[661-69-8] · C6H18Sn2 · Hexamethyldistannane · (MW 327.66)
(palladium-catalyzed substitution and addition reactions)
Physical Data: colorless oil, bp 62-63 °C/12 mmHg, 182 °C/756 mmHg; mp 23 °C; n
Solubility: sol most organic solvents.
Form Supplied in: colorless oil; readily available but highly expensive.
Analysis of Reagent Purity: 119Sn NMR recommended (d -109 ppm, 1J(119Sn-119Sn) 4404 Hz).8
Preparative Methods: although the preparation from Trimethylstannane and trimethyl(diethylamino)stannane affords an excellent yield (95%), it has the disadvantage of requiring two extremely air-sensitive starting materials.2 Better methods involve the use of Chlorotrimethylstannane with either Lithium or Sodium,3 or Trimethylstannyllithium (the latter from trimethylstannane and Lithium Diisopropylamide4).5 Catalytic elimination of hydrogen from trimethylstannane also affords hexamethyldistannane and can be effected either by various bases6a or by palladium catalysts such as Tetrakis(triphenylphosphine)palladium(0).6b This latter method can be recommended. The preparation of hexamethyldistannane starting from bis(trimethylstannyl) sulfide has also been described.7
Handling, Storage, and Precautions: the compound must be stored in the absence of oxygen, moisture, and light (preferably under argon). Very highly toxic. Use in a fume hood.
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 hexamethyldistannane is determined by the weakness of the tin-tin bond. The main reaction of synthetic importance is its use in palladium-catalyzed substitution reactions or addition reactions to multiply bonded systems.
While, in analogy to Hexabutyldistannane, Me6Sn2 dissociates on heating to give trimethylstannyl radicals, these are generally no longer used in organic synthetic transformations because of the toxicity of methyltin compounds and their cost. Me6Sn2 can also be used in transmetalation reactions, leading to the formation of other trimethylstannylmetal reagents.
These can be of two types, either substitution of a ligand (generally halide) by a trimethylstannyl group or addition to multiple bonds. These reactions have been reviewed by Stille9 and Mitchell.10
The substitution reactions provide an extremely useful alternative to the conventional use of Trimethylstannyllithium, which is a very strong base. The halides used have mainly been aryl or heteroaryl halides, while Tetrakis(triphenylphosphine)palladium(0), dichlorobis(triphenylphospine)palladium(II), and Bis(allyl)di-m-chlorodipalladium-Tetra-n-butylammonium Fluoride have generally been employed as catalysts. In more recent developments, couplings involving enol triflates11 and aryl triflates12 have been described.
The first example of a cine substitution was observed when a vinylstannane derived from a camphor triflate and hexamethyldistannane was allowed to react with bromobenzene (eq 1).13
The synthesis of symmetrical biaryls (using Bu6Sn2) has been described.14 This principle has recently been applied in an intramolecular manner and extended to include more complex cyclization reactions; hexamethyldistannane can be used as an alternative to hexabutyldistannane. 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 2).15
The reaction of Me6Sn2 with acyl halides provides an excellent method for the preparation of trimethylacylstannanes (eq 3).16
In the case of the addition reactions, hexamethyldistannane adds readily to a variety of allenes to give 2,3-distannyl-1-propenes; it is often possible to distinguish a kinetic and a thermodynamic product (eq 4).17
The addition to 1-alkynes proceeds in a quantitative manner at atmospheric pressure to give (Z)-1,2-distannylalkenes, which can in some cases undergo photochemical isomerization to afford the corresponding (E)-products in quantitative yields (eq 5).18
The presence of various functional groups is tolerated; thus the preparation of trimethylstannyl-substituted allylglycine derivatives19 and (Z)-4-trimethylstannyl-1,3-butadienes20 (via the enones) have been described (eqs 6 and 7). The first tris(trimethylstannyl)ethylenes were obtained by using a more active precatalyst system.21
In spite of the development of new catalyst systems, additions to nonterminal alkynes remain a virtually unsolved problem. Two groups have described the addition of hexamethyldistannane to 1,3-dienes; depending on the reaction conditions, the products are either (Z)-1,4-bis(trimethylstannyl)-2-butenes22 or the products of addition/dimerization (eqs 8 and 9).23
Thermolysis or photolysis of hexamethyldistannane generates trimethylstannyl radicals, which can in turn be used to generate other synthetically useful radicals. On this basis, Curran has developed the so-called
atom transfer method for radical cyclization, in which about 0.1 equiv. of the distannane is used. While in his original work24 on the cyclization of a-iodo esters, ketones, and malonates Curran used both hexamethyl- and hexabutyldistannane, later papers by his and other groups have reported only the use of Bu6Sn2 (for reasons of cost and toxicity). Me6Sn2 has been used to effect a selective one-electron radical chain reduction of the 10-methylacridinium ion to 10,10´-dimethyl-9,9´-biacridine.25 In combination with 1,1-Di-t-butyl Peroxide (as the radical initiator) it can generate sulfonyl radicals from sulfonate esters.26
Hexamethyldistannane can be cleaved by lithium metal27 or a lithium alkyl28 (e.g. MeLi) to give trimethylstannyllithium. These methods cannot be recommended: Me3SnLi can be better obtained either in a one-step process from Me3SnCl and Li (via Me6Sn2 which is not isolated) or from the reaction between Me3SnH and LDA (see Trimethylstannyllithium for details). Hexamethyldistannane can also serve as a source of stannylcuprates (Me3SnCu(CN)Li, (Me3Sn)2Cu(CN)Li2,29 Me3Sn(2-thienyl)Cu(CN)Li2).30
Terence N. Mitchell
University of Dortmund, Germany