Radical Translocation Reaction

Curran and co-workers have explored the use of silicon tethers to carry out these reactions. They have successfully demonstrated the 1,5 and 1,6 translocation of a radical that goes on to do intramolecular cyclization reactions. This method was used to synthesize natural product such as crinipellin A22 and 2-(o)-(2-bromoaryl)dimethylsilyl-a-methyl-D-mannopyranoside.24 One of the nice benefits to the use of silicon tethers is that they serve as a hydroxyl-protecting group before and after the reaction is performed. 

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 

Conditions A Substrate 1 equiv. nBu3SnD 1.3 equiv. Benzene [0.05 M] 

B Substrate 1 equiv. nBu3SnD 1.3 equiv. Benzene [0.01 M] C Substrate 1 equiv. nBu3SnD 1.3 equiv. Benzene [0.005 M] 

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

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. 

Radical translocation reactions in synthesis of heterocycles 01CSR94. 

Allylation of P-oxy-o-iodoanilides. Generation of an a-radical is via a radical translocation reaction. The allylation is subject to 1,2-asymmetric induction, giving predominantly the anti products. 

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... 

However, their intermolecular addition reactions with alkynes are mostly aimed at synthesizing substituted aLkenes, ° and only very few cascade reactions that are initiated by P radical addition to C = C triple bonds have been reported. Renaud and coworkers developed a simple one-pot procedure for the cyclization of terminal alkynes mediated by dialkyl phosphites (Scheme 2.35). In this radical chain procedure, dialkyl phosphite radicals, (R0)2P =0, undergo addition to the C = C triple bond in 190, which triggers a radical translocation (l,5-HAT)/5-eAO cyclization cascade. The sequence is terminated by hydrogen transfer from dialkyl phosphite to the intermediate 194 and regeneration of P-centered radicals. 

The oxidative carbonylation of saturated alcohols leading to (5-lactones can be achieved with the aid of the LTA (lead tetraacetate) induced one-electron oxidation system [83]. As exemplified by the case given in Scheme 4-47, an acyl radical resulting from carbonylation of a (5-hydroxyalkyl radical, formed by 1,5-radical translocation, undergoes the one-electron oxidation to form an acyl cation and subsequent deprotonative cyclization. The reaction can be extended to an LTA-free system, which uses photolysis of alkyl benzenesulfenates under CO pressure [83 b]. 

Radical translocation/cyclization reactions in syntheses of bridged azabicyclic compounds 03H(59)429. 

S02-extrusion affords the electrophilic radical 49 (Scheme 10). Intramolecular homolytic substitution eventually gives tetrahydronaphthalene 50 (92%). Beckwith showed that the A-(o-bromophenyl)amide 51 can be transformed into the corresponding oxindole 54 (70%) at high temperatures using BusSnH via tandem radical translocation of the initially formed aryl radical 52 to form 53 with subsequent intramolecular homolytic substitution [77]. The nucleophilic a-aminomethyl radical 55 reacted in a tandem addition/homolytic aromatic substitution reaction via radical 56 to tetrahydroquinoline 57 [78]. Radical 55 can either be prepared by oxida-... 

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