Divinylcyclopropanes isomerism

An interesting 1,2-divinylcyclopropane isomerization is observed when 5,5,10,10-tetrachlorotricyclo[7.1.0.09,6]deca-2,7-diene is subjected to flash vacuum pyrolysis at 700 °C and 10-4Torr a mixture of three isomeric dichloroazulenes is produced [215]. 

In general, this approach can be represented by equations 207 and 208 wherein the formation of cis- and fraws-substituted seven-membered rings (e.g. tropones or oxepines) is controlled by selection of appropriate isomeric divinylcyclopropanes or divinylepoxides as precursors. We will discuss here a series of examples which are not covered by a recent review260. 

Vogel, 1962 Vogel et al., 1961.) It would thus appear that the isomerization of the ci -l,2-divinylcyclopropane occiu s with a very low energy of activation. 

Cis-1,2-divinylcyclopropane also rearranges rapidly (Equation 12.106). The free energy of activation is 20 kcal mole-1, and A Hi is 19.4 kcal mole-1.172 The trans isomer, in contrast, rearranges only at 190°C, presumably the temperature required for its isomerization to the cis form.173... 

Under the same conditions, reaction of 1 with buta-1,3-diene afforded the isomeric 1-chloro-l, 2-divinylcyclopropanes 3. However, trans-3 with its two vinyl groups cis to each other immediately rearranges via a 1,3-sigmatropic shift to produce 2-chloro-l,4-cycloheptadiene (4) (see also Sections 1.2.1.2.2 and l.B.2.4.5.1 for similar examples of the formation of seven-membered rings). 

In general, the reaction is not stereoselective and EjZ mixtures of isomers are obtained. Thus, l,l-dichloro-2,3-diethylcyclopropane (11) yielded a 1 1 synjanti mixture of l-ethylidene-2-vinylcyclopropane (12). This example also illustrates the effect of reaction time on the products. Product 12 is obtained in 80-90% yield after 30 minutes at 25 °C. After 2.3 h of reaction time, however, the yield is drastically reduced by isomerization to 1,2-divinylcyclopropane (13) and a subsequent Cope rearrangement of the cw-isomer of 13, followed by isomerization, to cyclohepta-1,3-diene (14). Higher reaction temperatures also lead to rearrangements. ... 

Thermolysis of both (Z)- and ( )-l-(prop-2-enylidene)-2-vinylcyclopropane (1, R, R = CH = CHj, H) gave a mixture of 3-methylenecyclohepta-l, 4-diene (4) and 3-methylene-4-vinylcyclopentene (5) in the ratio of 80 20 and 86 14, respectively. This isomerization involves a methylenecyclopropane rearrangement to l-methylene-2,3-divinylcyclopropane, followed by a Cope rearrangement. 

Heating divinylcyclopropanes cw-74 in high-boiling solvents also leads to the formation of cycloheptadienes, accompanied in some cases by isomerization of the starting divinylcyclo-propane system. ... 

Rearrangements of divinylcyclopropanes in which one of the vinyl groups constitutes a part of an aromatic or heteroaromatic system, such as 3 or 6, are also known. The initially formed cycloheptadienes (e.g. 4) isomerizes into conjugation in those cases where a reconstitution of aromaticity results, thus providing cycloheptadienes 5 and 7, respectively, annulated with the aromatic ring. 

Fusion of a strained ring system to the 3,4 positions of a 1,5-hexadiene also activates the diene toward isomerization provided that the vie vinyl groups of the divinylcycloalkane are cis to one another. C/ -l,2-divinylcyclobutane isomerizes about a million times faster than 1,5-hexadiene, and all attempts to synthesize m-I,2-divinylcyclopropane yielded its isomerization product, 1,4-cycloheptadiene, instead . ... 

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