Methyl groups, scrambling

When these labeled oxaloacetates enter a second turn of the cycle, both of the carboxyl carbons are lost as CO2, but the methylene and carbonyl carbons survive through the second turn. Thus, the methyl carbon of a labeled acetyl-CoA survives two full turns of the cycle. In the third turn of the cycle, one-half of the carbon from the original methyl group of acetyl-CoA has become one of the carboxyl carbons of oxaloacetate and is thus lost as CO2. In the fourth turn of the cycle, further scrambling results in loss of half of the remaining labeled carbon (one-fourth of the original methyl carbon label of acetyl-CoA), and so on. 

Scrambling equilibria in mixtures of dimethyl selenide and dichloro-diselenide as well as dimethyl diselenide and dichloro diselenide have been investigated recently (103). The reactions observed at room temperature to occur in such mixtures are scrambling of methyl groups and chlorine atoms on selenium as well as condensation polymerization resulting from the elimination of methyl chloride. 

As a last comment, an attempt to dehydrocouple the labeled secondary silane, PhMeSiD2, to give D(PhSiMe)xD, with Cp2ZrCl2/2 BuLi demonstrated another process that was promoted by this catalyst system.50 Analysis of the oligomer mixture produced revealed that although each oligomer had approximately two deuterium atoms (determined by mass spectroscopy), the label had scrambled between the SiH and CH (methyl group) positions as determined by H and 2H NMR spectroscopy. 

The exchange results with 4-picoline generally resemble the data for 2-picoline with some significant differences. Borohydride-reduced cobalt shows appreciable isotope scrambling at the methyl group. In this respect the reactivity of the methyl groups in the picolines is 4> 2 > 3. This may reflect the increasing participation of species (25) such as is... 

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... 

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