Allyltin compounds radical reactions

Free-radical-mediated four-component coupling reactions are rare. However, when an allyltin-mediated radical carbonylation is conducted in the presence of electron-deficient alkenes, four-component coupling reactions take place efficiently to give good yields of p-functionalized <5,fi-unsaturated ketones [40]. The wide scope of this four-component coupling reaction is noteworthy Primary, secondary, and tertiary alkyl bromides and iodides can be used as well as aromatic and vinylic halides. A variety of electron-deficient alkenes, such as methyl vinyl ketone, ethyl acrylate, acrolein, acrylonitrile, and vinyl sulfone, can be used as the acyl radical trap (Scheme 6.23). Fluorous allyltin compounds can also be used in four-component coupling reactions [41]. 

Unsaturated ketones can be readily synthesized by a three-component coupling reaction, comprised of alkyl halides, CO, and allyltin reagents [53]. Because of the slow direct addition of alkyl radicals to allyltin compounds [54], radical carbonylation with allyltin can be conducted at relatively low CO pressures to give good yields of y ,y-unsaturated ketones (Scheme 4-29). 

Reactions involving organotin hydrides, or allyltin compounds, or hexaalkyldistannanes as reactants, and stannyl radicals as intermediates, now hold an important place in organic synthesis, to the degree that this dominance of organotin reagents in mediating free radical reactions has been referred to as the tyranny of tin.4... 

Allyltin compounds behave as excellent unimolecular chain transfer (UMCT) reagents [49] which serve as radical acceptors and sources of tin mediators [50]. Since acyl radicals are nucleophilic radicals, the addition reaction to allyltin, which is regarded as an electron rich alkene, is not a rapid process. Ryu, Sonoda, and coworkers found that unsaturated ketones can be synthesized by a three-component coupling reaction, comprised of alkyl halides, CO, and allyltin reagents [51]. Because of the slow direct addition of alkyl radicals to allyltin compounds [50b], radical car-bonylations with allyltin can be conducted at relatively low CO pressures, and high substrate concentrations (0.1-0.05 M) were used to ensure the chain length. 

The radical carbonylation of an alkyl iodide in the presence of Kim s sulfonyl oxime ethers [58, 59, 60] provides a new type of multicomponent coupling reaction where plural radical Cl synthons are consecutively combined [61]. In the transformation, allyltin was used to serve as a trap of benzenesulfonyl radical which converts sulfonyl radical to a tin radical, thus creating a chain. Scheme 14 illustrates such an example, where the product was easily dehydroxylated to give the corresponding tricarbonyl compound on treatment with zinc/AcOH. The radical acylation reaction by Kim s sulfonyl oxime ethers can be conducted under irradiation with the addition of hexamethylditin. This is an alternative path for achieving a similar transformation without the use of photolysis equipment. Scheme 15 illustrates several examples where carbon monoxide and Kim s sulfonyl oxime ethers are successfully combined to create new tandem radical reaction sequences [61],... 

Radical chain reactions can take place via organotin reagent without hydrogen donors such as allyltin [38, 39]. This methodology has been applied to synthesize some important chiral precursors of natural products (for example pseudomonic acid). Thus, the pentose derivative 35 affords the compound 36 with retention of configuration via the carbohydrate radical R- at C-4 position [Scheme 18]. 

The same strategy was applied to radical cabonylation with fluorous allyltin reagents. Propylene-spaced fluorous allyltin reagents 22 were tested as mediators for radical carbonylations the four-components coupling reaction with RX, CO, alkenes and the fluorous allyltin produced the -functionalized i8-aUylated ketones 23 (Scheme 28). Diphasic workup (acetonitrile and FC-72) was successfully carried out to separate the products from the tin compounds. Comparison experiments... 

Sugar allyltin derivatives, convenient precursors of dienoaldehydes such as compounds 31, have been obtained by Sr2 reaction of trialkyltin radicals with allylic thiocarbonates which were, in turn, produced by Wittig condensation followed by a 3 3 thermal rearrangement, as shown in Scheme 5. ... 

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