Chiral salen-metal complexes as novel catalysts for asymmetric phase transfer alkylations

Influence of aromatic substituents on metal(II)salen catalysed, asymmetric synthesis of α-methyl α-amino acids

“…In 1999, we reported that nickel(II)salen complex 1a (10 mol%) would catalyse the asymmetric benzylation of alanine enolate 2a leading to a-methyl phenylalanine 3a (Scheme 1) with 30% enantiomeric excess. 25 The corresponding copper(II)salen complex 1b was also studied and was found to be a far more effective catalyst. Just 2 mol% of complex 1b was sufficient to catalyse the formation of compound 3a with 88% enantiomeric excess.…”

“…26 Complex 1b also catalysed the asymmetric alkylation of compound 2a with other alkyl halides, giving a-methyl a-amino acids with 75-90% enantiomeric excess. 25 These reactions are carried out under solid -liquid phase transfer conditions with solid sodium hydroxide as the base, and both enantiomers of catalyst 1b are equally readily available, thus allowing the synthesis of either enantiomer of an a-methyl a-amino acid. In subsequent work, we have also demonstrated that under appropriate reaction conditions, it is possible to use the readily available methyl ester substrate 2b.…”

“…28 In this paper, we report the results of a study aimed at better understanding and optimizing the influence of various factors on the enantioselectivity of this reaction. In particular, the effect that substituents on the aromatic rings of catalyst 1b have on the enantioselectivity of the catalyst are reported 25,29 In addition, the influence of the structure of the imine within substrate 2b on the enantioselectivity of the reaction is studied. 29 Finally, a method for the preparation of the catalysts which avoids the use of size-exclusion chromatography on sephadex LH20 is described.…”

Contrast performance in catalytic ability—new cinchona phase transfer catalysts for asymmetric synthesis of α-amino acids

“…The unusual aromatic-F effect in cinchona based PTCs on the selectivity was also demonstrated by Park et al 8 Furthermore, non-cinchona chiral catalysts, such as phosphonium salts, 9 TADDOL, 10 and salen-metal complexes 11 have also been introduced for asymmetric PTC reactions. However, there has been little understanding over the arylmethyl groups employed for the quaternization of cinchonidine and their influence on asymmetric induction.…”

Glycine and Alanine Imines as Templates for Asymmetric Synthesis of α-Amino Acids

“…[26a] Similar conditions have been used for the enantioselective alkylation of the Wangresin-bound benzophenone imine of glycine 19 at room temperature affording α-amino acids in 51Ϫ89% ee. [26b] α-Methyl α-amino acids (AMAAs) [1l,1m] have been prepared by alkylation of aldimine Schiff bases of alanine esters 22 under PTC conditions by using TADDOL 23 [27a] (up to 82% ee and 68Ϫ96% yield), NOBIN and its derivatives 24 [27b] (up to 68% ee and 50Ϫ90% yield) or salenϪmetal complexes 25 [28] (up to 92% ee and 20Ϫ99% yield) as chiral catalysts. Activated alkyl bromides, with toluene as solvent and solid NaOH or NaH have been used at room temperature in all cases in the presence of 10% of catalysts 23؊25.…”