Tin Hydride Chemistry

Conjugated addition of trialkylbutyltin hydride is promoted by the presence of water, the former acts as a hydride donor whereas water participates to the reaction as a protonating partner. In this context, the role of the proton carrying reagent is further demonstrated by the optimalization of the yield when ammonium chloride is added to the reaction medium as [75]. 

Transition metal catalysis affords an efficient way for controlling the selectivity of these tin hydrides. For example, in the withanolide case, the selectivity of the reaction is completely modified by adding Pd(PPh3)4 to the medium. 

In fact, the enone system is reduced through a 1,4-addition mechanism in the absence of the transition metal catalyst. 

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Although tin hydride chemistry works very well in deoxygenation and in deamination reactions, the process has certain disadvantages. Tributyl- or triphenyltin hydrides have high molecular weights and are expensive for each hydrogen delivered. In addition, tin residues (usually ditin species) are always formed which are non-polar and difficult to remove from non-polar reaction products. Another disadvantage is that the quality of commercially available tin hydrides is variable. 

The original conception of reductive decarboxylation was in terms of tin hydride chemistry, simply because of previous experience. The earlier observations,7 in the case of dihydro-phenanthrene esters, led us to think of incorporating the C=N double bond to be formed (Scheme 2) by fragmentation, into an aromatic system. This was to provide an additional driving... 

The key propagation step in the mechanism is abstraction of hydride from the starting alkyl mercury. In the propagation step anything will do to cleave the weak Hg-H bond but once the chain is running it is an alkyl radical that does this job, just as in tin hydride chemistry. 

The radical chemistry of organotins is undoubtedly dominated by tin hydride chemistry, which has been at the source of radical chain reactions of valuable interest for organic synthesis. However, the demonstration of the ability of allyltin reagents to undergo homolytic cleavage of the carbon-tin bond goes back to the early and only ten years... 

The initiation step (i) producing the tributyltin radical can be performed by thermolysis of radical initiators such as AIBN. Complementarily, the use of a Et3B/02 system or of photochemical irradiation allows the reactions to proceed at low temperature. The reaction (ii) with the substrate forming the carbon radical applies the same substrates as those used in tin hydride chemistry, such as iodides, bromides, selenides xanthates and even thiocarbonates. Furthermore, the possible competitive addition of tributyltin radicals to the allyltin reagent is of no consequence due to the rapid /3-fragmentation of the resulting carbon-centred radical, which renders this reaction a degenerate one. It is noteworthy that less reactive radical precursors such as chlorides or phenyl thioethers can be used efficiently. The evolution of the radical (iii) via several intra- or intermolecular elementary... 

The radical carbonylation of alkyl and aryl radicals and the cyclization of the resulting acyl radicals onto tcrz-butyl sulfides leads to the formation of y-thiolac-tones with expulsion of the tert-butyl radical (Scheme 4-52) [89]. This process is applicable to a range of substituted 4-rerr-butylthiobutyl bromides and iodides giving moderate to excellent yields of the corresponding thiolactones. Using acyl selenide/tin hydride chemistry and competition kinetic methods, the rate constant for the cyclization was determined to be 7.5x10 s at 25 °C [89]. 

The radical chemistry of organotins is overwhelmed by the tin hydride chemistry. In the past decades, the knowledge of kinetic parameters authorized the expeditious construction of complex molecules by using cascade radical reaction based on BujSn" methodology. Moreover, these strategies also offered an excellent dia-stereocontrol, especially for the construction of polycyclic skeletons. These synthetic applications of BujSnH, which will not be covered in this chapter, were reviewed in recent years [279]. In addition to the tin hydride chemistry, there are several applications of organotins in radical syntheses involving mainly allylstan-... 

In contrast with the important amount of work done to solve the purification problems caused by organotin side products in tin-hydride chemistry, very little attention has been paid to the allyl transfer process. Nevertheless, as most of the radical reactions are conducted with an excess of the allyltin, new allyltin reagents were proposed to optimize the purification process. 

To be complete one should add the extremely rich tin hydride chemistry, or the halodestannylation reaction as well as the creation of various carbon-heterode-ment bonds. We should also add the use of tin oxides or hydroxides as protecting groups in polyol chemistry, the use of organotins as catalysts for many reactions such as transesterifications or amide formation. Finally, we could also mention the use of optically active organotin Lewis acids as efficient chirality inductors. All of these reactions, which are not covered in this chapter tend to prove that the use of organotins for organic synthesis is now undoubtedly established and is not limited to the carbon-carbon bond formation with the aforementioned reactions. 

The tin hydrides find important applications as reducing agents. Many of their reactions (particularly the reduction of alkyl halides and the hydrostannation of simple alkenes and alkynes) arc known to proceed through RaSn- intermediates, and this aspect of their chemistry is referred to in Section II,G. 

The versatility, predictability and functional-group tolerance of free radical methodology has led to the gradual emergence of homolytic reactions in the armory of synthetic chemistry. Tin hydrides have been successfully employed in radical chemistry for the last 40 years however, there are drawbacks associated with tin-based chemistry. Organotin residues are notoriously difficult to remove from desired end products, and this, coupled with the fact that many organotin compounds are neurotoxins, makes techniques using tin inappro-... 

Alkyl tin compounds react with tin tetrahalides to produce a range of products that have the general formula R SnX4 n. These compounds react with LiAlH4 to produce tin hydrides. Other aspects of the organic chemistry of tin are presented in Chapter 14. 

On the heels of work by Zhu and Horvath and Rabai, perfluorocarbon solvents and fluorous reagents have been used increasingly in organic syntheses. Ruorous compounds often partition preferentially into a fluorous phase in organic/fluorous liquid-liquid extraction, thus providing easy separation of the compounds. Tris[(2-perfluorohexyl)ethyl]tin hydride combines the favorable radical reaction chemistry of trialkyltin hydrides with the favorable separation features of fluorous compounds. 

Trialkyltin hydrides represent an important class of reagents in organic chemistry because of their utility in radical reactions. However, problems of toxicity and the difficulty of product purification made trialkyltin hydrides less than ideal reagents. Several workup procedures and structurally modified trialkyltin hydrides have been developed to facilitate the separation of tin residues from the reaction mixture. Tris(trimethylsilyl)silicon hydride has also been synthesized and is often used successfully in radical reactions. However, its reactivity is different from that of trialkyltin hydrides in a number of important respects. Other tin hydride surrogates are also available. ... 

Derivatives of trifluoroethanethiol have limited though interesting chemistry. Unfortunately, metallated difluorothioenol chemistry has not been reported, because rapid nucleophilic attack occurs even by hindered bases such as LDA. Nakai et al. exploited this high electrophilicity in a tandem addition/elimina-tion-rearrangement sequence [146], but more recent applications have concerned free radical chemistry (Eq. 46). Chlorination of trifluoroethyl phenyl sulfide followed by exposure to tin hydride in the presence of an allylstannane resulted in C-C bond formation with a reasonable level of stereocontrol [147]. 

The wide versatility of the tin hydride method in carbohydrate chemistry exists because anomeric radicals can be generated from many functional groups at the anomeric... 

Tin hydrides bearing highly fluorinated substituents (fluorous chemistry) were used in the radical-mediated reduction of 1-bromoadamantane. The reaction was complete within 3 min under 35 W microwave irradiation (Scheme 4.42)68. 

Trialkyltin hydrides are common reagents in organic radical chemistry, but the toxic by-products are extremely difficult to remove from reaction products. To facilitate their removal, a pyrene-functionalized tin hydride has been prepared (entry 20) 25 After standard solution-phase radical reactions with the stannane, filtration through activated carbon traps the tin species to afford pure products. 

Fluorous organometallic chemistry, examples, 1, 842 Fluorous solubles, in organometallic synthesis, 1, 81 Fluorous solvents, for hydroformylations, 11, 450 Fluorous tin hydrides, preparation and applications, 9, 346 Fluorovinyl groups, vinylic C-F bond activation, 1, 753 Fluoro vinyltitanocenes, synthesis, 4, 546 g tfZ-Fluorovinyltributylstannane, in carbonylative cross-coupling, 11,413... 

Porous polymers, supported tin hydrides, 9, 347 Porphyrinato complexes, with Zr(IV), 4, 809 Porphyrin-pyridylphosphines, inclusion chemistry, 12, 814 Porphyrins... 

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