BINAP.AgOTf complex

Enantioselective aldol reaction of tin enolates with aldehydes catalyzed by BINAP-AgOTf complex has been accomplished. This reaction proceeds through a cyclic transition state with the aid of chiral silver complex (Equation (67)).221... 

Besides allylation reactions, the BINAP/AgOTF complex effectively catalyzes the enantioselective aldol reaction of tin enolates with various aldehydes.315 Under these conditions, a wide range of aldol products were smoothly formed with good to high yields and enantioselectivities (Scheme 104). 

Table 20. Additions of allyl and methallyl tributyltin to aldehydes catalyzed by a (P)-BINAP-AgOTf complex.

Table 21. Additions of cis- and fra i-crotylstannane to benzaldehyde catalyzed by a (M-BINAP-AgOTf complex.

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 (( )-... 

Table 2. Diastereo- and enantioselective aldol reaction of tin enolates 74 with aldehydes catalyzed by 10 mol % (/ )-BINAP AgOTf complex in THF at -20 °C.

Silver complexes Allylation, The f-,3-pentadiene with al BINAP-AgOTf complex... 

Allylation. The y-selective reactions of crotylstannanes and 5-trimethylstannyl-1,3-pentadiene with aldehydes are highly enantioselective when promoted by a BINAP-AgOTf complex. 

Yamamoto reported that BINAP/AgOTf complexes are also competent asymmetric Lewis acids, generating the homoallylic alcohol in good yields and excellent enantioselectivities (Table 11). 

B) and methyl trichloroacetate. Then, the tin enolate (B) is allowed to add to an aldehyde under the influence of a BINAP-AgOTf complex to give a tin alkoxide of nonracemic aldol adduct (C). Finally, methanolysis of the alkoxide (C) gives the desired product (D) and regenerates the tin methoxide. 

An interesting allylation of carhonyl compounds has been introduced using a catalytic BINAP.AgOTf complex a mixture of allyltributyltin and various aldehydes gives good yields of alcohols with high enantiomeric excess (eqs 17 and 18). ... 

In a series of related reactions, BINAP-AgOTf complex has been used in the addition of tin enolates to nitrosobenzene (eq 63). This method afforded a-hydroxy ketones ora-amino ketone derivatives with high enantioselectivity. 

Optically active a-amino ketone derivatives have also been obtained by reacting trimethylsilyl enol ethers with azidodicarboxy-lates in the presence of BINAP-AgOTf complex. Although this reaction proceeded with high yield, the enantiomeric excess was poor (eq 64). ... 

Asymmetric Processes. BINAP-AgOTf complex has been used in several catalytic reactions since its first appearance in 1996. Aldol reactions afford -hydroxy ketones, which is a common structure found in natural products. New ways for forming such a motif are always of synthetic utility. Enol stannanes, existing in the O-Sn form and/or C-Sn form, reacted with aldehydes... 

The BINAP-AgOTf complex was also applied to Mukaiyama aldol reactions using trimethoxysilyl enol ethers instead of tin enol ethers. This reaction was only efficient in the presence of catalytic KF and 18-crown-6. Such conditions were also applied to asymmetric allylation (eq 65), increasing thus the scope of the AgOTf-catalyzed aUylations. ... 

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... 

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... 

Several other chiral Lewis acids have been developed for the addition of allyl and methallyl tributylin to aldehydes [28]. These additions usually proceed slowly with reaction times of days. Less reactive stannanes, for example crotyl tributyltin, require even longer times and diastereoselectivity is poor. The allyl and methallyl additions, however, afford products in high yield and ee. The most successful ligands are BINOL and BINAP as Ti or Zr complexes in the former case and an AgOTf complex in the latter. 

Reaction of aldehydes with 2,4-pentadienylstannanes is also catalyzed by BINAP sil-ver(I) complex, and the corresponding y-pentadienylated optically active alcohols are obtained with high enantioselectivity [31], When benzaldehyde is reacted with 1 equiv. pentadienyltributyltin (72, E/Z = 97/3) and 0.1 equiv. (5)-BINAP AgOTf at -20 °C, the y product 73 is obtained in 61 % yield with 90 % ee (Sch. 18). Pentadienyltrimethyltin affords chemical yield and enantioselectivity comparable with those of pentadienyltribu-tyltin (72). Ketones are inert under the standard reaction conditions. 

The asymmetric allylation of achiral aldehydes with a novel silver complex has recently been reported (Scheme 10-51) [90]. Initially, it was shown that the silver-promoted reaction of allyltributylstannane with benzaldehyde could be accelerated by triphenylphosphine. A survey of various chiral phosphine reagents and silver salts identified the combination of binap and AgOTf as optimal. The reaction of benzaldehyde and allyltributylstannane promoted by 5 mol% of the binap-AgOTf... 

Phosphine-AgOTf complexes are known to promote Michael addition reactions. Kobayashi and Shirakawa have achieved asymmetric Michael addition of P-ketoester to nitroalkenes catalyzed by (R)-Tol-BINAP-AgOTf in water [68]. When cyclopentanone-2-carboxylic acid tert-butyl ester (50) and trans- -nitrostyrene (51) were used as substrates, the Michael adduct (52) was obtained in 71% yield with 77 23 diastereomeric ratio. The major diastereomer showed 78% ee (Scheme 18.19). 

Shibasaki has examined catalysis of a complex, prepared in situ from PdCl2, AgOTf, (R)-or (S)-BINAP, 4 A molecular sieves, and H20, in the aldol addition reaction of enolsilanes by (Eq. 8B2.5) [13]. Under these conditions, aryl methyl ketone-derived trimethylsilyl enolates add to benzaldehyde and hydrocinnamaldehyde, affording adducts with up to 73% ee. 

Attempts to develop enantioselective protocols for the aza-Diels-Alder reaction were reported simultaneously with those described above. A first contribution in this area was the report by the. Mrgensen group,85 who studied the influence of salts of copper, silver, palladium, and zinc. Copper(I) perchlorate provides optimal yields and enantioselectivity, but complexes of BINAP (87) and Tol-BINAP (203) with AgSbF6, AgOTf, and AgC104 were able to catalyze the reaction, albeit with low enantioselectivity (Scheme 2.52). 

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. 

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. 

A chiral phosphine-silver(I) complex generated from BINAP and AgOTf-catalyzed asymmetric allylation of aldehydes with allylstannanes, resulting in high enantioselectivity. With 2-butenylstannane, the anti adduct was obtained preferentially irrespective of the double-bond geometry of the stannane (Scheme 12.28) [76]. 

Asymmetric Mukaiyama aldol reaction. The cationic Pd complex generated from [ff l-BINAP]PdCl2 and AgOTf in wet DMF in the presence of 4 A molecular sieves is highly effective for the aldol reaction at room temperature. A reaction proceeding via a Pd(II) enolate was identified for the first time. 

More recent work on the asymmetric PKR has focused on reactions catalyzed by rhodium and iridium complexes. Jeong and co-workers reported reactions catalyzed by a combination of [Rh(CO)jCl]2, (S)-BINAP, and AgOTf. Good to excellent ee s were obtained for a small range of substrates (Equation 17.79). After the observation that phosphine ligands improve the yield of the Ir-catalyzed PKR, Shibata reported intramolecular PKRs catalyzed by the combination of Tol-BINAP and [Ir(COD)Cl]2 in excellent yields and enantioselec-tivities (Equation 17.80). ... 

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