Intermolecular hydrogen transfer

In this case the parameters C and Q are of order of unity, and therefore they correspond to the intermediate situation between the sudden and adiabatic tunneling regimes. Examples are mal-onaldehyde, tropolon and its derivatives, and the hydrogen-oxalate anion discussed above. For intermolecular transfer, corresponding to a weak hydrogen bond, the parameters C, Q and b are typically much smaller than unity, and the sudden approximation is valid. In particular, carbonic acids fulfill this condition, as was illustrated by Makri and Miller [1989]. 

The key features of Curran s productive and elegant tandem radical cyclization strategy are illustrated in a retrosynthetic analysis for hirsutene (1) (see Scheme 27). The final synthetic event was projected to be an intermolecular transfer of a hydrogen atom from tri-rc-butyltin hydride to the transitory tricyclic vinyl radical 131. The latter can then be traced to bicyclic tertiary radical 132 and thence to monocyclic primary radical 133 through successive hex-5-enyl-like radical cyclizations. It was anticipated that the initial radical 133 could be generated through the abstraction of the iodine atom from... 

As with carbocation-initiated polyene cyclizations, radical cyclizations can proceed through several successive steps if the steric and electronic properties of the reactant provide potential reaction sites. Cyclization may be followed by a second intramolecular step or by an intermolecular addition or alkylation. Intermediate radicals can be constructed so that hydrogen atom transfer can occur as part of the overall process. For example, 2-bromohexenes having radical stabilizing substituents at C(6) can undergo cyclization after a hydrogen atom transfer step.348... 

The intramolecular hydrogen atom transfer occurs with lower activation energies in comparison with the intermolecular transfer (see the values of Ee for both types of reactions in Table 6.11). The values of the activation energies of intramolecular radical H-atom abstraction calculated by the IPM method are given in Table 6.15. 

There are few addition reactions to a,/J-disubstituted enoyl systems 151 that proceed in good yield and are able to control the absolute and relative stereochemistry of both new stereocenters. This is a consequence of problematic A1,3 interactions in either rotamer when traditional templates such as oxazolidinone are used to relieve A1,3 strain the C - C bond of the enoyl group twists, breaking conjugation which results in diminished reactivity and selectivity [111-124], Sibi et al. recently demonstrated that intermolecular radical addition to a,/J-disubstituted substrates followed by hydrogen atom transfer proceeds with high diastereo- and enantioselectivity (151 -> 152 or 153, Scheme 40). 

A mechanism possibly involving intermolecular hydride transfer in this promoted ruthenium system is thus very different from the reaction pathways presented for the cobalt and unpromoted ruthenium catalysts, where the evidence supports an intramolecular hydrogen atom transfer in the formyl-producing step. Nevertheless, reactions following this step could be similar in all of these systems, since the observed products are essentially the same. Thus, a chain growth process through aldehyde intermediates, as outlined earlier, may apply to this ruthenium system also. 

Both of the above reaction pathways are suppressed if 443-D and perdeuteriated solvents are used simultaneously in cyclization reaction 265. Mainly oligomer and less than 5% of alkane 450 were then produced. This has been explained by different reactivities of radical 445 formed directly from 444 or formed by abstraction of hydrogen atom from 443 through complex 453. The more energetic radicals formed by hydrogen atom transfer from 443, add intermolecularly to an alkene and initiate preferentially oligomerization512, while... 

The transfer of hydrogen to peroxyl radicals may proceed intra- or inter-molecularly. Intramolecular transfer reaction (isomerization) of peroxyl macroradicals of polypropylene occurs during the oxidation of the polymer in a solution of inactive solvent [75] while the intermolecular transfer is preferred during the oxidation in reactive solvent or in the crystalline state [76]. 

Such an enzyme might accept the hydrogen atom on one side of the NAD, and retransfer hydrogen to the enzyme-bound intermediate from the other face. Use of a substrate having a labeled hydrogen atom in the position to be transferred to NAD would then lead to labeling of the NAD (65), and, if a mixture of labeled and unlabeled molecules were used as substrate, intermolecular transfer of label would occur. The latter transfer may be detected by the use of per-... 

A considerable proportion of the investigations of catalytic reactions involve hydrogen or hydrogen-containing compounds. In all of the cases studied, intermolecular transfer of H atoms takes place and the evidence points to an intermediate chemisorption of H atoms (or ions) on the catalyst surface. 

Hydrogen chain transfer reaction, which may occur as intermolecular or intramolecular processes, leads to the formation of oleflnic species and polymeric fragments. Moreover, secondary radicals can also be formed from hydrogen abstraction through an intermolecular transfer reaction between a primary radical and a polymeric fragment. 

A. Intermolecular Proton and Hydrogen Atom Transfer in Guest-Host Systems, 184... 

The distinction between proton and hydrogen atom (i.c., proton plus electron) transfer is clear-cut for the intermolecular transfer of one proton or hydrogen atom, when either the ionic or radical character of the reaction partners is changed. In the case of intramolecular transfers, however, the rearrangement of the electronic structure allows for partial charge compensations, and this distinction may become arbitrary (see. e g., Michl and Bonacic [2], p. 405). This is also true for the exchange of two protons between two molecules. 

Electron spin resonance is commonly used to identify, monitor, and to structurally characterize radical pairs created by the intermolecular transfer of a hydrogen atom [15]. Nuclear magnetic resonance techniques are more... 

The two ketyl units that are part of the ion quadruplet undergo (1) interionic dimerization to give pinacolates, (2) intraionic disproportionation via hydrogen atom transfer from one unit to the other to give alcoholate and enolate, and (3) intermolecular hydrogen atom abstraction from the solvent molecule (THF) in the solvent shell to provide the alcoholate. Thus ketyl radicals appear to undergo the three basic reactions that are characteristic for free radicalsld. 

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