Carbon scrambling

De Jong, F. Sinnige, H.J.M. Janssen, M.J. Carbon Scrambling in Thiophene Under Electron Impact. Rec. Trav. Chim. Pays-Bas 1970, 89, 225-226. 

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... 

Carbon scrambling has also been observed in the cyclopentyl cation and a possible intermediate or transition state in this degenerate reaction is [22],... 

X = H] were able to demonstrate k t clled [2- C]-adamantane [196 occurred when the hydrocarbon reacLn degenerate carbon scrambling 0-1.---/i.,-. —---------.1. od with AlBrj at elevated temperatures. 

Ion [392] and the related ions [397]-[399] have been studied theoretically by MINDO/3 and perturbation methods (Jorgensen, 1977). No energy minimum was obtained for the trishomocyclopropenium ion [398]. The energy proximity of the ions in (255) suggests that complete carbon scrambling should be facile for isolated [392] in the gas phase. The energies have... 

Protonated methylcyclopropanes are considered to be involved in the carbon scrambling of the 2-butyl cation which occurs (Scheme 18) with a measured activation energy of 7.5 kcalmol" a value in good agreement with the calculated energy difference... 

When the isopropyl cation 2 was generated from 2-chloropropane with 50 percent 13C enrichment of C—2 in S02C1F—SbFs at 60 °C, equilibration of the 13C lable occurred with a half-life of 1 hour. After several hours, the 13C was distributed equally among the three carbons. This observation suggests involvement of proton-ated cyclopropanes in the carbon scrambling process (see subsequent discussion). Similar scrambling was observed in the secondary butyl 14 (sec-butyl) and r-amyl cations 4 (Saunders)33. ... 

One of the most striking differences between Winstein s and our work is the fact that the bicyclo[3.2.0] cations, reported in this chapter, are observable61. Although the [2.2.1] isomers have been observed in some cases, the thermodynamically more stable ions always turned out to have the [3.2.0] skeleton. The difference is nicely demonstrated in the five-carbon scrambling process. Winstein assumed a [3.2.0] cation 55 to be involved as an intermediate in the scrambling of the observed ion 37l3, (see also Scheme 11), whereas we assume that a [2.2.1 ] cation 56-d3 is an intermediate in the scrambling of the observed [3.2.0] carbon 51-d3 (Scheme 38). The position of the equilibrium is not dependent on the presence of the C=C double... 

The appearance of an intense M —1 peak in the mass. spectrum of benzocyclopropene was used as evidence for the formation of the benzocyclopropenyl cation upon electron impact. This view is, however, an oversimplification. It is now known from both and C labeling experiments that benzocyclopropene undergoes total hydrogen and carbon scrambling before fragmentation in the mass spectrometer. The nature of the rearranged species being unknown, the M —1 peak may not be attributed to the benzocyclopropenyl cation. 

Each of these units will still contain two C atoms no isotopic scrambling takes place. This statement is rigorously correct only if no internal rearrangement in the C4 monomers occurs prior to the formation of the Cg dimer. In reality, internal rearrangement is well documented. Its rate has, however, been found to be lower than the rate of isomerization for all catalysts. Randomization of the C label, as observed over sulfated zirconia would require an extremely fast internal atom rearrangement prior to the formation of the Cs intermediate, if only simple methyl shifts inside the dimer took place before p-fission. This model can thus be discarded. It follows that substantial carbon scrambling in the Cs carbocation is required to achieve randomization of the carbon atoms in the ultimate C4 entities. 

The reversibility of this process should lead to carbon scrambling in phenylcarbene [Eq. (3)]. 

We might further expect a Wolff-t5q)e Zf-migration interconverting the cyclo-heptatrienylidenes 30 and 31 [Eq. (4)]. The sum of Eq. (3) and Eq. (4) would be complete carbon scrambling, which has, in fact, been observed 6H. 

In the next section we shall consider some simple carbene expansions. The complete carbon scrambling is discussed in Section III.8 in connection with ring contraction. 

If carbon-scrambled phenylnitrene 166 is formed [Eqs. (61) - (62)] it must be possible to deactive some of these nitrenes and obtain scrambled aniline and azobenzene. Crow reported that aniline was not scrambled H7). To check the point, we... 

The molecular ion of 3-phenylbenzo[l>] thiophene gives the rearranged 1471 ion 2 prior to fragmentation.97 The mode of fragmentation of the 2,3-dideutero derivative suggested that all of the hydrogens in benzo[fo]thio-phene were randomized prior to the loss of ethyne from the molecular ion.98 However, later studies on benzo[b]thiophene-2-[13C] have shown that this randomization is undoubtedly due to carbon scrambling.99... 

Both vacuum pyrolysi.s and gas-phase pyrolysis give rise to some carbon scrambling [164 - 166]. for example a small amount of 5- C-azulene is obtained from vacuum thermolysis of 6- C-azulenc, which provides as the major product 6- C-naphthalene [166], These rcsult.s are explicable as follows [166] ... 

In a remarkable set of experiments, Scott subjected numerous aromatic hydrocarbons to high temperature - > I000°C - pyrolysis in a flow system and discovered carbon scrambling. With benzene-1,2-bis- C there was isomerization to the 1,3- and 1,4-isomers in a 6 1 ratio, respectively, when 72% of the 1,2-isomer was recovered (Scheme 7.10). ... 

Dimethylenecyclobutane (DMC) undergoes a degenerate carbon scrambling reaction at temperatures above 250°C at low pressures in the gas phase with log k = 14.45 - 46 800. Further, the isotopomers are formed kinetically as a 2 1 mixture of 1,3- and 3,3-shift products suggesting that an orthogonal or rapidly rotating 2,2 -bisallyl biradical is involved (Scheme 1.51) ... 

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