Atom migration

Upon prolonged standing or, more rapidly, upon refluxing for 4-18 hours, the sodium atom migrates and benzyl-sodium is formed, as is proved by the production of phenylacetic acid in good yield upon carbonation. 

The rearrangement (automerization) of Dewar thiophene 5-oxide (61), observed by NMR, occurs so much more rapidly than that of the corresponding episulfide that special mechanisms have been invoked. The one which involves a zwitterionic intermediate (Scheme 108) is favored over a pseudopericyclic sulfur-walk mechanism in which the electrons of the carbon-sulfur o--bond and the pair of electrons on sulfur exchange places as the sulfur atom migrates around the ring (80JA2861). 

The latter product is an example of a product formed by a concerted, photochemically allowed, electrocyclic reaction. A hydrogen-atom migration from a cyclopropyldimethyl... 

The cyclopropane bridge is formed only after hydrogen-atom migration. The driving force for this migration may be the fact that a more stable allylic radical results ... 

It is recalled that the elementary atomic migration by breaking bondings with surrounding atoms is also driven by thermal activation process. This is modeled through the incorporation of the activation barrier, AG, in the spin flipping event via the following equation. 

In actual oxidation, the cubic anion lattice becomes extended by the addition of new layers of close-packed 0 ions into which Fe atoms migrate to give rise to the appropriate stable structures. 

The taste of various amino acids, sugars, and aliphatic nitro compounds was studied, and it was concluded that the distance over which this hydrogen atom migrates, to give a second tautomeric form, determines the sweetness. In the case of saccharin, the sweetness was explained as due to two tautomeric forms. 

Finally, it is worthwhile to note that the methylene-transfer pathway does not involve direct transfer of a CH2 group. An H atom migration pathway is a more accurate description of this pathway, given the experimental and theoretical evidence. 

K (4). Stars show the beginning of sharp rise of the sensor signal under the influence of H-atoms migrating toward it. 

We have also observed competition between products resulting from C-C and C-H bond activation in reactions of Y with propene,138 propyne,143 2-butyric,143 four butene isomers,138 acetaldehyde,128 acetone,128 ketene,144 and two cyclohexadiene isomers,145 as well as for Zr, Nb, Mo, and Mo with 2-butyne.143 In this chapter, we use the term C-C activation to describe any reaction leading to C-C bond fission in which the hydrocarbon reactant is broken into two smaller hydrocarbon products, with one hydrocarbon bound to the metal. It is important to note, however, that C-C activation does not necessarily require true C-C insertion. As will be shown in this chapter, the reaction of Y, the simplest second-row transition metal atom, with propene leads to formation of YCH2 +C2H4. The mechanism involves addition to the C=C bond followed by H atom migration and C-C bond fission, rather than by true C-C insertion. 

Like other metal reactions studied previously in our laboratory, H2 elimination is initiated by insertion into one of the C-H bonds forming HMC3H5. The reaction rate constant for Y + cyclopropane was found to be very small at room temperature, 0.7 x 10 12 cm3 s 1, and it was suggested that the reaction most likely involved termolecular stabilization of C-H or C-C insertion complexes, rather than molecular elimination.22 By analogy with other systems studied, the dynamically most favorable route to H2 loss in this case is likely via H atom migration to the Y-H moiety, with concerted... 

These results combined with the total suppression of copolymerization in the presence of hydroquinone as inhibitor indicate that hydrostannylation takes place upon the polyaddition of diorganostannane to the epoxyolefine by a radical mechanism accompanied by hydrogen atom migration in each chain propagation, No addition of organostannanes to the oxirane ring was observed 98>. 

Silica-supported metal (e.g., Pd/Si02) catalysts also have surface silanol groups that can react with the alkoxysilane groups of the complexes. These combination catalysts consist of a tethered complex on a supported metal. A Rh complex was tethered to the surface of a Pd/Si02 catalyst, and the tethered catalyst was more active for the hydrogenation of aromatic compounds than the free complex or the supported catalyst separately.33 It is possible that the H2 is activated on the supported metal and the hydrogen atoms migrate to the silica, where they react with the reactant molecules coordinated by the tethered complex. 

Reaction of S-oxide of 1,4-dimethylthiobenzene 56 with trifluoroacetic acid provides another example of an oxygen atom migration that proceeds through a dication. The dication is in rapid equilibrium with the protonated sulfoxide 57 (Scheme 19).83... 

Most metals are covered by an oxide (impervious and insulating, or more often, as with Al, defect-ridden). In contrast, gold has no oxide, and has the advantage of making SAMs with thiols. However, Au atoms migrate somewhat after deposition to minimize total energy, and migrate even more under an electric field... 

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