Radical 1,5-translocation

Substituted 5, 8-cyclopurine and 5, 6-cyclopyrimidine nucleosides have been synthesized by combination of radical translocation/cyclization... 

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 following syntheses of five-membered carbocyclic systems involve radical-induced epoxide fragmentation with radical translocation and cyclization. The resulting bicyclic alcohols are formed as a mixture of two epimeric esters with cw-fused rings.[71]... 

A very attractive feature of radical chemistry is the generation of a novel radical after cyclization or any other radical translocation. This feature allows the inclusion of a second carbon—carbon bond-forming event and can, in principle, be extended even further. The resulting tandem reactions [38] can be extremely useful for the construction of complex molecules. Impressive early results have been reported by Stork in applications directed towards the synthesis of prostaglandins [39]. Our catalytic conditions also allow the realization of tandem reactions. An example including a mechanistic proposal is shown in Scheme 12.20. 

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... 

As for the a-radical participant in this coupling reaction, the main product is surely formed as a result of radical translocation. As for the cation-radical participant, the position of the coupling is explained as follows (Begley et al. 1994) ... 

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... 

Scheme 14. Pyrrolidine ring formation via cyclization involving radical translocation...

Gimisis T, Chatgilialoglu C (1996) 1,5-Radical translocation protocol for the generation of C-1 radicals in nucleosides. Synthesis of spiro nucleosides through a rare 5-e do-trig cyclization. J Org Chem 61 1908-1908... 

Synthesis of bridged azabicyclic compounds using radical translocation reaction followed by cyclization has been reported [95JCS(P1)1801]. Treatment of bromoamide 126 with... 

Radical translocation, an indirect method for the C-H transformation of ethers, has also been developed ... 

Spirocyclic pyrrolidin-2-ones (151) are prepared through the initial 1,5-radical translocation by an sp2 carbon-centered radical, followed by cyclization as shown in eq. 3.55 [161]. 

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.

Silyl ketals 7a, 7b and 7d were used to show that intramolecular radical translocation beyond the 1,5 atom was not only feasible, but efficient. 

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