Allyl bromides-tin

Mediated by Tin. In 1983, Nokami et al. observed an acceleration of the reaction rate during the allylation of carbonyl compounds with diallyltin dibromide in ether through the addition of water to the reaction mixture.74 In one case, by the use of a 1 1 mixture of ether/water as solvent, benzaldehyde was allylated in 75% yield in 1.5 h, while the same reaction gave only less than 50% yield in a variety of other organic solvents such as ether, benzene, or ethyl acetate, even after a reaction time of 10 h. The reaction was equally successful with a combination of allyl bromide, tin metal, and a catalytic amount of hydrobromic acid. In the latter case, the addition of metallic aluminum powder or foil to the reaction mixture dramatically improved the yield of the product. The use of allyl chloride for such a reaction,... 

Allylation. Carbonyl compounds are converted into homoallylic alcohols with allyl bromides-tin, diallyldibutylstannane, or allyltributylstannane-dibutyltin chloride. All three protocols require the presence of Me,SiCl (although acid chlorides may replace MCjSiCl in the last reaction). 

A-homoallylic alcohols were prepared in good yields by allylation of dialdehydes or their acetals with allyl bromide, tin(II) chloride, and potassium iodide in water or water/THF (Eq. 8.46). ° Under ultrason-ication, it was found that SnCU could efficiently mediate the aqueous Barbier reactions between carbonyl compounds and allyl bromide to give the corresponding homoallylic alcohols in high yields without using any Lewis-acid catalyst. 

In a typical example, using a platinum foil cathode, benzaldehyde, allyl bromide, tin powder and cyclohexene, dissolved in a 4 1 mixture of methanol and methanoic acid, were electrolyzed at 50-55 °C, affording the expected product in 91% yield ... 

A heterogeneous mixture of 1.46 mmol of hexanal, 1.96 mmol of allyl bromide, 0.85 mmol of tin powder. 1 mL of Et20, and 1 mL of water is stirred vigorously and 0.1 mL of 5% aq hydrogen bromide is added. After 2 h, the organic layer is separated, washed with NaCl and distilled yield 70%. 

Treatment of allyl bromides with the complex obtained from tin(II) chloride and the disodium salt of diethyl 2,3-dihydroxybutanedioate gives an intermediate which reacts with aldehydes to provide homoallylic alcohols with 50-65% ee. Lower enantiomeric excesses were obtained with bulky aldehydes and for allylstannanes with y-substituents. Pentacoordinated allyltin complexes may be involved97. 

In basic aqueous media, a kinetic study of the reaction between stannate(II) ions and alkyl halide shows that mono- and disubstituted organotin compounds are formed (Eq. 6.12a).27 The monosubstituted organotin compound is obtained after a nucleophilic substitution catalyzed by a complexation between the tin(II) and the halide atom. The disubstituted compound results from an electrophilic substitution coupled with a redox reaction on a complex between the monosubstituted organotin compound and the stannate(II) ion. Stannate(IV) ions prevent the synthesis of the disubstituted compound by complexation. Similarly, when allyl bromide and tin were stirred in D2O at 60° C, allyltin(II) bromide was formed first. This was followed by further reaction with another molecule of allyl bromide to give diallyltin(IV) dibromide (Eq. 6.12b).28... 

Later, Torii et al. found that the tin-aluminum-mediated allylation can be carried out with the less expensive allyl chloride, instead of allyl bromide, when a mixture of alcohol-water-acetic acid was used as the solvent.77 When combined with stoichiometric amounts of aluminum powder, both stoichiometric and catalytic amounts of tin are effective. As reported by Wu et al., higher temperatures can be used instead of aluminum powder.78 Under such a reaction condition, allyl quinones were obtained from 1,4-quinones, followed by oxidation with ferric chloride. Allylation reactions in water/organic solvent mixtures were also carried out electrochemically, with the advantage that the allyltin reagent could be recycled.79... 

Otera et al. extended the tin-mediated allylation to 2-substituted allyl bromides.80 When 2-bromo and 2-acetoxy-3-bromo-l-propene were used, the allylation with tin produced the corresponding functionalized coupling products (Eqs. 8.36 and 8.37). In the case of 2,3-dibromopropene, the reaction occurred exclusively through allylation in the presence of the vinyl bromo group. The presence of other electrophiles such as a nitrile (-CN) or an ester (-COOR) did not interfere with the reaction. 

The nature of the organotin intermediates has been studied. It was found that when allyl bromide and tin reacted in aqueous media, allyltin(II) bromide was first formed and then was followed by the formation of diallyltin(IV) dibromide (See also Section 6.4.1, Eq. 6.12b). Either of the two organotin intermediates can react with carbonyl compounds to give the corresponding homoallylic alcohols. However, the tin(II) species was found to be more reactive than the tin(IV) species (Eq. 8.38).81... 

The electrolysis of alkyl halides on platinum cathode and tin anode has been mentioned above. A completely different mechanism is associated with alkylation on tin cathodes. Electroreduction of allyl bromide on tin electrodes yields tetraallylstannane (Ca 90%). This is done in acetonitrile solutions with LiClC>4, Et4NBr or BujNBr as electrolyte and followed by CV with Ag/AgBr reference. Yields decrease to 78% in DMF. The proposed mechanism67 in this case is ... 

Carbanions, generated by the reaction of benzylsilanes with tetra-n-butylammo-nium fluoride react with non-enolizable aldehydes to produce the alcohol [67], When a stoichiometric amount of the ammonium fluoride is used, the methylarene corresponding to the benzylsilane is frequently a by-product and arises from formation of the hydrogen difluoride salt during the reaction. When only catalytic amounts of the ammonium fluoride initiate the reaction, the formation of the methylarene is suppressed. In a similar type of reaction (although the mechanism is not known) between aldehydes and ketones, allyl bromide, and tin in the presence of trimethylsilyl chloride the yield of the but-l-en-4-ol is raised significantly by the addition of tetra-n-butylammonium bromide, particularly in the reactions with... 

Metallic tin, Sn(0), is even more effectively employed. For example, in the presence of Sn(0), allyl bromide and a-halocarbonyl compounds afford nucleophilic organometallic species, which add to aldehydes in good yields to give homoallylic alcohols (12) and g-hydroxycarbonyl compounds (13,14) respectively. a-Diketones could be reduced by activated Sn(0), to give tin(II) enediolates which in turn undergo aldol reaction to form a,g-dihydroxyketones (15,16). This reaction was successfully applied to a stereoselective synthesis of methyl D-glucosaminate (17). 

Allylation of aldehydes. Aldehydes undergo selective allylation on reaction with allylic bromides and tin (1.2 equiv.) in H20/THF (5 1) in a sonicator.1 Similar results can be obtained with zinc powder in saturated aqueous NH4C1-THF (5 1).2 Example ... 

Monoetherification of polyols.12 Monobenzylation and monoallylation of polyols can be conducted conveniently under mild conditions by conversion to the stannylene derivative (dimeric) by di-n-butyltin oxide (5, 189 9, 141). The stannylene is then treated with benzyl bromide or allyl bromide and tetra-n-butylammonium iodide (1 equiv.) in benzene. The same conditions can be used to prepare monomethoxymethyl ethers. Quaternary ammonium bromides are less efficient catalysts than the iodides. These salts also accelerate reaction of stannylenes with acid anhydrides. The mechanism for this activation is not clear it may involve coordination of I" to tin. 

Otherwise, unsaturation may be introduced by use of carbonyl-containing carbohydrate derivatives and carbon nucleophiles that contain alkene (or, if desired, alkyne) functionality, a notable illustration being the tin-or indium-mediated C-l allylation of unprotected sugars. As an illustration, D-arabinose, treated with allyl bromide in aqueous ethanol in the presence of tin gives, after acetylation, 278 in 85% yield.258 In this procedure aldoses react better than do ketoses, and pentoses better than hexoses. More usual is the use of Grignard reactions to give, for example, the octynes 279. 

Tin metal also reacts directly with a number of activated organic halides, including allyl bromide, benzyl chloride, chloromethyl methyl ether, and p-halocarboxylic esters and nitriles giving fair-to-good yields of diorganotin dihalides (97,111—114). 

The common nonenolide core of ascidiatrienolide and the didemnilactones has been formed as shown in Scheme 5 (22). Specifically, a tin-mediated, ultrasound-promoted addition of allyl bromide to unprotected glyceraldehyde 37 furnished a mixture of the corresponding homoallyl alcohols 38 which were subjected to acetalization and esterification with 5-hexenoic acid under standard conditions. At that stage, the major jy/i-isomer 40 can be purified by flash chromatography. Exposure of this diene to a refluxing solution of the ruthenium... 

Cross-coupling of allyl bromides with allyltins (cf. 10, 26). The coupling of prenyl bromide with the tin reagent 1 in the presence of 10% ZnCl2 gives myrcene (2) in high yield. This coupling also provides a synthesis of vitamin K, (3).1... 

Certain Lewis acids are known to induce an epoxide-aldehyde rearrangement <01TL8129>, and this chemistry has recently been combined in tandem with metal-mediated allylations. For example, epoxides react with tetraallyltin in the presence of bismuth(III) triflate to give homoallylic alcohols 116. The reaction involves an initial 1,2-shift to form an aldehyde 115, which is then attacked by the allyl tin species <03TL6501>. A similar but operationally more straightforward protocol is available by combining allyl bromide with indium metal, followed by the addition of epoxide <03TL2911>. 

Allyltin compounds readily add to aldehydes and ketones. Allylic bromides react with tin to generate the organometallic in situ, which then adds to aldehydes. Allylic chlorides react with aldehydes in the presence of ditin compounds such as MesSn—SnMes and a palladium catalyst. Allyltrialkyltin compounds ... 

Reductive alkylation of aromatic nitro compounds is possible. The reaction of nitrobenzene with allylic or benzyl hahdes in the presence of an excess of tin metal in methanol, leads to the A,A -diaUyl or dibenzyl aniline. A similar reaction occurs with nitrobenzene, allyl bromide, and indium metal in aq. acetonitrile. 

Allylic organometallics modified at the metal center by chiral adjuvants add to aldehydes and ketones to provide optically active homoallylic alcohols. This process has been described for reagents containing boron, tin and chromium metal centers. Gore and coworkers have shown that a chromium-mediated addition reaction of allylic bromides to simple aldehydes that uses a complex of lithium N-methyl-norephedrine and chromium(Il) chloride occurs with modest (6-16% ee) enantioselectivity (equation 61, Table 8). 

A chiral reagent derived from tin(II) catecholate, DBU, allyl bromide, and (+)-di-/-butyl tartrate reacts with aromatic aldehydes in a Cul-catalyzed reaction, giving homoallylic alcohols sometimes in excellent optical purities. Allylation with allyltrichlorosilane has been conducted in the presence of 45 and 46 (cf. allenylation of aliphatic aldehydes ). Allylsilanes modified by a tartrate ester show a range of effectiveness in chirality transfer. ... 

Allyl bromide or benzyl chloride and various ring-substituted benzyltin chlorides however react with metallic tin in refluxing toluene (110 °C) to give good yields of the corresponding diorganotin dihalide (e.g. equation 4-40) 62 the tin can be activated with mercuric chloride and triethylamine 63 Pentamethylcyclopentadienyl bromide similarly reacts with tin to give the pentamethylcyclopentadienyltin bromides.64... 

Second, if tin(II) catecholate is ligated by an optically active dialkyl tartrate, it will react with an allylic bromide or iodide and an aldehyde, in the presence of Cul as catalyst, to give the optically active homoallylic alcohol. The enantioselectivity is highest when the alkyl groups of the tartrate are bulky (t-butyl or cyclohexyl). Aromatic aldehydes give higher enantioselectivities than do aliphatic aldehydes, and the reaction is also successful with a-carbonylketones 74... 

Trimethyl((V-phenylbenzimidoyl)stannane reacts with halides such as benzyl or allyl bromide, benzoyl chloride, or methyl iodide in the presence of KF and 18-crown-6 to give the corresponding ketimine presumably attack of the fluoride ion at tin gives an imidoyl anion equivalent73... 

The allylation of an aldehyde can be carried out by the direct reaction of tin with an allyl bromide in aqueous ethanol or in water,10 or in the absence of any solvent.11 Allylation can also be achieved by generating the allylstannane reagent in situ from tin(II) chloride and the appropriate allyl bromide.12 13 The reaction can be carried out with the more accessible but less reactive allyl chloride if sodium iodide is added, which converts the chloride into the iodide.14... 

Monoallylstannanes, AllylSnX[N(SiMe3)]2, are formed when the stannylene, Sn[N(SiMe3)]2, reacts with allyl bromide,15 17 and the reaction of allyl bromide with the tin(II) catecholate complex of a dialkyl tartrate,18 or with the oxathiastannolane complex of (R)-binaphthol19 gives chiral allyltin reagents. 

Keto stannylenolates can be prepared by the reaction of Sn-O or Sn-N bonded compounds with diketene, which can be regarded as a cyclic enol ester. The adducts formed from bis(tributyltin) oxide can undergo further reaction, with subsequent decarboxylation, to give the same products as those from the simple enolates. Alkylation with alkyl iodides or benzyl or allyl bromides is strongly catalysed by lithium bromide (e.g. Scheme 14-5). Double alkylation can be achieved with HMPA as solvent.120 The product of alkylation before the final hydrolysis is itself a tin enolate, which can be used in reactions with further carbon electrophiles. 

ICP-MS analysis of the homoallyl alcohols formed in these reactions indicates negligible tin and cerium contamination. The resin can be recovered and regenerated with allyl bromide and zinc or allylmagnesium bromide and reused several times without notable loss of activity. 

The achiral allylation of aldehydes has also been achieved in recyclable ionic liquids" and in water." Greener still is the corresponding Barbier allylation" of aldehydes and ketones with allylic bromides in water mediated by tin metal. The atom efficiency of this reaction is actually less than the corresponding tetraallyltin allylation of (say) benzaldehyde (65% and 83%, respectively) because of the loss of the heavy bromine atom, but this neglects the synthesis of tetraallyltin, which is prepared from allyl bromide or chloride. A particularly intriguing recent advance with this thoroughly studied reaction is the use of nano-tin " (Scheme 5.8.17). 

The reaction involves stirring aldehyde, allyl bromide, and stannous chloride in water and applying an electrolytic potential of 2.0 V with graphite electrodes. The cathode rednces Sn(ll or IV) salts to tin metal, which react with allyl bromide to generate allyltin(ll) bromide and diallyltin dibromide. These organotin species then allylate the aldehyde and the resulting homoallyl alcohol is extracted with diethyl... 

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