Vinylcyclopropanes concerted rearrangement

With the exception of vinylcyclopropane-cyclopentene rearrangements (cf. Section VII) radical reactions have so far rather rarely been used synthetically. This is also true for pericyclic modes of cyclopropane cleavage which are mainly restricted to divinylcyclopropane-cycloheptadiene type expansions. Cycloadditions, whether concerted or stepwise, occur only with very specific compounds. ... 

Houk, K. N., Nendel, M., Wiest, O., Storer, J. W. The Vinylcyclopropane-Cyclopentene Rearrangement A Prototype Thermal Rearrangement Involving Competing Diradical Concerted and Stepwise Mechanisms. J. Am. Chem. Soc. 1997, 119,10545-10546. 

Mazzocchi, P. H., Tamburin, H. J. Mechanism of the vinylcyclopropane-cyclopentene rearrangement. Evidence against a concerted process. J. Am. Chem. Soc. 1970, 92, 7220-7221. 

Low-temperature rearrangements of vinylcyclopropanes have been reported and reviewed. Remote-charge acceleration has been invoked, in some cases, to account for the decreased temperatures which are required to bring about the bond reorganization. Bicyclic cyclopen-tenols of type 1 have been prepared in excellent yields in a rearrangement that may constitute the only case of a concerted vinylcyclopropane-cyclopentene rearrangement. ... 

Offer an explanation for the facility of the reaction, as compared to the vinylcyclopropane rearrangement of hydrocarbons, which requires a temperature above 200°C. Consider concerted reaction pathways which would account for the observed stereospecificity of the reaction. 

The vinylcyclopropane rearrangement is of synthetic importance, as well as of mechanistic interest—i.e. the concerted vs. the radical mechanism. A reaction temperature of 200 to 400 °C is usually required for the rearrangement however, depending on substrate structure, the required reaction temperature may range from 50 to 600 °C. Photochemical and transition metal catalyzed variants are known that do not require high temperatures. 

Although it has been established that the HOMO (diazoalkane)-LUMO (alkene) controlled concerted cycloaddition occurs without intervention of any intermediate for the reactions of simple diazoalkanes with alkenes, Huisgen once proposed a mechanistic alternative 4 namely an initial hypothetical nitrene-type 1,1-cycloaddition reaction of phenyldiazomethane to styrene followed by a vinylcyclopropane-cy-clopentene-type 1,3-sigmatropic rearrangement Control experiments, however, excluded this hypothesis for the bimolecular 1,3-dipolar cycloaddition reaction of diazomethane (Scheme 60).204... 

Dinnocenzo and Conlon have described the remarkable effect of one-electron oxidation on the rate of certain vinylcyclopropane rearrangements. Exposure of several l-p-anisyl-2-vinylcyclopropane derivatives to a catalytic amount of tris(4-bromophenyl)aminium hexafluoroantimonate in acetonitrile at room temperature was found to induce ring expansion to form cyclopentenes (equation 25) temperatures in excess of 200 °C are required for the conventional thermal rearrangement cf these systems. At this time it is uncertain whether these reactions follow concerted mechanisms, or are stepwise processes involving trimethylene cation radical intermediates. 

Gajewski, J. J., Squicciarini, M. P. Evidence for concert in the vinylcyclopropane rearrangement. A reinvestigation of the pyrolysis of trans-1-methyl-2-(1-tert-butylethenyl)cyclopropane. J. Am. Chem. Soc. 1989, 111, 6717-6728. 

Homoazocines 49 and 50 (Section V,A) undergo thermal rearrangement to 3-azabicyclo[4.3.0]nonanes 217 and 218, respectively,405 most likely via a concerted or diradical [ 1,3]-sigmatropic vinylcyclopropane shift (Eq. 15),... 

The regioselectivity of the addition with respect to the arene has been interpreted in terms of both meta addition of the alkene to the Sj arene via an exciplex followed by formation of the vinylcyclopropane system,and initial rearrangement of the arene to a prefulvene species which then undergoes concerted alkene addition (see below). The orientation of the substituents on the wcfa-cycloadducts from some systems requires that both mechanisms operate in competition. The reaction shows no apparent regioselectivity with simple alkenes thus 2-methylpropene gives both wc/a-cycloadduct isomers in approximatively equal amounts. ... 

The mechanism of the thermal and photochemical rearrangement is believed to proceed as shown (1 - 2), with the dilemma of concerted vs. diradical nature not definitely resolved for all cases. A comparison of energy parameters for the thermolysis of the parent 1,2-divinyl-cyclopropane and of l-(hex-l-enyl)-2-vinylcyclopropane ( - and Z-isomers, tram- and cis-cyclopropanes) has been reported. When both alkenes bear a Z-positioned substituent, the rearrangement of the divinylcyclopropane system becomes very slow and other processes, such as the [1,5] sigmatropic shift of alkyl(vinyl)cyclopropanes or the vinylcyclopropane to cyclo-pentene rearrangement, may compete (see also Section 2.4.3.). Both the mechanism and the applications of this rearrangement have been reviewed. 

The photochemical production of vinylcyclopropane derivatives from compounds having two 7t-moieties bonded to an sp3-hybridized carbon648 is termed the din-methane rearrangement, also known as the Zimmerman reaction.649 A very broad spectrum of di-7t-systems can lead to photoproducts that are usually not obtainable by alternative routes.632,633 The reaction may be classified formally as a [l,2]-shift but, according to the proposed stepwise biradical mechanism,650 651 1,3- and 1,4-biradical (BR) intermediates and also the second 7t-bond may be involved652 (Scheme 6.29). A concerted (pericyclic) pathway for the di-7t-methane reaction from the excited singlet state is, however, not excluded. Typically, the singlet state reaction occurs upon direct... 

The alkyl substituents in 1-alkyl-l-vinyl and /rfl/7 -l-alkyl-2-vinyl-cyclo-propanes have little effect on reactivity. A cis alkyl substituent may have a profound effect for example, the energy of activation is nearly 20 kcal.mole smaller and the entropy of activation is 10 eu more negative, for isomerization of cis-1 -methyl-2-vinylcyclopropane than for typical vinylcyclopropane rearrangements. Further, the sole product of this reaction is cis-1,4-hexadiene. This and related reactions, which occur by a concerted mechanism involving 1,5-hydrogen migration, are discussed in the next section. 

The activation energy is only 7 kcal/mol less than that for the vinylcyclopropane rearrangement itself, consistent with roughly 20 kcal/mol for the pentadienyl radical resonance energy, and suggests that the reaction is not concerted. 

---