Divinylcyclopropanes, trans

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... 

T ABLE 3. Structural parameters of vinylcyclopropane (37) and trans-1,2-divinylcyclopropane (38)" ... 

Not unexpectedly, Irons-1,2-divinylcyclopropane (4) is much more stable than the cis isomer (1) and is a readily isolable compound. Nevertheless, at elevated temperatures, e.g. 190 C, (4) undergoes smooth bond reorganization to provide 1,4-cycloheptadiene (2) in essentially quantitative yield Thus, at the time that the Cope rearrangement of 1,2-divinylcyclopropane systems was discovered, it was already clear that both cis and trans isomers could in principle, serve as suitable substrates for the reaction. As it turns out, this is an important reaction characteristic, since, in most (but not all) cases, it makes uiuiecess-ary the stereoselective preparation of either the cis or trans starting material. 

The highly substituted cir-divinylcyclopropanes (20) and (21) do not undergo sigmatropic rearrangement at all. Apparently, the highly sterically congested nature of the transition states (F) precludes this possibility. Thermolysis of (20) and (21) at 170-180 C produces only equilibrium mixtures of these substances and the corresponding trans isomers (22) and (23), respectively (Scheme 3). ... 

From a synthetic viewpoint, a comparison of the thermolyses of the epimers (92) and (94) (Schemes 12 and 13) illustrates the point, previously mentioned, that, in certain cases in which alternative modes of rearrangement are possible, the stereoselective formation of the cts-divinylcyclopropane substrate is important. Thus, while thermal rearrangement of the cis substrate (92) provi s the Cope rearrangement product (98) in reasonable yield, thermolysis of the trans isomer (94) does not. 

The bicyclo[S.4.0]undecane sesquitetpenoid ( )-3-himachalene (109) was prepared via a route in which thermolysis of a 3-(2-vinylcyclopropyl) enone played a key role (Scheme 15). Reaction of the P-iodo enone (82) with the stereochemically homogeneous cyclopropylcuprate (106) provides the functionalized trans-divinylcyclopropane (107). Thermolysis of (107) gives exclusively the Cope reanange-ment product (108), which is converted into ( )-P-himachalene (1()9). Notably, in contrast to structurally related systems (vide supra), the p, y-unsaturated ketone function in (108) shows no inclination to rearrange to the corresponding a,P-unsaturated (conjugated) ketone. 

The comparison of the ionization potentials of identically substituted cyclopropenones, cyclopropenes and cyclopropanes is interesting, if not yet particularly informative to date. The ionization potentials of cyclopropane, cyclopropene and cyclopropenone are much closer, 9.86, 9.67 and 9.47 eV, than for their diphenyl derivatives. Diphenylcyclopropene has an adiabatic ionization potential of 7.45 eV while those of the cis and trans isomers of 1,2-diphenylcyclopropane (18) are 8.20 and 8.05 eV respectively. These latter values for the saturated species correspond to ring-opening to l,3-diphenylprop-l-yl-3-ium (19) (equation 24) a result corroborated by both experiment via solution phase chemi-ionization and ab initio calculations on the analogous divinylcyclopropane. (The... 

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). 

The Cope rearrangement of trans- and, notably, cu-l,2-divinylcyclopropanes 1 to 1,4-cyclohep-tadienes 2 (see Section 2.4.5) is a well-known reaction of importance in synthesis. i 11 , i s s. 159... 

Although further work in this area is desirable, it iqipears that in the Cope rearrangement of simple trans-divinylcyclopropanes, such as (33) and (34), enantioselectivity is poor. 

In general, rearrangement of stcreoproximal c/.v-l,2-divinylcyclopropanes immediately results in formation of the cycloheptadienes. whereas rearrangement of the stereodistal trans-... 

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