Scrambling of methyl groups

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. 

Thus the study of chemically generated silicon ions, stimulated by earlier recoil studies that raised the possibility of ionic intermediates, has provided mechanisms for their possible participation in recoil reactions. However, it is also possible that the apparent methyl group abstraction is actually the result of insertion followed by scrambling of methyl groups and hydrogens (32). 

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. 

In contrast, an extremely low activity was observed for the gallium-modified silicalite-1. scrambling started first at 723 K, which clearly indicates that Bronsted acid sites are necessary to activate propane adsorbed on zeolites Ga/ HZSM-5 179,181. A low activity was also observed for C-2-propane adsorbed on zeolite HZSM-5 in the absence of gallium. On this catalyst, C scrambling was observed after heating at 573 K for 20 min, and the theoretical 2 1 ratio of the signal intensities of methyl and methylene groups was reached after 80 min at 573 K. 

For years most Michael reactions were carried out under protic conditions so that rapid proton transfer was possible. However, in the early 1970s several groups performed Michael reactions under aprotic conditions and these processes are now quite common. Early attempts at trapping a kinetic enolate in aprotic solvents with simple enones such as methyl vinyl ketone or an a. -unsaturated ester such as acrylate led to a scrambling of the enolate (using the Michael product as proton source).8 However, the introduction of an a-trialkylsilyl group in the enone (93 Scheme 10) permitted the trapping of kinetic... 

The 1,1-cycloaddition process also occurs in nonphotolytic reactions involving azomethine ylides. Thermolysis of oxazolinone (147) led to a 3,5-fused bicyclic dihydropyrrole in 80% yield.72 The alkene stereochemistry was maintained in the product, although subsequent photolysis scrambled the methyl and trideuteromethyl groups. Nondeuterated oxazolinone gave the cyclization product which was converted to a dihydropyridine on warming with acid.74... 

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. 

Relatively few absolute rate constants have been reported for the silene rearrangements that are known to occur at or near room temperature, such as the interconversion of 1,1-di-terf-butyl-2,2-bis(trimethylsilyl)silene (4) and its kinetically stable isomer 5 via 1,3-methyl migration (equation 6)27-29. The analogous migration in the simpler dimethyl derivative of this type (6) occurs with a half-life ti/2 30 min at 120 °C, as measured by monitoring the scrambling of the —CD3 groups in 6-d(, (equation 7)30. 

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