Synthesis and X-ray Crystal Structures of Bis(oxazolinyl)phenyl-Derived Chiral Palladium(II) and Platinum(II) and -(IV) Complexes and Their Use in the Catalytic Asymmetric Aldol-Type Condensation of Isocyanides and Aldehydes

Abstract

The transmetalation of (Phebox)SnMe 3 (3; Phebox ) 2,6-bis(oxazolinyl)phenyl) with PdCl 2 -(PhCN) 2 or K[PtCl 3 (C 2 H 4 )](H 2 O) gave (Phebox)M II Cl complexes (4, M ) Pd; 5, M ) Pt) in modest to good yields. The (Phebox)Pt IV Cl 3 complexes 8 were also synthesized by the reaction of 5 with CuCl 2 (H 2 O) 2 as an oxidant. These chloride complexes, 4, 5, and 8, were in turn converted to the cationic complexes by treatment with the corresponding silver salts (AgX: X ) BF 4 , OTf, OCOCF 3 ). X-ray structure studies revealed that the octahedral structure of (Phebox)Pt IV Cl 2 fragment G is isosteric with the (Phebox)Rh III Cl 2 fragment D, and the (Phebox)M II fragment H (M ) Pd, Pt) is almost the same configuration with a square-planar structure. In these complexes, the cationic BF 4 aqua complexes (9, M ) Pd II ; 10, M ) Pt II ; 11, M ) Pt IV ) were found to act as chiral catalysts for the aldol-type condensation of isocyanides and aldehydes in the presence of i-Pr 2 NEt. Excellent trans diastereoselectivities and modest to good enantioselectivities were obtained in the reaction of tosylmethyl isocyanide (TosMIC, 1) by the use of [(i-Pr-Phebox)Pt II (H 2 O)](BF 4 ) (i-Pr-10). Scheme 2Scheme 3

Unsymmetrical, oxazolinyl-containing achiral and chiral NCN pincer ligand precursors and their complexes with palladium(II)

“…The angle of C-Pd-Cl (179.0°or 177.6°) is almost linear, while the N-Pd-N angle (around 161°) is small, which are in accordance with a pincer consisting two five-membered-ring palladacycles [8,19,20,24,26] and reflect a relative steric strain of the almost planar tetracyclic system. All the bond distances and angles around Pd(II) centre are similar to those observed in the related bis(oxazoline) pincer Pd(II) complexes [20,29]. In the crystal of complex 4e, chlorine atom forms hydrogen bond with the adja-…”

Synthesis and structural characterization of enantiopure exo and endo six-membered oxazoline-derived palladacycles

“…Metal complexes of unsymmetrically substituted 2-oxazolines obtained from chiral amino acids and their derivatives have found a variety of applications, particularly in asymmetric synthesis [47]. Recently, a number of C * -chiral enantiopure exo [48][49][50], endo [48,[50][51][52][53][54][55][56][57][58][59][60][61][62] and pincer-type [58,[63][64][65][66][67][68][69][70] cyclopalladated oxazolines with a five-membered metalacycle have been reported (Chart 2). Other cyclopalladated oxazolines with planar and central chirality have also been studied [38,[71][72][73][74][75][76][77][78][79][80].…”

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] .…”