Tributylstannyl ethers

The electrolysis of unsaturated a-tributylstannyl ethers 12 yields cyclic compounds by a similar reaction, involving in this case carbon-carbon bond formation. Here, too, the evidence points to a facile ET with formation of an a-stannyl ether radical cation. This in turn cleaves to the tributylstannyl radical and an unsaturated ene-oxonium residue. 

Bis(tributyltin) oxide, BtijSnOSnBuj, is a high-boiling oil, bp 180°C/2 mm, with a penetrating, unpleasant, sweetish smell. It is readily available. Mixing with an alcohol R-OH gives a tributylstannyl ether according to Eq. (5), a reversible reaction that may be drawn to completion by azeotropic removal of water. These ethers are moisture-sensitive. They are undoubtedly covalent derivatives, yet their reactions with electrophiles are, broadly speak-... 

The substitution of allyl and benzyl halides is also catalyzed by quaternary ammonium halides. In fact, the method was discovered with tributylstannyl ethers [15]. The conversion of tributylstannyl ethers to ketones with AMnomosuccinimide is an alternative to the bromine oxidation of stannylenes [12,13]. Again, the disposal of the tin by-product Bt SnX may be a problem in large-scale wotk. It may be fairly efficiently extracted with hexane from a solution of the reaction mixture in acetonitrile. 

After the nucleophilic attack of the bromide anion on the Sn atoms of the tributylstannyl ether 36 you get oxyanions at C-3 and C-6 as the reactive species. Now selective benzylation can proceed and the alcohol at C-4 is only benzylated as a by-product in low yield. 

Tributylstannyl ethers are prepared in the same manner by reaction of alcohols with hexabutyldistannoxane (more commonly known as bistribu-tyltin oxide). Holzapfel et al. noted that the reaction in benzene requires only 0.5 molar equivalents of bistributyltin oxide to go to completion but takes 16 h at reflux (Fig. 2). This is probably because the tin-containing by-product of the first half of the reaction, tributyltin hydroxide, reacts much more slowly than the initial reagent.18... 

Simple 2,2-dibutyl-l,3,2-dioxastannolanes form solid complexes of monomer units with certain nucleophiles, such as pyridine and dimethyl sulfoxide, that have 1 1 stoichiometry and pentacoordinate tin atoms.62 Such complexes are less stable for more-substituted stannylene acetals, such as those derived from carbohydrates.62 Unfortunately, the precise structures of these complexes have not yet been defined. Addition of nucleophiles to solutions of stannylene acetals in nonpolar solvents has been found to markedly increase the rates of reaction with electrophiles,63 and transient complexes of this type are likely intermediates. Similar rate enhancements were observed in reactions of tributylstannyl ethers.57 Tetrabu-tylammonium iodide was the nucleophile used first,57 but a wide variety of nucleophiles has been used subsequently tetraalkylammonium halides, jV-methylimidazole,18 and cesium fluoride64,65 have been used the most. Such nucleophilic solvents as N,N-dimethylformamide and ethers probably also act as added nucleophiles. As well as increasing the rates of reaction, in certain cases the added nucleophiles reverse the regioselectivity from that observed in nonpolar solvents.18,19... 

Alkylation requires more vigorous conditions. These reactions were originally performed on the stannylene acetal with the alkylating reagent in DMF at elevated temperatures (45°C for methyl iodide or 100°C for benzyl bromide)66 or on the tributylstannyl ether in neat benzyl bromide or allyl bromide at 80-90°C.67 It was then discovered that the presence of added nucleophiles markedly accelerates the reactions, so that alkylation of both tributylstannyl ethers and dibutylstannylene acetals in benzene, which is very slow at reflux with benzyl bromide alone, occurs at a reasonable speed at reflux in the presence of added tetrabutylammonium halides.57,63 Many other nucleophiles are also effective, including A-methylimidazole68 and... 

VII. Factors That Influence Reaction Regioselectivity 1. Tributylstannyl Ethers... 

When a polyol is reacted with bis(tributyltin) oxide, all tributylstannyl ethers are formed and these interconvert readily at the reaction temperatures.35-36 Ogawa and Matsui suggested that oxygen atoms of tributyltin ethers that have adjacent oxygen atoms in appropriate orientations to coordinate with the tin atom are activated toward electrophiles74,75 specifically, equatorial oxygen atoms that have an adjacent axial oxygen atom,... 

Fig. 10.—A mechanism proposed for activation of tributylstannyl ethers by nucleophiles.57...

Fig. 11.—An intermediate proposed for tributylstannyl ether reactions in the presence of A-methylimidazole.36...

Although the factors that influence the regioselectivities obtained from dialkylstannylene acetals are considerably more complicated than with tributylstannyl ethers, the outcome in most cases is surprisingly similar. The factor influencing regioselectivity in the absence of added nucleophiles will be discussed first. 

An alternative suggestion is that the coordinated monomer dissociates to give an oxyanion stabilized by the nucleophile s counterion, most commonly, tetrabutylammonium or cesium ions,69 by analogy to the mechanism proposed for activation of tributylstannyl ethers by nucleophiles.57 This proposal is summarized in Fig. 14, which also shows the product mixture obtained for methyl (2S,3R)-2,3-dihydroxybutanoate. For this compound, reaction on the oxygen atom of the inherently less acidic hydroxyl is favored. Both anions, E and F, are in equilibrium with the coordinated monomer, and the less populated (but more reactive) anion E reacts to a greater extent, or in other words, the difference in the rate constants for trapping is greater than the difference in equilibrium constants. 

Dibutylstannylene acetals or tributylstannyl ethers derived from isolated rrans-diols on pyranose or inositol rings that have one adjacent substituent axial and one equatorial react preferentially on the oxygen atom adjacent to the axial substituent, as shown by many examples in Table IV. Some... 

Fig. 24.—Regioselectivity in the benzoylation of the tributylstannyl ether of methyl a-o-glucopyranoside.74...

When reactions are performed on tributylstannyl ethers and dibutylstan-nylene acetals of hexopyranoside derivatives that have more oxygen atoms, including the primary one, unprotected, markedly different results are obtained, as shown in Figs. 24 and 25. Dibutylstannylene acetals favor reaction on the position in the 1,2-diol unit that is adjacent to an axial substituent, whereas tributyltin ethers prefer to react at the primary centers. However, this pattern was not observed for reactions of the tributylstannyl ether of l,2-(l-methoxyethylidene)-j3-D-mannopyranose, as shown in Fig. 26.122 Methylation of this compound through the tributylstannyl ether in toluene in the presence of added tetrabutylammonium bromide also gave substitution on 0-3 predominantly.123 These results may arise from distortion of the chair conformation by the fused isopropylidene acetal. 

Fig. 26.—Regioselective alkylation of a tributylstannyl ether of a mannopyranose derivative.122...

When the tributylstannyl ether of benzyl 3, 4 -0-isopropylidene-/3-lactoside (41) in toluene containing V-methylimidazole was treated with benzyl bromide, a mixture was obtained in which the 2-benzyl ether was the major product, isolated in 52% yield.142 A study of the alkylation of the tributyltin ether of benzyl jS-D-glucopyranoside has never been reported,... 

Formation of di-O- or tri-O-substituted derivatives from tributylstannyl ethers and dibutylstannylene acetals can often be performed regioselec-... 

Tributylstannyl ethers and dibutylstannylene acetals of a-D-glucopyranosides have been converted into 2,6-di-O-substituted derivatives in high yields with a wide range of electrophiles,74 107 151 152 but it should be noted that not all conditions result in selective reactions. 2,3,6-Tri-O-benzoyl derivatives can also be made in reasonable yields,74 153 constituting presumably another example of the electron-withdrawing substituent effect. 

Table VII lists the comparatively few di- or poly-substitution reactions that have been performed on dibutylstannylene acetals and tributylstannyl ethers of compounds containing cis-diols. Figure 40 (page 103) shows some of the...

The reactions of tributylstannyl ethers and dibutylstannylene acetals of diols or polyols containing one hydroxyl group that is at the end of a chain (terminal diols) are listed in Table IX. Some reactions on compounds of this type, primarily unprotected glycosides, have been discussed earlier, but for those examples, the terminal hydroxyl group was not critical in... 

From a synthetic point of view, the most important observation is that dibutylstannylene acetals and tributylstannyl ethers can be used to effect terminal substitution of diols in excellent yields, often better than can be obtained by direct reaction at low temperatures, even for benzoylation or p-toluenesulfonylation, where direct reaction is reasonably effective. Terminal O-alkylation, which cannot be performed directly, is routine through choice of the appropriate conditions, as outlined in the sections following. These types of reactions are considered first, followed by reactions where the nonterminal oxygen atom is favored. 

Both tributylstannyl ethers and dibutylstannylene acetals of terminal triols yield in most cases the product of reaction with 1,2-diols in preference to other hydroxyl groups, as shown in Figs. 47 and 48. Figure 47 also illustrates the tendency of t-butylchlorodimethylsilane to react with terminal 1,3-diols in preference to terminal 1,2-diols.87 Figure 48 shows that the preference for reaction at terminal oxygen atoms is considerably stronger than the preference for reaction next to unsubstituted centers, discussed in the previous section. As the numerous examples in Table IX demonstrate, this selectivity is maintained over a wide range of structural features. 

Dibutylstannylene acetals and tributylstannyl ethers of terminal 1,3-diols also react preferentially with most electrophiles on the terminal oxygen atom. The terminal 1,3-diol can be acyclic, or the secondary oxygen atom can be on a ring adjacent to an hydroxymethyl group, such as 0-4 and O-6 of aldohexopyranoses or 0-3 and 0-5 of aldopentofuranoses. Figures 50 to 53 show some examples. 

Fig. 51.—Benzylation of the tributylstannyl ether of a terminal 1,3-diol from a /3-d-glucopyranoside derivative.316...

Glycosyl acetates react with tributyltin methoxide to give glycosyl tributylstannyl ethers. As with glycosyl anions,337"339 the anomeric effect causes... 

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