1,2-Divinylcyclopropane

Some particularly striking examples of Cope nearrangemem can be found in the rearrangement of cis-divinylcyclopropanes. However, before we go into these, let us examine vinylcyclopropane itself, which is known to rearrange titennaOy to cyck ten-... 

A dramatic diflference in reactivity is evident when cb-divinylcyclopropane is compared wifli vinylcyclopropane. ciy-Divinylcyclopropane can only be isolated at low temperature because it very rapidly imdeigoes Cope rearrangement to 1,4-cycloh ta-... 

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),... 

In accordance with this, the reaction of the electron-donor-substituted butadienes 170 (R=Ph, OMe) with the arylcarbene complexes 163 yields divinylcyclopropane intermediates 168 with high chemoselectivity for the electron-rich double bond in 170, which readily undergo a [3,3]-sigmatropic rearrangement to give the as-6,7-disubstituted 1,4-cycloheptadiene derivatives... 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

An interesting example of l,3-cyclohexadiene-13 5-triene interconversion is the reaction of norcaradienes to give cycloheptatrienes. Norcaradienes give this reaction so readily (because they are cw-1,2-divinylcyclopropanes, see p. 1445)... 

Indeed, cw-l,2-divinylcyclopropanes give this rearrangement so rapidly that they generally cannot be isolated at room temperature,though exceptions are known. When heated, 1,5-diynes are converted to 3,4-dimethylenecyclobu-tenes. A rate-determining Cope rearrangement is followed by a very rapid electro-cyclic (18-27) reaction. The interconversion of 1,3,5-trienes and cyclohexadienes... 

The molecules taking part in a valence tautomerization need not be equivalent. Thus, NMR spectra indicate that a true valence tautomerization exists at room temperature between the cycloheptatriene 110 and the norcaradiene (111). In this case one isomer (111) has the cw-l,2-divinylcyclopropane structure, while the other does not. In an analogous interconversion, benzene oxide and oxepin exist in a tautomeric equilibrium at room temperature. 

In contrast to the synthesis of carbocyclic rings, the Cope rearrangement has been used sparsely for generating azepinones. Recently, the enantioselectivity of the conversion of 2-aza-divinylcyclopropane 286 has been investigated. The synthesis started from the optically active cyclopropanecarboxylic acid (90% ee), which had been converted into the isocyanate 286 by initial azidation to 285 and a consecutive Curtius rearrangement. Furthermore, the conditions of the iso-... 

Entry 2 illustrates the reversibility of the Cope rearrangement. In this case, the equilibrium is closely balanced with the reactant benefiting from a more-substituted double bond, whereas the product is stabilized by conjugation. The reaction in Entry 3 involves a cz s-divinylcyclopropane and proceeds at much lower temperature that the previous examples. The reaction was used in the preparation of an intermediate for the synthesis of pseudoguiane-type natural products. 

The 1 1 complex of ds-divinylcyclopropane 39 with bisethylenehexa-fluoroacetonatorhodium(I) [Rh(C2H4)2(acacF6)] rearranges to the cyclo-... 

The fragment of 1,4-PD is also present in 1,1-divinylcyclopropane (DVC), where the central methylene group is replaced by a three-membered ring. For this strained molecule a strong interaction between cyclopropane Walsh and vinyl 7r-orbitals was expected. The photoelectron spectra of DVC5 could be best understood with the assumption of optimal... 

FIGURE 2. Calculated high symmetry conformations (C2v, C2 and Dyd. S4, respectively) and experimentally determined molecular structures of 1,1-divinylcyclopropane (DVC) and tetravinylmethane (TVM) in Ci presentation with thermal probability plots of 50%... 

As a real example we show in Figure 2 the PE spectrum of 1,1-divinylcyclopropane (46 in Table 1), taken from the considerable number of diene and polyene PE spectra published by R. Gleiter and his coworkers. In the second column of the insert (5) are listed the / values in eV corresponding to the first bands of 46. 

A slight modification of the cyclopropyl conjunctive reagent transforms a cyclopentannulation into a cycloheptannulation. Thus, the 2-vinylcyclopropyllithium reagent 3, converted to its cuprate 4, generates a 1,2-divinylcyclopropane. Heating to only 180 °C leads to smooth Cope type rearrangement, driven by the release of the cyclopropyl strain, to create a perhydroazulene ring systerh of many sesquiterpenoids (Eq. 19) 20>. 

Cyclopropanation of l,3-dienes. a,0-Unsaturated carbenes can undergo [4 + 2]cycloaddition with 1,3-dienes (12, 134), but they can also transfer the carbene ligand to an isolated double bond to form cyclopropanes. Exclusive cyclopropanation of a 1,3-diene is observed in the reaction of the a,(3-unsaturated chromium carbene 1 with the diene 2, which results in a frans-divinylcyclopropane (3) and a seven-membered silyl enol ether (4), which can be formed from 3 by a Cope rearrangement. However, the tungsten carbene corresponding to 1 undergoes exclusive [4 + 2]cycIoaddition with the diene 2. 

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. 

It was emphasized that a particular advantage of this approach over other synthetic strategies based on Cope rearrangement consists in the facile way of selectively preparing c -divinylcyclopropane intermediates262. 

An interesting approach to form a divinylcyclopropane structure capable of rearranging into seven-membered functionalized derivatives consists of the silyloxylation of cyclic ketones 541 followed by a spontaneous Cope rearrangement to produce the cyclic enol esters 542 which then hydrolyzed to ketones 543 (equation 2 1 3)265. 

In principle, the divinylcyclopropane structure discussed here is incorporated into very well known systems such as bullvalene 547, barbaralane 548 and semibullvalene 549, which very easily undergo a Cope rearrangement. 

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