Palladium diaquo complexes

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. 

The formation of palladium enolates by cleavage of silyl enol ethers is a key step in palladium-catalyzed aldol reactions [170]. When, within the catalytic cycle, palladium diaquo complex 175 with chiral bis-phosphane ligands is allowed to react... 

An axially chiral NHC palladium diaquo complex derived from (5)-BEqAP promotes a-fluorination of oxindoles by selectfluor with modest enantioselectivity. TiCl2(TADDOLate)-catalysed asymmetric a-fluorination of a range of activated carbonyl compounds (with up to 91.5% ee) by selectfluor is also reported. ... 

The search for a catalyst suitable to promote addition of the less reactive silyl enol ethers of ketones has identified a novel class of cationic transition metal complexes in two independent laboratories. The use of a chiral palladium(II) di-aquo complex in the catalytic asymmetric addition of silyl enol ethers to aldehydes (first demonstrated by Shibasaki, Sodeoka et al. [52a, 52b]) provided a clear precedent for their subsequent use with a-imino esters [53] (Scheme 27). Initial experiments focused on the reaction of various a-imino esters 82a-c with silyl enol ether 83 (1.5equiv) in the presence of the Pd diaquo complex 80a (10 mol %) in DMF. Extensive experimentation led to the formation of 84c in 67% ee, and also underscored the importance of suppressing the generation of tetrafluoroboric acid during the course of the reaction. 

The Pd diaquo complex of BINAP 19 efficiently catalyzed the diastereoselective and enantioselective Michael addition of the y3-keto ester 20 to 3-penten-2-one (21), and the Michael adduct 22 was obtained in 89 % yield (diastereomeric ratio = 8/1) and the ee of the major isomer was 99%. Thus, congested vicinal tertiary and quaternary carbon centers were constructed. It is interesting to know that the Pd aquo complex 19 allows the successive supply of a Bronsted base and a Bronsted acid. The former activates the carbonyl compound to give the chiral palladium enolate and the latter cooperatively activates e enone [4]. 

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See also in sourсe #XX -- [ ]

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