Stannyl anions

The reduction of the stannyl radical (t-Bu2MeSi)3Sn with alkali metals produces a variety of structural modifications depending on the solvent used (Scheme 2.55). Thus, in nonpolar heptane, a dimeric stannyllithium species [58c Li ]2 (E = Sn) was formed, whereas in more polar benzene, the monomeric pyramidal structure 58c [Ti -Li (C6H5)] was produced. In the latter compound the Li+ ion was covalently bonded to the anionic Sn atom being at the same time n -coordinated to the benzene ring. A similar monomeric pyramidal CIP 58c [Li (thf)2] was prepared by reduction in polar THE the addition of [2.2.2]cryptand to this compound resulted in the isolation of the free stannyl anion 58c K+([2.2.2]cryptand), in which the ion lacked its bonding to the Sn atom. ... 

Stannyllithium compounds are important as sources of nucleophilic stannyl anions, and the dialkyltin lithium hydrides, R2SnLiH, have recently come to prominence as their reaction with electrophilic alkyl halides gives hydrides, R1R2SnH, with mixed alkyl groups (see Section 3.14.18.1).397... 

The first tin-centered radical has been isolated, though it is not strictly organometallic in the C-Sn bonded sense. It is prepared by the reaction shown in Equation (173), in which the SnCl2 acts both as a substrate for forming the stannyl anion and as a one-electron oxidizing agent. 

Fluoride ion first becomes coordinated with the silicon atom of tin reagent 33. Further coordination with the carbonyl oxygen atom of substrate 34 leads to an especially activated hypervalent silicon species 35.21 from which a stannyl anion is eliminated. I Ialogen-metal exchange between the tin anion and inyhc iodide 36 produces vinylic anion 37, which attacks in an intramolecular way the carbonyl group that has been activated through silicon coordination. Aqueous workup leads to alcohol 38. 

Bulky stannyl anions which are not available from stannyl hydrides (see Section I.A.l) were prepared by using the reaction of alkali metals with tin halides (equations 24 - 26)31,40 41. 

In a similar way, but in a one-pot experiment (equation 27)42, a stable silylated stannyl anion was prepared in 44% yield and isolated as white crystals. X-ray crystallography showed a distorted tetrahedral tin atom with a Sn-Li distance (2.87 A) which is shorter than those published for [Li(THF)3.Sn (4-MeC6H4)NSiMe2 3CH] (2.89 A) and [Li(PMDETA).SnPh3] (2.87 A), but longer than the Li-Mi4 distances in the Ge and Si analogues (2.67 A) and (2.64 A), respectively. 

Cleavage with cesium fluoride was used in the case of stannylsilanes (equation 34)49. The generated stannyl anions are very effective in synthetic applications, mainly in abstraction of halogen to initiate organic 4 + 2 cycloadditions (equation 35)49, a reaction which constitutes one of its chemical characterizations. 

A 1,2-oxastannetanide, which is an example of an anionic pentacoordinate tin compound with a four-membered ring oxygen and one organic substituent in apical position, was prepared at room temperature by deprotonation of the corresponding / -hydroxystannane using KH in THF and 18-crown-6 (equation 59)76. The presence of a stannyl anion was evident by the appearance of a double quartet with Sp centers at —75.09 ppm... 

Similar oxidative additions of BU4NF to tetracoordinate monoorganostannanes synthesized from a diaminostannylene also afforded pentacoordinate stannyl anions (equation 61)78. 

Stannyllithiums are synthetic equivalent of stannyl anions. The reaction of 2,6-di- 7 /-butyl-4-rncthylphcnyl(UI IT) esters with BuLi, followed by the addition of Bu3SnLi, and trapping of the resulting lithium enolate with phenyldi-methylchlorosilane leads to silyloxyvinylstannanes, which are allowed to the subsequent coupling with vinyl iodides (Equation (110)).279... 

When Me3SiSnBu3 is treated with CsF, the fluoride anion should coordinate to the silyl group and not the stannyl group to produce a hypervalent silicate, and as a result, a stannyl anion would be generated.282 The stannyl anion reacts with vinyl iodide to produce a vinyl anion via a halogen-metal exchange and it reacts with the carbonyl group intramolecularly (Equation (113)). Aryl halides or allyl halides are also used in similar cyclizations.283,284... 

A stannyl anion reacts with an unsaturated ester to give /3-stannylated esters. In this reaction, the intermediate is a silyl enolate. When a ketoester is employed, an intramolecular cyclization takes place (Equation (117)).289... 

Several publications about oxidative addition of metal or organometal derivatives to stannylenes describe a new and efficient way to stannyl anions . The reaction of CpLi with Cl2Sn resulted in a mononuclear complex as colorless cubic crystals obtained in 48% yield (equation 53) °. Its structure was resolved by X-ray diffraction and shows a complete separation of the ion-pair . 

Stannyl anions with a highly coordinated tin center are also known. A hydridostannyl anion in the shape of a trigonal bipyramid in which two iodine atoms occupy the apical positions was obtained by oxidative addition of lithium iodide to the corresponding tin hydride (equation 58) . It was characterized by Sn NMR. Since apical iodines are more nucleophilic than the hydrogen, in its reactivity with a-ethylenic carbonyl compounds, attack by iodine precedes reduction by hydrogen, achieving regioselective 1,4 reductions. 

Conjugate addition of stannyl anions to acyclic a,/ -unsaturated ketones and esters is also stereoselective, cf. 66, although in some cases (cf. 4 and 5s) the stereoselectivity is low5,6. 

This method was applied to the diastereoselective synthesis of vinyl ethers from readily available a-alkoxy aldehydes9. Reasonable selectivity in favor of the syn-adduct is observed when the stannyl anion is modified by softer metal salts such as zinc halides or copper(I) bromide. 

Trialkylsilyl higher-order cyanocuprates are prepared directly by trans-metallation of silylstannanes with higher-order dibutylcyanocuprates (Scheme 32) (101). In this method, steric bulkiness around silicion is essential for formation of the silylcuprates, so that the t-butyl or thexyl group is present on silicon otherwise, stannyl anion is formed. These reagents undergo typical silyl-cupration reactions, as mentioned earlier. 

Silyl and stannyl anions react with arsenic halides to give the corresponding As—Si and As—Sn compounds, respectively (equations 225 and 226 ). 

Potassium trialkyl- and triarylstannates are air and moisture sensitive. They are thermally instable, leading to R3Sn—SnR3, which is known to catalyse the decomposition of stannyl anions. They must be handled and stored in an inert atmosphere (nitrogen or argon) by normal vacuum line techniques. For a prolonged storage, it is desirable to store the substance at low temperatures (below -40°). 

Water and alcohol react quantitatively with potassium stannates, leading to the corresponding triaryl- or trialkylstannanes [Ph3SnH 95% yield, bp 165-168° (0.3 torr), mp 28° (/i-Bu)3SnH 90% yield, bp 68-74° (0.3 torr)]. Deuterolysis is well suited for determination of stannyl anion content. 

The rates of the reactions, however, depend on the structure of the stannyl anion and of the organic substrate, on the nature of the solvent, and on the presence or absence of radical initiators, particularly light. Four principal mechanisms have been identified. 

The stannylsilanes, particularly Bu3SnSiMe3, are finding increasing use in organic synthesis in the palladium- or platinum-catalysed stannylsilylation of alkynes and dienes, and in the reactions involving the halide-induced formation of stannyl anions.65... 

In order to provide a modular route compatible with different lengths of the spacer (>2 carbon units), another route has been developed consisting of initial grafting of a terminal chloroalkyl spacer (on Amberlite XE 305). The addition of an appropriate stannyl anion followed by a halogenation reaction affords the polymer C (Scheme 5.5.4). ° ... 

Stannylsilane (51) reacts selectively with fluoride ion on the silicon site, generating stannyl anion, which can cleave vinyl and aryl iodides under very mild conditions to give vinyl and aryl anions, respectively. Likewise, other halides and cyanide ions attack selectively at the harder silicon site of 51, chemoselectivity of which can be explained on the basis of HSAB theory. An application of the stannyl anion chemistry for the synthesis of (-)-cephalotaxine (56) via spirocyclization is shown in Scheme 3.28 [31]. 

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