Isomerization, of vinylcyclopropanes

The yield of isoprene was pressure independent and therefore presumably not an isomerization product of excited vinylcyclopropane, as found in the thermal isomerization of vinylcyclopropane.1 8... 

Similarly, the isomerization of vinylcyclopropane has a lower activation energy than the reactions of saturated cyclopropanes (Table 4) a similar result is obtained with other substituted vinylcyclopropanes . Moreover, these reactions have frequency factors nearly two orders of magnitude lower than the cis-trans isomeriza-tions of saturated cyclopropanes. These results can be explained by the formation of an allylically stabilized biradical as the intermediate, viz. 

The formation of the diene in the isomerization of vinylcyclopropane is analogous to the formation of propene from cyclopropane. The reaction has a higher energy of activation than the ring-closure reaction. Fig. 3 gives an interpretation of these results in terms of the biradical mechanism. 

Baldwin, J. E. Thermal isomerizations of vinylcyclopropanes to cyclopentenes. J. Comput. Chem. 1998,19, 222-231. 

Alkyl-allyl complexes of isomeric systems can be interconverted and thus be used in isomerization of vinylcyclopropanes. Ethyl 4-azabicyclo[5.1.0]octa-2,5-diene-4-carboxylate (20) reacts with pentacarbonyliron to give complex 21, which photochemically rearranges to complex 23. Carbonylation of both products 21 and 23 leads to ethyl 9-oxo-2-aza-bicyclo[3.3.1]nona-3,7-diene-2-carboxylate (22). While complex 21 upon heating regenerates the starting material, complex 23 gives the isomeric product 24. In contrast to iron, with rhodium only the endo-complex 25 is formed. ... 

Thermal isomerizations of vinylcyclopropanes usually yield cyclopentenes as the principal products, although acyclic dienes are sometimes among the products and in some instances are the major products. These reactions have been reviewed in part by Frey and Walsh - and by Sarel et al.. ... 

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 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... 

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. 

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. 

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... 

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... 

The thermal rearrangement of vinylcyclopropane to cyclopentene was uncovered in I96090 91. That vinylcyclopropanes, like other cyclopropanes, may undergo cis, trans iso-merizations was inferred in 1964 when trans-l-vinyl-2-methylcyclopropane was thermally converted to mostly (4Z)-1,4-hexadiene, a product formed at much lower temperatures from cw-1-vinyl-2-methylcyclopropane92. The reversible interconversion of the cis and trans isomers of l-vinyl-2-d-cyclopropane (equation 2) was reported soon thereafter, in 196793"96. Additional examples, including cases showing both geometrical isomerization and enantiomerization processes, soon followed. 

The final topic to be discussed in this section concerns the prospect of opening or breaking the weakened bond in the radical cations of the 2Aj structure type. This topic has been the subject of some controversy. As mentioned above, ab initio calculations fail to support the existence of such a species. Yet, they have been postulated to rationalize some ESR observations and also to explain the geometric isomerizations of 1-aryl-2-vinylcyclopropanes upon reaction with ami-nium radical cations [225]. 

The thermal isomerization of l-ethynyl-2-vinylcyclopropane 57 has been also reported. Thus, the dimer 59 was formed in the pyrolysis of the tosylhydrazone salt... 

A similar acceleration has most recently been observed in the rearrangement of vinylcyclopropanes of type (39 Scheme 8). This fluoride-mediated vinylcyclopropane-cyclopentene isomerization proceeds at -78 C to give (40) in 85% yield this is to date the mildest condition available. Two possible intermediates, the enolate anion (39a) or the diradical anion (39b), may be responsible for such acceleration in analogy to the enolate anion accelerated divinylcyclobutane rearrangement recently reported." The mechanism of this transformation is unclear but may involve anion acceleration similar to that observed in the rearrangement of sulfonyl anions derived from (42 Scheme 8). By comparison the thermolysis of (39) produced exclusively the endo isomer of (41) at 580... 

Vinylcarbene is also regarded as an intermediate in the catalytic isomerization of cyclopropene in the presence of [CuCl P(OPh)3 ]. 3-Methyl-3-cyclopropylcyclopropene (83a) and 3,3 dicyclo-propylcyclopropene (83b) react with cyclopentene derivatives under die influence of CuCl/P(OPh)3 to yield cntfo-vinylcyclopropane derivatives (84 equation 37) in 55-72% yields. 3-Methoxycarbonyl-l-propylcyclopropene also reacts with noibomadiene in the presence of CuCl to give the corresponding vinylcarbene adducts in 60% yield. 

MO calculations have been carried out on the isomerization of cyclopropane to propene, and the MNDO method has been used to study the reaction pathway and to optimize the structure of reactant, transition structure, and product of the ring opening reaction of bicyclo[1.1.0]butane. Various methods have been employed to estimate the rate constants for ring opening of the 2-cyclopropyl-2-propyl radical. 1-Acceptor-1-sulfenyl-substituted 2-vinylcyclopropanes of the type (430) have been found to afford 6-sulfenyl-a,jS y, -unsaturated carboxylic esters and nitriles (431) upon treatment with acid, by a process which involves C(l)—C(2) bond fission and a novel 1,5-sulfenyl rearrangement (see Scheme 110). It has been shown that the benzophenone-sensitized photolysis of vinyl norcaradiene derivatives, such as 5-(2-methylprop-l-enyl)-3-oxatricyclo[4.4.0.0 ]deca-7,9-dien-4-ones (432), results in the regioselective cleavage of only one of the cyclopropyl c-bonds to afford isochroman-3-one derivatives (433). It has been reported that the major product obtained from the reaction of structurally diverse a-diazo ketones with an electron-rich alkene in the... 

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