Silver© catalyst, chiral

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 the case of cyclic or cr-substituted tin enolates 358, it was found that E-enolates provide //-products 359, whereas the corresponding Z-tin enolates give syn-aldols (Scheme 105).315 Thus, the reaction seems to follow the classical Zimmerman-Traxler transition states 360/361 whereby the chiral silver catalyst activates the aldehyde. A possible alternative is a tin-silver exchange and formation of an analogous transition state of the silver enolate. 

In the area of [3 + 2]-cycloadditions (1,3-dipolar cycloadditions), chiral silver catalysts have been utilized extensively for the enantioselective formation of five-membered rings from prochiral substrates. For example, Zhang and co-workers360 have reported the highly enantioselective Ag(i)-catalyzed [3 + 2]-cycloaddition of azomethine ylides to electron-deficient alkenes. Thus, reaction of ct-imino esters 442 with dimethyl maleate in the presence of catalytic amounts of silver(i) acetate and the chiral bisferrocenyl amide phosphine 443 provided the chiral pyrrolidines 444 with high stereoselectivities and chemical yields (Scheme 131). Only the endo-products were isolated in all cases. 

Wadamoto and Yamamoto have modified the chiral silver catalyst system and achieved catalytic enantioselective addition of aUyltrimethoxysilane to ketones [45]. For example, the reaction of 1-naphthaldehyde (29) with 2 equiv of (27) in the presence of 5 mol% of (R)-DIFLUORPHOS, 5mol%ofAgF, and 1 equiv ofMeOH in THF at —78 °C for 12 hours provides the tertiary homoallylic alcohol (30) in 98% yield with 95% ee (Scheme 18.11). Use of y-substituted allylic trimethoxysilanes... 

A chiral silver-based catalyst - formed from an amino acid and AgF2 - promotes efficient enantioselective addition of enolsilanes to a-keto esters in THF at temperatures as low as -30 °C, with yields and ees in the high 90s.136... 

Yamamoto et al. reported the asymmetric (9-nitrosoaldol reaction using silyl enol ethers in the presence of the silver catalyst.32 In order to achieve this reaction, they developed a novel combination of silver and a chiral phosphite derived from BINOL. The disilanyl enol ether was used to ensure high yield and enantioselectivity. The reaction was conducted with disilanyl enol ether and nitrosobenzene in the presence of AgBF4 and the chiral phosphite ligand in THF to produce the O adduct with high regio- and enantioselectivity (Table 9.14). In addition, a chiral silyl enol ether could be used as a substrate. The reaction was conducted with chiral silyl enol... 

A similar chiral silver(I) catalyst 21 was applied to the asymmetric addition of allyltributyltin to various aldehydes in an aqueous medium [28]. Shi et al. have shown that chiral silver complex 22, prepared from chiral bidentate phospho-ramide and AgOTf, is also an effective chiral catalyst for the allylation [29]. Chiral bis(oxazoline) ligands have found widespread use in asymmetric reactions catalyzed by chiral metal complexes, and C2-symmetric chiral bis(oxazoline)-Zn(OTf)2 complex 23 has been applied to catalytic enantioselective allylation of aldehydes with allyltributyltin (44) however, satisfactory enantioselectivity was not observed [30]. 

This protocol could be directly applied in the addition of 2 silyloxy furans to keti mines and the generation of a quaternary chiral center. Using electronically highly activated a ketoimine esters 24 as substrates, the same chiral silver phosphine catalyst (10mol%) as previously used in reactions with aldimines delivered the desired Mannich products in good yields, good diastereoselectivity, and up to 94% ee (Table 5.3) [9]. 

Interestingly, the aldol-type condensation of tosyhnethyl isocyanide 15 with aldehydes is catalyzed by the silver(I)/ferrocenylphosphine 2c or 2e catalyst more selectively than it is catalyzed by the chiral gold catalyst (about 20% ee) under the standard reaction conditions (Scheme 5) [30]. Oxazoline 16 can be converted to optically active a-alkyl-P-(N-metylamino)ethanols by reduction with LiAlH4. 

The Widenhoefer group extended the method to the synthesis of enantiomerically enriched 2-vinylpyrrolidines from monosubstituted or trisubstituted y-aminoallenes, as well as allenic ureas, in the presence of chiral gold catalysts derived from [Au2 (S)-3,5-r-Bu-4-MeO-MeOBIPHEP Cl2] and a silver salt (Scheme... 

While effective bimetallic catalyst design has the potential to lead to an enhancement of the reaction rate, the use of chiral bimetallic catalysts has also been explored to enhance the enantioselectivity of a reaction. Such bimetallic chiral induction is excellently demonstrated by the use of digold catalysts for the hydroamination of prochiral substrates such as allenes and alkenes [59]. The bimetallic Au catalyst 66, for example, was shown to be an effective catalyst for the hydroamination of amino-allenes in the presence of a silver salt activator (Scheme 24) [106]. The highest enantioselective induction for this reaction was achieved with a 1 1 ratio of AgBp4 to 66 (51 % ee) suggesting that the monocationic... 

In analogy t 0 the Cu(II) complex systems, the silver(I) -catalyzed aldol reaction is also proposed to proceed smoothly through a Lewis acidic activation of carbonyl compounds. Since Ito and co-workers reported the first example of the asymmetric aldol reaction of tosylmethyl isocyanide and aldehydes in the presence of a chiral silver(I)-phosphine complex (99,100), the catalyst systems of sil-ver(I) and chiral phosphines have been applied successfully in the aldol reaction of tin enolates and aldehydes (101), Mukaiyama aldol reaction (102), and aldol reaction of alkenyl trichloroacetates and aldehydes (103). In the Ag(I)-disphosphine complex catalyzed aldol reaction, Momiyama and Yamamoto have also examined an aldol-type reaction of tin enolates and nitrosobenzene with different silver-phosphine complexes (Scheme 15). The catalytic activity and enantioselectivity of AgOTfi(f )-BINAP (2 1) complex that a metal center coordinated to one phosphine and triflate were relay on solvent effect dramatically (Scheme) (104). One catalyst system solves two problems for the synthesis of different O- and AT-nitroso aldol adducts under controlled conditions. 

In the same year, Roland and coworkers described a chiral silver-NHC complex with a tert-butyl substituted backbone for copper-catalyzed addition of Et2Zn to 2-cyclohexanone. However, the addition product was isolated in low enantioselectivity (23% ee) [77]. Later on, Alexakis et al. modified the carbene structure to improve the enantioselectivity (Scheme 3.51) [78]. By using the chiral silver-NHC salt 90 to transmetallate and generate the Cu catalyst, the asymmetric conjugate addition of diethylzinc to 2-cycloheptanone was achieved in good yield (95%) and enantioselectivity (93% ee). 

Silver carbene complexes act as very efficient NHC transfer reagents for the synthesis of different metal-NHC complexes. On the other hand, as far as we know, there is only one example described to date where a silver-NHC complex was used for asymmetric catalysis. In 2006, Ferndndez and coworkers reported the first and only asymmetric catalysis using a chiral NHC-silver catalyst. Enantioselective diboration of styrenes was realized by using silver complex 111 as the catalyst (Scheme 3.70) [99], However, the diol was obtained in low yield and low enantioselectivity (less than 10% ee). 

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... 

The transformation of ort/zo-alkynylaryl ketones (275) to isochromene derivatives (276) through a cyclisation/enantioselective-reduction sequence has been realised in the presence of a chiral silver phosphate catalyst (277). The reaction afforded the li -isochromene derivatives (276) in high yield with fairly good to high enantioselectivity (Scheme 75). ... 

Less Usual Elements.—Reviews have covered the applications of silicon/ selenium, and mercury in synthesis. Some specific examples of syntheses involving less usual elements have been queen substance (30) (palladium ), (+)-cataline (31) (vanadium ), ibogamine (32) (silver, palladium ), terpenoids chiral cobalt catalyst ), and prostaglandins (zirconium ). 

Recently, Barluenga and Fananas reported tandem intramolecular hydroaUc-oxylation/hydroarylation and hydroaUcoxylation/Prins-Type annulation reactions. In the first communication, they described the cycloisomerization of 5-alkynols with several gold, platinum, and silver catalysts and the application to the synthesis of enantiopure benzo fused cyclic ethers from the chiral pool [150] (Scheme 87). 

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