Rate Enhancement of Catalytic Asymmetric Reactions by Silver I Salts

5 Rate Enhancement of Catalytic Asymmetric Reactions by Silver(I) Salts 

The role of silver salts in the asymmetric Heck reaction is thought to be as in Sch. 29 [57b]. The Ag ion abstracts the iodide from the palladium intermediate 107 to produce the square-planar 16-electron Pd intermediate 108 which is indispensable for high asymmetric induction. 

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. 

3-Dipolar cycloaddition of nitrones to olefins is also catalyzed by chiral cationic palladium complexes [66]. When nitrone 119 is treated with 3-(( )-2-butenoyl)-l,3-oxazoli-din-2-one (120) under the influence of (5)-BINAP PdCla (10 mol %) and AgBF4 (20 mol %), isoxazolidine 121 is obtained in 61 % yield as a 45 55 mixture of endo and exo forms. The endo isomer is obtained in 91 % ee (Sch. 32). The chemical yield and endo selectivity are improved by employing (5 )-p-Tol-BINAP as chiral ligand. 

Mikami and Nakai have shown that chiral titanium perchlorate 123, prepared from chiral titanium dichloride 122 and AgC104 (2 equiv.), is an asymmetric superior catalyst to 122 in terms of the diastereo- and enantioselectivity of carbonyl-ene cycliza-tion [67], AgC104 alone does not catalyze the ene cyclization. One typical example is indicated in Sch. 33. Treatment of a-alkoxy aldehyde 124 with the chiral titanium catalyst 123 in the presence of 4-A molecular sieves in CH2CI2 at 0 °C gives the trans alcohol 125 selectively with 84 % ee. Employment of the titanium dichloride 122, in contrast, results in a nearly 1 1 mixture of trans-125 and cis-i25 with lower enantioselectivity. 

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