Enantioselective Base-Free Phase-Transfer Reaction in Water-Rich Solvent

Abstract

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.

A Base‐Free Neutral Phase‐Transfer Reaction System

“…Surprisingly, however, tetrabutylammonium bromide as a representative achiral phase‐transfer catalyst was less effective for the reaction under the same base‐free reaction conditions (Scheme ). Although some interesting observations have been reported in our preliminary study on the novel base‐free reaction system,4a the mechanism for this very attractive and interesting reaction system is totally unclear. To obtain mechanistic insight into the base‐free neutral phase‐transfer reaction system, we are interested in the effect of water and catalyst structure on the efficiency of the reaction.…”

The Design of Environmentally-Benign, High-Performance Organocatalysts for Asymmetric Catalysis

“…However, we recently discovered that some base free, neutral phase transfer reactions could be promoted by the use of chiral bifunctional ammonium bromides as catalyst (Scheme 27). 40,41 By using (S) 30b or (S) 30c, the asymmetric conjugate additions of 3 phenyloxindole or tert butylbenzyloxycarbamate to β nitrostyrene proceeded smoothly under neutral conditions in a water rich solvent. It should be noted that both reactions in toluene without water do not proceed at all, implying that water is essential for promotion of the reactions.…”

Atom-Economic Synthesis of 4-Pyrones from Diynones and Water

“…Generally, water offers several "green chemistry" benefits as a solvent in organic transformations, including high efficiency, lower cost, ease of process, green and environmental protection [3,4]. Recently, there are many reports of clean transformations in water medium [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], such as coupling reactions [20][21][22][23][24][25][26][27][28][29][30], cyclizations [31][32][33][34], Michael additions [35][36][37][38][39], and condensations [40,41]. Additionally, H2O also participates in organic reactions as a nucleophile [42,43] to provide various kinds of functional compounds such as imidazo [1,2-a]pyridines [44], amino acid salts [45], α-amino ketones [46], and 1,3-oxazinan-2-ones [47]…”