Tributyltin enolate

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Tin enolates of ketones can be generated by the reaction of the enol acetate 733 with tributyltin methoxide[60i] and they react with alkenyl halides via transmetallation to give 734. This reaction offers a useful method for the introduction of an aryl or alkenyl group at the o-carbon of ketones[602]. Tin enolates are also generated by the reaction of siiyl enol ethers with tributyltin fluoride and used for coupling with halides[603]. 

Note. Enol acetates undergo a similar oxidation, using the palladium species and tributyltin methoxide as dual catalysts. 

Although in the recent years the stereochemical control of aldol condensations has reached a level of efficiency which allows enantioselective syntheses of very complex compounds containing many asymmetric centres, the situation is still far from what one would consider "ideal". In the first place, the requirement of a substituent at the a-position of the enolate in order to achieve good stereoselection is a limitation which, however, can be overcome by using temporary bulky groups (such as alkylthio ethers, for instance). On the other hand, the ( )-enolates, which are necessary for the preparation of 2,3-anti aldols, are not so easily prepared as the (Z)-enolates and furthermore, they do not show selectivities as good as in the case of the (Z)-enolates. Finally, although elements other than boron -such as zirconium [30] and titanium [31]- have been also used succesfully much work remains to be done in the area of catalysis. In this context, the work of Mukaiyama and Kobayashi [32a,b,c] on asymmetric aldol reactions of silyl enol ethers with aldehydes promoted by tributyltin fluoride and a chiral diamine coordinated to tin(II) triflate... 

The reaction of alkyl dihalogenoacetate magnesium enolates with 2,3-isopropylidene-D-glyceraldehyde affords the expected /3-hydroxy-a-dihalogenoesters . The erythro isomer is obtained with isopropyl dichloroacetate magnesium enolate. This result is in agreement with theoretical models. 2-Deoxy-pentono-1,4-lactones are obtained after removal of the halogen atom by either Raney nickel or tributyltin hydride reduction (equation 89). 

Tributyltin hydride, 316 Tributyltinlithium, 319 Trichloroacetonitrile, 321 Other carbohydrates (Diethylamino)sulfur trifluoride, 110 Triethyloxonium tetrafluoroborate, 44 Carbonates (see also Enol carbonates) Carbon dioxide, 65 Di-/-butyl dicarbonate, 94 Carboxylic acids (see also Dicarbonyl compounds, Unsaturated carbonyl compounds)... 

Yamamoto has recently described a novel catalytic, asymmetric aldol addition reaction of enol stannanes 19 and 21 with aldehydes (Eqs. 8B2.6 and 8B2.7) [14]. The stannyl ketones are prepared solvent-free by treatment of the corresponding enol acetates with tributyltin methoxide. Although, in general, these enolates are known to exist as mixtures of C- and 0-bound tautomers, it is reported that the mixture may be utilized in the catalytic process. The complexes Yamamoto utilized in this unprecedented process are noteworthy in their novelty as catalysts for catalytic C-C bond-forming reactions. The active complex is generated upon treatment of Ag(OTf) with (R)-BINAP in THF. Under optimal conditions, 10 mol % catalyst 20 effects the addition of enol stannanes with benzaldehyde, hydrocinnamaldehyde, or cinnamaldehyde to give the adducts of acetone, rerf-butyl methyl ketone (pinacolone), and acetophenone in good yields and 41-95% ee (Table 8B2.3). 

Tributyltin enolates are useful radical mediators [47], although they generally exist in equilibrium with a-tributyltin ketones [48], Three-component coupling reactions proceed readily to give functionalized ketones in good to excellent yields, where an equilibrium shift to provide tin enolates operates efficiently (Scheme 6.28) [49]. Unlike the aforementioned case of allyltin-mediated reactions, acrolein is difficult to use in this reaction, since the Aldol reaction of the tin enolate with acrolein precedes the radical reaction. 

Ketone dilithio a,ft-dianion species (74) have been generated by the tin-lithium exchange reaction of the lithium enolate of /3-tributyltin-substituted ketones.291 Reaction with carbon electrophiles gives substituted ketones. 

Ketone dilithio a,f - and a -dianions have been generated by a tin-lithium exchange reaction of the lithium enolate of /3-tributyltin-substituted ketones.30 A chelation-aided approach, which employs /S-dichlorobutyltin-substituted ketones and n-BuLi, has been used for the generation of ketone a, f) -dianions having the Z-geometry at the alkene. The generated dianions have been transformed into ketones... 

An efficient and highly selective access to enantioenriched a-carbonyl all-carbon-substituted quaternary stereocentres has been provided by enantioselective alkylations of tributyltin enolates catalyzed by Cr(salen)Cl (13, M = Cr).33... 

In 1997, Yamamoto, Yanagisawa, and others reported the asymmetric reaction catalyzed by a BINAP-AgOTf catalyst (Table 9.7).16 The reaction was conducted with tributyltin enolate and aldehydes in the presence of the BINAP-AgOTf catalyst to afford the corresponding adduct with high enantioselectivity. The observed... 

In contrast, the reaction of tributyltin enol ethers and nitrosobenzene in the presence of a 1 2 mixture of BINAP and AgOTf in ethylene glycol diethyl ether afforded the N adduct predominantly with high enantioselectivity (Table 9.13). Momiyama and Yamamoto have determined the structures of silver-BINAP complex by an X-ray analysis and a 31P NMR study3015. 

Tsai and coworkers89,91,246,247 reported the synthesis of cyclic silyl enol ethers and silyl ethers by using a radical cyclization followed by the radical Brook rearrangement (equation 111). The cyclization of 4-bromo-4-stannylbutyl silyl ketones 188 in benzene with a catalytic amount of tributyltin hydride and AIBN gave cyclic silyl enol ethers 18989 91 247. The whole catalytic cycle proposed is shown in equation 112. 

The diastereoselective aldol reaction of the tributyltin enolate of cyclohexanone with benzaldehyde in the presence of a catalytic amount of various metal triflates has been studied. The highest A-selectivity is observed with Pd(OTf)2, while Zn(OTf)2 in THF shows moderate yy -selectivity (Equation (68)).222 OSnBu3... 

Copper(l) chloride in combination with tributyltin hydride shows unique character as an initiator of certain radical reactions. Hydrostannation of a,/3-unsaturated ketones with Bu3SnH is initiated by CuCl and the resulting tin enolates react with aldehydes under the influence of CuCl as a Lewis acid catalyst (Equation (81))/... 

The remarkable affinity of the silver ion for hahdes can be conveniently applied to accelerate the chiral palladium-catalyzed Heck reaction and other reactions. Enantioselectivity of these reactions is generally increased by addition of silver salts, and hence silver(I) compounds in combination with chiral ligands hold much promise as chiral Lewis acid catalysts for asymmetric synthesis. Employing the BINAP-silver(I) complex (8) as a chiral catalyst, the enantioselective aldol addition of tributyltin enolates (9) to aldehydes (10) has been developed." This catalyst is also effective in the promotion of enantioselective allylation, Mannich, ene, and hetero Diels-Alder reactions. 

Thiophene halides have been coupled with in situ generated tributyltin enolate of acetone in the presence of a palladium catalyst (Pd2(dba)3) along with 2-dimethylamino-2 -diphenylphosphino-l,r-biphenyl as the phosphine ligand) to give the corresponding arylacetone 125-126 in good yields <03TL8869>. 

The BINAP silver(I) complex can be further applied as a chiral catalyst in the asymmetric aldol reaction. Although numerous successful methods have been developed for catalytic asymmetric aldol reaction, most are the chiral Lewis acid-catalyzed Mukaiyama aldol reactions using silyl enol ethers or ketene silyl acetals [32] and there has been no report which includes enol stannanes. Yanagisawa, Yamamoto, and their colleagues found the first example of catalytic enantioselective aldol addition of tributyltin enolates 74 to aldehydes employing BINAP silver(I) complex as a catalyst (Sch. 19) [33]. 

The tributyltin enolates 74 are readily prepared from the corresponding enol acetates and tributyltin methoxide in the absence of solvent [34]. The tin enolates thus obtained occur in the 0-Sn form and/or the C-Sn form, and both species can be used for the aldol reaction of this system. Although the tin enolates themselves have adequate reactivity toward aldehydes [34c], in the presence of the BINAP silver(I) catalyst the reaction proceeds much faster even at -20 °C. Optimum conditions entail the use of THF as solvent and the results employing these conditions in the catalytic enan-tioselective aldol reaction of a variety of tributyltin enolates with typical aromatic, a,/3-unsaturated, and aliphatic aldehydes are summarized in Table 2. TTie characteristic features are (i) All reactions proceed to furnish the corresponding aldol adducts 75 in moderate to high yield in the presence of 10 mol % (i )-BINAP AgOTf complex at -20 °C for 8 h, and no dehydrated aldol adduct is observed (ii) with an a,j3-unsaturated aldehyde, the 1,2-addition reaction takes place exclusively (entry 3) (iii) a bulky alkyl substituent of tin enolate increases the enantioselectivity of the aldol reaction. For instance, the highest ee (95 % ee) is obtained when the tin enolate prepared from pinacolone 77 or rert-butyl ethyl ketone 79 is added to aldehydes (entries 2, 7, and 8) (iv) addition of the cyclohexanone-derived enol tributylstannane 78 (( )-... 

Scheme 11.77 shows that the two subunits 372 and 373 were linked by a silyl ketal tether to give 374. An anomeric radical was then produced by treatment with tributyltin hydride. This radical reacted with the enol ether acceptor to give the cyclic derivative 375 as the major product in a yield of 43% together with two of the three possible isomers in yields of 6 and 13%. The combined yield shows that more than 56% of the radical attack occurred from the a face of the gluco residue. However, the intermediate radical, located at C4, is mainly trapped by the a face of the furanose moiety. Although this approach is attractive, further elaboration to the... 

The phosgenation of the tributyltin enolate of acetone led to the same results although some trials performed at low temperature (below - 20°C) showed the possibility to obtain small amounts of isopropenyl chloroformate. It is noteworthy that, in a recent work devoted to a new synthesis of retinal (Vitamin A aldehyde), Bienayme from Rhone-Poulenc (Ref. 117), obtained isopropen-1-yI chloroformate in low yield through phosgenation of the reaction product of tributyltin methoxide with 1-acetoxy isoprene as depicted in scheme 92. 

Enol siiyl ethers undergo Pd-catalyzed coupling with aromatic bromides in the presence of tributyltin fluoride, which converts the enol silyl ethers into the stannyl ethers or a-stannyl ketones regarded as real active species chemoselective a-arylation of terminal ketones is possible (equation 110). ... 

Michael reactions. anri-Selectivity is observed in the reaction of tributyltin enolates with enones. On the other hand, using butyldichlorotin diisopropylamide as enolization agent the major products are the sy/t-isomers. 

Diketene similarly reacts readily with tributyltin oxide in THF at room temperature47 to give the stannyl 3-stannyloxybut-3-enoate which rearranges to the 2-enolate this enolate will then give an ene reaction with an aldehyde,48 or react with an alkyl halide,49 followed by decarboxylation, providing a route to ketones, enones, and ketoaldehydes. Similar reactions with bromomethyl P- or y-lactones result in the formation of P-hydroxy-y-lactones or y-hydroxy-5-lactones. 

A number of methods are now available for preparing the tin enolates, which are often not isolated, but are generated and caused to react further in situ. Tributyltin methoxide reacts exothermically with enol acetates without a solvent, to give the corresponding stannyl enolate, and methyl acetate which can be removed under reduced pressure or by slow distillation (b.p. 57 °C) (e.g. equation I4-54).37-104-105... 

The reaction of less electrophilic halides, particularly aryl and vinyl halides, can be catalysed with a palladium compound (equations 14-63 and 14-64),115 and the tin enolate can be prepared in situ from the enol acetate and tributyltin methoxide, or lithium enolate and tributyltin trifluoroacetate, or silyl enolate and tributyltin fluoride. 

Evidence that the enol form is more reactive than the keto form is shown in Scheme 14-4. Tributyltin ethoxide reacts with ketene at -78 °C to give the ketene acetal, which at the same temperature adds to benzaldehyde to give the hydroxyester. However, if the enolate is allowed to stand at 0 °C for several hours before the aldehyde is added at that temperature, no reaction occurs.116... 

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. 

Enol ethers of alkyl cyclopropyl ketones are alkylated or carboxylated under the correct conditions. Methyl ann, c ,cw-2,9,9-trimethyl-5-trifluoromethylsulfonyloxytricyclo[5.3.0.0 " ]dec-5-ene-4-carboxylate on treatment with palladium(II) acetate in methanol containing triethylamine under a carbon monoxide atmosphere resulted in diester formation giving dimethyl fln ,c .5,cw-2,9,9-trimethyltricyclo[5.3.0.0 ]dec-5-ene-4,5-dicarboxylate in excellent yield (93%). The same substrate was converted to methyl anr/,cw,d. -5-formyl-2,9,9-trimethyltricyclo[5.3.0.0 ]dec-5-ene-4-carboxylate in 88% yield on carbonylation in the presence of tetrakis(triphenylphosphane)palladium, tributyltin hydride, and lithium chloride. ... 

Reductive removal of the halogen was achieved with tributyltin hydride and subsequent ozonolysis gave aldehyde 221. An aldol condensation of 221 with the trimethylsilyl enol ether of methyl propionate, followed by Jones oxidation... 

Yamamoto has pioneered the use of Ag(I) complexes as Lewis acids for aldehyde allylation [122] and aldol addition [123]. For the aldol addition process, ketone-derived tributyltin enolates have been employed as the nucleophiUc component (Eq. 48). These enolates are readily prepared from the corresponding enolace-tates upon treatment with BujSnOMe. Importantly, although the resulting BujSn-enolates are known to exist as a mixture of C- and 0-bound tautomers 230/231,... 

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