a diradical. From allene, in a flow reactor, a mixture of the head-to-head dimer 25 and the head-to-tail dimer 26 are obtained. At 500 °C (9 % conversion) a mixture of 34% of 25 and 13% of 26 are formed. The formation of the cyclodimer 26 is unusual, because substituted allenes afford only the head-to-head dimers. 

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. 

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