A practical and efficient method for the synthesis of cyclic carbonates from epoxides and CO2 under mild reaction conditions was achieved via the use of a potassium iodide (KI)–tetraethylene glycol complex as a readily available and economical catalyst. The effects of glycols and alkali metal salts were investigated in the present work to clarify the importance of both KI and tetraethylene glycol. Scalability and reusability of this catalytic system were also demonstrated.

Although the hydrogen-bonding ability of the α-hydrogens on tetraalkylammonium salts is often discussed in the chemistry of phase-transfer catalysts, the catalysis that utilizes the hydrogenbond donor properties of tetraalkylammonium salts remains unknown. In the present work, we demonstrated hydrogen-bonding catalysis with newly designed tetraalkylammonium salt catalysts in Mannich-type reactions. Both the structure and the hydrogenbonding ability of the new ammonium salts were investigated via Xray diffraction analysis and NMR titration studies.Tetraalkylammonium salts are recognized as representative organocatalysts, [1] and are often used as phase-transfer catalysts for the activation of anionic nucleophiles through the formation of an ion pair with an ammonium cation. [2] Although the structures of tetraalkylammonium salts are commonly expressed as 1a shown in Figure 1, the actual ionic structures are discussed differently. [3][4][5] The positive charge of ammonium salts delocalized on the α-hydrogen atoms, which are known to interact with an anionic counterion through hydrogen bonding, as shown in 1A. Reetz proved the delocalization of the positive charge in tetraalkylammonium salts by X-ray crystal analysis of tetrabutylammonium salts such as tetrabutylammonium enolate and phenoxide. [3] Furthermore, DFT calculations support the delocalized structures of ammonium salts, which include chiral ammonium salts. [4,5] The interaction between α-hydrogens on the chiral tetraalkylammonium salt catalyst and the enolate oxygen was thought to be important in the transition-state model of asymmetric phase-transfer reactions. [5] However, despite the interesting hydrogen-bonding ability of the α-hydrogens on tetraalkylammonium salts, the catalysis that could utilize such properties is, to the best of our knowledge, still unknown. Herein, we report a new dimension of tetraalkylammonium salt as a hydrogen-bonding catalyst that utilizes the characteristic properties of the α-hydrogens on the catalyst. [6] Figure 1. Structures of tetraalkylammonium salt.To realize the efficient hydrogen-bonding catalysis of a tetraalkylammonium salt, we designed a new tetraalkylammonium salt, 2, which was readily prepared via the methylation of a commercially available 2,6-piperidinecarboxylate, 3 (Scheme 1). The ammonium salt 2 possesses carboxylate groups at the α-carbon, which enhance the hydrogen-bonding ability of the α-hydrogens. Furthermore, the six-membered structure of the piperidine backbone fixes the acidic α-hydrogens to a position that is appropriate for bidentate binding to an anionic group. [7] Scheme 1. Design and synthesis of a new tetraalkylammonium salt for use as a hydrogen-bonding catalyst.To obtain structural information of newly prepared tetraalkylammonium salts 2, we performed an X-ray diffraction COMMUNICATIONanalysis of ammonium iodide 2a (Figure 2). [8] The crystal structure of 2a provided important structural information. As expected, the hydrogen-bonding interactions between the α-hydrogens and the...

Phase-transfer catalysis has long been recognized as a versatile method for organic synthesis. In particular, during more than the past three decades, asymmetric phase-transfer catalysis based on the use of structurally well-defined chiral catalysts has become a topic of great scientific interest. Although various effective chiral catalysts have already been reported and these catalysts were utilized for practical asymmetric transformations, further design and development of new chiral phase-transfer catalysts are still attractive research subjects in organic chemistry due to the high utility and practicability of phase-transfer-catalyzed reactions. This review focuses on the recent examples of newly designed effective chiral phase-transfer catalysts.

Axially, planar, and helical chiral compounds are indispensable building blocks in modern organic synthesis. A wide variety of chiral ligands and catalysts were designed based on these chiral scaffolds, and these chiral ligands and catalysts were used for various catalytic asymmetric transformations to produce important chiral compounds in an optically enriched form. Furthermore, these chiral skeletons are found in the structure of biologically active natural products. Thus, the development of efficient enantioselective methods for the synthesis of these chiral compounds is an important task in the field of organic chemistry. In the last few years, organocatalyzed approaches, which are one of the most reliable catalytic asymmetric methods, became a hot topic. This Focus Review summarizes asymmetric organocatalytic methods for the synthesis of axially, planar, and helical chiral compounds as useful chiral building blocks.

Although quaternary ammonium and phosphonium salts are known as important catalysts in phase-transfer catalysis, the catalytic ability of tertiary sulfonium salts has not yet been well demonstrated. Herein, we demonstrate the catalytic ability of trialkylsulfonium salts as hydrogen-bonding catalysts on the basis of the characteristic properties of the acidic α hydrogen atoms on alkylsulfonium salts.The importance of onium salt compounds has been established in the field of organic chemistry. 1 Alkyl-ammonium, phosphonium, and sulfonium salts are some of the most important and reliable onium salt reagents in organic synthesis (Fig. 1). These compounds are often utilized as very useful reagents in the construction of organic building blocks, and the reactions using these reagents appear in textbooks of organic chemistry as important, named reactions. 2,3 Furthermore, quaternary ammonium and phosphonium salts are also known as reliable catalysts, which are used to promote a wide variety of organic transformations as phase-transfer and/or base catalysts. 4 Despite the wide synthetic utility of onium salt compounds as reagents and catalysts, the catalytic ability of tertiary sulfonium salts has not yet been demonstrated well in organic synthesis. 5 The limitation of sulfonium salt catalysts has been attributed mainly to the high reactivity and instability of the compounds with acidic α hydrogen atoms. To create new possibilities for sulfonium salts as catalysts, we focused on the hydrogen-bonding abilities of α hydrogen atoms on alkylsulfonium salts when we reported the use of type 1 tetraalkylammonium salts as hydrogen-bonding catalysts on the basis of the characteristic properties of the α hydrogen atoms (Fig. 2). 6,7 Herein, we demonstrate the catalytic ability of trialkylsulfonium salts in hydrogen-bonding catalysis. 8 Fig. 1 Onium salts in organic synthesis.Based on the design of type 1 tetraalkylammonium salts as effective hydrogen-bonding catalysts, 6 we focused on simple type 2 trialkylsulfonium salts (Fig. 2). The structures of the α hydrogen atoms that binded to the iodide anion compared favorably to the X-ray crystal structures of ammonium iodide 1a and sulfonium iodide 2a. 9,10 Furthermore, we expected the acidity of the α hydrogen atoms of 2a to approximate the acidity of 1a, based on the reported pKa values. 11 Fig. 2 Comparison between ammonium iodide 1a and sulfonium iodide 2a.

A piperidine-derived tetraalkylammonium salt with a non-coordinating counteranion worked as an effective hydrogen-bonding catalyst in an aza-Diels-Alder reaction of imines and a Danishefsky diene. The hydrogen-bonding interaction between the ammonium salt and an imine was observed as part of a (1) H NMR titration study.

An efficient synthesis of cyclic thiocarbonates from epoxides and CS2 under mild reaction conditions was achieved when a KI–tetraethylene glycol complex was used as a readily available and economic catalyst. The effects of various glycols and alkali metal halides were investigated in the present work to clarify the importance of both KI and tetraethylene glycol. The reaction mechanisms for the synthesis of cyclic thiocarbonates are discussed on the basis of the stereochemistry of the products.

Although the hydrogen-bonding ability of the α-hydrogens on tetraalkylammonium salts is often discussed in the chemistry of phase-transfer catalysts, the catalysis that utilizes the hydrogenbond donor properties of tetraalkylammonium salts remains unknown. In the present work, we demonstrated hydrogen-bonding catalysis with newly designed tetraalkylammonium salt catalysts in Mannich-type reactions. Both the structure and the hydrogenbonding ability of the new ammonium salts were investigated via Xray diffraction analysis and NMR titration studies.Tetraalkylammonium salts are recognized as representative organocatalysts, [1] and are often used as phase-transfer catalysts for the activation of anionic nucleophiles through the formation of an ion pair with an ammonium cation.[2] Although the structures of tetraalkylammonium salts are commonly expressed as 1a shown in Figure 1, the actual ionic structures are discussed differently. [3][4][5] The positive charge of ammonium salts delocalized on the α-hydrogen atoms, which are known to interact with an anionic counterion through hydrogen bonding, as shown in 1A. Reetz proved the delocalization of the positive charge in tetraalkylammonium salts by X-ray crystal analysis of tetrabutylammonium salts such as tetrabutylammonium enolate and phenoxide.[3] Furthermore, DFT calculations support the delocalized structures of ammonium salts, which include chiral ammonium salts. [4,5] The interaction between α-hydrogens on the chiral tetraalkylammonium salt catalyst and the enolate oxygen was thought to be important in the transition-state model of asymmetric phase-transfer reactions.[5] However, despite the interesting hydrogen-bonding ability of the α-hydrogens on tetraalkylammonium salts, the catalysis that could utilize such properties is, to the best of our knowledge, still unknown. Herein, we report a new dimension of tetraalkylammonium salt as a hydrogen-bonding catalyst that utilizes the characteristic properties of the α-hydrogens on the catalyst. To realize the efficient hydrogen-bonding catalysis of a tetraalkylammonium salt, we designed a new tetraalkylammonium salt, 2, which was readily prepared via the methylation of a commercially available 2,6-piperidinecarboxylate, 3 (Scheme 1). The ammonium salt 2 possesses carboxylate groups at the α-carbon, which enhance the hydrogen-bonding ability of the α-hydrogens. Furthermore, the six-membered structure of the piperidine backbone fixes the acidic α-hydrogens to a position that is appropriate for bidentate binding to an anionic group. [7] Scheme 1. Design and synthesis of a new tetraalkylammonium salt for use as a hydrogen-bonding catalyst.To obtain structural information of newly prepared tetraalkylammonium salts 2, we performed an X-ray diffraction COMMUNICATIONanalysis of ammonium iodide 2a (Figure 2). [8] The crystal structure of 2a provided important structural information. As expected, the hydrogen-bonding interactions between the α-hydrogens and the counteranion (I -) were clearly observed, and the iodid...