Protonated Cyclopropane 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... 

Weitkamp, J. (1980) New evidence for a protonated cyclopropane mechanism in catalytic isomerization of n-alkane, in Studies in Surface Science and Catalysis, vol. 17 (eds... 

Protonated cyclopropane has been considered to be a real intermediate since the work of Aboderin and Baird (1964), and protonated alkylcyclopropanes have also recently come to be considered intermediates as opposed to transition states in many rearrangements occurring in strong acid media where SbFs is the Lewis acid. Thus Brouwer and Oelderik (1968a) suggested that protonated methyl-cyclopropane is an intermediate in the isomerization of sec-butyl-1- C to sec-butyl-2- cations in the HF-SbFj system, and Saunders et al. (1968) implicated this species in the rearrangement of... 

Saunders and co-workers have recently reported results strongly supporting the protonated cyclopropane intermediate in the isotope scrambling process.114 They found no isomerization of the 2-butyl-1,2-13C2 cation (19a) at —78°C, whereas the isotopomer 2-butyl-2,3-13C2 cation (19b) showed rapid formation of all other iso-topomers except 19a (Scheme 3.2). These results are consistent with the involvement of protonated cyclopropane with the interchange of either C(l) and C(2) or C(3) and C(4) with the breaking of only the C(2)-C(3) bond. 

Isomerization of n-hexane 6 in superacid proceeds by three steps formation of the carbenium ion (step 1, Scheme 5.10), isomerization of the carbenium ion via hydride shift, alkyde shift, and protonated cyclopropane (for the branching step) (step 2,... 

Whereas step 1 is stoichiometric, steps 2 and 3 form a catalytic cycle involving the continuous generation of carbenium ions via hydride transfer from a new hydrocarbon molecule (step 3) and isomerization of the corresponding carbenium ion (step 2). This catalytic cycle is controlled by two kinetic and two thermodynamic parameters that can help orient the isomer distribution, depending on the reaction conditions. Step 2 is kinetically controlled by the relative rates of hydrogen shifts, alkyl shifts, and protonated cyclopropane formation, and it is thermodynamically controlled by the relative stabilities of the secondary and tertiary ions. (This area is thoroughly studied see Chapter 3.) Step 3, however, is kinetically controlled by the hydride transfer from excess of the starting hydrocarbon and by the relative thermodynamic stability of the various hydrocarbon isomers. 

The skeletal isomerization of C4 and C5 n-olefins is an acid-catalyzed reaction requiring relatively strong acid sites that proceeds via carbenium ion intermediates formed upon protonation of the double bond (17). Double bond cis-trans isomerization usually occurs on the acid sites before skeletal isomerization. The general reaction mechanism for branching isomerization is depicted in Fig. 2 2. Protonation of the double bond leads to a secondary carbenium ion, which then rearranges into a protonated cyclopropane (PCP) structure. In the case of n-butenes,... 

Cyclohexane is known to be isomerized to methylcyclopentane when catalyzed by strong acids. In fact, the SO jT rOi catalyst converts cyclohexane into methylcyclopentane and methylcyclopentane into cyclohexane [119, 142, 143]. The reactions proceed by the monomolecular mechanism via the intermediacy of secondary and tertiary carbenium ions followed by protonated cyclopropanes. 

Concerning step c, the reaction product distribution of C7-C17 n-paraffins on Pt/H-USY catalysts has led to a generalized reaction scheme (297-299) involving (1) alkyl shifts, also called type A isomerization (2) branching via protonated cyclopropane (PCP) intermediates, or type B isomerization and (3) five types of /3-scission reactions, denoted A, Bi, B2, C, and D types of hydrocracking. 

Branching isomerization of long-chain n-alkanes on Pt/H-USY zeolites occurs primarily via substituted protonated cyclopropane (PCP) intermediates (300), with a minor contribution of larger protonated rings (301). The di- and tribranched carbocations are particularly susceptible to undergo /3-scission to cracked products. Various paths of isomerization and /3-scission are outlined in Fig. 24. 

The CA spectra of gaseous ions formed by the protonation of cyclopropane and propene confirm that the isomerization of protonated cyclopropane to sec-propyl cation occurs in less than 10-5 sec31. On the other hand, the isomerization is slow in comparison to the 10-7 to 10-8 sec-between collisions in the high pressure studies mentioned above26. The apparent low activation energy suggests that the isomerization of protonated cyclopropane does not proceed by way of the n-propyl cation. 

For skeletal rearrangements over zeolite, the nonclassical protonated cyclopropane intermediate could account for the experimental observations. Theoretical studies of the reaction mechanism indicated that protonated cyclopropane-type species do not appear as intermediates but rather as transition states. Considering all zeolite-catalyzed hydrocarbon reactions (hydride transfer, alkylation, disproportionation, dehydrogenation), only carbocations in which the positive charge is delocalized or sterically inaccessible to framework oxygens can exist as free reaction intermediates. In theoretical studies on the mechanism of the superacid-catalyzed isomerization of n-alkanes (ab initio and DFT calculations), protonated cyclopropanes were found to be transition states for the branching of both the 2-butyl cation and the 2-pentyl cation. ... 

Key Words Acid Catalysts, Bifunctional Catalysts, Sulfated Zirconia, Metal-Proton Adducts, Carbenium Ions, Protonated Cyclopropane, Butane Isomerization, Collapsed Bifimctional Sites, Electron Deficiency. 

Most feeds contain some olefin as an impurity moreover many sulfated zirconia catalysts contain traces of iron or other transition metal ions that are able to dehydrogenate hutane. In the presence of such sites, the olefin concentration is limited by thermodynamics, i.e a high pressure of H2 leads to a low olefin concentration. That aspect of the reaction mechanism has been proven in independent experiments. The isomerization rate over sulfated zirconia was dramatically lowered by H2. This effect is most pronounced when a small amount of platinum is deposited on the catalyst, so that thermodynamic equilibrium between butane, hydrogen and butene was established. In this way it was found that the isomerization reaction has a reaction order of +1.3 in -butane, hut -1.2 in hydrogen [40, 41]. The byproducts, propane and pentane, are additional evidence that a Cg intermediate is formed in this process. As expected, this kinetics is typical for butane isomerization only in contrast pentane isomerization is mainly a monomolecular process, because for this molecule the protonated cyclopropane ring can be opened without forming a primary carbenium ion [42]. 

Even isomerizations of the Cg carbocation via protonated cyclopropane intermediates can lead to isomers which, after p-fission, each have two C atoms. Scrambling, leading to binomial distribution, requires that an additional condition be fulfilled an isomer which could undergo P-fission must isomerize further, before breaking into two C4 entities. This is illustrated in reaction scheme 11 ... 

Nucleophilic capture of the spirooctadienyl cation opens the 3-member ring. This behavior characterizes many reactions of many other cyclopropane-containing carbocations, as well, y-radiolysis of perdeuterated propane forms CsD ions, most of which either transfer D or form isopropyl adducts. As the propane pressure is raised from 1000 mbar to 2000 mbar, however, the isopropyl/ -propyl adduct ratio falls from 30 1 to about 5.5 1. This implies the formation of corner-protonated cyclopropane, which reacts with nucleophiles as though it were an -propyl cation. With increased pressure, vibrationally excited protonated cyclopropane experiences more frequent nonreactive collisions, which deactivate it and slow down its rate of unimolecular isomerization to isopropyl cation. 

Condon (130), in discussing the formation of 2,5-dimethylhexane from the reaction of isobutane with A1C13, postulates a protonated cyclopropane intermediate to explain the skeletal isomerization of the 2,2,4-trimethylpentyl ion, which he presumes was formed by attack of f-butyl ion on isobutylene. In this reaction the isobutylene was produced by removal of a proton from the initially formed f-butyl ion. The cyclic intermediate can be avoided if part of this isobutylene was first converted to butenyl ions which then added to the remaining isobutylene to provide the 2,5-dimethylhexyl skeleton directly. 

The interaction of many hydrocarbons (both aliphatic and aromatic) with zeolites has been investigated. HY zeolite catalyses the conversion of cyclopropane at room temperature to isobutane. The proposed mechanism involves a non-classical protonated cyclopropane ion intermediate. At 200 cyclopropane isomerizes to propene and also forms aromatic species. Adsorption and transformation of but-ene has been widely studied. It is useful to draw a distinction between hydroxylated and dehydroxylated samples. On hydroxylated samples but-ene isomerizes and also oligo-The -OH groups vibrating at 3640 cm" were found to be... 

In the case of the propyl ion, it could be shown that there is almost complete isomerization of M-C3H7 and cyclo-CsH to sec-CaHv before ion-molecule reactions can take place 221). For protonated cyclopropane, however, an alternative explanation is possible if AH/(c-C3Ht) is higher than AH/(sec-C3H7). 

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