Radical translocation abstraction

The C-centered radical is thought initially to rearrange to a S-centered radical via a 1,3-hydrogen shift, followed by a radical translocation from sulfur to the silicon surface on the neighboring row. The abstraction of a H-atom from... 

The reaction is formally a hydrosilylation process analogous to the homogeneous reactions described in Chapter 5. Scheme 8.11 shows the proposed H—Si(lll) surface-propagated radical chain mechanism [48]. The initially formed surface silyl radical reacts with alkene to form a secondary alkyl radical that abstracts hydrogen from a vicinal Si—H bond and creates another surface silyl radical. The best candidate for the radical translocation from the carbon atom of the alkyl chain to a silicon surface is the 1,5 hydrogen shift shown in Scheme 8.11. Hydrogen abstraction from the neat alkene, in particular from the... 

Intramolecular H-abstraction (radical translocation) has attracted a lot of attention this year. 1,2-1,5 H-atom transfer reactions have been studied theoretically using UHF-AM1 methods. The predicted activation energies were compared to experimentally measured data.91 Ah initio studies into the 1,2-1,6 translocation of the 2-methylhexyl radical predicted that 1,5-H-transfer would be die fastest isomerization process.92 The effects of various groups (dioxolane, acetoxy, TBS ether) on the relative ability of 1,5-1,7 radical translocation have been examined.93... 

Our success in synthesizing silyl ketals containing an aryl halide with (+)-ethyl lactate led us to explore the intramolecular radical translocation reaction (Scheme 29). The term radical translocation is described by Robertson et al. as the intramolecular abstraction of an atom (usually hydrogen) or group by a radical center this results in a repositioning of the site of the unpaired electron which can lead to functionalization at positions normally unreactive towards external reagents or whose selective modification is difficult In the most common cases the abstraction occurs at a site that is five atoms away from the radical 1,6 atom abstraction are less common, and l,n-abstractions where n > 6 are rare. This is because the shortest chain length that can accommodate the trajectory for atom abstraction contains six atoms, as in the case of the 1,5 atom abstraction. Entropic factors usually result in the failure of the process in the cases where n > 6 atoms. 

We explored three examples of this reaction. In our first two examples, the abstraction occurs at the 7th position away from the initial radical center (Table 10, entries 1 and 2), and in another example, it occurs at the 6th position away from the radical center (Table 10, entry 3). It is a well established fact that radical quenching is a very fast process however in all our examples where n > 6 atoms, radical translocation was faster than radical quenching. This result... 

Scheme 29. Radical translocation and hydrogen atom abstraction.

Table 10. Hydrogen Atom Abstraction versus Radical Quenching for 1,7 and 1,8 Atom Radical Translocation.

Translocation of radicals has been shown to be a remarkably powerful means of introducing functionality at unactivated sites. The diversity of the examples presented in this chapter proves the generality and usefulness of hydrogen atom abstraction. Based on simple rules, it is possible to plan radical translocation and to incorporate them into useful synthetic strategy. 

Interestingly, another example of a [1,5]-radical translocation coupled with an unusual 5-endo-trig radical cyclization was reported in a structurally different system (Scheme 10). In this case, the a-bromovinyl radical abstracted a hydrogen either from (TMSjsSiH to give vinyl bromide in 25% yield or from the anomeric position to generate the C-1 radical which underwent an unusual 5-endo-trig cyclization onto the proximal double bond to generate an anomeric mixture of spironucleosides after bromine atom ejection. 

The anion translocation can be considered the anionic equivalent of the well-known radical translocation, the intramolecular radical abstraction of a hydrogen atom, which is a key step in some important radical reactions. 

We observed that vinyl radicals show exquisite reactivity for efficient and chemoselective H-abstraction [64]. The resulting translocated radical can then undergo various types of inter- or intramolecular transformations. Interestingly, we have also shown that the hydrogen transfer step can serve as a driving force for the unfavorable 4-exo-dig mode of cychzation [65]. 

The currently accepted chemical mechanism of action of EAL, shown in Figure 17 for the reaction of ( 5)-2-aminopropanol, follows the pattern of other coenzyme Bi2-dependent isomerases. The chemical mechanism begins by homolytic scission of the Co—C5 bond in adenosylcobalamin with the substrate bound at its site translocation of the ribosyl moiety of the 5 -deoxyadenosyl radical by torsion about the ALribosyl linkage to place the 5 -methylene radical in contact with Cl of the substrate, in analogy with DDH ° abstraction of C 1-hydrogen from the substrate by the 5 -deoxyadenosyl radical to form 5 -deoxyadenosine and the... 

A very important tactic for a-amino radical generation involves translocation via hydrogen abstraction by an aromatic free radical generated from a halogen five bonds away. Thus chain extension at the a-position of an amide or amine can be accomplished with great success. 

H-abstraction is generally a facile process, which often competes with desired transformations. However, in this case it can be utilized favorably to translocate the radical to the desired position for further reaction. The radical precursor in 186 is just a modified phenyldimethylsilyl ether and is much more readily incorporated than the alternative product, which would possess the radical precursor at the site of cyclization. Exposure of (o-bromophenyl)dimethylsilyl ether 186 to Corey s catalytic tin hydride conditions [73] formed cyclopentane 187 as a 1 1.1 mixture of cisltrans isomers, in addition to some directly reduced product 188 (Scheme 10-60). 

Transpositionalalkylations. A -(o-iodobenzyl) heterocycles generate radicals that are translocated through hydrogen abstraction. The resulting a-amino radicals then react with carbonyl compounds. This process has previously been developed with BUjSnH as initiator. 

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