Isomerizations cyclopropane—propene

In the following examples ET reactions to cyclopropanes have been presumed to account for the observed cyclopropane - propene isomerizations. 

One can therefore conclude that the base-catalyzed cyclopropane propene isomerization 170a - 173a is induced by an acid-base and not by an ET reaction. 

While the cyclopropane to propene isomerization is not observed in the thermal rearrangements of gem-difluorocyclopropanes (vide supra), this is the principal reaction of polyfluorocy-clopropanes in the presence of Lewis acids.23 Perfluoro(alkylcyclopropanes) 27 isomerize to perfluoroalkenes 28 in high yield, despite the harsh conditions (100°C) required in some cases. It should be noted that double-bond migration under these reaction conditions occurs readily (see Section 5.1.2.2.). 

Self-Test 13.6B Cyclopropane gas isomerizes to propene gas in a first-order process. How long is needed for the concentration of cyclopropane to decrease from 1.0 mol-L 1 to 0.0050 mol-L 1 at 500°C Use the data in Table 13.1. 

However, this cyclopropane to propene isomerization is the only reported preparative method for the synthesis of tetracyclo[3.3.3.0. 0 ]undec-9-ene 4) from pentacyclo-[3.3.3.0 -. 0 - .0. "]undecane (13). ... 

Excited sulphur atoms S( D2), from the u.v. photolysis of COS in situ, react with ethene to yield thiiran and vinylthiol in 100% yield.The vinylthiol is formed via direct insertion into a C—H bond and by unimolecular isomerization of the chemically activated ground-state thiiran. This novel thiiran vinylthiol rearrangement is analogous to the cyclopropane propene rearrangement in the sense that both proceed concertedly via a bicyclic activated complex. 

Table 5.1. Relative Efficiency of Various Buffer Gases in Maintaining the Rate of Isomerization in the Cyclopropane-Propene Reaction" ...

What is the enthalpy of isomerization of propene (CLiH ) to cyclopropane at 298 K by a semi empirical calculation ... 

Calculate the energy change En tor isomerization of cyclopropane to propene in the gaseous state at 0 K... 

We have seen before that such diradicals can close up to give cyclopropanes (17-36). Therefore, pyrolysis of cyclopropanes can produce not only propenes but also isomerized (cis trans or optically active inactive) cyclopropanes. See, for example, Berson, J.A. Balquist, J.M. J. Am. Chem. Soc., 1968, 90, 7343 Bergman, R.G. Carter, W.L. J. Am. Chem. Soc., 1969, 91, 7411. 

C15-0086. For the isomerization that converts cyclopropane to propene, A -f reaction — 33 kJ and... 

An example of a first-order elementary reaction is the isomerization of cyclopropane to propene, for which the reverse reaction hardly proceeds, and we may write the rate as... 

Sketch plausible transition states for (a) the dissociation of a molecule in the gas phase (b) the reaction of cyclopropane to give propene (c) the isomerization of CH3CN to CH3NC (d) the desorption of an atom from a surface (e) the dissociation of an adsorbed molecule such as CO on a metal surface. 

Cyclopropane isomerizes into propene when heated to 500°C in the gas phase. The amount converted at various times with various initial pressures has been followed by gas chromatography. In the table, t is in sec and pressures are in Torr. Find the rate equation. 

A second example of an inverse statistical weight isotope effect is that of the secondary H/D KIE on C-C bond rupture during the gas phase unimolecular isomerization of cyclopropane to propene. Theory and experiment are compared in Fig. 14.2 for reactions 14.37 and 14.38. 

The thermolysis of cyclopropane, cyclobutane and their derivatives has received considerable attention. The thermal rearrangement of cylcopropane to propene is a clean, first-order process.79 Information concerning the course of the reaction was provided by a study of the thermal isomerization of cis- and mmr-1,2-dideuteriocyclopropane (18).80 The process occurs significantly faster than conversion to propene, suggesting a propane-1,3-diyl 19 as an intermediate. 

Photostationary cis/trans ratios vary with sensitizer structure in a manner not yet perfectly understood, and both the time required for attainment of a photostationary state and actual flash spectroscopic quenching rates indicate that energy transfer to diphenylcyclopropane is quite inefficient. Such should be the case if nonvertical energy transfer with the production of a biradical is occurring. When care is taken so that only the sensitizer absorbs light, only cis-trans isomerization is observed. Direct excitation of the cyclopropane produces 1,3-diphenyl-propene and 1-phenylindane as well.298... 

Gamma irradiation of benzene solutions of 1,2-diphenylcyclo-propane also results in cis-trans isomerization, presumably by energy transfer from triplet benzene.300 G values for isomerization are close to one. Gamma irradiation of the neat cyclopropane produces high G values for geometric isomerization and only traces of 1,3-diphenyl-propene. 

The cis/trans isomerization of l,l-difluoro-2,3-dimethylcyclopropane (2) has an activation energy about lOkcal mol-1 lower than that of the parent hydrocarbon.5 Unlike the parent hydrocarbon, competing cyclopropane to propene rearrangement is not observed, a generalization that extends to other gem-difluorocyclopropanes studied in the Dolbier laboratories.1... 

If cyclopropane may be thermochemically compared with ethylene, then methylcyclo-propane may likewise be compared with propene. In the same way, the isomeric 1,1-, cis-1,2- and turns-1,2-dimcthylcyclopropanc 21a, 21b, 21c with X = Me) are relatable in turn to the isomeric dimethylethylenes isobutylene, (Z)- and ( )-2-butene. But what is meant by thermochemically compared We start with the observation that cyclopropane and ethylene have fortuitously nearly identical gas-phase enthalpies of formation, (53.3 0.6) and (52.5 0.4) kJmol1. Can we then conclude that gas-phase methylcyclopropane and propene (methylethylene) have nearly the same enthalpy offormation, and that the former is higher than the latter by (0.8 0.7) kJmol-1 In fact, methylcyclopropane has a higher enthalpy of formation23 than propene by 24.3 [20.0 ( 0.8)] = [4.3 ( >0.8)] kJmoT1. The two differences, [0.8 ( 0.7)] and [4.3 ( >0.8)] kJmol1, are hardly the same. 

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. 

Self-Test 13.4B Calculate the concentration of cyclopropane, C3HA (1), remaining after the first-order isomerization to its isomer propene (2) ... 

Avery (43) and Ward (71) have shown that at room temperature closely similar spectra are obtained by the adsorption of cyclopropane or propene on Pd/Si02 and Ni/Si02, respectively. Isomerization of cyclopropane to propene most probably occurs as an intermediate situation. 

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