Equilibrating Alkyl Cations

An example of rapidly equilibrating alkyl cations is the tetramethylethyl cation 154. It shows a PMR spectrum, in which all four methyl groups are equivalent and thus does not allow a clear distinction to be made between a rapidly equilibrating pair of ions or a static hydrogen bridged ion (which may be formulated either as a a-com-plex 155 or atkenomium ion 156). 

We consequently undertook the infrared and Raman26, 34 spectroscopic study of the tetramethylethyl cation 154 and for comparisons a series of alkyl cations with known static structure, such as the f-butyl, f-amyl, and isopropyl cations 1, 3 and 2. The nearly identical spectra of the ions and the evident planarity (or close to planarity) of the carbocation centers suggest that the tetramethylethyl cation is classical , similar to the static ions used for comparison. 

The proton spectrum of the sec-butyl cation at —120 0 consists of two resonances at 6 3.2 and 6 6.7 of relative area 2 119S This is a result of a degenerate 1,2 hydride shift which at this temperature is fast with respect to the NMR time-scale. 

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In contrast to this mechanism, the one proposed in our work operates direct from the oxidation state of the alkane feedstock. The same alkyl cation intermediate can lead to both alkane isomerization (an alkyl cation is widely accepted as the reactive intermediate in these reactions) and we have shown in this paper that a mechanistically viable dehydrocyclization route is feasible starting with the identical cation. Furthermore, the relative calculated barrier for each of the above processes is in accord with the experimental finding of Davis, i.e. that isomerization of a pure alkane feedstock, n-octane, with a dual function catalyst (carbocation intermediate) leads to an equilibration with isooctanes at a faster rate than the dehydrocyclization reaction of these octane isomers (8). 

A catalytic cycle is composed of a series of elementary processes involving either ionic or nonionic intermediates. Formation of covalently bound species in the reaction with surface atoms may be a demanding process. In contrast to this, the formation of ionic species on the surface is a facile process. In fact, the isomerization reaction, the hydrogenation reaction, and the H2-D2 equilibration reaction via ionic intermediates such as alkyl cation, alkylallyl anion, and (H2D)+ or (HD2)+ are structure-nonrequirement type reactions, while these reactions via covalently bound intermediates are catalyzed by specific sites that fulfill the prerequisites for the formation of covalently bound species. Accordingly, the reactions via ionic intermediates are controlled by the thermodynamic activity of the protons on the surface and the proton affinity of the reactant molecules. On the other hand, the reactions via covalently bound intermediates are regulated by the structures of active sites. 

As mentioned earlier (Section 3.4.2), the cyclopentyl cation 33 shows a single peak in the H NMR spectrum of 8 H 4.75 even at - 150°C.143 In the 13C NMR spectrum,856 a 10-line multiplet centered around 95.4 ppm with/c H = 28.5 Hz was observed. This is in excellent agreement with values calculated for simple alkyl cations and cyclopentane and supports the complete hydrogen equilibration by rapid 1,2-shifts [Eq. (3.127)]. 

The AcBr-2AlX3 (X = Cl, Br) complexes display high activity in the alkylation of adamantane with alkanes to form poly alkylated adamantanes (Cn < C < C33) and bisadamantylalkanes (C23 < C < C50)119 [Eq. (5.70)]. The suggested pathway includes the 1-adamantyl cation and alkyl cations generated by hydride removal by the superacidic complexes. The 1-adamantyl cation then alkylates alkenes equilibrating with the alkyl cations. Various transformations may follow, resulting in the formation of additional products. 

Olah GA, Donovan DJ. C Nuclear magnetic resonance spectroscopic study of alkyl cations. The constancy of C nuclear magnetic resonance methyl substituent effects and thier application in the study of equilibrating carbocations and the mechanism of some rearrangements. J Am Chem Soc 1977 99 5026-39. 

Unsubstituted thiinium cation is stable up to pH 6 in aqueous solution at higher pH it ring opens to the aldehyde (219). Methoxide adds to 4-alkyl-2,6-diphenylthiinium cation to give a mixture of the 4-methoxy-4H- and 2-methoxy-2//-thiins under kinetic control (cf the oxygen analogues 217, 218). The mixture then equilibrates toward the thermodynamically favored 2//-system. 

Cyanide ions react with the soft alkyl halides in SN2 reactions and with the hard carbocations in SnI reactions to give, almost always, the nitrile 4.27, which is thermodynamically preferred. Isonitrile products are formed along with the nitrile products when the cation is so reactive that the rate has reached the diffusion-controlled limit, and the reversible reaction that would equilibrate the products is too slow. One consequence when reactions are as fast as this is that there is a barrierless combination of ions, and selectivity is not then controlled by the kinetic factors associated with the principle of hard and soft acids and bases. 

Three basic types of MCRs can take place.MCRs of type I are equilibria that form mixtures of reactants, intermediate products, and final products, whereas the reactants and intermediate products of MCRs of type II equilibrate, but their final products are in practice formed preferentially and irreversibly. MCRs of type III are sequences of irreversible subreactions from the reactants toward their final products. Usually, only rather low yields of products are formed by MCRs of type I. In 1850, StreckerP introduced the formation of a-amino-substituted alkyl cyanides 3 from ammonia, hydrogen cyanide, and the corresponding carbonyl compound 1, via the cationic intermediate 2 (Scheme 1). The introduction of the Strecker three-component reaction (S-3CR), an MCR of type I, heralded the beginning of MCR chemistry. 

We have previously mentioned (p. 236) that stable tertiary carbocations can be obtained, in solution, at very low temperatures. The NMR studies have shown that when these solutions are warmed, rapid migrations of hydride and of alkyl groups take place, resulting in an equilibrium mixture of structures. For example, the tert-pentyl cation (5) equilibrates as follows ... 

Protonation and alkylation of arenes afford cyclohexadienyl cations (arenium ions) which are also of importance in electrophilic aromatic substitution. The hepta-methylbenzenium ion (16) is a very stable species30-, but even the parent benzenium ion (17a) has been observed as have most of its alkyl, halo, and alkoxy derivatives51. The benzenium ion (17a) undergoes a rapid degenerate rearrangement which equilibrates the seven protons over six carbons. Data for the monosubstituted benzenium ions show that (17) is the most stable of the possible isomeric forms. Positive charge... 

For the norbomyl cation (22), on the other hand, the average 13C chemical shift for C—1,2 was found at 126.1 ppm (at -150°C where all other rearrangements except for the degenerate alkyl shift are frozen out )55)- Thus, C-1,2 absorb by ca. 80 ppm further upfield that the corresponding carbons of the equilibrating ion (25). A similar discrepancy (85 ppm) is estimated on the basis of isopropyl cation and C—1 of norbomane as a model. Some doubt, however, remains whether these acyclic ions are good approximations of the hypothetical 2-norbornyl cation. 

It is surprisingly also possible to trap an external electrophile in this kind of sequence. The amino alcohol 122 from phenylglycine (chapter 23) was alkylated and the unsaturated amine 123 combined with glyoxal. This gives an equilibrating mixture of various cations such as 124 and 125. If a large excess of a nucleophile such as azide ion is present, bicyclic products like 126 are formed in reasonable yield.19... 

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