Tin bonding

Compounds with Tin—Tin Bonds. The most important class of catenated tin compounds is the hexaorganoditins. The ditin compounds are usually prepared by reductive coupling of a triorganotin haUde with sodium in Hquid ammonia ... 

The silyl and stannyl substituents are crucial to these reactions in two ways. In the electrophilic addition step, they act as electron-releasing groups promoting addition and also control the regiochemistry. A silyl or starmyl substituent strongly stabilizes carboca-tion character at the /3-catbon atom and thus directs the electrophile to the a-carbon. The reaction is then completed by the limination step, in which the carbon-sihcon or carbon-tin bond is broken. 

The mechanism of the reaction was postulated as being either synchronous attack of the RO" ion (R = Me or H) at tin and breaking of the aryl-tin bond, or rapid formation of a pentacoordinate tin intermediate [ROMe3SnPh-] followed by rate-determining breaking of the aryl-tin bond. In either case the phenyl-carbanion could separate as such and then react rapidly with solvent, or it could react with solvent as it separated. 

This prediction has now been confirmed by the observation (R. Alexander, C. Eaborn and T. G. Traylor, /. Organometal. Chem., 21 (1970) P65) that a solvent-isotope effect is obtained in base cleavage of benzyl- and aryl-tin bonds by NaOH in MeOH-MeOD. 

Second, the theory of hybrid bond orbitals was utilized recently to discover a new type of chemical bond involving the resonating unshared electron pair.30 31 For example, bis(bistrimethylsilylmethyl) tin(II), (CH3)3 Si 2HCSnCH Si(CH3)3 2, forms dimers in the solid state having a tin-tin bond characterized by resonance of an unshared electron pair or... 

The earlier work on acidolysis of the aryl-tin bond is reviewed in reference (97). Attachment of the proton to the aryl ring is rate-determining, and the Hammett p-factor for the reaction has been shown to... 

Acidolysis of the alkyl-tin bond provides a useful route from tetraal-kyltins to alkyltin carboxylates, and is discussed in Section II,C. 

The mechanism of the cleavage of the alkyl-tin bond by mercuric halides and carboxylates has been thoroughly investigated, and the evidence is in favor of an open Se2 transition state (104-110). 

The reactivity of various groups (R) follows the sequence allyl, benzyl > aryl > alkyl, and usually proceeds readily to the stage of RaSnlOSORlj (121,122), but pentafiuorophenyl- and trifluorovinyl-tin bonds are usually unreactive. The reactivity is enhanced by such ligands as bipyridyl (123). 

Photoelectron spectroscopy shows that the carbon-tin bond prefers that orientation in which it lies parallel to the pir orbitals of the double bond, to permit carbon-metal hyperconjugation (146). 

The chemistry of cyclopentadienyltin compounds is reviewed in references (267) and (26S). The ready disproportionation of (CsH5)4Sn with SnCU (269), and the sensitivity of the cyclopentadienyl-tin bond to acidolysis and to photolysis (270) suggests that these compounds may find application in synthesis. 

Tin-tin bonds are usually best prepared by reducing an Sn-0 or Sn-N bonded compound with a tin hydride. For example, trimethyl (diethylamino)tin is reduced by alkyltin trihydrides to give decaorganotetratins (256). 

The tin-tin bond is also cleaved by alkylmercuric halides, triethyl-... 

A single-crystal. X-ray diffraction analysis of the structure has recently been performed that shows that the compound is, in fact, a tin-tin bonded dimer, having an Sn-Sn bond length of 276 pm, similar to that in hexaphenylditin this was interpreted in terms of overlap of a filled spaPy orbital with the vacant p orbitals on the other tin atom resulting in a "bent, weak, Sn-Sn double bond (332). 

The trifluoromethyl-tin bond is, however, much less stable chemically (24). Reaction of, e.g., (CF3)2SnBr with an excess of the relatively covalent, methylating agent (CHaljCd results in the very slow substitution for one of the Sn-CFg bonds, but the reaction of CFaSnBr3 with an excess of the more powerful, more ionic reagent methyllithium results in the displacement of all of the ligands, and the formation of (CH3)4Sn as shown in Equations 16 and 17. 

Organotin compounds are characterized by the presence of a carbon-tin bond and have the following general formula ... 

The classical route to synthesize organic tin(II) compounds is the thermolysis or photolysis of tetravalent tin compounds. The driving force of these reactions may be either weak tin-tin bonds or the formation of stable compounds besides the desired... 

Peddle and Redl,0) were still rather pessimistic in 1970 Thus while it should be possible to resolve an optically active organotin compound with four carbon-tin bonds, it seems unlikely that such a compound would be very useful in investigating the stereochemistry of substitution at the tin atom 10). 

The problems associated with the use of this classical method in organotin chemistry are essentially due to the fact that the carbon-tin bond can sometimes very easily be cleaved by electrophiles or by nucleophiles. The crucial step is therefore the elimination of the auxiliary group without the cleavage of any of the carbon-tin bonds. This cleavage could for instance not be achieved successfully in the case of /7-(z -propylmethylphenylstannyl)-N,N-dimethylaniline [formula (60) in... 

Another point which has already been mentioned before, and which is not specific for organotin compounds, is that the diastereomers should be distinguishable by a spectroscopic method (for instance NMR) to be sure that their separation was sufficient enough to give at least one of them in pure form. Till now, this has been the case only once. The methoxy signal seems to be a good probe for this purpose (see above), but unfortunately, the anisyl-tin bond is cleaved much more rapidly than the phenyl- or naphthyl-tin bond, which might sometimes cause problems in the crucial step. 

The trityl-tin bond is cleaved by triphenylstannyllithium (+)-(67) reacts with PhjSnLi to give 54% PhgSr, 50% optically inactive (72) and 30% racemic Me(PhMe2CCH2)PhSnSnPl (75)66). 

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. 

This reaction has been studied, and it was assumed that the hydride attacks electrophili-cally on nitrogen in a polar reaction forming a tin-tin bond. This means that the amine behaves as a catalyst. No formation of hydrocarbons or ammonia was observed in this reaction. 

It is proposed that the reaction proceeds through (i) oxidative addition of a silylstannane to Ni(0) generating (silyl)(stannyl)nickel(n) complex 25, (ii) insertion of 1,3-diene into the nickel-tin bond of 25 giving 7r-allylnickel intermediate 26, (iii) inter- or intramolecular allylation of aldehydic carbonyl group forming alkoxy(silyl)nickel intermediate 27, and (iv) reductive elimination releasing the coupling product (Scheme 69). 

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