Cyclopropane isomerisation

Cyclopropane isomerisation - demonstration of falling off in the first order rate constant with decrease in pressure, 149-150, 150-151... 

Cyclopropane isomerises to propene at 1000°C. The rate is 0.92 s when the concentration of cyclopropane is 0.1 mol dm , and 0.46 s when the concentration of cyclopropane is 0.05 mol dm. Deduce the relationship between rate and cyclopropane concentration, giving the value of any constants. 

Mitsch and Neuvar have shown that perfluorovinyl cyclopropane isomerises at 131-183 °C to perfluorocyclopentene by a clean unimolecular reaction. The activation energy is about 15 kcal.mole less than for the reaction involving the parent hydrocarbon - . Perfluoroallylcyclopropane decomposes unimolecu-larly at 196-250 °C to perfluoro-l,4-pentadiene and perfluorocyclopropane plus perfluoroethylene (the latter two products presumably from difluorocarbene formed in the initial reaction). Arrhenius parameters are. <4 = 6.3 x 10 sec , E = 42.7 kcal.mole . Placzek et 499,500 reported on the rates of... 

We can carry this comparison with experiment a little further by measuring the rate of the cyclopropane isomerisation at sufficiently low pressures that the mean free path exceeds the diameter of the vessel [63.K2]. A 11 spherical flask was used, and the results are shown in Figure 5.2 the solid theoretical line is calculated on the assumption that the internal energy relaxation rate is given by... 

Figure 22.11 Measurement of half-life for cyclopropane isomerisation.

The gas-phase isomerisation reaction of cyclopropane to propylene satisfies the following rate expression ... 

When the cyclopropane ring of this interesting reaction product is broken as a result of isomerisation two substances may he formed ... 

Since the addition of methylene to an olefin should be exothermic, with the evolution of about 90 kcal/mole, isomerisations of the initially formed cyclo-propanes are very likely, since they only need about 64 kcal/mole. RRKM-studies demonstrate that this isomerisation should be faster than the rearrangement of cyclopropanes 32, 33 to the pentenes 34, 35 Numerous studies of the photochemical generation in the gas phase provided conclusive evidence in favour of these findings uo.iii.iis). 

So the cyclopropane 36 isomerises at high temperature, no doubt via the [1,3] shift to 37, formation of the extended enol 38 and movement of the alkene into conjugation follows. The product 39 was used in a synthesis of the natural product zizaene.10... 

Jean, Y., Thesis Isomerisation geometrique du cyclopropane. Orsay 1973. 

Chapuisat, X., Thesis Etude dynamique des ructions chimiques isomerisations optique et geometrique du cyclopropane. Orsay 1975. 

In contrast to the cyclobutane systems, fluorinated cyclopropanes show a lesser thermal stability than the parent hydrocarbon. Trotman-Dickenson et al. (refs. ° )have studied thethermally induced unimolecular isomerisations of partly fluorinated cyclopropanes to mixtures of the corresponding fluoro-propenes. At 450 °C the rate of decomposition of monofluorocyclopropane is about three times that of cyclopropane products formed are 1-fluoropropene (79 %) cis-trans mixture), 2-fluoropropene (9 %) and 3-fluoropropene (11 %). The substitution of further fluorine atoms in the cyclopropane ring further increases the rate of isomerisation ° (see Table 2). Unlike the partly fluorinated cyclopropanes, perfluorocyclopropane does not isomerise to propene compounds but decomposes at 250-300 °C by a first-order reaction to perfluoroethylene . The rate coefficient is expressible as, k = 1.78 x 10 exp (-38,600//JjT) sec , and the decomposition is consistent with the mechanism... 

Table 2 summarises Arrhenius parameters for the unimolecular isomerisations of fluorinated cyclopropanes. 

Reaction of 1-alkenyltriphenylbismuthonium salts (85) with potassium ferf-butoxide led to the carbene derivatives, 1-alken-l-ylidenes, which can either be trapped by styrene to give cyclopropanes or isomerise to give the corresponding acetylenes. ... 

Irradiation of electron deficient arenes in the presence of cis-l,2-diphenylcyclopropane leads to formation of the trans isomer by an electron transfer mechanism. The reaction occurs by way of the radical cation of the cyclopropane which isomerises prior to back electron transfer. It has now been examined using menthyl and bornyl esters of benzene tetracarboxylic acid as chiral electron transfer sensitisers. °° Slight excesses of one of the enantiomers of the trans-1,2-diphenylcyclopropane were observed. The dicyanoanthracene sensitised reactions of 1,1,2,3-tetra-arylcyclopropanes have been studied.Depending on the substituents present on the arene rings these compounds rearrange to 1,1,3,3-tetra-arylpropenes. The rearrangement occurs in a ring opened radical cation intermediate. 

Photolysis of permethrin (34) in various solvents and on soil in sunlight resulted primarily in cyclopropane ring isomerisation and ester cleavage to yield... 

On heating, vinyl cyclopropanes (22) isomerise to cyclopentenes (23). This is a disallowed (1,3) sigmatropic shift which occurs because the reaction is so favourable in enthalpy terms that the symmetry barrier is overcome. ... 

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