Silver-BINAP catalyst

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... 

This silver-BINAP catalyst can be applied for the reaction of 2,4-pentadienyl-stannane and aldehydes.5 Generally, the reaction of 2,4-pentadienylstannane and aldehydes in the presence of a Lewis acid catalyst afforded the corresponding 8 adduct. On the other hand, the reaction using the silver catalyst gave the corresponding y adduct predominantly (Table 9.3). These results suggested that the reaction catalyzed by silver could proceed through a six-membered cyclic transition state (Fig. 9.1). 

An alternative system for this reaction that gave the product in a comparable yield and similar enantioselectivities has been reported, and uses trialkyltin methoxide and a BINAP-silver(l) catalyst (BINAP = 2,2-bis(diphenyl-phosphanyl)-1,1-binaphthyl Equation 17) <1999JA892>. 

Najera reported that silver perchlorate forms a catalytically active complex with (5)-BINAP (87).55 This complex efficiently catalyzes the reaction of aryl iminoesters 108 with /V-methylmaleimide 114 in the presence of the external base triethylamine (Scheme 2.30, Table 2.8). High enantioselectivity is obtained in this reaction. As a consequence of the low solubility of the silver BINAP complex in the reaction medium toluene, it can be quantitatively recovered from the reaction mixture by simple filtration. The recovered catalyst could be recycled at least 4 times with no apparent loss in activity. Carretero reported that /V-phenylmaleimide also reacted with... 

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... 

Silver BINAP complex was used as well by Yanagisawa and coworkers . The particular feature of this catalyst is the marked anti selectivity when nsing crotyltins, regardless of whether the donble bond is (E) or (Z). This selectivity is explained by a fast transmetaUation step between the chiral complex and the organotin reagent, followed by the addition on the carbonyl gronp via a cyclic transition state. This reaction has been extended to other organometallics snch as 2,4-pentadienylstannanes . 

In 1996 Yanagisawa, Yamamoto, and their colleagues first reported the asymmetric allylation of aldehydes with allylic stannanes catalyzed by a BINAP silver(I) complex [29]. The chiral phosphine-silver(I) catalyst can be prepared simply by stirring an equimolar mixture of BINAP and silver(I) compound in THF at room tempera-... 

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 zinc- and silver-based catalysts BINOL-Zn(U) 453 and BINAP-Ag(O) 454, developed by Tagliavini and Yamamoto, respectively, are more reactive than 451 (Keck s original procedure) and show equal or improved efficiency, compared to 451, with a, ff-unsaturated and aromatic aldehydes, but lower efficiencies and enantioselectivities with aliphatic aldehydes [291, 293, 302]. 

The first example of the intermolecular AHR was reported by Hayashi et al. and involved the asymmetric arylation of 2,3-dihydrofurans using aryl triflates [18]. Although little or no enantiomeric excess was obtained when aryl iodide/silver salt combinations were used, the use of triflates along with the familiar Pd(OAc)2/ BINAP catalyst system resulted in the formation of the 2-aryl-2,3-dihydrofuran product 54, together with minor amounts of the 2, 5-dihydrofuran isomer 55. The rationale proposed by the authors for this outcome is shown in Scheme 12 it is hypothesized that addition of the catalytic complex to either face of the sub-... 

We found that a BINAP silver(I) complex also catalyzes the asymmetric al-lylation of aldehydes with allylic stannanes, and high y-, anti-, and enantioselec-tivities are obtained by this method [31,32,33]. The chiral phosphine-silver(I) catalyst can be prepared simply by stirring an equimolar mixture of chiral phosphine and silver(I) compound in THF at room temperature. Scheme 10 shows the results obtained by the reaction of a variety of aldehydes with allyltributyltin (33) under the influence of 5 to 20 mol % of (S)-BINAP silver(I) triflate (enf-12) in THF at -20 °C [31]. The reaction furnishes high yields and remarkable enan-tioselectivities not only with aromatic aldehydes but also with a,P-unsaturated aldehydes, with the exception of an aliphatic aldehyde which gives a lower chemical yield. In the reaction with a,P-unsatuxated aldehydes, 1,2-addition takes place exclusively. Enantioselective addition of methallyltributylstaimane to aldehydes can also be achieved using this method [31,32]. 

In contrast, Yanagisawa, Yamamoto, and coworkers have studied diverse combinations of BINAP-silver(I) catalysts and silyl enolates and found that high levels of asymmetric induction and isolated product yields were attained in the p-Tol-BINAP-AgF-catalyzed aldol reaction of trimethoxysilyl enolates (38) with aldehydes (39) in methanol (Scheme 18.15) [54]. Table 18.2 summarizes the results... 

Asymmetric Mannich-type reaction is a versatile method to prepare nonracemic P-amino carbonyl compounds, which can be transformed into P-lactams or related compounds. Lectka and coworkers have shown that a BINAP-silver(I) complex acts as asymmetric catalyst in the reaction of a-imino esters for the first time in 1998 [58]. The chiral silver (I) catalyst has been further applied to asymmetric ene reactions of a-imino esters [59, 60]. In contrast, Hoveyda and co workers have developed a new chiral silver(I) catalyst using iso-Leu-derived phosphine (44) as a chiral ligand, which promotes asymmetric Mannich reaction of silyl... 

It was demonstrated that BINOL catalysts authorize the /3- and /-functionalizations of the allyltin reagents without lowering the enantioselectivity level [223], and such a strategy was used in the total syntheses of macrolides [224] or substituted tetrahydropyran units [225]. It was noteworthy that the BINOL-Ti catalysis was extended to the enantioselective allylation of alkyl and aromatic ketones in good yields with up to 96% ee [226]. Silver/BINAP was used as well, with a marked anti selectivity, when using crotyltins whatever is the nature, (E) or (Z) of the double bond [227]. This reaction was extended to other organometallics such as 2,4-pentadienylstannanes [228] or buta-2,3-dienylstannanes [229] (Scheme 6.26). 

Besides the silyl enolate-mediated aldol reactions, organotin(IY) enolates are also versatile nucleophiles toward various aldehydes in the absence or presence of Lewis acid.60 However, this reaction requires a stoichiometric amount of the toxic trialkyl tin compound, which may limit its application. Yanagisawa et al.61 found that in the presence of one equivalent of methanol, the aldol reaction of an aldehyde with a cyclohexenol trichloroacetate proceeds readily at 20°C, providing the aldol product with more than 70% yield. They thus carried out the asymmetric version of this reaction using a BINAP silver(I) complex as chiral catalyst (Scheme 3-34). As shown in Table 3-8, the Sn(IY)-mediated aldol reaction results in a good diastereoselectivity (,anti/syn ratio) and also high enantioselectivity for the major component. 

J0rgensen and coworkers reported the preparation of A-tosyl aziridines 19-20 by the net carbene addition (via a diazo compound) to A-tosyl iminoesters with either copper or silver catalysts.13,14 It was noted that the copper catalysts were generally superior, although a catalyst derived from AgSbF6 and (R)-Tol-BINAP provided the corresponding aziridine 19 from 16 and trimethylsilyl diazomethane 17 (R = TMS) in excellent chemical yield with high levels of diastereoselectivity, but unfortunately the enantioselectivity was poor (Scheme 8.3). This success with trimethylsilyldiazo-... 

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... 

Silver salts or reagents have received much attention in preparative organic chemistry because they are useful catalysts for various transformations involving C-G and C-heteroatom bond formation.309 Especially, the silver(i)/ BINAP (2,2 -bis(diphenylphosphino)-l,T-binaphthalene) system is a very effective catalyst for a variety of enantio-selective reactions, including aldol, nitroso aldol, allylation, Mannich, and ene reactions. Moreover, silver salts are known to efficiently catalyze cycloisomerization and cycloaddition reactions of various unsaturated substrates. Recently, new directions in silver catalysis were opened by the development of unique silver complexes that catalyze aza-Diels-Alder reactions, as well as carbene insertions into C-H bonds. 

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. 

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]. 

This BINAP silver(I) complex was subsequently used by Lectka and coworkers as a catalyst for Mannich-type reactions [35]. Slow addition of silyl enol ether 49 to a solution of tosylated a-imino ester 80 under the influence of 10 mol % (i )-BINAP AgSbFg at -80 °C affords the corresponding amino acid derivative 81 in 95 % yield with 90 % ee (Sch. 20). They reported, however, that (R)-Tol-BINAP-CuC104-(CH3CN)2 was a more effective chiral Lewis acid for the reaction and gave the highest yield and ee at 0 °C. 

The BINAP silver(I) complex was further applied to ene reactions of a-imino esters independently by two groups [36,37]. For example, treatment of a-imino ester 80 with a-methylstyrene (82) in the presence of 5 mol % (R)-BINAP- AgSbFg in ben-zotrifluoride at room temperature leads to ene adduct 83 with 71 % ee (Sch. 21) [36]. Both groups have reported that the Tol-BINAP Cu(I) complex is superior to BINAP Ag(I) complex as a chiral catalyst. 

The famous ligand BINAP controls an intramolecular Heck reaction to give decalin derivatives with good enantiomeric excess. BINAP is the optically pure phosphine built into the palladium catalyst. The presence of silver ions accelerates the reaction as well as preventing double bond isomerization in the original substrate. This time the chiral ligand selects which double bond is to take part in the reaction. The vinyl palladium species is tethered to the alkene and can reach only the same face. The faces of the alkenes are diastereotopic but the two alkenes are enantiotopic and you must know your right from your left to choose one rather than the other. 

---