1.2- Divinylcyclopropanes cycloheptadiene ring

When ring strain is relieved, Cope rearrangements can occur at much lower temperatures and with complete conversion to ring-opened products. A striking example of such a process is the conversion of cw-divinylcyclopropane to 1,4-cycloheptadiene, a reaction which occurs readily at temperatures below —40°C.139... 

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). 

Formation of Seven-Membered Rings The Divinylcyclopropane to Cycloheptadiene (Cope) Rearrangement... 

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. 

A detailed description of the numerous examples of vinylcyclopropropanes used in transition metal mediated organic synthesis is far beyond the scope of this section and can be found in several reviews. Prominent examples are conversions to open-chain products, as well as formation of four-, five-, six- and seven-membered rings via ring expansion or incorporation of other substrates such as carbon monoxide, alkenes or alkynes. Thus divinylcyclopropanes, obtainable via transition metal catalyzed cyclopropanation reactions, undergo a facile thermal Cope rearrangement to form cycloheptadienes. ... 

The thermolytic transformation of tra s-1,2-divinylcyclopropane (4) into 1,4-cycloheptadiene (2) probably proceeds via the pathway shown in equation (2). Homolytic cleavage of the cyclopropane ring of (4) provides the resonance stabilized diradical (5), which, in addition to reverting to (4), can undergo bond rotation and subsequent ring closure to give cis-l,2-divinylcyclopropane (1). The latter substance then rearranges, by way of conformation (lb), into (2). 

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 . ... 

Carbene transfer to the more electron-rich C=C bond in diene 31 generates the cfr-divinylcyclopropane 32 which undergoes a spontaneous ring expansion to give cycloheptadiene 33 in a regio- and stereoselective one-pot reaction. A similar reaction sequence has been applied in the synthesis of dihydroazepines from aza-l,3-dienes. [38]... 

When the C-3 and C-4 carbons of 1,5-dienes are connected to a cyclopropane ring, the reaction rates are accelerated due to favorable interactions of the diene termini resulting in the reduction of the enthalpy of activation. For example, the conversion of cfr-divinylcyclopropane 61 to 1,4-cycloheptadiene 62 occurs readily at temperatures below -40 °C [37]. 

R = R = H) underwent equilibration with its cis-fuscd isomer and only a trace of the cycloheptadiene was observed presumably steric interactions make the Cope rearrangement very difficult. The results suggest an initial epimeriza-tion of irons- to cis-fused isomers followed by a normal Cope rearrangement. An analysis of the kinetics of conversion of ( — )-trans-divinylcyclopropane into cyclohepta-1,4-diene indicates that the ring-opening is not a concerted pro-cess. 

The divinylcyclopropane unit is converted to a cycloheptadiene in this 3,3-sigmatropic rearrangement. Interestingly, this rearrangement also opens one ring (cyclopropane) and forms a new ring (cycloheptadiene). 

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