Protonated cyclopropyl intermediate

A fourth mechanism has been suggested by several researchers in order to explain the formation of near-linear polyethylene type oligomers produced on modified[22] and unmodified1231 ZSM-5 zeolite from olefins such as propene, isobutene and 1-decene. They propose a mechanism which proceeds by losing methyl branches via protonated cyclopropyl intermediates as presented in Scheme 6.4. 

The fourth mechanistic approach was suggested by various investigators, and it explains the linear structure of the products obtained by the olefinic oligomerization reaction.The authors have proposed a scheme for an oligomerization reaction in the propene HZSM-5 system, proceeding via protonated cyclopropyl intermediates as shown in Figure 3. " ... 

Synthetic oils with high viscosity indices were produced in the oligomerization of isobutene using HZSM-5 catalyst, surface-deactivated with 2,6-di-/ert-butyl-pyridine. Chen and Bridger proposed a protonated cyclopropyl intermediate for the isobutene-HZSM-5-system in the formation of linear oligomers. 

Ciner, J.-L. and Djerassi, C. (1991) Biosynthetic studies of marine lipids. 31. Evidence for a protonated cyclopropyl intermediate in the biosynthesis of 24-propylidenecholesterol. I. Am. Chem. Soc., 113, 1386-1393. 

A mechanism for the origin of skeletal isomers observed in the cationic dimerization of 1-decene is proposed to involve a protonated cyclopropyl intermediate formed directly by reaction of a 2-decyl carbocation with 1-decene. This intermediate is said not to occur via ring closure of a branched secondary carbocation. Rapid repeated isomeriza-tions of the protonated cyclopropyl intermediates lead to a multitude of skeletal isomers (Scheme 77). ... 

Spirodienone 28 arose through decarboxylation of the corresponding P-keto acid 27, which was obtained by proton loss from C9 and concomitant cleavage of the 1,10-bond — a process that would be favored in the chelated conjugated cyclopropyl intermediate 26b (Scheme 4). 

When dienones 39 and 40 are photolyzed in sulfuric acid they both rearrange to the same product, 2-methyl-5-hydroxybenzaldehyde (41) (Filipescu and Pavlik, 1970). The mechanism for this photorearrangement is consistent with that of the protonated cyclohexadienones already discussed, i.e., disrotatory closure to afford the intermediate bicyclic cations 42 and 43. In this case it is conceivable that the electron-withdrawing effect of the dichloromethyl group forces the subsequent thermal cyclopropyl migration entirely in the direction of the most stable cation 44 to yield the observed product. 

The elusive diazoalkenes 6 and 14 are unlikely to react with methanol as their basicity should be comparable to that of diphenyldiazomethane. However, since the formation of diazonium ions cannot be rigorously excluded, the protonation of vinylcarbenes was to be confirmed with non-nitrogenous precursors. Vinyl-carbenes are presumedly involved in photorearrangements of cyclopropenes.21 In an attempt to trap the intermediate(s), 30 was irradiated in methanol. The ethers 32 and 35 (60 40) were obtained,22 pointing to the intervention of the al-lylic cation 34 (Scheme 10). Protonation of the vinylcarbene 31 is a likely route to 34. However, 34 could also arise from protonation of photoexcited 30, by way of the cyclopropyl cation 33. The photosolvolysis of alkenes is a well-known reaction which proceeds according to Markovnikov s rule and is, occasionally, associated with skeletal reorganizations.23 Therefore, cyclopropenes are not the substrates of choice for demonstrating the protonation of vinylcarbenes. 

On the other hand, cyclization of 6-cyclopropyl-5-hexenyllithium, derived from iodide 71, is accompanied by ring opening of the three-membered ring in the intermediate organolithium 72. However, in this case it is not possible to functionalize the putative 3-butenyllithium intermediate, probably due to competing protonation by solvent (Scheme 19)45. 

In CIDNP studies the pattern of nuclear spin polarizations is used to deduce the structures of free radical intermediates, and if there is a simultaneous NOE this can affect the conclusions drawn. (344) In the photolysis of [10] to yield [12] the biradical intermediate [11] should not cause significant polarization of the olefinic protons of [10] but in fact these are observed to have weak emission. This raises the possibility that the intermediate is really the biradical [13], but a homonuclear double resonance experiment which destroyed the cyclopropyl spin polarization of [10] eliminated the olefinic emission which was evidently solely due to the NOE. Thus it is confirmed that [11] is indeed the intermediate in the reaction. (344)... 

Consequently the formation of both these compounds can be visualized by the pathways shown on the previous page (with R changing from methoxycarbonyl to the carboxy functionalities in the final products). Direct attack of the intermediate cyclopropylmethyl cation C by water and loss of a proton leads to the nonisolated intermediate E which subsequently lactonizes as a consequence of the cis relationship of the cyclopropyl substituents. Similarly, the indirect attack of the nucleophile leads to the nonisolated hydroxy acid corresponding to F which on workup is transformed into a tetrahydrofuran derivative. 

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