Hexabutyldistannane1

Bu3Sn-SnBu3

[813-19-4]  · C24H54Sn2  · Hexabutyldistannane  · (MW 580.20)

(source of tributylstannyl radicals; used in palladium-catalyzed tin-carbon bond formation; used for deoxygenation and desulfurization reactions)

Alternate Name: hexabutylditin.

Physical Data: bp 147-150 °C/0.2 mmHg, 198 °C/10 mmHg; d 1.1520 g cm-3; n20D = 1.5120.

Solubility: sol most organic solvents.

Form Supplied in: colorless oil; readily available and not expensive.

Analysis of Reagent Purity: 119Sn NMR recommended (d -83 ppm, 1J(119Sn-119Sn) 2748 Hz).9

Preparative Methods: various methods are available. The classical method uses Tri-n-butylstannane and either Bis(tri-n-butyltin) Oxide (0.5 equiv), tributyldiethylaminotin, or Tri-n-butyl(methoxy)stannane in yields of 90-98%.2 Since hexabutyldistannoxane is commercially available and not air-sensitive, its use is preferable. A second method involving the stannoxane involves its treatment with metals (Mg, Na, K, Ti/K); the yields lie between 70 and 80%.3

It can also be obtained from Tri-n-butylchlorostannane and either lithium, sodium,4 or magnesium,5 from tributyltin chloride and Tri-n-butylstannyllithium7 (the latter from tributyltin hydride and Lithium Diisopropylamide6), or by catalytic elimination of hydrogen from tributyltin hydride. This can be effected either by various bases8a or by palladium catalysts such as Tetrakis(triphenylphosphine)palladium(0).8b

Purification: reversed-phase flash chromatography using C-18.10

Handling, Storage, and Precautions: must be stored in the absence of oxygen, moisture, and light (preferably under argon). Highly toxic. Use in a fume hood.

Introduction.

General aspects of the preparation and chemistry of hexaalkyldistannanes are reviewed in the Houben-Weyl volume on organotin compounds (literature coverage up to 1977).1

The chemistry of hexabutyldistannane is determined by the weakness of the tin-tin bond and dominated by three aspects:

  • 1)it dissociates on heating to give tributylstannyl radicals, which are of considerable importance in organic synthetic transformations;
  • 2)under the influence of palladium catalysts it can be used for tin-carbon bond formation in the sense of either substitution reactions or addition reactions to multiply bonded systems;
  • 3)it can readily take up oxygen or sulfur and can thus be used for deoxygenation and desulfurization reactions.

    It can also be used for the preparation of other organometallic reagents via transmetalation.

    Use as a Source of Tributylstannyl Radicals.

    Thermolysis or photolysis of hexabutyldistannane generates tributylstannyl radicals; these can in turn be used to generate other synthetically useful radicals, for example carbon radicals from reactions with organic halides or sulfides.11 This reaction has the advantage over generation of stannyl radicals from triorganotin hydrides in that the latter can act as powerful hydrogen donors and thus strongly influence subsequent reactions.

    The distannane can also be used in less than stoichiometric amounts, the reactants being irradiated in the presence of a small amount of the distannane (generally 0.1-0.3 equiv). This so-called atom transfer method has been developed by Curran and also applied by other authors. Examples include the radical cyclization of a-iodo ketones (or esters with a suitably placed double bond) (eq 1),12 annulation reactions of iodomalonates (eq 2),13 cyclizations of unsaturated a-iodocarbonyls,14 and radical cyclization of a-fluoro-a-iodo and a-iodo esters and amides (eq 3).15 A novel cyclization of an iodoepoxide to a cyclopentanol possessing the hydrindane skeleton has also been reported (eq 4).16 This type of reaction may be subject to a temperature effect; thus the cyclization of allylic a-iodo esters and amines proceeds much more efficiently at 80 °C than at 25 °C.17

    An in situ generation of nitrile oxides via photolysis of hexabutyldistannane has been reported (eq 5).18

    Palladium-Catalyzed Reactions.

    These can be of two types, either substitution of a (generally halide) ligand by a tributylstannyl group or addition to multiple bonds. These reactions have been reviewed by Stille19 and Mitchell.20

    The substitution reactions provide a useful alternative to the conventional use of Tri-n-butylstannyllithium, which is a very strong base. The halides used have mainly been aryl or heteroaryl halides, while Tetrakis(triphenylphosphine)palladium(0), dichlorobis(triphenylphosphine)palladium(II), and Bis(allyl)di-m-chlorodipalladium/Tetra-n-butylammonium Fluoride have been employed as catalysts.

    The synthesis of symmetrical biaryls has been described.21 This principle has recently been applied in an intramolecular manner and extended to include more complex cyclization reactions. Thus as well as two (symmetrical or mixed) aryl and benzyl halide moieties, a combination of two aryl iodide moieties with a carbon-carbon double or triple bond can be used (eq 6).22

    Unfortunately the reaction of Bu6Sn2 with acyl halides is not suitable for the preparation of tributylacylstannanes, in contrast to the corresponding reaction of Hexamethyldistannane.23

    A further interesting development is the use of three-component systems: a mixture of hexabutyldistannane, an allyl halide or carbonate, and a heteroaryl bromide leads to allylation of the heteroaromatic moiety; the catalyst used was the somewhat exotic Dichloro[1,1-bis(diphenylphosphino)ferrocene]palladium(II).24 This method avoids the prior preparation of organotin reagents (eq 7).

    In the case of the addition reactions, hexabutyldistannane adds readily to a variety of allenes; it is often possible to distinguish a kinetic and a thermodynamic product (eq 8).25

    On the other hand, the addition to alkynes does not proceed in a quantitative manner at atmospheric pressure (in contrast to the behavior of hexamethyldistannane), though it can be forced to do so by the application of high pressure.26

    Deoxygenation and Desulfurization Reactions.

    The deoxygenation of amine oxides has been described,27 as has the photo-desulfurization of 1,3-dithiole-2-thiones to give tetrathiofulvalenes.28

    Use as a Source of Other Tributylstannylmetal Compounds.

    Hexabutyldistannane can be cleaved by lithium metal29 or a lithium alkyl (e.g. MeLi)30 to give tributylstannyllithium. These methods cannot be recommended: Bu3SnLi can be better obtained either in a one-step process from Bu3SnCl and Li (via Bu6Sn2 which is not isolated) or from the reaction between Bu3SnH and LDA6 (see Tri-n-butylstannyllithium for details). Hexabutyldistannane can also serve as a source of stannylcuprates (Bu3SnCu(CN)Li, (Bu3Sn)2Cu(CN)Li2, Bu3Sn(R)Cu(CN)Li2).31

    Use in Electron-Transfer Reactions.

    It has been shown in a very recent development that Bu6Sn2 (and in addition Bu4Sn) can take part in novel substitution reactions when allowed to react with pyridine derivatives.32 Thus treatment of 4-cyanopyridine with Bu6Sn2 leads to a new type of ipso substitution accompanied by a substitution of the type observed when lepidine reacts with Bu6Sn2 and t-butyl bromide (eqs 9 and 10)


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    2. Neumann, W. P.; Schneider, B.; Sommer, R. LA 1966, 692, 1.
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    Terence N. Mitchell

    University of Dortmund, Germany



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