Ethyl shift

Pines and Csicsery (90, 90a) proposed three and/or four-membered cyclic intermediates in the isomerization of various branched alkanes over non-acidic chromia-alumina. A similar, 1,3-methyl shift has recently been reported with an oxygenated reactant (tetramethyloxetane) over supported Pt, Pd, and Rh (90b). Future experiments are necessary to elucidate whether hydrocarbons, too, can form C4 cyclic intermediates over metal catalysts. Some products assumedly formed via ethyl shift could be interpreted by C4 cyclic isomerization. 

When 3-methyl[3-13C]pentane (17 ) is isomerized to 2-methylpentane, the label distribution shows that the isomerization cannot be explained by a simple 1,2-methide shift 30% of the 2-methylpentane has the 13C label in a position that can be best explained by the ethyl shift (Scheme 5.12). The recovered 3-methylpentane (96%) also shows a very large degree of internal shift [Eq. (5.45)]. 

The rate of ethyl shift in this reaction is three times as high as the apparent rate of conversion of 3-methylpentane to 2-methylpentane. A simple calculation shows that... 

Results of similar experiments by Gault and his co-workers (93) with a 10-wt % Pt/Al203 catalyst (mean crystallite size 150-200 A) required the assumption that several successive rearrangements took place in the adsorbed phase before desorption. A model was developed in which either a dehydrocy-clization-hydrogenolysis event or a methyl or ethyl shift involving a tertiary atom competed with desorption. By assuming that the isomeric hexanes had the same desorption probability (d) and the different bond-shift processes proceeded with the same chance (r), it was found possible to reproduce the observed initial product distributions with these two independent parameters. In general, values of d 0.5 and t = 0.10-0.20 fitted the results best. As an additional refinement, the ratio of the C2—C3 and C3—C4 bond scission probabilities for methylcyclo-pentane (0) was taken to be 3.3, rather than the statistical value of 2, to improve further the fit. 

The deamination of the diastereomeric 2-amino-l-chloro-3-methylpentanes (222)203 provides data on a purely aliphatic system. Although hydrogen migrates preferentially the ratio of methyl and ethyl shifts depends strongly on the choice of... 

Additional evidence for open secondary cations comes from the stereochemistry of 1,2-alkyl shifts from a secondary to a primary carbon. The ethyl shift induced by diazotization of 2-methyl-butylamine (635) gives 2-pentanol (640) with 54% inversion of configuration469. 69% inversion was observed in the formation of 3-hexanol (644) from 2-methylpentylamine (641) by a 1,2-methyl shift while the 1,2-propyl shift leading from (641) to 2-hexanol (648) proceeds with 65% inversion457. (These... 

Proton shifts within protonated cyclopropanes also explain the rather low stereo-specificity at the origin of 1,2-ethyl shifts. (15, 2S)-(661) gives 3-ethyl-2-pentanol (674) with 54% inversion and with substantial redistribution of a label, as required by the conversion of the protonated cyclopropane (672c) to its enantiomer (tf72d)4S7-47a ... 

A slight modification of this mechanism, involving a dissociatively adsorbed cyclopropane intermediate (63) and stabilization of this intermediate by methyl substituents explains the predominance of ethyl shift (Path B) over methyl shift (Path A) in the isomerization of isopentane to n-pentane (40) (Scheme 20). 

The Fig. 15 indicates the change in the number of species generated (paraffins, olefins, ions, in Fig. 15a) and the number of reactions (hydrogenations, pro-tonations, HS, MS, ethyl shiftes (ES), PCP branching, PCB branching, beta scissions, in Fig. 15b) according to the number of carbons of a single hydro-cracked normal paraffin. 

Side chains. The speed of the hydride, methyl and ethyl shift reactions is the basis of the lumping by number of carbon atoms and by number of branches. Consequently, as soon as a compound with a given number of branches is formed, the other isomers with the same number of branches are formed immediately. This is also true for the side chains of an activated complex, enabling us to define the notion of equivalent side chains. 

As expected, within the limits of the alkyl series the effects are not so great, the ethyl group displays an accelerating effect on the methyl migration (AAG 78-c = 1.2 kcal/mole) in comparison with the methyl one as has been established for ions (cf. similar data for the 1,2-ethyl shift in ions (20) as well as those... 

Judging by the estimates made in Ref. the pushing effect of the ethyl group on the 1,2-ethyl shift in 9,9,10-trialkylphenanthrenonium ions (the alkyls are CHj and CjHj) is also far lower than that of the methyl one. A similar order of influence seems to be also characteristic of the 1,2-methyl shift in ions of this type (cf. the data of Ref. and 2.244) ... 

Note also, that the 1,2-ethyl shift does not involve the states that could result in the randomization of the hydrogen atoms of the methyl and methylene fragments. This follows from the retention of multiplicity of the signals of the ethyl... 

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