Cyclopropane protonated intermediate

A number of other reactions have been postulated to involve protonated cyclopropanes as intermediates.122 For example, nmr studies of the, e-butyl cation in superacid show that from — 100 to -40°C a process, with an activation energy of 7 kcal mole-1, occurs that scrambles all the protons. The activation energy is too low for the scrambling to occur by the mechanism shown in Equa-... 

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 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... 

Also, he introduces an additional ion III which he calls C-ethenemethonium (methyl-bridged) ion. The latter, he believes, is not formed on cyclopropane protonation but results as intermediate in a rearrangement of the initially formed ions of type II. Ion III, unlike the two others, >ntains tetracoordinated methine atoms and a pentacoordinated carbon of the methyl group. Olah gives the following equilibrium scheme of cyclopropane protonation ... 

Protonated cyclopropane (PCP) intermediate mediated branching of carbenium ion on acid sites,... 

Interconversions of 2-methylpentane 2,3-dimethylpenttuie as well as 3-methylpentane 3= 2,3-dimethylpentane proceed probably via protonated cyclopropane-type intermediates (IV) ... 

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

Along with the minimal barrier for H shift, the 2-butyl to t-butyl rearrangement gives the energy surface shown in Fig. 5.9. This diagram indicates that the mechanism for C-3/C-4 scrambling in the 2-butyl cation involves the edge-protonated cyclopropane intermediate. 

These results indicate an energy profile for the 3-methyl-2-butyl cation to 2-methyl-2-butyl cation rearrangement in which the open secondary cations are transition states, rather than intermediates, with the secondary cations represented as methyl-bridged species (comer-protonated cyclopropanes) (Fig. 5.10). 

Even more scrambling was found in trifluoroacetolysis of 1-propyl l- " C-mercuric perchlorate. " However, protonated cyclopropane intermedi... 

It is likely that protonated cyclopropane transition states or intermediates are also responsible for certain non-1,2 rearrangements. For example, in superacid solution, the ions 14 and 16 are in equilibrium. It is not possible for these to interconvert solely by 1,2 alkyl or hydride shifts unless primary carbocations (which are highly unlikely) are intermediates. However, the reaction can be explained " by postulating that (in the forward reaction) it is the 1,2 bond of the intermediate or transition state 15 that opens up rather than the 2,3 bond, which is the one that would open if the reaction were a normal 1,2 shift of a methyl group. In this case, opening of the 1,2 bond produces a tertiary cation, while opening of the 2,3 bond would give a secondary cation. (In the reaction 16 14, it is of course the 1,3 bond that opens). 

However, the same reaction applied to 2-methyl-2-butanol gave no 32, which demonstrated that 35 was not formed from 34. The conclusion was thus made that 35 was formed directly from 33. This experiment does not answer the question as to whether 35 was formed by a direct shift or through a protonated cyclopropane, but from other evidence" it appears that 1,3 hydride shifts that do not result from successive 1,2 migrations usually take place through protonated cyclopropane intermediates (which, as we saw on p. 1382, account for only a small percentage of the product in any case). However, there is evidence that direct 1,3 hydride shifts by way of A may take place in superacid solutions." ... 

A mechanistic interpretation for the formation of 35 is depicted in Scheme 5. Deprotonation of an allylic proton yields ylide intermediate 36. This then adds to methyl acrylate to give intermediate 37, which cyclizes to construct a cyclopropane ring together with the fission of the S-C bond to afford the final adduct 35. 

The Lewis acid-Lewis base interaction outlined in Scheme 43 also explains the formation of alkylrhodium complexes 414 from iodorhodium(III) meso-tetraphenyl-porphyrin 409 and various diazo compounds (Scheme 42)398), It seems reasonable to assume that intermediates 418 or 419 (corresponding to 415 and 417 in Scheme 43) are trapped by an added nucleophile in the reaction with ethyl diazoacetate, and that similar intermediates, by proton loss, give rise to vinylrhodium complexes from ethyl 2-diazopropionate or dimethyl diazosuccinate. As the rhodium porphyrin 409 is also an efficient catalyst for cyclopropanation of olefins with ethyl diazoacetate 87,1°°), stj bene formation from aryl diazomethanes 358 and carbene insertion into aliphatic C—H bonds 287, intermediates 418 or 419 are likely to be part of the mechanistic scheme of these reactions, too. 

It is supposed that the nickel enolate intermediate 157 reacts with electrophiles rather than with protons. The successful use of trimethylsilyl-sub-stituted amines (Scheme 57) permits a new carbon-carbon bond to be formed between 157 and electrophiles such as benzaldehyde and ethyl acrylate. The adduct 158 is obtained stereoselectively only by mixing nickel tetracarbonyl, the gem-dibromocyclopropane 150, dimethyl (trimethylsilyl) amine, and an electrophile [82]. gem-Functionalization on a cyclopropane ring carbon atom is attained in this four-component coupling reaction. Phenyl trimethyl silylsulfide serves as an excellent nucleophile to yield the thiol ester, which is in sharp contrast to the formation of a complicated product mixture starting from thiols instead of the silylsulfide [81]. (Scheme 58)... 

Table II also lists several isomerizations and skeletal rearrangements (examples 4-7) which are related to butadiene-ethylene dimerization. Protonation of phosphorus-containing nickel(O) complexes is sufficient to achieve skeletal rearrangement of 1,4-dienes in a few seconds at room temperature, probably via cyclopropane intermediates (example 6, Table II). For small ring rearrangements see Bishop (69).

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... 

Figure 16. Protonated cyclopropane intermediates 28 from 2-octyl cation.

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