Tri-n-butyl(methoxy)stannane1

n-Bu3SnOMe

[1067-52-3]  · C13H30OSn  · Tri-n-butyl(methoxy)stannane  · (MW 321.14)

(mild methoxide source with strong nucleophilic properties but with reduced basicity, making it less likely to induce elimination reactions; precursor to other alkoxytributylstannanes)

Alternate Names: tributyltin methoxide; methoxytributylstannane.

Physical Data: bp 90 °C/0.1 mmHg; d 1.129 g cm-3; 1H NMR (CDCl3) 3.54 ppm (OMe); 119Sn NMR 83 ppm.

Solubility: sol common organic solvents.

Form Supplied in: colorless liquid; widely available.

Handling, Storage, and Precautions: toxic, as are most organotin reagents; should be manipulated in a fume hood and handled with gloves. Readily hydrolyzable, it must be stored in the absence of moisture and CO2. Exposure to moist air leads to hexabutyldistannoxane and tributylstannyl carbonate.

Introduction.

This reagent can be used in substitution, oxidation, or addition reactions. It is also employed as a catalyst for urethane, polylactone, polylactide, and polycarbonate synthesis.

Substitution Reactions.

Methoxytributylstannane and related compounds react with acyl chlorides or anhydrides to give esters under very mild conditions (eq 1).2 This smooth reaction is particularly useful in carbohydrate chemistry.1d

Methoxytributylstannane is also useful for the preparation of peresters, as reactive stannyl peroxide intermediates are quantitatively obtained from the corresponding hydroperoxides.3 Methoxytriethylstannane can be used under palladium catalysis to give aryl esters from aryl iodides and carbon monoxide (eq 2).4

These esterifications have been generalized to organosulfur and organophosphorus chemistry. With sulfenyl chlorides,5 Bu3SnOMe gives access to sulfenates in better yields than does Sodium Methoxide (eq 3). O-Alkyl S,S-diaryl phosphorodithioates are converted to O,O-dialkyl S-aryl phosphorothioates (eq 4)6 and dinucleoside S-aryl phosphorothioates give dinucleoside O-methyl phosphates (eq 5).7

In the presence of Lithium Chloride as an activating agent, methoxytributylstannane promotes mild, chemoselective debenzoylations. Thus debenzoylation of 7,13-diacetylbaccatin III (a precursor of taxol) occurs only at C-2 (eq 6). In a highly polar solvent, NMP, none of the four other acyl groups is affected to any appreciable extent.8 O-Deacetylation of sugars is readily accomplished with methoxytributylstannane. Anomeric acetates are more reactive than primary or secondary acetates, thereby enabling selective removal.9

Dioxybis(tributylstannane), prepared from methoxytributylstannane and Hydrogen Peroxide, allows the preparation of primary, secondary, and tertiary dialkyl peroxides from the corresponding triflates in good yields.10

The modulation of the nucleophilicity of an alkoxy group by the tin atom is particularly useful in methoxylation reactions,11 where the absence of base even allows the use of base sensitive halides (eq 7).

Alkyl glycosides can be prepared by using alkoxytributylstannanes. Per-O-acetyl-a-D-glucopyranosyl bromide reacts with methoxytributylstannane in the presence of catalytic amounts of Tin(IV) Chloride to give methyl per-O-acetyl-b-D-glycoside (eq 8). Replacement of tin(IV) chloride by 0.5 equiv of triethylammonium bromide completely changes the course of the reaction; epimerization to the more reactive b-anomeric configuration occurs during the reaction, affording an exo-orthoester (eq 9).12

When 2-, 3-, 4-, or 5-halogenated alcohols are used, methoxytributylstannane first reacts with the hydroxyl in an exchange reaction. Then the alkoxide intermediates undergo intramolecular alkoxylation to give oxiranes, oxetanes, tetrahydrofurans, or tetrahydropyrans, respectively (eq 10).13 The displacement is stereospecific, as erythro-3-bromo-2-butanol leads mainly to (E)-butene oxide and threo-3-bromo-2-butanol to the (Z)-isomer (n = 1).

Methoxylation with methoxytributylstannane is not limited to halogenated derivatives. Under palladium catalysis, allyl acetates are transformed into allyl methyl ethers. This chemoselective approach to allyl etherification is often stereoselective and tolerates other electrophilic functional groups such as primary halides. It has been applied to carbohydrate chemistry (eq 11).14

b-Lactams are stereospecifically methoxylated at the 4-position by methoxytributylstannane when a phenylsulfinyl group is used as leaving group (eq 12). Thus 3-substituted or unsubstituted 4-phenylsulfinyl-2-azetidinones lead to the corresponding 4-methoxy-2-azetidinones in the presence of catalytic amounts of Trimethylsilyl Trifluoromethanesulfonate at rt.15

Enoxytributylstannanes (tin enolates) can be regiospecifically prepared by transesterification of enol acetates derived from aldehydes16 or ketones17 with methoxytributylstannane. The formation of new carbon-carbon bonds can be accomplished with primary alkyl halides, allyl halides, or functional halides. This coupling is regiospecific (eqs 13 and 14).

Under palladium catalysis, aryl18, vinyl,19 or heteroaryl20 halides, or allyl acetates21 may be used instead of allyl or alkyl halides (eqs 15 and 16). This reaction is also regiospecific.22 It allows arylation of chroman-4-ones into isoflavanones.23 In contrast, a different reaction takes place in acetonitrile, which leads to a-enones. This process has been applied to the synthesis of a,b-unsaturated large-ring ketones from enol esters (eq 17).24

Metalation of alkynes by methoxytributylstannane gives alkynyltributylstannanes, which are able to undergo further reactions with acyl chlorides, leading to alkynyl ketones (eq 18).25 With 1,3-dienes, [4 + 2] cycloadditions give cyclic vinylic organotins which are easily transformed into functional cyclohexadienes (eq 19) through vinyllithium intermediates.26

Oxidation Reactions.

Alkoxystannanes, conveniently prepared from methoxytributylstannane and alcohols, are oxidized under very mild conditions by bromotrichloromethane,27 1,1-Di-t-butyl Peroxide,28 Bromine,29 N-Bromosuccinimide,30 or Nitronium Tetrafluoroborate (eq 20).31 Secondary alcohols are selectively transformed into ketones, even in the presence of primary alcohols.32

Disulfides are obtained when thiols are treated with methoxytributylstannane and anhydrous Iron(III) Chloride, through an oxidation reaction involving thiostannane intermediates (eq 21). Tertiary, as well as primary, secondary, or aryl thiols may also be employed. Various functionalities, such as hydroxy, amino, amido, or ester groups, remain intact under the reaction conditions.33

Addition Reactions.

Enoxytributylstannanes, prepared from methoxytributylstannane, add to aldehydes34 to give predominately the anti-aldols at low temperature, whereas syn selectivity is observed at higher temperature (eq 22).35

Intramolecular opening of oxiranes by tin alkoxides leads to cyclic ethers. After treatment by methoxytributylstannane, substituted 3-epoxybutanols stereospecifically give either oxetanemethanols (eq 23) or 3-oxolanols (eq 24) on heating, depending on the substituents.36

Nucleophilic additions of methoxytributylstannane to isocyanates or ketenes lead to reactive intermediates giving heterocycles after further condensations. Mixed isocyanurates are obtained when different isocyanates are involved in the reaction (eq 25).37

Alkoxides resulting from the opening of halogenated lactones by methoxytributylstannane add to isocyanates to give 2-oxazolidones (eq 26),38 or 2-oxazinones,39 depending on the ring size of the starting lactones. Other heterocumulenes, such as diimides, isocyanates or carbon dioxide, react equally well (eq 27).

A Darzens reaction can be performed under mild, neutral conditions with the stannyl carbamate resulting from the addition of methoxytributylstannane to ethyl isocyanate (eq 28). This adduct selectively generates organotin enolates from a-halo ketones even when enolizable a-hydrogens are present in the halo ketone.40

Methoxytributylstannane adds to silylketenes under very mild conditions (eq 29). Condensation of the adduct with aldehydes in the presence of titanium tetrachloride gives b-hydroxy-a-silyl esters with very high syn selectivity. A stereocontrolled elimination in these esters subsequently leads to (E)- or (Z)-a,b-unsaturated esters, depending on the conditions used.41

Stannyl quinones are formed by the thermal isomerization of 4-stannyloxy-4-alkynylcyclobutenones (eq 30) via a ketene intermediate. Once this intramolecular rearrangement is accomplished, the stannyl group can be substituted under palladium catalysis by various organic halides to give substituted quinones.42

Iminocarbonylation of aryl bromides, involving the palladium-catalyzed condensation of t-butyl isocyanide, methoxytributylstannane, and an aryl bromide, leads to imidates (eq 31).43

Catalysis.

Methoxytributylstannane is used as a catalyst for polymerization of lactones,44 lactides,45 and 1,3-dioxan-2-one.46 The reaction is suggested to proceed by a coordinative insertion mechanism involving the cleavage of the acyl-oxygen bond of the monomer, and not by an ionic mechanism.

Related Reagents.

Di-n-butyltin Oxide; Sodium Methoxide.


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Bernard Jousseaume

Université Bordeaux I, Talence, France



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