Base-Free Neutral Phase-Transfer Reaction

Base-Free Neutral Phase-Transfer Reaction 

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Base-Free Neutral Phase-Transfer Reaction... 

Although quaternary onium bromides and chlorides as phase-transfer catalysts are generally beUeved to require base additives for phase-transfer reactions of active methylene and methine compounds, which were discussed above, we have recently discovered an hitherto unknown base-free neutral phase-transfer reaction system in asymmetric conjugate additions (Scheme 14.5) [23]. The reactions were efficiently promoted by chiral bifunctional ammonium bromide (S)-7 under neutral conditions with water-rich biphasic solvent. The role of hydroxy groups in the bifunctional catalyst was clearly shown in the transition-state model of the reaction based on the single-crystal X-ray structure of ammonium amide [23b] and nitro-nate [23c]. 

Recently, Maruoka and coworkers presented an asymmetric aldol reaction of a-substituted nitroacetates with aqueous formaldehyde under base-free neutral phase-transfer conditions [127]. In the presence of 0.1mol% (S)-43a, the aldol products were obtained in 62-86% yield with 74-91% ee (Scheme 12.21). Two of the aldol products were treated with zinc and acetic acid in isopropanol to give the corresponding a-methylserinates 96, which are core structure of biologically active natural products such as conagenin and piperazimycins. 

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