Hydrogen atom migrations

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 ... 

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. 

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. 

As already noted, the carbonylation of bis(pentamethyl-cyclopentadienyl) actinide hydrocarbyls is irreversible in the cases studied thus far. Thus, thermolysis does not result in CO loss, but rather in interesting chemical reactions. Thermolysis of 1 ( 5) in toluene solution results in hydrogen atom migration to yield an enolate (eq.(5)). NMR studies establish that eq.(5) is essentially quantitative, and that the stereochemical course... 

The hydrogen atom migration observed on thermolysis of is reminiscent of 1,2-hydrogen atom migrations in carbene chemistry (45,46,47). The stereochemistry of such processes is now relatively well-understood and involves initial hyperconjugative interaction between a gauche C-H bond and the carbene unoccupied p atomic orbital, followed by a low activation energy 1,2 shift (eq.(6)) ( 7,48,49,50). 

Application of this picture to the present thorium acyl system (J,) suggests that a hydrogen atom migration to the electron deficient carbenoid p orbital must occur from a conformation (J) other than that found in the crystal structure (H) (Figure 1) to yield the cis product. These relationships are illustrated in eqs.(7) and (8) ... 

A [1,3]- or [1,5]-H shift is formally required for the rearrangement of 162 to benzene (Scheme 6.40). Quantum-chemical calculations predict that the hydrogen atom migrates in two steps, that is, in consecutive [1,2]-H shifts, with the species 177 being the intermediate, which has to be described either as a diradical [117] or a car-bene [116, 117]. The experimental activation enthalpy for the conversion of 176 into benzene [112] was correctly simulated by the energy of the transition state separat-... 

The isomerization processes described above do not imply a change in the oxidation state of the metal but only a reorganization of the electron density within the organic ligand with the hydrogen atom migrating as a proton. This is particularly favored for d6 metal centers not prone easily to oxidation (for instance Ru11 RuIV). 

In contrast to hydrocarbon radicals where hydrogen atom migration is common, fluorine atom migration is rare.105 Pcrfluoroalkyl radical disproportionation is as yet unknown and cannot... 

Figure 4.42 Supra (1) and antara (2) pathways of hydrogen atom migration as an example of a sigmatropic isomerization...

The initial reaction of C11 with olefins is perhaps an addition to the double bond followed by electronic rearrangement, rupture of a C—C bond, and hydrogen atom migration in the resulting hot intermediate to give the observed products. Further experimental work may provide conclusive evidence for such a mechanism. 

The reaction of t with 1-methylimidazole is reported by Kaupp and Gunter (136) to yield the addition product 77. Unlike the reaction of It with 1-methylpyrrole, the formation of 77 does not require acid catalysis. The formation of 77 is proposed to occur via the 1,2-biradical intermediate 78, which undergoes a 1,3-hydrogen atom migration to yield 77 (eq. 27). The reaction of It with benzothiazole also yields an acyclic adduct 79 and a second adduct 80 as the major products. Both 79 and 80 are proposed to arise via a 1,4-biradical intermediate similar to 78. Irradiation of t-1 with caffeine (81) yields a [2+2] cycloadduct 82 and an acyclic adduct 83, analogous to 80, as the major products. 

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