Tin radicals, generation

These results are in accordance with the findings of Boothe and coworkers26, who found that the reactions of four diastereomeric 2-bromo-3-phenylsulfinylbutanes with tributyl-tin radicals generate /3-phenylsulfinyl sec-butyl radicals (8) which eliminate PhSO radicals to form the 2-butenes in a stereoselective manner. The stereoselectivities observed in this free radical elimination must result from the fact that the rate constant for elimination is greater than that for rotation about the C—C bond. Furthermore, a neighboring phenyl group on the radical center seems to stabilize the radical enough so that the internal rotation can compete with the -elimination reaction. It is also noteworthy that the small... 

Owing to the expense, toxicity, and purification problems associated with use of stoichiometric amounts of tin hydrides, there has been interest in finding other hydrogen atom donors.205 The trialkylboron-oxygen system for radical generation (see Part A, Section 11.1.4) has been used with fra-(trimethylsilyl)silane or diphenylsilane as a hydrogen donor.206... 

A range of examples of alkylation reactions via radicals generated through electron transfer sensitization is available in the literature, and a few of them are reported in Figure 3.12. Alkyl tin derivatives can be used as precursors, but in many cases these highly toxic reagents can be advantageously substituted by... 

Results of a chemical activation induced by ultrasound have been reported by Nakamura et al. in the initiation of radical chain reactions with tin radicals [59]. When an aerated solution of R3SnH and an olefin is sonicated at low temperatures (0 to 10 °C), hydroxystannation of the double bond occurs and not the conventional hydrostannation achieved under silent conditions (Scheme 3.10). This point evidences the differences between radical sonochemistry and the classical free radical chemistry. The result was interpreted on the basis of the generation of tin and peroxy radicals in the region of hot cavities, which then undergo synthetic reactions in the bulk liquid phase. These findings also enable the sonochemical synthesis of alkyl hydroperoxides by aerobic reductive oxygenation of alkyl halides [60], and the aerobic catalytic conversion of alkyl halides into alcohols by trialkyltin halides [61]. 

For secondary alkyl iodides, the two one-electron polarographic waves are more separated. Reduction of 2-iodooctane at the potential of the first wave alfords the dialkylmercury and 7,8-dimethyl-tetradecane by reactions of the sec-octyl radical. At the potential of the second wave only octane and octenes are isolated [37]. 2-Bromooctane shows only one polarographic wave and yields octane and octene on reduction at any potential [37]. Radicals generated by reduction of primary and secondary iodoalkanes will react with other cathode materials including tin, lead and thallium to form metal alkyls [48,49],... 

Alternative chemical reactions are available for tire generation of phenyl a-radicals. These include the diazonium salt reactions mentioned earlier and also reaction of aryl bromides and iodides with tributyltinhydride or triphenyltinhydride in the presence of a radical initiator [174], Electrogeneration of the tin radical by... 

Tin radicals were generated by Noltes et al. in 1956, and their applications in organic chemistry were pioneered by Kuivila and co-workers. These species have proven to be of tremendous utility in organic chemistry, although their toxicity and other unfavorable properties have led to a search for substitutes. Interestingly, in the initial publication it was proposed that the reaction (equation 64) did not involve a free radical mechanism, as no inhibition by hydroquinone was detected. 

Of all the radical reactions, the exo-l,5-cycIization of a hex-5-enyl radical to cyclopen-tylmethyl radical and its subsequent trapping by various reagents have attracted the most attention from synthetic chemists (Scheme 1) [4-7]. Starting materials that are most often used for the tin method (initiation of the chain by trialkyl tin radical) are halides, sulfides, selenides, or thionocarbonates. The generation and cyclization of the radical proceeds under exceptionally mild neutral conditions, and these conditions are compatible with a wide variety of common functional groups. A prototypical example of an application in carbohydrate chemistry is shown in Scheme 2 [8]. Readily available 2,3-di-O-isopropyl-ideneribonolactone 1 was converted into the bromoacrylate 2 in three steps. Radical... 

As the rate of cyclization becomes slower, the reactivity of the precursor becomes more important. To ensure that the radical generation step does not break the chain, it is important to use the most reactive precursor available. For very slow cyclizations, the advice is simple use iodides whenever possible. The purity of the precursor is also critical for slow cyclizations because tin hydride can sometimes react with impurities to generate hydrogen atom sources that are much more reactive than itself. Any impurities that might generate thiols or selenols may cause undue amounts of reduction (thus, the purity of phenyl sulfides and selenides is especially important). Metal impurities, which may form transition metal hydrides, can be devastating, even for fast cyclizations.41 Empirically, it seems that breaking of the chain is less of... 

Each of the syntheses of seychellene summarized in Scheme 20 illustrates one of the two important methods for generating vinyl radicals. In the more common method, the cyclization of vinyl bromide (34) provides tricycle (35).93 Because of the strength of sjp- bonds to carbon, the only generally useful precursors of vinyl radicals in this standard tin hydride approach are bromides and iodides. Most vinyl radicals invert rapidly, and therefore the stereochemistry of the radical precursor is not important. The second method, illustrated by the conversion of (36) to (37),94 generates vinyl radicals by the addition of the tin radical to an alkyne.95-98 The overall transformation is a hydrostannylation, but a radical cyclization occurs between the addition of the stannyl radical and the hydrogen transfer. Concentration may be important in these reactions because direct hydrostannylation of die alkyne can compete with cyclization. Stork has demonstrated that the reversibility of the stannyl radical addition step confers great power on this method.93 For example, in the conversion of (38) to (39), the stannyl radical probably adds reversibly to all of the multiple bond sites. However, the radicals that are produced by additions to the alkene, or to the internal carbon of the alkyne, have no favorable cyclization pathways. Thus, all the product (39) derives from addition to the terminal alkyne carbon. Even when cyclic products might be derived from addition to the alkene, followed by cyclization to the alkyne, they often are not found because 0-stannyl alkyl radicals revert to alkenes so rapidly that they do not close. 

The propagation steps for this reaction are outlined in Scheme 4.15. The tin-centered radical abstracts X from R-X, generating a carbon-centered radical, R , which subsequently abstracts hydrogen from the tin hydride, yielding the organic product, and completing the chain by regenerating the tin radical. 

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). 

Reaction of the unsaturated bromoethyl glycoside 216 in the foregoing manner gives the bicyclic product 217,218 and similar treatment of the propargyl ether 219 with a tributyltin radical results in carbon-radical generation and cyclization to afford the tin-containing adduct 220 in 90% yield. On oxidation with osmium tetraoxide and periodate ion, the Sn-C bond is cleaved, and the corresponding ketone 221 is produced in excellent yield.219... 

The development of new radical initiators is an area of active investigation. Tada et at. have described a (triphenyltin)cobaloxime complex as a reagent for radical generation from bromides [95JOC6635]. This is an alternative to reductive processes involving tin hydride and allows for the introduction of an additional functional group in the product. 

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