Generation of Reactive Onium Carbanion Species

In the Starks extraction mechanism, the phase-transfer catalyst moves back and forth across the organic and aqueous phases. The onium salt equilibrates with the inorganic base in the aqueous phase, and extracts hydroxide into the organic phase. 

The onium hydroxide then abstracts hydrogen from the acidic organic compound to give the reactive intermediate Q 1 R.  

The advocated pathway of the interfacial mechanism is the first formation of metal carbanion at the interface of organic and aqueous phase in the absence of phase-transfer catalyst, followed by the extraction of the formed metal carbanion species from the interface into the organic phase by the action of phase-transfer catalyst. 

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. 

Clearly, the area of the interface and the basicity of the inorganic salt affect the amount of available onium carbanion. It should be also noted that an excessively lipophilic phase-transfer catalyst would hardly access the interface, and consequently the use of such a catalyst would result in an insufficient reaction. 

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