Vinylcyclopropane isomerization

It will also be noted from the results in Table 4 that, unlike the saturated cyclopropanes, the vinylcyclopropanes isomerize with normal preexponential factors. Consideration of the postulated transition complex and the reactant molecule makes it clear why this is so. In the reactant the vinyl group can undergo essentially free rotation. In the transition complex the allylic part of the biradical is rigid and cannot rotate. Thus the entropy contribution of this free rotation in the reactant is lost on forming the transition complex. As a result of ring rupture one new centre of free rotation is produced which is not present in the reactant. The result of these effects is that on passing from the reactant to the... 

Whereas the parent difluoro-vinylcyclopropane isomerizes to difluorocyclopentene under pyrolysis conditions, the corresponding alkyl compounds also lead to acyclic dienes. The activation energy for the difluoro-vinylcyclopropane isomerization is practically identical with that observed for the unsubstituted hydrocarbon [211, 212], If the alkyl group is oriented cis to the vinyl substituent, only dienes are isolated, and the process occurs at much lower, temperatures. Presumably these stereoisomers rearrange by a different mechanism (a 1,5-homodienyl hydrogen shift [213]). When the dichlorocyclopropane XVII is subjected to flash vacuum pyrolysis it isomerizes to 9,9-dichloro-bicyclo[5.3.0]dec-l(7)-ene [214],... 

Most vinylcyclopropane isomerizations have energies of activation near 50 kcal.mole" and entropies of activation near zero. Activation parameters and reaction products of vinyl cyclopropane isomerizations are summarized in Table 9. 

According to this mechanism, the initial step in typical vinylcyclopropane isomerizations is cleavage of the cyclopropane ring to form an allylic diradical, which undergoes intramolecular reorganization more rapidly than it enters into bimolecular reactions with other radicals, viz-... 

In contrast to the behavior of homoallylic alcohol (70a) when treated with methanesulfonyl chloride is pyridine, heating A -19-methanesulfonate (68b) in pyridine gives the 5)5,19-cyclo-6-ene (72). Vinylcyclopropane (72) is inert to the conditions used for converting vinylcyclopropane (73) to the A ° -B-homo-7)5-ol (70a). The latter results are only consistent with the existence of two discrete isomeric carbonium ion intermediates which give rise to isomeric elimination products. °... 

The photochemical isomerization of 1,4-dienes 1, bearing substituents at C-3, leads to vinyl-cyclopropanes 2, and is called the di-n-methane rearrangement This reaction produces possible substrates for the vinylcyclopropane rearrangement. 

The rearrangement proceeds from the Si-state of the 1,4-diene 1. The Ti-state would allow for different reactions like double bond isomerization. Rigid systems like cyclic dienes, where EfZ -isomerization of a double bond is hindered for steric reasons, can react through the Ti-state. When the rearrangement proceeds from the Si-state, it proves to be stereospecific at C-1 and C-5 no -isomerization is observed. Z-l,l-Diphenyl-3,3-dimethyl-l,4-hexadiene 5 rearranges to the Z-configured vinylcyclopropane 6. In this case the reaction also is regiospecific. Only the vinylcyclopropane 6 is formed, but not the alternative product 7. ... 

The direction of ring opening by homolytic cleavage of a cyclopropane bond is controlled by the stability of the diradical species formed. Upon heating of the mono-deuterated vinylcyclopropane 3, a mixture of the two isomeric mono-deuterated cyclopentenes 4 and 5 is formed ... 

Some care must be taken in drawing conclusions from the E/Z or syn/anti selectivity of a given catalyst/alkene combination. The intrinsic stereoselectivity may be altered in some cases by subsequent isomerizations initiated by the catalyst. For example, epimerization of disubstituted vinylcyclopropanes is effectively catalyzed by palladium compounds the cis - trans rearrangement of ethyl chrysanthemate or of chrysanthemic acid occurs already at room temperature in the presence of PdCl2 L2 (L = MeCN, EtCN, PhCN)96 Oxycyclopropane carboxylic esters undergo metal-... 

The first examples of transition metal-catalyzed [5 + 2]-cycloadditions between vinylcyclopropanes (VCPs) and 7r-systems were reported in 1995 by Wender and co-workers.10 This [5 + 2]-reaction was based conceptually on the Diels-Alder reaction, replacing the four-carbon, four-7r-electron diene with a five-carbon, four-electron VCP (Scheme 1). Although the [5 + 2]-reaction of VCPs and 7r-systems can be thought of as a homolog of the Diels-Alder [4 +21-reaction, the kinetic stability of VCPs (activation barrier for the thermal isomerization of VCP to cyclopentene has been reported as 51.7 kcal mol-1)11 makes the thermal [5 + 2]-reactions involving VCPs and 7r-systems very difficult to achieve. A report of a thermal [5 + 2]-cycloaddition between maleic anhydride and a VCP has been published,12 but this reaction has not been reproduced by others.13 14 Based on the metal-catalyzed isomerization of VCPs to cyclopentenes and dienes,15-20 Wender and co-workers hypothesized that a metal might be used to convert a VCP to a metallocyclohexene which in turn might be trapped by a 7r-system to produce a [5 + 2]-cycloadduct. Based on its previous effectiveness in catalyzed [4 + 2]-21 and [4 + 4]-cycloadditions (Section 10.13.2.4), nickel(0) was initially selected to explore the potential of VCPs as four-electron, five-carbon components in [5 + 2]-cycloadditions. 

Overberger and Borchert (1960) were the first to report that the P3u olysis of vinylcyclopropane yielded cyclopentene as the major product. Independently Flowers and Frey (1961b) studied this isomerization and found that it was homogeneous and kinetically first order and almost certainly unimolecular. The Arrhenius equation for the isomerization was found to be... 

It will be noted that the isomerization to cyclopentene proceeds with a considerably lower energy of activation than the other cyclopropane isomerizations so far discussed. As a result these reactions have been investigated kinetically at temperatures about 100° lower than those not having a vinyl substituent. A number of substituted vinylcyclopropanes have been studied and the Arrhenius parameters for their isomerizations to substituted cyclopentenes determined. The results are shown in Table 4. From the results in Table 4 it can be seen that the isomerizations... 

Thus the rotation of the allylic system out of the plane (required before ring closure can occur) results in the reduction of the available allylic resonance energy by about 5 kcal mole". This particular steric requirement does not apply to the isomerizations to the dienes, and these have much the same values as for vinylcyclopropane itself, viz. ... 

Since the isomerization to the cyclopentene is made more difficult by the substitution, but the isomerizations to the dienes are tmaffected, the result is that (unlike the case of vinylcyclopropane itself) the dienes now constitute major products of the reaction. 

A number of other vinylcyclopropanes have been studied qualitatively. Some of them are shown on p. 159. In all cases the isomerizations occur at 300° C which indicates that their energies of activation are probably close to that for vinylcyclopropane (Doering and Roth, 1963). 

Frey and Ellis, 1965). The Arrhenius parameters are very close to those obtained for the cia-l-methyl-2-vinylcyclopropane and support the l>ostulated similarity of the two transition states. Further evidence that the low A factor arises mainly from the loss of internal rotations in the transition complex, comes from the work of Grimme (1965) on the thermal isomerization of bicyclo[5,l,0]octene-2 to eyclo-octa-1,4-diene. 

Rearrangement of non-donor-substituted vinylcyclopropanes can be induced by transition metals [1421-1423] or thermally [1424-1427]. The thermal isomerization, however, requires high reaction temperatures (typically 200-500 °C). 

A quantum-chemical study has been undertaken on the isomerization of ctx-1-vinyl-, -1-formyl-, -1-thioformyl-, and -l-iminomethyl-2-vinylcyclopropane to cyclohepta-1,4-diene, 2,5-dihydrooxepine, 2,5-dihydrothiepine, and 2,5-dihydroaze-pine, respectively. Reaction pathways for circumambulatory rearrangements of main group migrants (NO, PO, NCS, SCN, NCO, OCN, SR, Cl, Br, and XX where X... 

Unlike the well-known chemistry of the vinylcyclopropane-cyclopentene rearrangement, there is no general method for the rearrangement of alkynyl-cyclopropane to cyclopentene derivatives. One specific example is the pyrolysis of l-ethynyl-2-methylcyclopropane to methylenecyclopentene and other compounds [5]. At 530°C, l-ethynyl-2-methylcyclopropane (1) undergoes a [1,5]-hydrogen shift to give hexa-l,2,5-triene (2), which further isomerizes to methy-lenecyclopentenes 3 and 4 in 38 and 29% yield, respectively (Scheme 1). 

In contrast to the isomerization of 39, the retention of optical activity for the conversion of 41 to 42 is very small. However, the loss of activity cannot be explained by a degenerate vinylcyclopropane rearrangement, since the recovered starting material ([(X]54g = +32 ) shows essentially complete retention of chirality. An alternative explanation involves competing stereoselective processes in a chiral intermediate (41a ). For 41a a hydride shift from the allylic position, C2, might compete with the migration from C6 to Cl. This possibility can be probed by... 

If the double bond is connected directly to the three-membered ring, i.e. if the substrate is a dihalo-vinylcyclopropane derivative, a vinylcyclopropylidene to cyclopentadiene isomerization, the so-called Skattebol rearrangement, takes place upon treatment with alkyllithium. Again, this ring-forming step competes with a ring-destruction process leading to allenic hydrocarbons (vinylallenes,... 

Skatteb0l rearrangements in more complex dibromo-vinylcyclopropanes have also been reported. Thus, either a system consisting formally of two double bonds and one dibromocyclopropane unit or an educt containing two dibromo-cyclopropane moieties and one double bond undergo several carbene-carbene isomerizations upon treatment with methyllithium. In both cases, complex product mixtures arise. 

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