Trialkyltins

Trialkyltin substituents are also powerful ipso-directing groups. The overall electronic effects are similar to those in silanes, but the tin substituent is a better electron donor. The electron density at carbon is increased, as is the stabilization of /S-carbocation character. Acidic cleavage of arylstannanes is formulated as an electrophilic aromatic substitution proceeding through an ipso-oriented c-complex. ... 

Another reagent system that has been recently employed in the Paal synthesis of thiophenes is the combination of bis(trialkyltin)- or bis(triaryltin) sulfides with boron trichloride. Known as the Steliou reagent,it has been utilized in the transformation of 1,4-diketone 11 to thiophene 12. Higher yields are obtained in shorter reaction times in contrast to the use of Lawesson s reagent. Additionally, others have noted the relative ease of the work-up procedure using the Steliou conditions, and the fact that the tributyltinchloride byproduct of the reaction is reusable. Similarly, the combination of the bis(trimethylsilyl)sulfide has been used in conjunction with trimethylsilyltriflate for the preparation of thiophenes in an analogous manner. ... 

In NMR speetra of trialkyltin derivatives of 1,2,3-triazoles, C(4) and C(5) atoms show one eommon signal at the lowest attained temperature of solution, -50°C [77JOM(132)69]. This observation was explained by the symmetrie strueture 42c of the eompounds (R = COOMe) [77JOM(127)273]. However, rapid annular tautomerism explains this observation equally well and eannot be ruled out. 

Trialkyltin groups p to a carbon-centered radical are readily eliminated... 

Enantiomerically enriched l-(diisopropylaminocarbonyloxy)allyllithium derivatives (Section 1.3.3.3.1.2.) add to carbonyl compounds with syn-l,3-chirality transfer21, giving good evidence for a pericyclic transition state in the main reaction path (Section 1.3.3.1.). However, since the simple diastereoselectivity and the degree of chirality transfer are low, for synthetic purposes a metal exchange with titanium reagents or trialkyltin halides (Section D.1.3.3.3.8.2.3.) is recommended. 

Finally, the in situ formation and thermolysis of trialkyltin ethers derived from tertiary homoal-lylic alcohols has been developed into a useful allylstannane synthesis39,40. This fragmentation is the reverse of the addition of an allylstannane to a ketone41. 

A similar elimination in which the tin is attacked by fluoride anions (cf. the reaction of silanes with F ) has been used179 to synthesize terminal methylene compounds as in equation (75). An analogous reaction sequence using a trimethylsilyl group in place of the trialkyltin group has been published by Hsiao and Shechter180 as part of a synthesis of substituted 1,3-butadienes. 

Allyltin compounds can be prepared by simple modifications of the usual reaction involving allyl Grignard reagents (139), by the 1,4-addition of trialkyltin hydrides to 1,3-dienes 140,141), or by the reaction of an aldehyde or ketone with the appropriate, tin-carrying, Wittig reagents (142). 

The trialkyltin alkoxides can often be prepared more conveniently by azeotropic dehydration of the appropriate bis(trialkyltin) oxide and alcohols, or by heating together the bis(trialkyltin) oxide and dialkyl carbonate (192). The latter reaction involves formation, and then decarboxylation, of the alkyl trialkyltin carbonate, e.g.,... 

There is a growing interest in the tin enolates that can be prepared by treating enol acetates with trialkyltin methoxides, e.g. (193),... 

The lower trialkyltin hydroxides and oxides, which are usually readily interconverted, have been characterized by IR and Mossbauer spectroscopy (212). The dimer of di-n-butyltin oxide (Bu2SnO)2 has been reported to be formed as a crystalline solid when dibutyltin dichloride is hydrolyzed with ammonium hydroxide (213). 

Trialkyltin methoxides react with anhydrous hydrogen peroxide in ether to give the rather unstable bis(trialkyltin) peroxides, RsSnOOSnRs (214). Under the same conditions, dialkyltin dimethox-ides give polymeric peroxides, (RzSnOO), but, if an aldehyde is present, monomeric peroxides of the following structure are obtained (215). 

A cyclic mechanism has been tentatively proposed for the reaction involving trialkyltin halides. 

The preparation of trialkyltin compounds of lithium, RgSnLi, and their use for preparing the organotin compounds, RaSnR, has been discussed in a previous section. 

Grignard reagents having bulky alkyl groups react with trialkyltin hydrides to give compounds having a Sn-Mg bond, and are synthetically useful as a source of nucleophilic RsSn in particular, they react with carbonyl compounds, oxiranes, and oxetanes to give the -, jS-, or... 

The bis(trialkyltin) compounds of mercury are formed when trialkyl-... 

The products are yellow or red solids when R = Me, Et, Pr, or Bu, they decompose below —10°, but when R = Ph, or, particularly, when R = Me iCH, the products are more stable. They are oxidized immediately in air to the corresponding distannoxanes, readily exchange the trialkyltin group with trialkyltin hydrides, and add across polar-substituted alkynes or azo compounds. 

Trialkyltin radicals are important intermediates in the reduction of alkyl halides, and in the hydrostannation of alkenes (1, 287). 

The last two reactions are useful for esr studies involving free radicals. Until recently, the only trialkyltin radical that had been observed directly, in solution, by esr was MesSn- (295), but many more have now been reported (e.g., EtsSn-, PrsSn-, and BusSn ) (296). Bulky ligands [e g., (PhCMejCHjlaSn ] increase the persistence of the radicals, so that esr observation is easier (297), and tris(2,3,5-trimethylphenyl)tin and tris(2,3,5-triethylphenyl)tin radicals, at 180° and 100°, respectively, are in thermal equilibrium with the corresponding hexaaryldi-tins (298). 

On a macromolecular scale, trialkyltins are known to bind to a number of proteins (see Table III). 

It may, therefore, be seen that the mammalian toxicity of the lower dialkyltin compounds, unlike that of their trialkyltin counterparts, is markedly dependent upon the nature of the X groups this is probably true for species other than mammals (e.g., fungi) if the mode of action is similar. 

The earliest work in this field, by Cremer (466), showed that tetra-ethyltin is metabolized in vitro and in vivo (rats) to a triethyltin derivative and, later, it was further demonstrated that triethyltin compounds are converted in vitro into diethyltin derivatives (500). The latter are broken down in vivo to monoethyltins, which are eliminated from the body within a short time (501). Other trialkyltins appear to behave similarly (500). 

Mechanistic studies have shown that TBT and certain other forms of trialkyltin have two distinct modes of toxic action in vertebrates. On the one hand they act as inhibitors of oxidative phosphorylation in mitochondria (Aldridge and Street 1964). Inhibition is associated with repression of ATP synthesis, disturbance of ion transport across the mitochondrial membrane, and swelling of the membrane. Oxidative phosphorylation is a vital process in animals and plants, and so trialkyltin compounds act as wide-ranging biocides. Another mode of action involves the inhibition of forms of cytochrome P450, which was referred to earlier in connection with metabolism. This has been demonstrated in mammals, aquatic invertebrates and fish (Morcillo et al. 2004, Oberdorster 2002). TBTO has been shown to inhibit P450 activity in cells from various tissues of mammals, including liver, kidney, and small intestine mucosa, both in vivo and in vitro (Rosenberg and Drummond 1983, Environmental Health Criteria 116). 

Trialkylstannyl enolates can be prepared from enol acetates by reaction with trialkyltin alkoxides and are sufficiently reactive to add to aldehydes. Uncatalyzed addition of trialkylstannyl enolates to benzaldehyde shows anti stereoselectivity.31... 

These results suggest that the solubility of trialkyltin fluoride in nonpolar solvents is considerably increased by the introduction of a trimethylsilyl group at the end of the chain but very much so at the expense of decreasing their ability to viscosify these solvents. 

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