Phase-transfer catalysis has been recognized as a powerful method for establishing practical protocols for organic synthesis, because it offers several advantages, such as operational simplicity, mild reaction conditions, suitability for large-scale synthesis, and the environmentally benign nature of the reaction system. Since the pioneering studies on highly enantioselective alkylations promoted by chiral phase-transfer catalysts, this research field has served as an attractive area for the pursuit of "green" sustainable chemistry. A wide variety of asymmetric transformations catalyzed by chiral onium salts and crown ethers have been developed for the synthesis of valuable organic compounds in the past several decades, especially in recent years.

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.

A protocol for the dehydrative nucleophilic substitution of benzyl alcohols with a variety of carbon- and heteroatom-centered nucleophiles using dodecylbenzenesulfonic acid (DBSA) as a surfactant-type Brønsted acid catalyst in water has been developed. The reaction system can be applied to the stereoselective C-glycosylation of 1-hydroxy sugars in water. [reaction: see text].

Phase-transfer catalysis (PTC) has been recognized as a convenient and highly useful tool in academia and industry because it offers several advantages for practical organic synthesis, such as operational simplicity, mild reaction conditions in aqueous media, environmental benefits, and suitability for large-scale reactions. [1,2] Also the development of efficient methods for the preparation of natural and nonnatural a-alkyl-and a,a-dialkyl-a-amino acids, especially in their enantiomerically pure forms by asymmetric PTC, has become very important because of their high synthetic utility. [3, 4] Accordingly, several phase-transfer catalysts have been developed that lead to products with excellent enantioselectivities in high yields.[4] However, despite numerous studies, truly efficient catalytic systems with high enantioselection at very low catalyst loading (e.g., < 0.1 mol %) are still rare in asymmetric carbon-carbon bond formation, and major progress in terms of catalyst loading is still desirable for practical asymmetric synthesis. Since our recently developed, chiral spiro-type (R,R)-or (S,S)-3,4,5-trifluorophenyl-NAS bromide 1 shows exceedingly high enantioselectivity in asymmetric alkylation of a-amino acid derivatives, [4d,e,m] our next target was the design of a very active catalyst. Considering the highly lipophilic nature of 1 and the generation of a metal enolate in an interfacial layer, [5] such lipophilic 1 (QX) must move to the interfacial layer to induce a facile exchange reaction with a metal enolate (Scheme 1). Based on this assumption, our strategy was to replace the rigid binaphthyl moiety in 1 by flexible straight-chain alkyl groups to furnish a new catalyst of type 2, which substantially accelerates the enolate exchange with 2 because of the increasing polarity of the dialkylammonium moiety. Herein, we report that such a designer chiral quaternary ammonium salt 2 behaves as a very powerful chiral phase-transfer catalyst for the highly practical, enantioselective alkylation of protected-glycine and aalkyl-a-amino acid derivatives.The requisite catalyst (S)-2 can be readily prepared from the commercially available (S)-1,1'-binaphthyl-2,2'-dicarboxylic acid (3) [6] in a six-step sequence as outlined in Scheme 2.[7]Thus, (S)-dicarboxylic acid 3 was transformed with iPrBr, catalytic Bu 4 N·HSO 4 , and KF·2 H 2 O to the corresponding diisopropyl ester 4 in 95 % yield. Treatment of 4 with freshly prepared Mg(TMP) 2 (TMP = 2,2,6,6-tetramethylpiperidide) in THF and subsequent additon of bromine gave rise to (S)-3,3'-dibromo-1,1'-binaphthyl-2,2'-dicarboxylic ester 5 in 91 % yield. Suzuki-Miyaura cross coupling of 5 with 3,4,5-trifluorophenylboronic acid in the presence of catalytic Pd(OAc) 2 , PPh 3 , and K 2 CO 3 in N,N-dimethylformamide (DMF) afforded (S)-3,3'-bis(3,4,5-trifluorophenyl)-1,1'-binaphthyl-2,2'-dicarboxylic ester (6) in 94 % yield. Reduction of 6 with Scheme 1. Proposed mechanism for the generation of chiral ammonium enolate.Scheme 2. a) iPrBr (10 equiv), Bu 4 N·HSO 4 (20 mol %), KF·2 ...

Reinjection- and MIBI-g-SPECT provide clinically satisfactory various functional data. These functional data in combination with the perfusion information will improve diagnostic and prognostic accuracy without an increase in cost or the radiation dose to the patients.

An efficient synthesis of cyclic carbonates from epoxides and CO 2 under mild reaction conditions was achieved via the use of triethylamine hydroiodide as a simple yet effective bifunctional catalyst. The importance of the bifunctional feature of the catalyst was clearly demonstrated in the present work via control experiments and 1 H NMR studies. The scalability of this catalytic system was also demonstrated.

An efficient synthesis of cyclic carbonates from epoxides and CO 2 under mild reaction conditions was achieved via the use of a newly designed bifunctional quaternary phosphonium iodide catalyst. The importance of the bifunctional design of the catalysts was clearly demonstrated in the present work by control experiments. Furthermore, a chiral version of the bifunctional phosphonium salt could be applied to a kinetic resolution of this reaction.Carbon dioxide (CO 2 ) is recognized as an unfavorable industrial waste due to its greenhouse effect, and the reduction of CO 2 emissions has become integral to the creation of a sustainable society.1 From a different point of view, however, CO 2 is also recognized as an ideal C1 source for chemical synthesis because it is inexpensive, abundant, and nontoxic. In this context, the development of chemical transformations to useful compounds via the use of CO 2 as a reactant has become a hot topic in the field of green sustainable chemistry. 2 Among these transformations, the synthesis of cyclic carbonates 2 from epoxides 1 and CO 2 has been the most extensively studied due to their high utility in industrial processes. 3Although a wide variety of metal catalysts and organocatalysts such as quaternary onium salts have been developed to promote this reaction, high temperature (>100 °C) and pressure (>10 atm) are often required to produce the products 2 in satisfactory yields. 4 Therefore, the development of catalysts that can be effective for this reaction under mild conditions continues to be highly desired. To achieve this desirable task, we became interested in the design of new bifunctional quaternary onium salt catalysts possessing a phenolic hydroxy group when a binary catalyst system with onium salts and phenols was reported to have effectively promoted the reaction under relatively mild conditions (Scheme 1). 5,6 Herein, we report the development of effective bifunctional quaternary phosphonium salts 7 for the synthesis of cyclic carbonates 2 with epoxides 1 under atmospheric CO 2 pressure, mild temperature (60 °C), and low catalyst loading (1 mol %). The utility of a bifunctional design for the catalyst in this reaction was clearly demonstrated in the present work. Furthermore, a chiral version of the bifunctional organocatalyst was also developed, and we applied the chiral catalyst to a kinetic resolution in this reaction. 9Scheme 1 Synthesis of cyclic carbonates 2 under mild reaction conditions.To achieve an efficient synthesis of cyclic carbonates 2 under mild reaction conditions, we designed bifunctional quaternary phosphonium bromides 3 possessing a phenolic hydroxy group. The phosphonium salts 3 with a biphenyl backbone were readily prepared via the reaction with triphenylphosphine and corresponding arylmethyl bromides. There was a concern that the distance between the phosphonium salt moiety and the hydroxy group on catalyst 3 could affect the catalytic activity, and therefore, we prepared regioisomers: o-3, m-3, and p-3 (Scheme 2). The structure of the phosph...

The development of enantioselective phase-transfer catalysis for preparing important natural products or physiologically active compounds is quite attractive and challenging in terms of environmental consciousness. Although quaternary ammonium salts as phase-transfer catalysts are believed to require base additives for phase-transfer reactions, we have discovered that even without any base additives, the enantioselective phase-transfer conjugate addition of 3-phenyloxindole to beta-nitrostyrene proceeded smoothly in the presence of a chiral bifunctional ammonium bromide under neutral conditions in water-rich solvent with both high diastereo- and enantioselectivity.