Hydrogen shifts degenerate

The degenerate [1,5] hydrogen shift reaction of (Z)-l,3-pentadiene has been studied by a number of ab intio [67-69] and semiempirical [70-72] methods. Since the energy of concert [73] for this reaction is approximately 40 kcal/mol, there is no doubt that this reaction has a concerted mechanism with an aromatic... 

Saunders and Stofko (1973) also studied the hydrogen shift of this next higher homologue ion [13] under conditions of long life. The H-nmr spectrum of [13] shows a single methyl peak at —95°C, indicating rapid degenerate... 

The 2,5-difluorobenzenium ion [298] was studied by Olah and Mo (1973) who observed degenerate intramolecular 1,2-hydrogen shifts. Above 0°C an intermolecular reversible deprotonation occurred equilibrating the H, and Hb protons thus making all protons of [298] equivalent. However, below... 

Alkyl and hydrogen shifts are normally fast on the NMR time scale down to -150°C. Selected degenerate rearrangements have been discussed in the preceding paragraph. Additional examples include the 2-butyl cation (34), in which the secondary-secondary hydrogen shift is faster than 104 sec 1at -130°C (AG <5.5 kcal/... 

The NMR spectrum of 2,4-dimethyl-2-pentyl cation (42) in the temperature range from -110 to -70°C indicates rapid exchange of the nonequivalent methyl groups (Ea=8.5 kcal/mol)71. As the signal of the methylene protons remains unaffected, a direct 1,3 shift of hydrogen is the most plausible mechanism. Similar degenerate 1,4- and 1,5-hydrogen shifts have been shown to occur in (43) and (44), respectively. The relative rates decrease in the order 1,5>1,3>1,4—H shift. 

The cisjtrans isomerization which accompanies the 1,5-hydrogen shift in trans-X -methyl-2-vinylcyclopropane is degenerate and hence invisible in l,l-dimethyl-2-vinylcyclopropane this hydrocarbon has been shown to isomerize thermally to (Z)-2-methylhexa-1,4-diene with log A = 11.41, and 33.5 kcal moP ) ... 

The simplest sigmatropic hydrogen shift reaction is the degenerate positional migration of a hydrogen atom across the allylic carbon skeleton in propene from C-l to C-3 (1 < 2), and vice versa. 

The value of the intrinsic barrier is naturally assumed to grow with increasing steric strain in the transition state of rearrangement. It is probably this factor which accounts for the retardation of the degenerate 1,2-hydrogen shift in the ac naph-thylenonium ion (SOa) ... 

According to the available data the medium change insensibility of the rate is also characteristic of the degenerate 1,2-hydrogen shifts. For instance, the rearrangement rate of the hexamethylbenzenonium ion and some other benzenonium ions is the same for the acid systems HF—BFj and HF—SbFj The rearrangement... 

If Ri = R3 and R2 = R, then the structure of the ion before and after the rearrangement is the same, so such processes are termed d enerate. The principal tool for fast degenerate processes is at present the dynamic NMR. Its aj lication to ionic systems started from the study of fast 1,2-hydrogen shifts. Conader, e.g., the changes in the spectrum of the l-H-l,2,3,4,S,6-hexamethylbenzaiium km generated in HF—BF3 At —85 °C the PMR spectrum of this cation (see Fig. la)... 

In protonated o-xylene and o-difluorobenzene, the degenerate 1,2-hydrogen shifts have about the same rates... 

Later it was shown that taking in addition the data on the degenerate 1,2-hydrogen shift in the 2-H-l,2,3,4-tetramethylnaphthalenium ion and in the l-H-l,2-di-methyl-3,4,5,6-tetrachlorobenzenium ion yields the following expresaon ... 

Just as in degenerate rrarrangements due to the 1,2-hydrogen shifts, the temperature changes in the NMR spectra reveal the fast reversible rearrangements of ions connected with the shifts of other substituents. The first to be studied was the degenerate rearrangement of the heptamethylbenzenium ion (see also... 

The data obtained from the degenerate rearrangements of benzenium ions testify that the rate coefficients of intraionic 1,2-hydrogen shifts are usually by several... 

An explanation for the orbital symmetry-forbidden stereochemistry of the [1,51-sigma-tropic shift of substituted norcaradienes has been reported, and ab initio methods have been used to study the [l,5]-hydrogen shift in cycloheptatriene and the [1,51-carbon shift in norcaradiene. A degenerate rearrangement consisting of a formal... 

The degenerate [l,5]-hydrogen shift reaction of (Z)-l,3-pentadiene is one of the classical examples of a concerted mechanism with a aromatic transition state. The energy of concert for this reaction has been estimated to be about 40 kJ mol . Because of this unequivocally concerted character of the reaction, it has been studied as a model system by a number of ab initio and semiempirical methods. 

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