Hydrogen atom transfer intramolecular

Cases of addition-abstraction" polymerization have also been reported where propagation occurs by a mechanism involving sequential addition and intramolecular 1,5-hydrogen atom transfer steps (Section 4.4.3.4). 

It was reported that Pd(0)-catalyzed coupling reactions of allenic alcohols, amines and acids with hypervalent iodonium salts afforded cyclized heterocyclic tetrahydrofurans, tetrahydropyrans, pyrrolidines, piperidines, or lactones under mild conditions <99SL324>. Intramolecular 1,5-hydrogen atom transfer radical cyclization reaction of pyrrolidine derivatives was examined. Reaction of 3,4-dialiyloxy-JV-(0-bromobenzyl)pyrtolidine gave hexahydro-... 

Intramolecular 1,5-hydrogen atom transfer cyclizations.3 In reactions with low concentrations of the tin hydride (14,313), an original vinyl radical can undergo a 1,5-hydrogen shift to produce a new radical from a C—H bond, which then undergoes cyclization (equation I). 

Hofmann-Ldffler-Freytag reaction Formation of cyclic amines from N-halogenated amines via an intramolecular 1,5-hydrogen atom transfer to a nitrogen radical. 208... 

Beaulieu, F. (1994) The thiocarbamate as a dual directed metalation and cross coupling group intramolecular 1,5-hydrogen atom transfer in aromatic amides. Ph.D. Thesis. University of Waterloo, Canada. 

The bicyclic product is formed by coupling of the two radical sites, while the alkene results from an intramolecular hydrogen-atom transfer. These reactions can be sensitized by aromatic ketones and quenched by typical triplet quenchers and are therefore believed to proceed via triplet excited states. 

There are no proven examples of 1,2-hydrogen atom shifts this can be understood in terms of the stereoelectronie requirements on the process. The same limitations are not imposed on heavier atoms (e.g. chlorine). The postulate309 that ethyl branches in reduced PVC are all derived from cbloroethyl branches formed by sequential 1,5-intramolecular hydrogen atom transfers as described for PE (Section 4,4.3.1) has been questioned.,6,6 It has been shown that many of these ethyl branches are derived from dichloroethyl groups. The latter are formed by sequential 1,2-chlorine atom shifts which follow a head addition (Section 4.3.1.2). 

The vinylsilanes (e.g. 40, 41) do not readily homopolymerize. Forsyth et al.Mj explored the mechanism of grafting these monomers using dodecane as a model for PE. Their work suggests that multiple monomer units are attached through a sequence of addition and intramolecular hydrogen atom transfer steps by a mechanism analogous to that shown in Scheme 7.33 on page 394. 

Carrington, T., Jr., Miller, W. H., 1986, Reaction Surface Description of Intramolecular Hydrogen Atom Transfer in Malonaldehyde , J. Chem. Phys., 84, 4364. 

Ikegami M, Arai T (2002) Photoinduced intramolecular hydrogen atom transfer in 2-(2-hydroxyphenyl)benzoxazole and 2-(2-hydroxyphenyl)-benzothiazole studied by laser flash photolysis. J Chem Soc Perkin Trans 2 1296-1301... 

Intramolecular Hydrogen Atom Transfer in Peroxyl Radical... 

Alkoxyl radicals are very active and rapidly enter into bimolecular reaction (see Chapter 2). Moreover, alkoxyl radicals with sufficiently long alkyl substituents react with intramolecular hydrogen atom transfer, for example [37] ... 

Enthalpies, Activation Energies, and Rate Constants of Intramolecular Hydrogen Atom Transfer in Peroxyl Radicals Calculated by the IPM Method [36], (Ais Increment of Polar Interaction in the Transition State)... 

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. 

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. 

The position of the equilibrium (equation 65) depends upon the nature of X, and the intramolecular hydrogen atom transfer can be monitored by H NMR magnetization transfer. This provides a model for the generalized imide-amide-amine interconversion by proton transfer. 

Bansal KM, Patterson LK, Schuler RH (1972) Production of halide ion in the radiolysis of aqueous solutions of the 5-halouracils. J Phys Chem 76 2386-2392 Barnes JP, Bernhard WA (1994) One-electron-reduced cytosine in acidic glasses conformational states before and after proton transfer. J Phys Chem 98 887-893 Barvian MR, Greenberg MM (1992) Independent generation of the major adduct of hydroxyl radical and thymidine. Examination of intramolecular hydrogen atom transfer in competition with thiol trapping.Tetrahedron Lett 33 6057-6060... 

Barvian MR, Greenberg MM (1992) Independent generation of the major adduct of hydroxyl radical and thymidine. Examination of intramolecular hydrogen atom transfer in competition with thiol trapping. Tetrahedron Lett 33 6057-6060... 

Scheffer et al. provided another unimolecular asymmetric transformation involving the Norrish type II reaction, a well-known excited state process of ketones that is initiated by an intramolecular hydrogen atom transfer from carbon to oxygen through a six-membered transition state (Scheme 5). [19a] An adamantyl ketone derivative 27 was found to crystallize from ethanol in very large prisms in the chiral space group P2 2 2. Upon irradiation of these crystals to approximately 10% conversion, the chiral cyclobutanol derivatives 28 were afforded as the major products in 80% ee. 

Radical polymerization can lead to branched polymers by intramolecular hydrogen atom transfer, a process sometimes called backbiting. Removal of H through a six-membered transition state moves the growing radical atom five atoms back down the chain, and leads to butyl side-chains. A more stable secondary radical is produced and chain growth then occurs from that point. 

Scheffer et al. provided another unimolecular asymmetric transformation involving the Norrish type 2 reaction, a well-known excited state process of ketones that is initiated by an intramolecular hydrogen atom transfer from carbon to... 

Intramolecular hydrogen atom transfer in hypervalent onium radicals is another reaction that has implications in the chemistry of gas-phase peptide and... 

Similar reactions occur with TV-nitrosoamines R2-NNO and /V-nitrosoamides RC0(R )NN0.70 Apart from the intramolecular hydrogen atom transfer, diverse intramolecular radical additions (cyclizations) and other rearrangements have also been used to obtain cross-reaction products between NO and the radicals resulting from the transformation of the primarily formed R to another radical R. 69 70 Giving evidence for the persistent radical effect, the yields of the desired products are large and those of the selftermination of the transient intermediates are low,... 

I, 4-dehydrobenzene intermediates. Trapping by inter- and intramolecular hydrogen atom transfer and observation of high-temperature CIDNP. 

As described above for a-carbon-centered radicals, where it is not feasible to access a side-chain amino acid radical by direct manipulation of a functional group, radical translocation may be employed instead, as illustrated in Scheme 10 [60]. Regiocontrol may be achieved by exploiting product radical stability and the preferred geometry of intramolecular hydrogen atom transfer. 

The efficiency of photoreduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen atom transfer, becomes the dominant process. Hydrogen abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process, known as photoenolization, ° can be detected by photolysis in deuterated hydroxylic solvents even though there is no net transformation of the reactant. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent. 

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