Enolates tributyltin

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. 

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. 

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... 

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 (( )-... 

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. 

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. 

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,... 

Yamamoto and coworkers have developed an asymmetric a-aminoxylation of cyclic tributyltin enolates using nitrosobenzene (5.85) as oxygen source in the presence of Lewis acidic 1 1 silver-BINAP complexes. For example, the enolate (5.86) is converted into the a-aminoxyketone with high ee using this procedure. Cleavage of the N-0 bond is then effected with no racemization, using catalytic copper sulfate in methanol. 

Reaction of n-butyl-lithium with tetrahydrofuran at 25 C for 16 h affords the lithium enolate of acetaldehyde. This enolate O-acylates and O-silylates readily C-alkylation is accomplished by initial formation of the tributyltin enolate followed... 

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. 

There are only scattered reports of catalytic enantioselective processes for the alkylation of enolates. Most notably in this respect, Jacobsen has reported that Cr-salen complex 195 mediates the alkylation of a range of tributyltin enolates. Thus, treatment of 194 with 2.5 mol % of catalyst 195 and benzyl bromide affords 196 in 91 % yield and 93% ee (Equation 15) [22, 109]. 

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