Cationic [2,6-Bis(2‘-oxazolinyl)phenyl]palladium(II) Complexes:  Catalysts for the Asymmetric Michael Reaction

Abstract

Reaction of 1,3-dicyanobenzene with β-amino alcohols (S)-H 2 NCHRCH 2 OH (R ) i Pr, i Bu, t Bu, CH 2 Cy, CH 2 Ph) and (R)-H 2 NCHPhCH 2 OH gave new 1,3-bis(2′-oxazolinyl)benzenes (30-51%). These, together with 1,3-bis(4′,4′-dimethyl-2′-oxazolinyl)benzene, were treated with LDA/TMEDA followed by the addition of PdBr 2 (1,5-COD) to give [2,6-bis(2′-oxazolinyl)phenyl]palladium(II) bromide complexes (21-41%). In two cases no complexes were obtained (R ) Ph, CH 2 Ph) due to ring opening of the oxazolines by LDA/TMEDA. Treatment of the palladium complexes with AgBF 4 , AgOTf, or AgSbF 6 in wet CH 2 Cl 2 provided a series of cationic [2,6-bis(2′-oxazolinyl)phenyl]palladium complexes (28-87%) containing water coordinated to palladium, as established by an X-ray crystal structure analysis of (S,S)-[2,6bis(4′-isopropyl-2′-oxazolinyl)phenyl]aquopalladium(II) trifluoromethanesulfonate. All of the cationic complexes proved to be efficient catalysts for the Michael reaction between R-cyanocarboxylates and methyl vinyl ketone and between acrylonitrile and activated Michael donors. Selectivities of up to 34% ee were obtained for the formation of (R)-ethyl 2-cyano-2-methyl-5-oxohexanoate.

Development of Pincer Catalysts with Selected Group 8～10 Metals

“…[29] . [34] ; 用于催化异腈与醛的 aldol 反应 时, 最高可获得 75% ee [35] ; Phebox-Pd 用于催化 Suzuki 偶 联 反 应 , ee 值 最 高 也 只 有 49% [36] ; 阳 离 子 型 的 Phebox-Pd 化合物在催化 α-氰基丙酸酯与多种 Michael 受体的不对称 Michael 加成反应中, 最高仅得到 34% ee [37] . [39] .…”

Deuterium as a Stereochemically Invisible Blocking Group for Chiral Ligand Synthesis

“…The latter likely arises from lithiation at position 4 of the oxazoline followed by ring-opening and nitrogen protonation on work-up, a process reported previously with another phenylsubstituted oxazoline substrate. 12 Thus prevention of the favored lithiation pathway by deuterium introduction can lead to an undesirable reaction pathway, in addition to the formation of the alternative diastereoisomer.…”

Organocatalysis in Enantioselective α‐Functionalization of 2‐Cyanoacetates

“…[19] When 2-chloroacrylonitrile was used as the Michael acceptor two stereogenic centers were created. Modified Cinchona alkaloids and thioureas derived from alkaloids have been used to promote this reaction.…”

“…Complexes derived from rhodium and chiral diphosphines, [28] palladium and chiral ligands with trans-chelating donor sites, [19,29] iridium [27] and platinum complexes [30] with chiral phosphanyloxazolines, chiral salen-aluminum complexes [10e] and, more recently, a ferrocenyl-1,1'-diyl bis-imidazoline bis-palladacycle complex [31] have been used to promote this reaction. The first efficient organocatalyzed conjugate additions using a,b-unsaturated ketones as Michael acceptors were not described until 2007.…”