Makosza interfacial mechanism

The exact pathway for generating the reactive onium carbanion species remains the subject of controversy, typically among Starks extraction mechanism (Scheme 1.2) and the Makosza interfacial mechanism (Scheme 1.3). 

Since asymmetric phase-transfer catalysts normally contain highly lipophilic chiral organic frameworks, and are reluctant to enter the aqueous phase, the Makosza interfacial mechanism seems plausible. 

Figure 4.28 Different pathways for PTC a the classic Starks extraction mechanism b the Makosza interfacial mechanism.

Charles Liotta modified the Makosza interfacial mechanism. In this modification deprotonation takes place at the interfacial region and is assisted by the quaternary cation. 

Not only does the solvent affect the reaction rate, but it also determines the reaction mechanism. In Starks extraction mechanism of PTC, most reacting compound transfers to the bulk phase. However, reaction may occur at the interface of the two phases. For example hexachlorocyclotriphosphazene has been reported to react very slowly with 2,2,2-trifluoroethanol in an alkaline solution of NaOH/C HjCl two-phase system in the absence of phase-transfer catalyst.Since sodium 2,2,2-trifluoroe anoxide is not soluble in chlorobenzene, the process probably proceeds at the interface region of the system. Similar is the reaction of benzylation of isobutyraldehyde in the presence of tetra-n-butylammonium iodide in an alkaline solution of NaOH/toluene, which is a two-phase system. Makosza interfacial mechanism was employed to rationalize the experimental results. The main reason is that the ammonium salt of the nucleophilic reagent is not soluble in toluene. 

The Makosza interfacial mechanism begins with the formation of a metal carbanion species at the interface between the organic and aqueous phases in the absence of the phase-transfer catalyst. Extraction of this species from the interface to the organic phase then takes place, mediated by the phase-transfer catalyst (Figure 14). ... 

A combination of the Makosza and modified interfacial mechanisms of hydroxide-promoted reactions of organic acids was discussed by Juang and Liu [2] ... 

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