Topics in Current Chemistry (2019) 377:31
1 3
1 Introduction
Chiral non-racemic onium ions have been widely exploited as effective organic
catalysts for enantioselective carbon–carbon or carbon–heteroatom bond-forming
reactions. The mode of catalysis most widespread for chiral onium ions is phasetransfer catalysis [1–6]. An essential elementary event in asymmetric phase-transfer catalysis is the generation of well-defined ion pairs consisting of chiral onium
ions and anionic intermediates, which subsequently undergo bond formation with
electrophiles in an efficient and stereoselective manner. The features of such ionpair catalysis are dictated primarily by the electrostatic interactions between cations and anions. These attractive forces are compatible with other non-covalent
interactions, thus ensuring the feasibility of designing hybrid catalytic systems
composed of ionic organic molecules and metal complexes, of which the latter
generally harness coordinative interactions in exerting catalytic performance. The
cooperation with metal catalysis has led to a broader scope of substrates in asymmetric transformations than that allowed by ionic organic catalysts alone. On the
other hand, the development of chiral ligands that incorporate ionic molecular
components provides a unique opportunity to impart distinct stereocontrolling
ability to the corresponding metal complexes being applied as catalysts for the
target transformations. The focus of this chapter is to outline the characteristic
reactivity and selectivity attained in the methodology development based on the
combined use of chiral cationic organic catalysts and metal catalysts or the elaboration of chiral ligands embedded with cationic functionalities.
2 Asymmetric Transformation Using Chiral Onium Salts with Metal
Catalysts
2.1 Asymmetric Allylic Alkylation of α‑Amino Acid‑Derived Schiff Base
Asymmetric substitution of alkyl halides with active methylene compounds, i.e.,
the alkylation reaction, occupies the central position in the field of asymmetric
phase-transfer catalysis and one such representative reaction is the enantioselective alkylation of glycine Schiff base for the efficient asymmetric synthesis of
α-amino acids. The first example was introduced by O’Donnell and co-workers,
who used cinchonine- or cinchonidine-derived quaternary ammonium salts as catalysts for promoting and controlling the target asymmetric alkylation [7, 8]. For
example, the reaction of glycine-derived Schiff base 1 with 4-chlorobenzyl bromide proceeded smoothly under the influence of a catalytic amount of N-benzylcinchoninium chloride 3 in the biphasic system consisting of 50% aqueous NaOH
and CH 2 Cl 2 to afford the alkylated product 2a in good yield with moderate enantioselectivity (Fig. 1). Following this pioneering work, enormous efforts towards
the development of structurally well-defined new chiral onium salts, such as
Reprinted from the journal
132
1 3
1 Introduction
Chiral non-racemic onium ions have been widely exploited as effective organic
catalysts for enantioselective carbon–carbon or carbon–heteroatom bond-forming
reactions. The mode of catalysis most widespread for chiral onium ions is phasetransfer catalysis [1–6]. An essential elementary event in asymmetric phase-transfer catalysis is the generation of well-defined ion pairs consisting of chiral onium
ions and anionic intermediates, which subsequently undergo bond formation with
electrophiles in an efficient and stereoselective manner. The features of such ionpair catalysis are dictated primarily by the electrostatic interactions between cations and anions. These attractive forces are compatible with other non-covalent
interactions, thus ensuring the feasibility of designing hybrid catalytic systems
composed of ionic organic molecules and metal complexes, of which the latter
generally harness coordinative interactions in exerting catalytic performance. The
cooperation with metal catalysis has led to a broader scope of substrates in asymmetric transformations than that allowed by ionic organic catalysts alone. On the
other hand, the development of chiral ligands that incorporate ionic molecular
components provides a unique opportunity to impart distinct stereocontrolling
ability to the corresponding metal complexes being applied as catalysts for the
target transformations. The focus of this chapter is to outline the characteristic
reactivity and selectivity attained in the methodology development based on the
combined use of chiral cationic organic catalysts and metal catalysts or the elaboration of chiral ligands embedded with cationic functionalities.
2 Asymmetric Transformation Using Chiral Onium Salts with Metal
Catalysts
2.1 Asymmetric Allylic Alkylation of α‑Amino Acid‑Derived Schiff Base
Asymmetric substitution of alkyl halides with active methylene compounds, i.e.,
the alkylation reaction, occupies the central position in the field of asymmetric
phase-transfer catalysis and one such representative reaction is the enantioselective alkylation of glycine Schiff base for the efficient asymmetric synthesis of
α-amino acids. The first example was introduced by O’Donnell and co-workers,
who used cinchonine- or cinchonidine-derived quaternary ammonium salts as catalysts for promoting and controlling the target asymmetric alkylation [7, 8]. For
example, the reaction of glycine-derived Schiff base 1 with 4-chlorobenzyl bromide proceeded smoothly under the influence of a catalytic amount of N-benzylcinchoninium chloride 3 in the biphasic system consisting of 50% aqueous NaOH
and CH 2 Cl 2 to afford the alkylated product 2a in good yield with moderate enantioselectivity (Fig. 1). Following this pioneering work, enormous efforts towards
the development of structurally well-defined new chiral onium salts, such as
Reprinted from the journal
132