Tin hydride-mediated reaction

In contrast to the tin hydride-mediated reaction (Giese reaction) [90], no slow addition of the chain carrier is necessary. This is easily understandable... 

Radical carbonylation can also be conducted in a zinc-induced reduction system. A similar three-component transformation reaction to that illustrated in the second equation of Scheme 6.14 can be attained using zinc and protic solvents (Scheme 6.38) [59]. The observed stereochemical outcome is identical to that for the tin hydride-mediated reaction, providing a additional evidence for free-radical generation, radical carbonylation, and acyl radical cyclization taking place simultaneously, even in the zinc-induced system. In this system, however, the final step is reduction to form a carbanion and protonation. 

Neutral aminyl radicals generated from tin hydride-mediated reactions of sulfenamides (Section II,F) have been shown to undergo cyclizations when energetically favored by addition to a strained alkene or by formation of a stabilized intermediate benzylic radical. In both cases, the reverse reaction, cleavage of the /3-amino radical, apparently did not occur (92TL4993). 

Some degradation of the flnorons tin hydride does appear to occnr with multiple recycling. The first four reductions of bromoadamantane were complete in 4 honrs while the last two required 5 hours. Nevertheless, the ability to use 1-10% tin catalyst with almost complete recovery substantially improves the green credentials of tin hydride redactions. Most importantly, tin contamination of the prodnct of fluorous tin hydride-mediated reactions cannot be detected to an estimated detection limit of 1%. 

Contrary to popular tin-hydride-mediated reactions, which are realized in the presence of a catalytically generated initiator and are conducted under atom/group... 

Tin Hydride-Mediated Reaction Allyltin Method Photochemical Fragmentation of Barton Esters Direct Photolysis of Cobaloximes Direct Photolysis of Organotellurium Compounds Miscellaneous... 

Scheme 16 summarizes the results obtained by enantioselective radical reduction of a-bromoester by chiral binaphthyl-derived tin hydride. The reactions were generally performed at - 78 °C. An increase in the temperature resulted in the lowering of the selectivity. All reactions mediated by (S)-configured chiral tin hydride showed an (R)-selective preference in the product. The use of the opposite enantiomer of the chiral stannane resulted in a quantitative reversal of the selectivity (not shown). The selectivity remained modest on addition of magnesium Lewis acids. These reductions were also feasible when a catalytic amount of chiral tin hydride (1 mol %) was employed in combination with an excess of achiral hydride NaCNBH3, providing similar results. 

A conceptually different C-C bond forming reaction involving intramolecular radical cyclisation of allylic esters of bromodifluoroacetic acid was described by Itoh et al. [200]. Reduction to the acetal sets the stage for a tin hydride-mediated cyclisation (Eq. 65). 

A detailed mechanistic analysis of the factors affecting the success of these tin hydride mediated addition reactions has been provided by Giese.3 This analysis, which is especially illustrative of how experimental conditions for free radical reactions are planned, is summarized in Scheme 27. Three intermediate radicals, (8), (9) and (10), are involved. As is characteristic of all radical reactions, these radicals are simultaneously exposed to the same reagent pool and each can potentially undergo an addition reaction or an atom transfer reaction. The required reaction of the tributyltin radical (8) is atom ab-... 

In addition to alkyl radicals, other nucleophilic radicals can participate in tin hydride mediated addition reactions (Scheme 31). Phenyl radicals are highly reactive112 but give only modest yields of addition products113 and one suspects that vinyl radicals should behave similarly.114 a-Oxy radicals have been... 

Radical cyclizations may occur by different means including hydride-mediated additions, radical addition of RX across a multiple bond and radical addition followed by fragmentation. Of these, the tin hydride mediated addition is the most common. Additionally, radical cyclizations may occur in tandem or three reactions may occur together within the same molecule. 

Vinyl radicals also add to carbon-carbon double bonds intramolecularly to give 2,6-cw-disubstituted cyclic ethers (Equation (5)).41 In the tin hydride-mediated cyclization of the substrates including alkynes, alkyl radicals attack to carbon-carbon triple bonds leading to uco-alkylidene allylic alcohols (Equation (6)).42 The coupling reaction between alkyl radicals may afford cyclization products. Thus, the reduction of 1,3-diiodopropane derivatives with a tin hydride provides substituted cyclopropanes.4... 

The next four examples shown in Table 3 demonstrate that tin hydride-mediated radical carbonylation can be efficiently combined with intramolecular Sh2 type reaction of acyl radicals at sulfur, providing good yields of y-thiolactones [37]. The ability of the ert-butyl radical as an Sh2 type leaving group is inferior to that of the benzyl radical [38]. Nevertheless, the fer/-butyl radical in this case has an advantage over the benzyl radical, since the starting benzylthiobutyl radical suffers an unde-... 

Miscellaneous reactions. The Heck reaction is accomplishable in perfluorinated solvents while employing a fluorous triarylphosphine ligand. A minimally fluorous diaryl diselenide used to assist tin hydride mediated cyclization can be recovered. ... 

Kotani M, Yamago S, Satoh A, Tokuyama H, Fukuyama T (2005) Synthesis of 2,3-dialkylindoles from 2-aUcenylphenylisocyanides and imines by silyltelluride- and tin hydride-mediated sequential radical reactions. Synlett 1893-1896... 

During the past years this method has been used successfully in modern biaryl synthesis, with several groups having shown its potential for C(sp )-C(sp )-bond formation [35], Heteroarenes such as substituted pyrroles, indoles, pyridones and imidazoles can act as radical acceptors in these processes [36], In addition, aryl bromides, chlorides and iodides have been used as substrates in electrochemically induced radical biaryl synthesis [37]. In a series of elegant studies, Harrowven recently applied the tin hydride-mediated radical biaryl synthesis to the preparation of helicenes [38], These reactions comprise a double C(sp )-C(sp )-bond formation an example-the impressive transformation of diiodide 10 to [7] helicene 11-is presented in Scheme 13.4. 

All of these tin hydride-mediated intramolecular arylations involve an initial radical generation by reaction of BusSn with precursor 25 to afford the corresponding Cintramolecular addition onto an arene to give a stable cyclohexadienyl radical 27 which is then oxidized ( ) to the product 28 (Scheme 13.7). It should be noted that this oxidation must occur under reducing tin hydride conditions. When debate centered on the mechanism of the homolytic aromatic substitution, a number of hypotheses was proposed. Thus, in... 

Free radicals are neutral species and therefore cannot normally be detected by ESI-MS. However, it is well known that radical reactions can be mediated by Lewis acids if the substrate acts as a Lewis base by chelating the metal atom of a Lewis acid in solution. Fiirmeier and coworkers studied tin hydride-mediated radical additions to dialkyl 2-alkyl-4-methyleneglutarates in the presence of Lewis acids (Scheme 5.5) [28,... 

Remarkably, only a small number of intramolecular reactions utilize Barton esters " - likewise, the use of the Barton chemistry in making CC bonds in tandem or domino reactions is far less exploited compared to the tin-hydride-mediated free-radical sequences. The following example is taken from the biomimetic conversion of isoflavone to the rotenoid, where the reaction affords the 6-endo product (Scheme 24). ... 

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