Cyclopropanes isomerization reactions

R. C. Pink (Queens University, Belfast, Northern Ireland) Turkevich has said that only the D-H exchange reaction can, with certainty, be attributed to the Lewis acid activity. The cyclopropane isomerization reaction, however, seems to respond to both the Bronsted and the Lewis activity. NH4" Y zeolites activated at different temperatures show two temperature regions of activity, one corresponding closely to the Bronsted activity of the catalyst, the other at a much higher temperature ( — 650°) corresponding to the temperature at which the electron donor-acceptor properties are at a maximum. 

P3.03.23. EFFECT OF PRESSURE ON UNIMOLECULAR REACTIONS. CYCLOPROPANE ISOMERIZATION. 

It will be noted that the isomerization to cyclopentene proceeds with a considerably lower energy of activation than the other cyclopropane isomerizations so far discussed. As a result these reactions have been investigated kinetically at temperatures about 100° lower than those not having a vinyl substituent. A number of substituted vinylcyclopropanes have been studied and the Arrhenius parameters for their isomerizations to substituted cyclopentenes determined. The results are shown in Table 4. From the results in Table 4 it can be seen that the isomerizations... 

No complete kinetic experiments have yet been described for the isomerization reactions of tricyclic systems containing two cyclopropane rings. A number of preliminary observations have been reported and some of the isomerizations noted are given below. 

Calculations based on this second model give the observed value for the entropy of activation. In addition, this model may be used to account for the observed isotope effect (Benson and Nangia, 1963). If the tetra-methylene biradical is involved then it is to be expected that appropriately substituted cyclobutanes might undergo cis-trans isomerization reactions. This will be referred to again later. One final point should be mentioned in connection with biradical intermediates in both cyclopropane and cyclobutane reactions. This concerns the absence of any effect of radical inhibitors on these systems, when it might be expected that they would interact with the biradicals. In fact calculations show that, under the conditions of formation, the biradicals have extremely short lifetimes sec) and hence, unless radical inhibitors are... 

We first describe our representation of chemical reactions. Consider the isomerization reaction of cyclopropane to propylene,... 

The results presented in this section all depend strongly on the assumption which allowed us to terminate the trajectories. For instance, certain reactive trajectories, if they were free to go on, could come back to the startii point of the reaction. Conversely, certain non-reactive trajectories, after the first process of ring opening and closure, could yield a cyclopropane molecule possessing a more suitable amount of CH2 vibration energy and the isomerization reaction could now be possible (Fig. 12a). Furthermore, the treatment of the dynamical problem in its full dimensionality might well make the unreactive region between the two reactive bands disappear. 

The dynamical study of the coupling between the modes of ring opening (and closure) and the modes of rotation of the methylene groups of a cyclopropane molecule in the course of isomerization reactions confirms essentially the two main conclusions of the static study ... 

The formation of the diene in the isomerization of vinylcyclopropane is analogous to the formation of propene from cyclopropane. The reaction has a higher energy of activation than the ring-closure reaction. Fig. 3 gives an interpretation of these results in terms of the biradical mechanism. 

Isomerization reactions of ionized cyclopropane derivatives indeed seem to be ubiquitous and many further examples are reported in Refs 49 and 50. However, a careful examination of randomly selected paperindicates that the generality of these processes is not likely to be recognized, because the results and strange interpretations presented in these articles could easily be accounted for by mechanisms described in this chapter instead of the ones originally proposed ... 

Cyclopropane isomerizes to propylene according to a first-order reaction ... 

The kinetics of the thermal cyclopropane isomerization cis-78a - trans-79a extrapolate to a half-life at 0 °C of 109 years60). In the presence of Na/K cis-78a disappears almost completely at the same temperature after 120 min clearly, this is not a thermal reaction. 

SET Induced Reactions - Further studies on ring opening reactions of 1,2-diarylcyclopropanes have focused on compounds (83) bearing an acetyl functional group.All of the cyclopropane derivatives studied show efficient cis.trans isomerism with reasonable quantum efficiency. The isomerism reaction involves an acetophenone-like triplet state with lifetimes shorter than 1 ns. The formation of ring opened products also takes place but inefficiently to give a mixture of the two alkenes (84) and (85). A study of the electron-transfer-induced photochemical reactivity of the cyclopropane (86) has been reported in the co-sensitisation system (biphenyl/phenanthrene/DCA). ... 

Photolysis of a-diazo esters in the presence of benzene or benzene derivatives often results in [2-1-1] cycloaddition of the intermediate acylcarbene to the aromatic ring, thus providing access to the norcaradiene (bicyclo[4.1.0]hepta-2,5-diene)/cyclohepta-l,3,5-triene valence equilibrium. The diverse effects that influence this equilibrium have been discussed (see Houben-Weyl, Vol. 4/3, p509). To summarize, the 7-monosubstituted systems obtained from a-diazoacetic esters exist completely in the cycloheptatriene form, whereas a number of 7,7-disubstituted compounds maintain a rapid valence equilibrium in solution. On the other hand, several stable 7-cyanonor-caradienes are known which have a second 7t-acceptor substituent at C7 (see Section 1.2.1.2.4.3). Subsequent photochemical isomerization reactions of the cycloheptatriene form may destroy the norcaradiene/cycloheptatriene valence equilibrium. Cyclopropanation of the aromatic ring often must compete with other reactions of the acylcarbene, such as insertion into an aromatic C H bond or in the benzylic C H bond of alkylbenzenes (Table 7). 

The reaction of bicyclo[4.1.0]heptane with diborane and subsequent treatment with hydrogen peroxide produced predominantly cyclohexylmethanol (5b) in high yield.Under modified experimental conditions small amounts of isomeric methylcyclohexanol (6b) and cycloheptanol (7b) were isolated. In contrast to the hydroboration of alkenes, the cyclopropane cleavage reaction is inhibited by ethereal solvents such as diethyl ether, tetrahydrofuran or 2-methoxy-ethyl ether. Bicyclo[3.1.0]hexane reacted with diborane in a similar fashion to give mainly cyclopentylmethanol (5a).The reaction of l-methylbicyclo[4.1.0]heptane gave a mixture of cis- and tram-(2-methylcyclohexyl)methanol (5c) in an initial ratio of 60 40. Spiro[2.5]octane reacted with diborane to yield only products derived from scission at the spiro carbon. The main product was 2-cyclohexylethanol (8). ... 

---