Protonated cyclopropane isomers

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 presence of other hexane isomers and a typical hexane isomer distribution of 26% 2,3-dimethylbutane, 28% 2-methylpentane, 14% 3-methylpentane, 32% n-hexane, far from equilibrium, indicate that the 1-propyl cation (although significantly delocalized with protonated cyclopropane nature) is also involved in alkylation. It yields n-hexane and 2-methylpentane through primary or secondary C—H bond insertion, respectively (Scheme 5.3). 

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 alkyl carbonium ions which result from these reversible, relatively unselective hydride abstractions then undergo a series of 1,2- (Wagner-Meerwein) or 1,3- (protonated cyclopropane) rearrangements which eventually result in the formation of the thermodynamically most stable products. The number of different reaction sequences by which one may rationalize the formation of a given products is, of course, necessarily large. A variety of independent pathways are generally available for the interconversion of the isomers of a given species by successive alkyl shifts. 

The carbenium ion intermediates rearrange via hydride transfer and alkyl shift steps and through substituted protonated cyclopropane intermediates (8). All possible isomers form and the corresponding carbenium ions crack into smaller fragments, as illustrated for several isomers (10) ... 

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 ... 

There is rapid interconversion of alkenes and carbocations over acidic zeolite catalysts, and the carbocations permit skeletal rearrangements and hydride transfer reactions. These reactions proceed in the direction of formation of more stable isomers. The rearrangements probably proceed through protonated cyclopropanes (see Section 4.4.4). 

Protonated cyclopropane has been reported in the gas phase" "" to be ca 8 kcal mol" in energy above the isopropyl cation. The bent bonds of the cyclopropane ring are susceptible to electrophilic attack leading to the expectation that cyclopropane will be more basic than saturated alkanes and that protonation will occur on the C—C bond, i.e. the edge-protonated isomer will have the lowest energy. There is, however, considerable evidence from solution chemistry that corner-protonated cyclopropanes exist as intermediates in 1,2-alkyl shifts in carbocations. There have been several reviews of protonated cyclopropanes " and, in the current work, only the very recent theoretical work will be reviewed. 

The observation that the trans isomer affords a different mixture of products excluded a freely rotating, equilibrating corner-protonated cyclopropane structure. It is interesting to note that in the proposed... 

In decoupling the methyl protons, the NOE difference spectrum shows a nuclear Overhauser enhancement on the cyclopropane proton at = 1.60 and on the terminal vinyl proton with trans coupling at <5// = 5.05 and, because of the geminal coupling, a negative NOE on the other terminal proton at Sh= 4.87. This confirms the trans configuration G. In the cis isomer H no NOE would be expected for the cyclopropane proton, but one would be expected for the alkenyl-// in the a-position indicated by arrows in H. 

An interesting variant of a geometric isomerization was observed for the 7,7-dimethylbicyclo[4.1.0]hept-2-ene system. The electron transfer reaction of the highly strained rrans-fused isomer (30) with 1-cyanonaphthalene rapidly converted it to the cw-fused system (31) [224], The observed rearrangement requires inversion at one of the tertiary cyclopropane carbons. This can be accomplished either by removal of a hydrogen (proton) or by cleavage of a cyclopropane or an allylic bond. 

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