Degenerate hydride shifts

In contrast to the isomerization of 39, the retention of optical activity for the conversion of 41 to 42 is very small. However, the loss of activity cannot be explained by a degenerate vinylcyclopropane rearrangement, since the recovered starting material ([(X]54g = +32 ) shows essentially complete retention of chirality. An alternative explanation involves competing stereoselective processes in a chiral intermediate (41a ). For 41a a hydride shift from the allylic position, C2, might compete with the migration from C6 to Cl. This possibility can be probed by... 

Rapidly equilibrating (degenerate) carbenium ions are characterized by averaged carbon-13 shifts and carbon-proton coupling constants. 2,3-Dimethylbutyl in comparison with 2-propyl cation is an example [497]. Only two instead of four signals are observed at — 80 °C due to a rapid 1,2-hydride shift. 

Carbon shifts 228 U-, 1,3-, 1,4- and 1,5-Hydride shifts 229 Homoallylic-, cyclopropylcarbinyl- and homocyclopropylcarbinyl rearrangements 230 Other degenerate reactions 231... 

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... 

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 2-brexyl cation (2-tricyclo[4.3.0.0 - ]nonyl cation, [215]) has an intriguing structure containing two norbornyl moieties. Of the two obvious rearrangement paths possible for ion [215], Wagnei-Meerwein shifts (139) and 1,3-hydride shifts (140), only the former is degenerate (Nickon et ai, 1965). The latter gives the isomeric 4-brexyl cation [216], which can undergo further... 

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). 

Furthermore, Yannoni and coworkers succeeded in freezing out the degenerate hydride shift in the 34 cyclopentyl cation in the solid state at -203°C. The observed chemical shifts at 5 C 320.0,71.0, and 28.0 indicate the regular trivalent nature of the ion and are in good agreement with the estimated shifts in solution based on the average shift data. 

Model calculations of the dimethyl vinyl cation system at the MP2/6-31G level show that the hydrogen-bridged cation is more stable than the secondary cation and that the barrier for the degenerate hydride transfer is very low (< 1 kcalmol Fig. 11). The isomeric allylic cation, which could be generated by a 1,2-hydride shift across the single bond of the secondary vinyl cation, is much more stable than the vinyl cation, but the barrier for this 1,2-hydride shift is very high since allylic conjugation cannot contribute to the stabilization of the transition state for the hydride shift. 

The effects ob rved might be gssociat l with the d a ase in the el tron deficiency of the carbonium centre in the above series of carbocations, but a comparison of these data with those on the rates of degenerate 1,2-hydride shifts in some other carbocations suggests that other factors, too, seemingly steric, act here. Indeed, as distinct from ion (64c), the 9-H-9,10-dimethylphenanthrenonium ion (JSb) undergoes a very rapid rearrangement by 1,2-hydride shift 10 s , AG so-c 8.5... 

According to the authors of Ref. in passing from the gas phase to the solution the rates of degenerate 1,2-hydride shifts in ethyl and cyclohexadienyl ions should decrease due to more effective solvation of the carbocation in the ground state, see also Ref. cf., however. Ref. This conclusion agrees with the results of quantum chemical calculations. The experimental data available, however, are either insufficient for verifying this conclusion or contradict it. For example, for the 1,2-hydride shifts in the ethyl cation only the upper limits of the activation barrier are known g 1.9kcal/mole for the solution in SbF,—SO (cf. Ref. >)... 

The p-hydrido bridged cation [90] is equivalent to a resonance system such as exists in allyl cations (single energy minimum) and can be distinguished from a set of two degenerate cycloalkyl cations [88] undergoing a fast hydride shift (double energy minimum) as shown in (70) by comparison with suitable model cations. 

In fact, if the equilibration occurred only by a consecutive Wagner-Meerwein shift/6,2-hydride shift/reverse Wagner-Meerwein shift (82) between [109] and [112] (Sorensen et al., 1972), the C-5,C-7 positions would be symmetrical in the degenerate cations. Deuterium at either the C-5 or C-7 position should lack an equilibrium isotope effect in the equilibration process. 

The same direction of the equilibrium isotope effect was observed in the nondegenerate 1,2-hydride shift equilibrium of 2-(4 -trifluoromethyl-phenyl)-3-methyl-2-butyl cation [142] with one trideuteriomethyl group at C-2 or C-3 respectively (Forsyth and Pan, 1985). The isotope shifts in the spectrum are much smaller (1.3 ppm-1.45 ppm) than in degenerate cations like [141] because Ky is very much in favour of the benzylic cation structure for [142]. 

Recently Saunders and Kates (1978) have been successful in measuring the rates of degenerate 1,2-hydride and 1,2-methide shifts of several simple tertiary alkyl cations employing high field (67.9 MHz) C-nmr spectroscopy. From band broadening in the fast exchange limit the free energies of activation (AC ) were determined to be 3.1 0.1 kcal mol at —138°C for [10] and 3.5 0.1 kcal mol at —136°C for [5]. 

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