A new generation of chiral phase-transfer catalysts

Abstract

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.

Bifunctional Ammonium Salt Catalyzed Asymmetric α‐Hydroxylation of β‐Ketoesters by Simultaneous Resolution of Oxaziridines

“…[4,5] We have recently introduced an ew class of bifunctional( thio)-urea-containingc hiral quaternary ammonium salt catalysts 1,which can be easily obtained in amodular fashion startingf rom simple trans-1,2-cyclohexanediamine. [6,7] These hybrid catalysts were found to be very promising for av ariety of reactions like asymmetric a-halogenations of b-ketoesters 2 or cascade reactions, and the modulars ynthesis approach allows for ar apid fine-tuning of catalyst structure for ag iven target reaction. [7] Keeping in mind the high value of chiral a-hydroxy-1,3-dicarbonyl compounds and the resulting currenti nteresti nt he development of asymmetric synthesis methods, we becamei nterested in testing the potential of our catalysts ystem for the asymmetric a-hydroxylation of b-ketoesters 2.…”

The Synthesis of Hydrobenzoin-Based Monoaza Crown Ethers and Their Application as Recyclable Enantioselective Catalysts

“…First, the reaction conditions were optimized, and the effect of catalysts was evaluated. The MIRC reaction of 2-benzylidene-1,3-indandione (7) and diethyl bromomalonate (8) was chosen as the model reaction to test the utility of catalysts (R,R)-4a-d (Table 4, entries 1-4) [78,79]. The reactions took place in diethyl ether using Na 2 CO 3 as the base, and 5 mol % of the catalyst in 1-2 h. In this instance, the side arm of lariat ethers (R,R)-4a-d did not have a significant effect on the enantioselectivity as compared to the above-mentioned epoxidation.…”

“…Asymmetric phase transfer catalysis has become a topic of great scientific interest in the last 30 years. A great number of chiral catalysts have been synthesized and used with excellent enantioselectivities in different reactions [1][2][3][4][5][6][7][8]. Despite all advantages, most of the applied methods have the shortcomings that the recovery of the catalyst is not solved, or cumbersome purification (e.g.…”

Chiral Tertiary Sulfonium Salts as Effective Catalysts for Asymmetric Base‐Free Neutral Phase‐Transfer Reactions

“…Chiral onium salts are recognized as some of the most reliable compounds in asymmetric synthesis (Figure 1). [1][2][3][4][5] Va rious types of chiral quaternary ammonium and phosphonium salts have been synthesized, particularly over the past two decades. [1,2] These compounds have been employed as chiral phase-transfer catalysts,a nd have enabled aw ide variety of efficient enantioselective transformations.…”

“…[1,2] These compounds have been employed as chiral phase-transfer catalysts,a nd have enabled aw ide variety of efficient enantioselective transformations. [3] On the other hand, the catalytic ability of chiral tertiary sulfonium salts has yet to be demonstrated in asymmetric synthesis, despite the utility of these chiral compounds as reagents in stereoselective reactions. [4,5] Thel imitations of chiral sulfonium salt catalysts have been attributed mainly to the high reactivity and instability of the compounds with acidic ahydrogen atoms.…”