Alkyl tin bond

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). 

Addition reactions at the alkyne bonds are dealt with in the section on alkenylstannanes that are produced. The alkynyl-tin bond is more readily cleaved by both electrophiles and nucleophiles than is the alkenyl- or alkyl-tin bond. Strong electrophiles such as halogens or halogen acids attack at the z/Mzi-position of the triple bond to give a /3-stannyl cation that is stabilized by C-Sn hyperconjugation, but this is followed by cleavage of the C-Sn bond (Equation (83)). 

Alkyl-tin bonds can be cleaved with lead tetraacetate, the reactivity falling in the order Ar m Me > Et > Bu >> Pr1 (Equation (130)) and the reaction of ArSnBu3 provides a convenient route to the aryllead triacetates, ArPb(OAc)3.344... 

In the stannole series, the reactions could be explained by the extraordinary reactivity of the exocyclic tin-carbon bonds with regard to lithium reagents. Thus, the reaction of 1,4-dilithio-1,2,3,4-tetraphenyl-1,3-butadiene with 1,1 -dialkyl-2,3,4,5-tetraphenylstannole leads to alkyl-tin bond cleavage-cyclization by the dilithium reagent33 (equation 61). 

RELATIVE SECOND-ORDER RATE COEFFICIENTS AND ACTIVATION PARAMETERS FOR THE CLEAVAGE OF ALKYL-TIN BONDS BY MERCURIC IODIDE IN SOLVENT 96% METHANOL-4 % WATER AT 298 °K.18... 

The brominolysis of a number of unsymmetrical tetraalkyltins was later investigated3 5, and rate coefficients for cleavage of given alkyl-tin bonds determined by the methods described previously. In this later work35, a value of k°2s = 1.42... 

The brominolysis of a number of unsymmetrical tetraalkyltins was also studied35 and second-order rate coefficients for the cleavage of given alkyl-tin bonds determined by combination of rate studies with product analyses. Details are given in Table 21 and it should be noted that for all of the unsymmetrical tetraal-kyls listed in this table only the second-order term in equation (40) is of importance35. It is convenient now to discuss the two terms in equation (40) separately. 

Table 24. Faleschini and Tagliavini36 stated that for all of the unsymmetrical tetraalkyls they studied, only the smaller alkyl group in each tetraalkyl was split off as RBr. Such selectivity is rather unexpected in view of the poor selectivity observed for brominolysis in solvent chlorobenzene. Indeed, Gielen and coworkers37 have shown that in the brominolysis of EtSnBu"3 in carbon tetrachloride at 35 °C, both the ethyl and the n-butyl groups are split off as alkyl bromides, with a ratio EtBr/Bu"Br of 0.43. Similarly, the ratio Pr"Br/BunBr was 1.3 in brominolysis of Pr"SnBu"3. In view of this work, the rate coefficients recorded by Faleschini and Tagliavini are valid only as overall rate coefficients and (until further work is done) cannot be used as rate coefficients for the cleavage of particular alkyl-tin bonds. Only for the two sets of compounds examined by Gielen and co-workers37 can the rate coefficient data be used to deduce these alkyl-tin rate coefficients, and values calculated by the author are given in Table 24.

These mechanisms are discussed, together with the reactions, below. By most of these mechanisms, the alkyl-tin bond is less reactive than the aryl-, alkenyl-, alkynyl-, allyl-, or benzyl-tin bond, and when two types of group are bonded to tin, reactions are selective for this second class of group. 

The reactivity of the alkyl-tin bond towards protic acids is rather low, and allyl-, vinyl-, or phenyl-tin bonds are cleaved preferentially. Towards both hydrogen halides and carboxylic acids, the reactivity drops as the size of the alkyl groups increases, and the removal of subsequent alkyl groups becomes successively more difficult. Some examples are shown in equations 5-21-5-24. 

The electrophilic cleavage of an alkyl-tin bond can be assisted by a suitably positioned intramolecular ligating group. Thus in 5-3, the ease of the cleavage of the methyl-tin bond is ascribed to nucleophilic assistance by the sulphoxide group.38... 

The aryl-tin bond, like the alkyl-tin bond, is stable to air and moisture, but it is more susceptible to cleavage by electrophiles. Melting points of some aryltin compounds are given in Table 7-1. 

Cleavage of an alkyl-tin bond by an Sh2 reaction (see Section 5.3.6) occurs more readily with the alkyltin halides (and carboxylates) than with the tctraalkylslannancs.87 88 These reactions were first identified by ESR spectroscopy in the photolysis of di-t-butyl peroxide in the presence of organotin halides, which provides a very convenient technique for ESR studies of alkyl radicals, for example equations 11-28 and 11-29.89... 

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