Interfacial Processes—The Key Steps of Phase Transfer Catalyzed Reactions

Abstract

After short historical introduction, interfacial mechanism of phase transfer catalyzed (PTC) reactions of organic anions, induced by aqueous NaOH or KOH in two-phase systems is formulated. Subsequently experimental evidence that supports the interfacial deprotonation as the key initial step of these reactions is presented.

Technological Aspects of Highly Selective Synthesis of Allyloxyalcohols—New, Greener, Productive Methods

“…Interestingly, the activity of the revealed PT catalyst was substantially higher than Bu 4 N + HSO 4 − , postulated by us previously [44]. Although the study on the mechanism of PTC mono-O-allylation was not planned in this work, our experimental results suggested that the reaction of butane-1,4-diol with allyl chloride in the studied PTC/concentrated NaOH(aq) system proceeds mainly via the interfacial mechanism, proposed by M ąkosza [54,55]. However, considering the lower pKa value of butane-1,4-diol of 14.5 [57], and according to the general instructions that organic acids with a pKa range 18 < x < 25 undergo PTC reaction in an interfacial manner, the Starks' extraction mechanism cannot be ruled out.…”

“…For these purposes, the effect of concentrated aqueous solutions of NaOH on the O-allylation reaction course was first studied-with respect to the other crucial reaction parameters. Generally, although the mechanisms of substitution reactions in the PTC/NaOH system are more complicated, the role of NaOH(aq) in L-L PTC is relatively well known [54][55][56]. In short, the NaOH(aq), located in the inorganic phase, is essential to the deprotonation initial step of appropriate organic substrates for the generation of organic anions (the acid-base equilibrium) in the interface between the immiscible phases and their subsequent ion exchange with TAA salt.…”

“…Surprisingly, the ammonium cation with methyl, ethyl, butyl, or phenyl groups, significantly differ in their lipophilicity, with practically no effect on the TAA catalytic activity. The visible higher ABO yields were determined for TAA with trioctylammonium cation, but the TAA with longer chains of twelve units and above sixteen do not act as a PT catalyst (probably due to high lipophilicity of ammonium cations, they remain preferentially in the organic phase, resulting in unfavorable extraction capability and, in consequence, decelerating or stopping the PT catalysis [53][54][55][56]). The PT catalyst with the highest activity (66% of the yield) was MeOc 3 N + Br − (4).…”

“…However, considering the lower pKa value of butane-1,4-diol of 14.5 [57], and according to the general instructions that organic acids with a pKa range 18 < x < 25 undergo PTC reaction in an interfacial manner, the Starks' extraction mechanism cannot be ruled out. However, the full understanding of the structural features that lead to PT catalytic activity and selectivity of TAA salts are much more complicated and require detailed research on both physical (mass transport between phases) and chemical (the interaction and correlation of reactivity of all ion reagents) aspects [53][54][55][56]. Instead, our attention became focused on the real effectiveness of the discovered MeOc 3 N + Br − (4) in the studied model reaction.…”

Assisted by Hydrogen-Bond Donors: Cinchona Quaternary Salts as Privileged Chiral Catalysts for Phase-Transfer Reactions

“…Scheme 11 Enantioselective cyclization to indole type products using hybrid PTC catalystsIn the discussed work, the authors postulated two reaction mechanisms (Scheme 11), initiated by deprotonation according to the Mąkosza30 interphase mechanism. In both versions, the key stage is the formation of an ionic pair between the catalyst and the substrate.…”