Distannanes

Examples BjHg, diborane Bj(,Hj4, decaborane(14) BjqHjj, decaborane(16) P2H4, diphos-phane SnjHg, distannane HjSej, diselane HjTej, ditellane H2S5, pentasulfane and PbH4, plum-... 

Catenation is well established in organotin chemistry and distannane derivatives can be prepared by standard methods (see Ge, p. 396). The compounds are more reactive than organodiger-manes e.g. Sn2Meg (mp 23°) inflames in air at its bp (182°) and absorbs oxygen slowly at room temperature to give (Me2Sn)20. Typical routes to higher polystannanes are ... 

The disproportionation reactions of organotin compounds may also be regarded as alkylations by organometallic compiounds, as they involve transfer of an alkyl group from one tin atom to another. An ingenious application of this has been described in which a,ct>-distannanes are caused to disproportionate into the corresponding tetraalkyltins and 1,1-dialkyIstannacycloalkanes (65). 

Transient dialkylstannylene intermediates R2Sn (reviewed in ref. 333) can be prepared by the thermolysis of distannanes ClR2SnSnR2Cl, R3SnSnR2Cl, or HR2SnSnR2H, e.g., (334, 335)... 

ESR spectrum of the RjSn radical (R = 2,4,6-triisopropylphenyl), spontaneously generated upon dissolving distannane RBSn-SnRj in deoxygenated toluene, revealed at -140°C in the solid state the Sn hfcc of 163 Such a radical is more planar than the PhjSn radical ( Sn hfcc = 186.6mT) but less planar than the MejSn radical C Snhfcc = 161.1 mT)3° ... 

The mercury atom is four-coordinate, both nitrogen atoms being involved in intramolecular coordination. The reaction of (Cabc,N)SnMe2Br with Na afforded the bis(o-carboranylamino)distannane [(Cabc,N)SnMe2]2 53. The two tin atoms exhibit approximately trigonal-bipyramidal geometry. 

It should be noted that 1,1-organoboration of Me3SiC=CMe with Et3B requires harsher conditions 100 °C and 24 h. Bis(trimethylsilyl)ethyne does not react with Et3B, but readily (20 °C) inserts into the 1-boraadamantane core giving rise to an equilibrium mixture containing c. 80% of 92 (Scheme 35). Treatment of this mixture with bis(trialkylstannyl)ethynes leads to the formation of distannane derivatives 93 (R = Me, Et) which seem to be more stable than 92 <2001JOM(620)51>. 

These reactions to form aryl tin bonds could occur by initial oxidative addition of the aryl halide or the distannane. The stoichiometric reaction between [(PPh3)2Pd(Ph)(I)] and Me3SnSnMe3 in the presence of chloride generated good yields of the aryltin product. This result suggests that the reactions occur by initial oxidative addition of aryl halide. 

Table 15 lists bond parameters for the simple digermanes, distannanes, diplumbanes and mixed dinuclear species. Some disilanes and the gas-phase value for Ge2Hg are included for comparison. The set of Si—Ge examples has increased since Sheldrick s review, and all the other E—E combinations are now represented. 

Upon exposure to 10% Pd/C in pentane, the nonracemic stannyl hydride 37 is converted to a mixture of meso and enantioenriched distannanes 4625. Dimerization of 37 can also be achieved by treatment with LiAlFLt or Me2Hg, but these reactions lead to mixtures of meso and racemic distannanes (equation 12). 

The molecular structure of compound 84 in the gaseous phase was analysed by electron diffraction. It has chair conformation with Cs symmetry. The axial methyl groups are twisted outwards to relieve steric repulsion259. The symmetry of gaseous tetraphenyl-germane is debatable on the basis of its electron diffraction spectrum260. Hexamethyl-distannane (MesSnSnMes) has D3 symmetry in the gas phase, as shown by electron diffraction261. 

Using alkyllithium compounds, a distannane with a very long Sn—Sn bond, 3.03A, and a syw-peripheral conformation122 was prepared. Thus, reaction of di-t-butyl(chloro)2,4,6-triisopropylphenyl)tin, 78, with f-butyllithium gives 1,1,2,2-tetra-f-butyl-l,2-(2,4,6-triisopropylphenyl)ditin, 79, which displays a restricted rotation about the Sn—C bond as well as the Sn—Sn bond, even at high temperatures. 

Besides the addition of distannanes to allenes where both Z- and -isomers are generated, the same type of reaction occurs between RC=CH and hexaalkyldistannanes but, in this case, only the Z-isomer is formed. 

Addition of distannane to alkenes has been achieved only with strained cyclopropenes (Equation (60)).158 3,3-Disubstituted cyclopropenes undergo highly face-selective distannation in the presence of the palladium-isocyanide complex to afford m-adducts. 

The intermediate Sn-Pd-Sn compound in distannation (Equation (34)) has recently been isolated in the reaction of an r -distannane.126... 

The distanna[2]ferroceneophane319-320 shows the characteristic behavior of a distannane, inserting platinum into... 

Hexabutyldistannane, which is an important reagent in many organic syntheses, can be prepared very conveniently by reducing bis(tributyltin) oxide with sodium borohydride in ethanol at room temperature. After 5 min, the only tin species present is tributyltin hydride, but in 2 h, its decomposition is catalyzed by the ethoxide ion that is formed to give the distannane in 83% yield.444 Hexaalkyldistannanes, R3SnSnR3 (R = Et, Pr, or Bu), are obtained in ca. 50% yield when the corresponding trialkyltin halides are treated with zinc powder in THF.445... 

Recent interest has been focused more on the formation of distannanes by treatment of tin hydrides with transition metals, rather than on these more classic methods.443... 

A number of transition metal complexes will catalyze the dehydrogenative coupling of organotin tin hydrides, R SnI I, to give the distannanes, RjSnSnRj.443 These metals include palladium,449 gold,450, hafnium,451 yttrium, and ruthenium.452 The catalyst that is most commonly used is palladium, often as Pd(PPh3>4, and the most active catalysts appear to be the heterobimetallic Fe/Pd complexes, in which both metals are believed to be involved in the catalysis.443... 

Most of the reactions that can be used to prepare distannanes can be extended to the preparation of oligo- and polystannanes, which have attracted interest for potential use in electronic and optical devices.455 The structures of the products are not necessarily those of completely linear catenanes, (R2Sn) , and different degrees of branching confer different properties. The formation of polymers is also frequently accompanied by the formation of cyclic pentamers or hexamers, and samples prepared by different methods may show substantially different properties. 

Stannyl radicals are usually generated by homolytic substitution at hydrogen in a tin hydride, or at tin in a distannane, or, conjugatively, at the y-carbon atom in an allylstannane.453 The initiator is commonly AIBN at ca. 80 °C. In the presence of a trace of air, organoboranes are oxidized by a radical chain mechanism, and triethylborane is now commonly used as an initiator at temperatures down to —78°C,519 and it can be used in aqueous solution.520 9-Borabicyclo[3.3.1]nonane (9-BBN) has similarly been used to initiate the reaction of tin hydrides at 0 and —78°C,521 and diethylzinc works in the same way.522... 

When the distannyl reagent 7 is employed, the fate of the reaction apparently depends on the reaction conditions. Indeed, Eisch observed that the reaction of 7 with boron trichloride at —40 °C in hydrocarbon solvent is not always selective and leads to the formation of both a monoborylated intermediate (10) that slowly converts into 6 (Scheme 5). The outcome of these reactions is apparently further complicated by a fast methyl group transfer from the tin to the boron centers yielding the undesired monoborylated derivative 11. Eisch also found that such reactions are not limited to the case of boron trialide but can be performed with dialkyl boron halide starting materials. For example, the reaction of the distannane... 

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