2- Butyl cation 1,2-hydride shift

In the case of the -butyl cation, a shift of hydrogen yields the more stable m-butyl cation migration of an ethyl group would simply form a different -butyl cation. In the case of the 2-methyl-1-butyl cation, a hydride shift yields a tertiary cation, and hence is preferred over a methyl shift, which would only yield a secondary cation. In the case of the 3,3-dimethy 1-2-butyl cation, on the other hand, a methyl shift can yield a tertiary cation and is the rearrangement that takes place. 

The 2-butyl cation can be observed under stable-ion conditions. The NMR spectrum corresponds to a symmetrical species, which implies either very rapid hydride shift or a symmetrical H-bridged structure. 

The isobutyl cation spontaneously rearranges to the tart-butyl cation by a hydride shift. Is the rearrangement exergonic or endergonic Draw what you think the transition state for the hydride shift might look like according to the Hammond postulate. 

When -butenes are used, the initiation produces a secondary carbenium ion/butoxide. This species may isomerize via a methyl shift (Reaction (2)) or accept a hydride from isobutane to form the tertiary butyl cation (Reaction (3)). Isobutylene forms the tertiary cation directly. 

This reaction gives two products 21 and 22 but neither contains the /-butyl group. Both contain instead the z-butyl group. The intermediate complex rearranges by hydride shift 19 into the t-butyl cation 20 as the primary cation 17 is too unstable. 

Protonated pyrroles are currently believed to be kinetic intermediates, precursors of a-protonated species. However, it has been found that some A -vinylpyrroles in superacids afford exclusively 3-protonated species (Scheme 65) <1998MI30>. For example, 2-/-butyl-l-vinylpyrrole 288 in the superacid at —70°C gives a stable cation 289 having the proton attached to the position 3 and the double bond intact. It requires several hours at 0 °C to rearrange this ion, via a [l,2]-hydride shift, to the a-protonated isomer 290 with an unexpected structure (with a proton at the position where the highly branched substituent is attached). 

Sometimes intermediates like [6] undergo hydride shift to another comer protonated cyclopropane (7). This is another mechanism by which carbon scrambling can be achieved, i.e. through a combination of methide- and hydride shifts. Together with 1,2-hydride shifts, such a mechanism accounts for the complete degeneracy of the 2-butyl cation [7]. Vinyl cations substituted... 

Degenerate 1,2-hydride shifts as in 2,3-dimethyl-2-butyl cation [10] take place with low barriers, ca 3-4 kcal mol". The 1,3-hydride shift in 2,4-dimethyl-2-pentyl cation [11] also takes place through a nonlinear transition state (TS) and with a higher barrier = 8.5 kcal mol" (Brouwer and van... 

This ion appears to have a classical structure, which is in equilibrium with a bridged ion under conditions of low nucleophilicity, but undergoes extremely fast 1,2-hydride shifts which have not been frozen out. It also shows complete hydrogen and carbon degeneracies which most probably involve corner protonated cyclopropane intermediates. At higher temperatures it rearranges to t-butyl cation. 

The 2-methyl-2-butyl cation [68] also shows degenerate properties. Prepared by hydride abstraction from n-pentane or isopentane in FSOjH-SbFj, its H-nmr spectrum was observed by Olah and Lukas (1967). The temperature dependence of the spectrum indicated scrambling of the methyl groups. A mechanism was proposed with an initial 1,2-hydride shift to the secondary... 

The isobutyl cation spontaneously rearranges to the tert-butyl cation by a hydride shift ... 

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. 

Fast Wagner-Meerwein rearrangement and degenerate 1,2-hydride shifts have been extensively investigated under superacid conditions to probe the nature of intermediate carbocations. The 2-butyl cation (3) has been prepared from 2-chlorobutane in SbF5-SO2ClF at -100°C in a vacuum line by Saunders et al. 2 with very little contamination from the rprFbutyl cation (4) (Scheme 6.13). 

Initiation (or olefin protonation) In this step, a f-butyl cation is formed from isobutene. A sec-butyl cation is formed from 1-C4= or 2-C4=. The sec-butyl cation can form a f-butyl cation by methyl shift, or it can undergo hydride transfer from isobutane, forming n-C and a t-butyl cation. 

The 2-butyl cation has been extensively investigated both computationally and experimentally. The 2-butyl cation can be observed under stable ion conditions. C(2) and C(3) are rapidly interconverted by a hydride shift. The NMR spectrum corresponds to a symmetrical species, which implies either a very rapid hydride shift or a symmetrical H-bridged structure. A maximum barrier of 2.5kcal/mol for hydride shift can be assigned from the NMR data. °... 

The occurrence and extent of rearrangement of the 2-butyl cation during solvolysis has been studied using isotopic labeling. When 2-butyl tosylate is solvolyzed in acetic acid, only 9% hydride shift occurs in the 2-butyl acetate that is isolated.Thus, under these conditions most of the reaction proceeds by direct nucleophilic participation of the solvent. 

It is not excluded that cyclic intermediates are formed in the process of 1,2-hydro-gen shifts as well. For instance, according to the authors of Ref. from the analysis of tlK NMR spectra it follows that the 1,2-hydride shift in jec-butyl cation occurs with the formation of an interm liate bridged ion ... 

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