Trialkyltin radicals

Trialkyltin radicals are important intermediates in the reduction of alkyl halides, and in the hydrostannation of alkenes (1, 287). 

The last two reactions are useful for esr studies involving free radicals. Until recently, the only trialkyltin radical that had been observed directly, in solution, by esr was MesSn- (295), but many more have now been reported (e.g., EtsSn-, PrsSn-, and BusSn ) (296). Bulky ligands [e g., (PhCMejCHjlaSn ] increase the persistence of the radicals, so that esr observation is easier (297), and tris(2,3,5-trimethylphenyl)tin and tris(2,3,5-triethylphenyl)tin radicals, at 180° and 100°, respectively, are in thermal equilibrium with the corresponding hexaaryldi-tins (298). 

The propensity of trialkyltin radicals to add to the terminal position of allenes has been applied in the synthesis of substituted hydroxylamines from N-oximinoallenes,... 

The reaction process, outlined in Scheme 7, involves the initial formation of the radical 11 by atom abstraction of the trialkyltin radical 15 from the glycosyl precursor 17. 

J. Dupuis, B. Giese, J. Hartung, M. Leising, H.-G. Korth, and R. Sustmann, Electron transfer from trialkyltin radicals to nitrosugars The synthesis of C-glycosides with tertiary anomeric carbon atoms, J. Am Chem. Soc. 107 4332 (1985). 

The same group also studied the addition of trialkyltin radical to an acetylene and cyclization of the resulting vinyl radical in the context of serial radical cyclizations [Eq. (6) ... 

Besides new insight into the reactivity of free radicals, methods for die production of carbon-centered free radicals have also seen major improvements in die last several years. One very common new mediod is to use tin-based reagents as radical chain carriers. Trialkyltin radicals readily abstract bromine or iodine from carbon to produce a carbon-centered free radical. Placement of a bromide or iodide substituent on a substrate dius permits formation of a carbon-centered free radical at diat position using tin-based mediodology. This process was initially developed for die reduction of alkyl halides, and it remains an excellent synthetic method for diat purpose. The complete chain mechanism for die reduction is shown. 

Finally, it is noteworthy that various deoxypyranos-2-, -3-, and -4-yl radicals64,66,67,72 generated in benzene by means of trialkyltin radicals, either... 

A novel radical-induced C-N bond formation has been reported [95TL323], The reaction involves the treatment of an alkyl iodide 51 with iso-amylnitrite and a trialkyltin radical precursor. The reaction is postulated to proceed through the formation of trialkyltin nitrite. 

Lythgoe first observed that certain groups which form stabilized free radicals when substituted fl to the thionocarbonyl derivative of a hydroxyl moiety undergo smooth elimination upon treatment with trialkyltin radicals to give the olefin.155 This important observation charted a course for a series of... 

Besides oxidation to give 153, tetrathiafulvalene has been shown to undergo attack by trialkyltin radicals in an Sh2 process which results in extrusion of a molecule of acetylene and formation of persistent radicals of type 155. ... 

In 1962, Kuivila showed that the reaction of trialkyltin hydrides with alkyl halides (hydrostannolysis) (equation 1-6) was a radical chain reaction involving short-lived trialkyltin radicals, R3Sn, 12 and in 1964, Neumann showed that the reaction with non-polar alkenes and alkynes (hydrostannation) (equation 1-7) followed a similar mechanism,13, 14 and these reactions now provide the basis of a number of important organic synthetic methods. 

Unfortunately, the radical chain reaction of simple S-phenyl thiol esters was not efficiently triggered by trialkyltin radicals [58]. The homolytic bond dissociation of S-2-naphthyl and other thiol esters by photolysis also seems to be a low quantum yield process ( =0.08-0.10) [59]. 

Most of the useful radical reactions in synthetic chemistry involve a chain mechanism, in which radical species are continually regenerated and trapped. Such propagation steps are illustrated for reduction of a substrate RX (4.4). The feasibility of this sequence depends on the relative reaction rates which themselves are determined by the structures of the radicals (including that used to initiate the reaction). In reactions such as this, the trialkyltin radical is sometimes referred to as the chain carrier as it is continuously regenerated to propagate the cycle. 

Trialkyltin hydrides are effective hydrogen atom transfer agents, and trialkyltin radicals wiU abstract chlorine, bromine, or iodine atoms from alkyl halides. Together with a radical initiator, such as AIBN and trialkyltin hydrides, alkyl halides can give reduction or other radical-derived products (Scheme 4.27). 

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. ... 

The study of the reactivity of trialkylsilyl radicals in solution has been placed on a firmer foundation by the measurement of absolute rate constants for some reactions of triethylsilyl radicals (generated by the reactions of tert-butoxyl radicals with triethylsilane), with some organic halides and benzil/ These data show for example, the greater reactivity of Et3Si in halogen abstraction than that of trialkyltin radicals. Kinetic isotope effects (kn/fco) for the insertion of photochemically generated dimethylsilylene and methylphenylsilylene into Si-H and 0-H single bonds are about 1.3 and 2.1 - 2.3, respectively. The preferred mechanism for insertion of silylenes into O-H bonds is shown in equation (1). Other workers haye shown that dimethylsilylene inserts preferentially into O-H bonds of alcohols compared with S-H bonds in silanes or Si-O bonds in alkoxy-silanes. ... 

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