Divinylcyclobutanes

The Cope rearrangement is of great importance as a synthetic method e.g. for the construction of seven- and eight-membered carbocycles from 1,2-divinylcyclopropanes and 1,2-divinylcyclobutanes respectively (e.g. 11 12),... 

When buta-1,3-diene 3 is irradiated in the presence of a photosensitizer (e.g. benzophenone), the isomeric divinylcyclobutanes 6 and 7 are formed via the intermediate diradical species 4 and 5 respectively in addition the [4 + 2] cycloaddition product 4-vinylcyclohexene (see Diels-Alder reaction) is obtained as a side product ... 

The cis-, 2-divinylcyclobutane in the reaction mixture rearranges rapidly under reflux to the higher-boiling 1,5-cyclo-octadiene. 

If s-l,2-divinylcyclobutane is desired, it can be isolated in 7—8% yield from the reaction mixture by preparative gas chromatography with the Beckman Megachrom instrument, using columns packed with Apiezon J. 

The trans isomer of 1,2-divinylcyclobutane may be isolated in low yield from the mixture formed by thermal dimerization of butadiene.5 The cis isomer has been prepared by a sequence of reactions.6... 

Not all Cope rearrangements proceed by the cyclic six-centered mechanism. Thus c/5-1,2-divinylcyclobutane (p. 1445) rearranges smoothly to 1,5-cycloocta-... 

The thermal isomerization of syn-2 has been studied in detail.68 When a solution of syn-2 in decahydronaphthalene was heated at 220 °C, it isomerized to anti-2 quantitatively (Scheme 15). When anti-2 in decahydronaphthalene was likewise heated at 220 °C, no isomerization of anti-2 to syn-2 was observed, and anti-2 was recovered. The thermolysis pathway is partially dilferent from that of the corresponding ladderane syn-tricyclo[4.2.0.02,5]octane was reported to give 1,5-cyclooctadienes, anti-tricyclo[4.2.0.02,5]octane, and 1,2-divinylcyclobutanes in 51, 41, and 8% yields, respectively.29c,d... 

SCHEME 25. Activation and reaction volumes of the ring enlargment of trans-l,2-divinylcyclobutane... 

Photolysis, apparatus for, 47, 65 of butadiene to cis- and Irans-1,2-divinylcyclobutane, 47, 65 of dispiro[5.1.5.l]tetradecane-7,14-dione to cyclohexylidenecyclo-hexane, 47, 34... 

An alternative mode to release the strain of the four membered ring of vinylcyclobutanones arises from the addition of vinylorganometallics to the carbonyl group whereby a divinylcyclobutane is created 78,120,121). Since equilibration of the stereoisomers 78 and 79 is fast relative to rearrangement78), both stereoisomers nicely rearrange even though a cis divinyl orientation is required (Eq. 95). 

The Cope and oxy-Cope rearrangement are very useful in organic synthesis, particularly when the 1,5-diene system is incorporated in a ring, then intringuing cyclic compounds may result. Also, the Cope and oxy-Cope rearrangements are greatly facilitated for a cw-1,2-divinylcyclobutane (Eq. 15)), resulting in the formation... 

Similarly the conversion of cis 1, 2 divinylcyclobutane also involves a boat transition state. 

Another available stereoselective synthetic pathway, starting from ris-1,2-divinylcyclobutanes produced in the solid state, is that of ring expansion to 1,5-... 

The dicyclopenta[a,characteristic element of some terpenoids was obtained in 100% yield by a very facile Cope rearrangement of the highly functionalized divinylcyclobutane derivative 533 on heating in benzene at 55 °C for 4 h. The mild conditions can be due to participation of the lone pair of the sulfur atom or to the strain energy of the divinylcyclobutane fragment (equation 206)259. 

The relatively easy rearrangement of cis-l,2-divinylcyclobutane was first reported by Vogel (1958) who found that at 120° ci5,cis-l,5-cyclo-octadiene was the only product. 

More recently, kinetic data have become available for the isomerization of this compound and also for the iraws-l,2-divinylcyclobutane (Hammond and DeBoer, 1964). From determinations of the rate of formation of the cia-cis-l,5-cyclo-octadiene from the cis-l,2-divinylcyclobutane at four temperatures between 65 and 108° C, an enthalpy of activation of... 

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