BINAP-silver complexes

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 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. 

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 . 

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. 

In asymmetric reactions, chiral phosphine ligands such as BINAP derivatives are used as effective chiral ligands in silver complexes. In particular, an Agr-BINAP complex activates aldehydes and imines effectively and asymmetric allylations,220-222 aldol reactions 223 and Mannich-type reactions224 proceed in high yield with high selectivity (Scheme 51). 

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. 

Enolates with Aldehydes Catalyzed by BINAP-Silver(I) Complex, J. Am Chem Soc 1997,119, 9319-9320. (d) S. E Denmark, K-T. Wong, R. A Stavenger, The Chirality of Trichlorosilyl Enolates. 2. Highly-Selective Asymmetric Aldol Additions of Ketone Enolates, J. Am Chem. Soc 1997,119,2333-2334, and references cited therein. 

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. 

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 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. 

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]. 

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]. 

Reactions of aldehydes with 2,4-pentadienylstaimanes are also catalyzed by the BINAP silver(I) complex, and the corresponding y-pentadienylated optically active alcohols are obtained with high enantioselectivity [33]. When benzaldehyde is reacted with 1 equivofpentadienyltributyltin(43,E/Z=97/3)and0.1 equiv of (S)-BINAP AgOTf (enf-12) at -20 °C, the y-product 44 is obtained in 61% yield with 90% ee (Scheme 12). Pentadienyltrimethyltin offers a chemical yield and enantioselectivity comparable to those of pentadienyltributyltin. Ketones are inert under the standard reaction conditions. 

Several stable silver triflate complexes with (R)-Binap as Ugand have been isolated at very low temperatures, used in aldol reactions and characterized by X-ray diffraction analysis by Yamamoto s group [43]. However, at room temperature, the binap-AgC104 complexes 26 and 27 could not be differentiated by NMR spectroscopy and could not be characterized by X-ray diffraction analysis [42]. The 1 1 complex (Sa)-26 was obtained in quantitative yield from Binap and AgC104 and further characterized by ESI-MS... 

The silver complexes containing chiral ligands are scarcely investigated as chiral catalyst. Recently, cationic silver(I)-BINAP complex has been shown to be an effective catalyst in the asymmetric Mukaiyama aldol reaction (316). A similar system derived from AgC104 and BINAP showed high enantioselectivity in asymmetric amination of silyl enol ethers (317). 

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. 

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... 

For their formal synthesis of (5E,9Z)-(+)-indolizidine 223AB ent 1780), Sulikowski and his team devised an interesting intramolecular Heck cyclization of the enamide 1791, which was promoted by a palladiu-m(II)-(R)-BINAP complex with silver phosphate as haHde scavenger (Scheme 226). When performed in dimethylformamide as solvent, the reaction produced the (R)-(+)-8,8a-dihydroindoUzin-3(7H)-one 1792 in 64% yield and an ee of 85%. Reduction of the conjugated alkene followed by treatment with methanol in trifluoroacetic acid produced the... 

Silver(I) complexes with Tol-BINAP (270) were used by Yamamoto and coworkers for mediating diastereoselective and enantioselective Mukaiyama aldol additions. According to the authors conclusion, the mechanism does not involve transmetallation to silver enolates but follows the usual carbonyl group activation [135]. Hoveyda and coworkers used silver(II) fluoride in the presence of a dipeptide-type ligand for enantioselective additions of silyl enol ethers to a-keto esters [136]. The reaction of 2-trimethylsilyloxyfuran with aromatic and aliphatic aldehydes was catalyzed with chromium salen complex in the presence of protic additives like isopropanol [137]. Various protocols of enantioselective Mukaiyama aldol reactions that use water as cosolvent have been elaborated ... 

A chiral BINAP-diphosphine complexed to silver , with fluoride as counterion, catalyses the enantioselective protonation of TMS-enolates, giving ketones with a tertiary asymmetric a-carbon in up to 99% ee.294... 

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). 

The Shibasaki cychzation of mc.vo-cyclohexa-1,4-dienes in the presence of a chiral palladium complex and silver carbonate in l-methyl-2-pyrrolidinone is probably the first example of an enantioselective Heck reaction (Scheme 10.34).60 The enantiomeric excess could be slightly improved by replacing the chiral phosphine ligand BINAP with the corresponding less coordinating arsine ligand, as well as replacing silver carbonate with silver phosphate.61,62... 

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... 

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. 

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