Trialkyltin hydrides hydride

Allyltin compounds can be prepared by simple modifications of the usual reaction involving allyl Grignard reagents (139), by the 1,4-addition of trialkyltin hydrides to 1,3-dienes 140,141), or by the reaction of an aldehyde or ketone with the appropriate, tin-carrying, Wittig reagents (142). 

Grignard reagents having bulky alkyl groups react with trialkyltin hydrides to give compounds having a Sn-Mg bond, and are synthetically useful as a source of nucleophilic RsSn in particular, they react with carbonyl compounds, oxiranes, and oxetanes to give the -, jS-, or... 

The products are yellow or red solids when R = Me, Et, Pr, or Bu, they decompose below —10°, but when R = Ph, or, particularly, when R = Me iCH, the products are more stable. They are oxidized immediately in air to the corresponding distannoxanes, readily exchange the trialkyltin group with trialkyltin hydrides, and add across polar-substituted alkynes or azo compounds. 

In the general context of donor/acceptor formulation, the carbonyl derivatives (especially ketones) are utilized as electron acceptors in a wide variety of reactions such as additions with Grignard reagents, alkyl metals, enolates (aldol condensation), hydroxide (Cannizzaro reaction), alkoxides (Meerwein-Pondorff-Verley reduction), thiolates, phenolates, etc. reduction to alcohols with lithium aluminum hydride, sodium borohydride, trialkyltin hydrides, etc. and cyloadditions with electron-rich olefins (Paterno-Buchi reaction), acetylenes, and dienes.46... 

Reactions of highly electron-rich organometalate salts (organocuprates, orga-noborates, Grignard reagents, etc.) and metal hydrides (trialkyltin hydride, triethylsilane, borohydrides, etc.) with cyano-substituted olefins, enones, ketones, carbocations, pyridinium cations, etc. are conventionally formulated as nucleophilic addition reactions. We illustrate the utility of donor/acceptor association and electron-transfer below. 

Several workers have investigated the reduction of halogens ft to a carbon-carbon 7r bond. For example, Fantazier and Poutsma94 have found that a halogen ft to a carbon-carbon triple bond can be reduced easily with a trialkyltin hydride (equation 70). 

While Baldwin and Barden119 found that triphenyltin deuteride added to the carbon-carbon triple bond of phenylacetylene in a stereo specific reaction, several workers120-124 have found that the addition of a trialkyltin hydride to a carbon-carbon triple bond gives a mixture of the cis and trans isomers (equation 89). The more stable trans isomer is produced in the highest yield. 

Obviously, the stereochemistry of these addition reactions is controlled by other factors. For instance, the mixture of cis and trans products obtained depends on the amount of the trialkyltin hydride and the temperature. Generally, a greater excess of the trialkyltin hydride and a higher temperature increase the yield of the more stable tram isomer123. 

Cobaloxime(I), electrochemically regenerated from chloro(pyridine)-cobaloxime (III) (232), has been employed as a mediator in the reductive cleavage of the C—Br bond of 2-bromoalkyl 2-alkynyl ethers (253), giving (254) through radical trapping ofthe internal olefin (Scheme 95) [390]. An interesting feature of the radical cyclization (253) (254) is the reaction in methanol, unlike the trialkyltin hydride-promoted radical reactions that need an aprotic nonpolar solvent. An improved procedure for the electroreductive radical cyclization of (253) has been attained by the combined use of cobaloxime(III) (232) and a zinc plate as a sacrificial anode in an undivided cell [391]. The procedure is advantageous in terms of the turnover of the catalyst and the convenience of the operation. 

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. ... 

Higher group IV hydrides also autoxidize readily. Since Curtice, Gilman, and Hammond have shown that the autoxidation of triphenyl-silane is a chain process (6), an initial hydroperoxide chain seems plausible. Trialkyltin hydrides similarly take up oxygen on exposure to air (13), yielding species with Sn—O bonds, but the reactions have not been studied in much detail. 

Trialkyltin hydrides are converted rapidly to trialkyltin iodides with molecular iodine. An ether solution of iodine is added drop wise until the iodine color just persists. 

Trialkyltin hydrides can serve as chain carriers in radical reactions. The low Sn-H bond energy (74 kcal/mol) facilitates easy homolytic cleavage by an initiator radical to produce tin radicals that are in a position Lo cleave a Cl-C bond with formation of a carbon radical. The resulting carbon radical cycli/es stereoselectively to pyrurt 6 in a b-exo-trig ring closure. ... 

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. 

Preliminary results confirm the expected utility of direct ionic substitution reations for the preparation of derivatives of diamantane. In refluxing bromine, diamantane gives 1-bromodiamantane (74) 26S> as well as a variety of disubstituted bridgehead bromides (Eq. (64)) 266 If, on the other hand, the reaction is carried out in the presence of AlBr3, significant amounts of 4,9-dibromodiamantane, 77, are also observed (Eq. (65) 265l Treatment of this dibromide with one equivalent of trialkyltin hydride yields the 4-mono-substituted diamantane, 78. 

The chemistry of R3MH (M = Ge, Sn, Pb) is highlighted firstly by the dominance of trialkyltin hydrides in synthesis, and secondly by the prevalence of free-radical chemistry in the large majority of transformations of synthetic significance reported in the literature. 

New methods for the preparation of germanes and stannanes reported since 1995 are dealt with in Section n. In Section III, radical chain chemistry involving trialkyltin hydrides is examined. In particular, the synthetic utility of tributyltin hydride will be reviewed, as well as that of other stannanes. Recent advances in the area of asymmetric radical chemistry involving chiral non-racemic stannanes are also included. Section IV details a limited number of examples of non-radical stannane chemistry, while Section V covers recent advances in germane and plumbane chemistry. While we have restricted ourselves largely to the literature since the beginning of 1996, some salient features of earlier work are included when relevant to the discussion. 

III. TRIALKYLTIN HYDRIDES AS REAGENTS IN RADICAL CHAIN REACTIONS... 

As already demonstrated, stannane chemistry often involves the intermediacy of free radicals. There are some notable examples, however, of non-radical transformations involving trialkyltin hydrides. This (much smaller) subset of reactions is dominated by transition metal catalysed hydrostannylation chemistry551,808-860, chemistry that rivals the free-radical examples provided above. In addition, there are a few examples of ionic reduction chemistry involving these reagents861,862. 

Sml2-mediated radical cyclisations involving alkyl, alkenyl and aryl radical intermediates can be used to construct efficiently five-membered and, in certain cases, six-membered ring systems. This approach provides a useful alternative to trialkyltin hydride-mediated methods as toxic reagents and problematic tin byproducts are avoided. In addition, the use of Sml2 to induce radical cyclisations has led to the development of a number of powerful, radical/anionic sequential processes for the construction of complex systems. Sequential reactions involving radical-alkene/alkyne cyclisations are discussed in Chapter 6. 

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