BINAP-AgOTf catalyst

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... 

Aldol condensation of the tin enolates with aldehydes often takes place spontaneously at room temperature, but Lewis acids (e.g. TiCL, BF3.OEt2, ZnCl2, CuCl2) or PdCl2[(o-C6H4)3P]2 can be used as catalysts, and enantioselective addition can be achieved with an (/ )-BINAP-AgOTf catalyst.101 The stereoselectivity is dependent on the reaction conditions, and the high threo selectivity at low temperature is ascribed to the presence of a cyclic transition state 14-20.104... 

Catalytic asymmetric aldol addition of tributylstannyl enol ethers to aldehydes is achieved in the presence of (R)-BINAP AgOTf catalyst.The catalyst is considered to enhance the electrophilic reactivity of aldehydes to give diastereomeric adducts with high selectivities from -enol ethers, awft -aldol products are produced, whereas yn-aldol products result from Z-enol ethers. The stereochemical outcome is understood in terms of 6-membered cyclic transition states (Scheme 3-195). 

It should be added that an analogous, previously developed [15], metal-catalyzed synthesis, based on use of BINAP-AgOTf as catalyst, is also available. This effi-... 

This catalytic system could be applied to the enantioselective hydroxymethylation of silyl enol ethers with aqueous formalin (Scheme 9.12).21 Doyle and coworkers have successfully applied the catalyst system of BINAP-AgOTf, KF, and %-crown-6 for Mukaiyama aldol reaction of a-diazo silyl enol ether (Scheme 9.13).22... 

The asymmetric synthesis of jV-adduct is accomplished by BINAP-AgOTf (0.4/1) complex. The complex has the structure with 1 1 P-Ag moieties which is confirmed by NMR. The in situ generated catalyst gives hydroxyamino ketones in high enantioselectivity (Equation (721).225... 

The reaction was started by use of 2 equiv. allyltributyltin (68) and 0.1 equiv. (/ )-BINAP-AgOTf, and 0.1 equiv. catalyst was added after 4 h. 

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 (( )-... 

Wadamoto, M., Ozasa, N., Yanagisawa, A., Yamamoto, H. BINAP/AgOTf/KF/18-Crown-6 as New Bifunctional Catalysts for Asymmetric Sakurai-Hosomi Aiiyiation and Mukaiyama Aidoi Reaction. J. Org. Chem. 2003, 68, 5593-5601. 

In 1996, Yamamoto and Yanagisawa reported the allylation reaction of aldehydes with allytributyltin in the presence of a chiral silver catalyst.2 They found that the combination of silver and a phosphine ligand accelerates the allylation reaction between aldehydes and allyltributyltin. After this discovery, they screened several chiral phosphine ligands and found that chiral silver-diphosphine catalysts can effect the reaction in an enantioselective fashion (Table 9.1).2 For example, when benz-aldehyde and allyltributyltin were mixed in the presence of 5 mol% of AgOTf and (S)-2,2 -bis(diphenylphosphino)-1,1 -binaphthyl (BINAP), the corresponding homoallyl alcohol was obtained with 96% ee and 88% yield (Table 9.1). Generally, the reaction with aromatic aldehydes afforded the corresponding homoallyl alcohols in excellent... 

In addition, this silver-BINAP catalyst has been applied to the crotylation reaction between aldehydes and 2-butenylstannanes (Scheme 9.1).3 The addition of (E)-2-butenylstannane to benzaldehyde in the presence of 20 mol% AgOTf/BINAP gave the corresponding adduct with 94% ee with a high anti syn ratio. In contrast to other... 

To overcome this limitation, Yamamoto and coworkers investigated the reaction conditions and found that the use of KF and %-crown-6 in the presence of AgOTf and BINAP was effective in promoting the allylation reaction (Table 9.6).11 Thus, the reaction was conducted with aliphatic aldehydes and allytrimethoxysilane in the presence of catalyst to give the corresponding adducts with satisfactory yields and high enantioselectivities. Moreover, this catalytic system can be applied to the reaction between aromatic aldehydes and allyltrimethoxysilane. 

More recently, the catalyst that was prepared from AgOTf and Tol-BINAP was applied to the synthesis of 2,3-dihydrobenzofnrans. The reaction of 2,3-dihydroben-zoxasilepine and an aromatic aldehyde was carried out in the presence of the silver catalyst, KF, and 18-crown-6 to give tra s-2,3-disubstituted 2,3-dihydrobenzofuran... 

The use of Lewis acid drastically changes the regioselectivity. The highly enantioselective and O-selective nitroso aldol reactions of tin enolates with nitrosobenzene have been developed with the use of (i )-BINAP-silver complexes as catalysts. AgOTf and AgCICL complexes are optimal in the O-selective nitroso aldol reaction in both asymmetric induction (up to 97% ee) and regioselection (0/N= > 99/1), affording amino-oxy ketone. The product can be transformed to a-hydroxy ketone without any loss of enantioselectivity (Equation (71)).224... 

Lewis acid catalysts activate the aldehyde by coordination to the carbonyl oxygen. Shibasaki et al. [13] were able to demon,strate that the activation of the enol ether is possible too. The reaction of the aldehyde 37 with the silyl enol ether 38 in the presence of the catalyst 39 proceeds with good, but still not excellent enantioselectivity to yield the aldol adduct 40. Only 5 mol % of the chiral palladium(II) complex 39 was used (Scheme 6a). Activation of the Pd(lI)-BINAP complex 39 by AgOTf is necessary. Therefore, addition of a small amount of water is important. 

Cationic Pd complexes can be applied to the asymmetric aldol reaction. Shibasaki and coworkers reported that (/ )-BINAP PdCP, generated from a 1 1 mixture of (i )-BINAP PdCl2 and AgOTf in wet DMF, is an effective chiral catalyst for asymmetric aldol addition of silyl enol ethers to aldehydes [63]. For instance, treatment of trimethylsi-lyl enol ether of acetophenone 49 with benzaldehyde under the influence of 5 mol % of this catalyst affords the trimethylsilyl ether of aldol adduct 113 (87 % yield, 71 % ee) and desilylated product 114 (9 % yield, 73 % ee) as shown in Sch. 31. They later prepared chiral palladium diaquo complexes 115 and 116 from (7 )-BINAP PdCl2 and (i )-p-Tol-BINAP PdCl2, respectively, by reaction with 2 equiv. AgBF4 in wet acetone [64]. These complexes are tolerant of air and moisture, and afford similar reactivity and enantioselec-tivity in the aldol condensation of 49 and benzaldehyde. Sodeoka and coworkers have recently developed enantioselective Mannich-type reactions of silyl enol ethers with imi-nes catalyzed by binuclear -hydroxo palladium(II) complexes 117 and 118 derived from the diaquo complexes 115 and 116 [65]. These reactions are believed to proceed via a chiral palladium(fl) enolate. 

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