Silver-catalyzed aldolization

On this basis, silver-catalyzed aldolization and allylation reactions have been developed, expanding the technology for creating a C-C bond using silver (see Chapter 9). Asymmetric versions have also been developed.74,37... 

TABLE 8B1.8. Silver-Catalyzed Asymmetric Aldol Reaction of Isocyanoacetate 3a with Aldehydes (Scheme 8B1.7)... 

Silver(I)-Catalyzed Aldol Reaction. In 1991 the silver(I)-catalyzed aldol reaction of an aldehyde with an a-isocyanoacetate ester was reported, analogous to the above mentioned gold(I)-catalyzed reaction. The catalyst was prepared in situ from (2) and Silver(I) Perchlorate. The stereoselectivity of the silver(I)-catalyzed reaction was shown to be temperature dependent, which was attributed to the variation of the degree of metal coordination with temperature. Slow addition of the a-isocyanoacetate ester to a mixture of the aldehyde and catalyst, which favored the preferred tricoordinate Ag, gave high diastereo- and enantioselec-tivity (eq 3). 

Gold and Silver-Catalyzed Asymmetric Aldol Reactions of a-Isocyanocarboxylates... 

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 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... 

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. 

It is interesting that aldol-type condensation of tosylmethyl isocyanide (16) with aldehydes is catalyzed by the silver catalyst more stereoselectively than that catalyzed by the gold catalyst under the standard reaction conditions (Scheme 8B1.9) [26], Elucidation of the mechanistic differences between the gold and silver catalysts in the asymmetric aldol reaction of 16 needs further study. Oxazoline 17 can be converted to optically active a-alkyl-p-(A-methyl-amino)ethanols. 

On the other hand, silver salts can act as a mild Lewis acid, promoting various reactions such as allylations, aldolizations, cycloadditions, and cyclizations. Silver salts can also be used as halide scavengers, acting as cocatalysts in cross-coupling reactions catalyzed by other metals, especially palladium. In the latter context, the exact role of silver salts is far from clear and may be more complex than just halide... 

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... 

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]. 

Ito and coworkers found that chiral ferrocenylphosphine-silver(I) complexes also catalyze the asymmetric aldol reaction of isocyanoacetate with aldehydes (Sch. 26) [51]. It is essential to keep the isocyanoacetate at a low concentration to obtain a product with high optical purity. They performed IR studies on the structures of gold(I) and silver(I) complexes with chiral ferrocenylphosphine 86a in the presence of methyl isocyanoacetate (27) and found significant differences between the iso-cyanoacetate-to-metal coordination numbers of these metal complexes (Sch. 27). The gold(I) complex has the tricoordinated structure 100, which results in high ee, whereas for the silver(I) complex there is an equilibrium between the tricoordinated structure 101 and the tetracoordinated structure 102, which results in low enantioselectivity. Slow addition of isocyanoacetate 27 to a solution of the silver(I) catalyst and aldehyde is effective in reducing the undesirable tetracoordinated species and results in high enantioselectivity. 

The asymmetric aldol-type addition of tosylmethyl isocyanide (99) to aldehydes can also be catalyzed by the chiral silver(I) complex, giving almost exclusively trans-S-alkyl-4-tosyl-2-oxazolines 98 [E = S02(p-Tol)] with up to 86 % ee as shown in Sch. 26 [52]. The slow addition method described above is not necessary for this reaction system. 

Silver(i) complex coordinated with the ferrocenylbisphosphine ligand 8g is also effective as a catalyst for the asymmetric aldol reaction of isocyanoacetate when the isocyanoacetate is kept in low concentration in the reaction system (Scheme 2-58) [82], Thus, by the slow addition of isocyanoacetate over a period of 1 h to a solution of aldehyde and the silver catalyst, iranj-oxazolines are formed in 80—90% ee, the enantioselectivity being only a little lower than that observed in the gold(i)-catalyzed... 

The ferrocenylphosphine-silver complex catalyzes the aldol-type reaction of tosylmethyl isocyanide 71 with aldehydes with higher stereoselectivity than the gold complex (Scheme 2-59) [84]. The reaction with several aldehydes produces trans-4-tosyloxazolines 72 in up to 86% ee, which can be converted into optically active l-alkyl-2-aminoethanols by reduction with LiAlH4. 

Transition-metal complexes bearing chiral ligands catalyze an asymmetric aldol condensation of isocyanoacetates with aldehydes to afford a mixture of cis- and /rons-4,5-disubstituted-4,5-dihydrooxazoles (188) in high optical purity (Equation (27)). Both gold and silver ferrocenylphos-phine complexes are effective <94TL2713>. 

Ferrocenyldiphosphines 3.41 (R = Me2NCH2CH2) are used as ligands in gold-catalyzed asymmetric aldol reactions of a-isocyanoesters or -amides [408, 752, 858, 950], Silver complexes can also be used with the modified phosphine 3.41 (R = (CH2)5 or (CH g) [951, 952]. 

The most important source of acetone is the Hock process for phenol production. In this process acetone is obtained as stoichiometric coupling product. If acetone needs to be produced deliberately, it can be obtained by oxidative dehydrogenation or dehydrogenation of isopropanol. Oxidative dehydrogenation proceeds at 400-600 °C at silver or copper contacts, direct dehydrogenation is carried out at 300-400 °C using zinc contacts. Alternatively, acetone can also be obtained by a Wacker-Hoechst oxidation of propylene. Acetone is used industrially as solvent. Moreover, the aldol condensate products of acetone (diacetone alcohol) are used as solvents. Acetone is also converted in an add catalyzed reaction with two moles of phenol for the synthesis of bisphenol A. Bisphenol A is an important feedstock for the production of epoxy resins and polycarbonates. 

Fig. 6 Catalytic asymmetric dihydropyranone synthesis through copper(l)-catalyzed asymmetric aldol reaction and subsequent silver(I)-catalyzed oxy-Michael reaction...

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 ... 

Scheme 3-195. Silver/(R)-binap-catalyzed asymmetric aldol reaction of stannyl...

Simple chiral bisphosphine silver(I) complexes have been reported by Yamamoto to catalyze enantioselective addition reactions of trimethoxysilyl enolates [149, 150] and diketenes [150] to aldehydes. The aldol addition reaction with diketene 301 in Equation 28 is illustrative [150]. The requisite tin enolates are generated in situ from the corresponding enol acetates or diketenes upon exposure to Me3SnOMe or Bu2Sn(OMe)2. This represents a noteworthy example of a transition-metal-catalyzed process involving aldol addition that can be carried out successfully in a polar, protic solvent (methanol). 

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