Reaction mechanisms vinylcyclopropane rearrangement

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. 

Mild Ni(0)-catalysed rearrangements of l-acyl-2-vinylcyclopropanes to substituted dihydrofurans have been developed.86 The room temperature isomerizations afford dihydrofuran products in high yield. A highly substituted, stereochemically defined cyclopropane has been employed in the rearrangement to evaluate the reaction mechanism. The Cu(II)-catalysed cycloisomerization of tertiary 5-en-l-yn-3-ols with a 1,2-alkyl shift affords stereoselectively tri- and tetra-cyclic compounds of high molecular complexity (Scheme 29).87 A proposed mechanism has been outlined in which... 

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. 

The formation of pyrroles by the reaction of the cyclopropylketimine (63) with HBF4 has been studied from a mechanistic point of view using 3-chloropropionyl chloride enriched with at the carbonyl carbon atom The C-NMR spectrum of the pyrrole formed from the cyclopropyl ketimine (63) shows enhancement of the peak at the 5-carbon in accord with mechanism A outlined in Scheme 27. Operation of process B analogous to the vinylcyclopropane rearrangement 63a would have led to enrichment at the 3-carbon of the pyrrole product. 

Dinnocenzo, J. P., Conlon, D. A. The cation radical vinylcyclopropane —> cyclopentene rearrangement reaction mechanism and periselectivity. Tetrahedron Lett. 1995, 36, 7415-7418. 

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. 

Studies carried out by different research groups for more than 30 years have shown that this reaction is very general and usually takes place with high chemical and quantum yields affording vinylcyclopropanes that in many instances are difficult to obtain, or not available, by alternative routes. A biradical mechanism has been proposed to account for the rearrangement as... 

The term di-ir-methane rearrangement is meant to describe the photoisomerization of 1,4-dienes (i.e. two TT-systems separated by a methane unit) leading to vinylcyclopropanes. The reaction can be generalized in schematic form as in equation (1). As will be discussed more specifically in the section dealing with mechanism, such a simplistic presentation is not intended as the actual reaction path, but it gready helps in predicting the reaction products. 

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 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... 

Three more reactions, which presumably proceed via carbopalladation steps without subsequent dehydropalladation, should be mentioned (Scheme 24). The first one is the diastereoselective formal [3 + 2] cycloaddition of the chiral nonracemic (/3-sulfinyl)vinylcyclopropane derivative 109 onto acrylonitrile (110). The second example is the Pd-catalyzed asymmetric vinylcyclopropane to cyclopentene rearrangement of the chiral nonracemic E)- and (Z)-arylsulflnyl-l,3-butadienylcyclopropane derivatives 112 [66] Plausible mechanisms that can rationalize the stereochemical outcome of the reactions were proposed in both publications. ... 

A common reaction that has shown some synthetic utility is the vinylcyclopropane to cyclopentene rearrangement. Ej, for the parent system is 49.6 kcal / mol. Write a mechanism for this reaction, propose an inlermed iate, and include appropriate electron pushing. How might you test your mechanistic postulate ... 

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