Tributyltin enol ethers

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. 

Addition of Bu3SnCl or EtjAl to the enolate solutions prior to alkylation suppresses polyalkylation (141). Ketone enolates generated by cleavage of tributyltin enol ethers (142) appear to behave analogously. 

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]. 

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... 

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 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]. 

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... 

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... 

This chiral catalyst was then found to effect aldol reactions of aldehydes with the silyl enol ether of S-cthyl propancthioatc (equation II). In this case dibutyltin diacetatc is somewhat superior to tributyltin fluoride as the cocatalyst. With this chiral promotor, chemical yields arc high, and only the syn-aldol is formed in >98% cc. This high stereoselectivity obtains with aliphatic and aromatic aldehydes and a, /3-enals. 

A series of aryl radical cyclizations were reported by a group at Novartis [10], and some of these processes were also compared with bond formation by Pd-mediated Heck cyclization of the same substrates. The tributyltin hydride-mediated reaction of iodo alkenes 7 (Scheme 3), immobilized on polystyrene resin through a linker, gave dihydrobenzofurans 8 [11]. It was also possible to perform a tandem cyclization using allyltributyltin to give the allylated product 9, although the yields were less satisfactory. The radical cyclization onto enol ethers was demonstrated [12] by the conversion of 10 to 11. For best results, the tributyltin hydride and AIBN were added portionwise every 5-8 h. The impressive 95% yield was in fact higher than that for the solid-phase Heck cyclization of 10. Similarly, cyclization of anilide 12 afforded the phenanthridine 13. 

In 1989, a highly enantioselective aldol reaction of achiral silyl enol ethers of thiol esters with achiral aldehydes was developed by using a novel chiral promoter system consisting of chiral diamine-coordinated tin(II) triflate and tributyltin fluoride (or dibutyltin diacetate) [23]. When the silyl enol ether 16 of S-ethyl ethanethioate was treated with PhCHO in the presence of stoichiometric amounts of tin(II) triflate, (S)-l-methyl-2-[(piperidin-l-yl)-methyl]-pyrrolidine (18), and tributyltin fluoride, the aldol reaction proceeded at -78 °C to afford the corresponding adduct 17 in 78% yield with 82% ee (Scheme 4). 

Radical Reactions. The a-p-tolylsulfonylmethyl radical, generated using either tributyltin hydride, azobisisobutyro-nitrile, or hexabutyldistannane with UV irradiation, adds to enol ethers, enatnines, and silyl enol ethers. The addition to enam-mes shows considerable syn diastereoselectivity which can be explained on the basis of an allylic 1,3-strain model (eq 11). When an alkenic or alkynic moiety is present in the a-substituent of the a-p-tolylsulfonylmethyl radical, cyclization can occur to give five-membered ring compounds (eqs 12 and IS). ... 

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). ... 

Addition of tin hydride to a,p-unsaturated carbonyl compounds is a general method of obtaining enol stannyl ethers. With triphenyltin hydride, the resulting tin enolate cannot be isolated and the reaction leads to the saturated carbonyl compounds. With tributyltin hydride, a mixture of O- and C-stan-nylated adducts is obtained (Scheme 28). 

Finally, a method of generating enol stannyl ethers in situ is to start from enol silyl ethers by an exchange reaction with tributyltin fluoride. An example is given in Scheme 29. It has been reported that this process occurs preferentially at the less-substituted enol silyl ether site if several are available. ... 

Not only silyl end ethers but also enol acetates prepared from saturated ketones give a,/3-unsaturated ketones by heating with allyl methyl carbonate in the presence of Pd(II)(OAc)2 and dppe with tributyltin methoxide as a bimetallic catalyst (Scheme 12). Regioselective generation of palladium(II) enolate intermediate is simply carried out by treatment of allyl enol carbonates, which are prepared by trapping of ketone enolates with chloroformate, with Pd(II)(OAc)2 in the presence of dppe (Scheme 13). 7r-Allylpal-ladium(II) enolates thus generated provide a,/3-nnsatnrated ketones. 

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