Cyclopropane structural isomerization

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

The rate constants were determined at a series of pressures in the fall-off region, and the fall-off curve was very similar to that obtained for the structural isomerization to propylene. The similarity of the two sets of data suggests that both reactions may proceed through similar reaction paths. One obvious possibility is that once again the trimethylene biradical is formed, which can undergo internal rotation followed by recyclization. An alternative transition state has been suggested which involves, as an activated complex, a much expanded cyclopropane ring in which hindered internal rotation occurs (see also Smith, 1958). 

The cis-trans isomerization of cyclopropanes is not restricted to the deuterium-substituted molecules, cis- and traws-l,2-Dimethylcyclo-propane have been shown to imdergo reversible geometrical isomerization as well as slower structural isomerization. All the processes are homogeneous and kinetically first order, and almost certainly unimolecular. The reaction scheme is shown below. 

The structural isomerization of a number of alkyl substituted cyclopropanes has been investigated. In all cases the reaction is probably unimolecular. In general several olefins are formed. The results obtained are shown in Table 2. It is to be expected (especially if a diradical is an... 

The reaction of CH2 with cyclopropane1617 46177 gives excited methyl-cyclopropane which is deactivated or undergoes structural isomerization to butene-1, butene-2 (m and trans), and isobutene. The lifetime of methylcyclopropane wa6 found to depend on the CH2 source. 

Early kinetic studies on the structural isomerization of cyclopropane to propene provided estimates of activation parameters73 75 and prompted speculation that the reaction might well involve a trimethylene diradical intermediate. This possibility seemed reinforced when the thermal interconversion of the els and trans isomers of l,2-d2-cyclo-propane at 414 to 474 °C (equation 1) was reported in 195876. This structurally degenerate isomerization was found to be substantially faster than conversion to deuterium-labeled propenes—about 24 times faster at the high pressure limit76 77. 

Though not of primary importance to the topic of this chapter, the cyclopropane to propene structural isomerization, it must be noted, has played a prominent role in the development of theory for unimolecular isomerizations and it continues to attract experimental work. The marked dependence of rate on gas pressure and on isotopic substitution exhibited by this isomerizaton provides fundamental insights on intramolecular and vibrational energy transfers and redistributions97"114. 

Thus a substantial body of experimental evidence shows that 1,2-disubstituted cyclopropanes, including vinylcyclopropanes, react thermally to give isomeric cyclopropanes through both one-center and two-center epimerizations, with (kt + k2) kl2 ratios from 1.4 to 4. Rate constants for both (, + k2) and kl2 events respond to the capacity of substituents to stabilize adjacent radicals in a regular fashion consistent with trimethylene diradical transition structures. Rate constants for vinylcyclopropane structural isomerizations do as well, thus reinforcing the notion that these reactions are nonconcerted diradical mediated reactions. 

The Arrhenius parameters for the gas-phase unimolecular structural isomerizations of 1,1,2-trimethylcyclopropane28 to three isomeric methylpentenes and two dimethyl-butenes, and of 1,1,2,2-tetramethylcyclopropane29 to 2,4-dimethylpent-2-ene have been determined over a wide range of temperatures. Despite previous reports on substantial decreases in activation energies for structural isomerizations of methyl-substituted cyclopropanes, this study has revealed that the trend does not continue beyond dimethylcyclopropane isomerization. 

With the initial synthesis of cyclopropane in 1882 , and the report of its thermal structural isomerization to propene in 1896, this simplest of cyclic hydrocarbons began its extraordinarily fruitful stimulation of fresh insights on fundamental problems in organic chemistry, ranging from basic concepts of ring strain and structural isomerism to questions of thermochemistry and reactivity and of a aromaticity And from the beginning there was controversy, extending a few years before suitably authoritative commentators confirmed the fact that cyclopropane is indeed converted thermally to propene. ... 

The structural isomerization of cyclopropane to propene does not involve radical chain processes if a trimethylene diradical intermediate is involved, it must be of such a short lifetime that it may not be easily trapped through gas-phase intermolecular reactions Yet the trimethylene diradical hypothesis does account for the thermal interconversion of cis and trans 1,2-d2-cyclopropanes in a most plausible manner. Homolytic cleavage of one carbon-carbon bond would form a 1,3-diradical intermediate rotations of terminal methylene groups in this trimethylene diradical followed by reformation of the cyclopropane ring would rationalize the isomerization According to this model, the net outcome of a stereomutation process would be dictated by the relative magnitudes of rate... 

Several different mechanisms have been proposed for the structural isomerization of cyclopropane into propylene. Chambers and Kistiakowsky suggested the following two possibilities ... 

Finally, it should be noted that geometrical and structural isomerizations of substituted cyclopropanes by means of ET-catalyzed reactions, via intermediate trimethylene radical anions, is only one pathway to perform these reactions. Other possibilites are the thermal reaction via trimethylenes , the light induced reaction the photosensitized reaction via trimethylene radical cations the Pd/C-catalyzed reaction S and the base-catalyzed reaction ... 

The mechanism of the formation of the ring-opened species 1,3-diphenylpropene 84a and 1,3-diphenylpropane 82a from the cyclopropanes cis-78a and trans-79a has been outlined in Scheme 5, route A, page 18. Common intermediate is the 1,3- dianion 81 a which in the case of the formation of 84 a first loses hydride and then is protonated. The l,3- dianion 81a is formed by electron transfer from the trimethylene radical anion 80a which thus is the key intermediate 1) in the ET-catalyzed-stereoisomeriza-tion and 2) in the structural isomerization of the cyclopropanes cis-78 and trans-79. Importantly, the structural isomerization is not an ET-catalyzed reaction. 

The competing side reaction is a cyclopropyl anionic ring opening (for a review see ref 13). In the case of isomeric l-benzoyl-2,3-diphenylcyclopropanes, no base-induced isomerization (MeOH/ NaOMe, 65 °C, 4 h) was detected. In many cases these rearrangements require rather drastic conditions but the interconversion can be facilitated by the participation of an adjacent substituent. For example, with an acyl-substituted cyclopropane, enolization permits cisitmns isomerization to proceed without ring opening and therefore without competing anionic structural isomerization. 

Structural Isomerization Reactions.—In earlier work Kropp and his co-workers19 suggested that the unsaturated and cyclopropane products formed by the irradiation of olefins in solvents of low nucleophilicity were produced by initial rearrangement to carbenes. In a later study Kropp and Fields20 have demonstrated that carbene intermediates are indeed formed in the irradiation of olefins... 

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