Starks extraction mechanism

Mechanistically, typical PTC reactions take place via two main mechanisms in liquid-liquid systems. The extraction mechanism (Starks, 1971) is the most commonly accepted one for simple nucleophilic substitution under neutral conditions for reactions with a range of anions (e.g halides, cyanide, thiocyanate, sulfite, nitrite, acetate, carbonate, etc.). According to this mechanism (Figure 16.2a), the PT catalyst, denoted by Q+X , is a vehicle to transfer the reactive anion Y of the metal salt from the aqueous phase into the organic phase, where it reacts with the organic substrate, RX, to give the desired product RY and regenerating Q+X , which can continue the PTC cycle. Typically, the active form of... 

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). 

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. 

With regards to the mechanism of the generation of onium anion, the Starks extraction mechanism and interfacial mechanism (Brandstrom-Montanari modification) are suggested (Scheme 1.7). As in the above-described case, the interfacial mechanism seems to be operative in the asymmetric phase-transfer catalysis. 

Starks Extraction Mechanism Brandstrom-Montanari Modification... 

Phase-transfer-catalyzed reactions proceed via two main pathways. In the classic extraction mechanism proposed by Starks (Figure 4.28a), the quat Q extracts the... 

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

Two limiting mechanistic models describing liquid-liquid PTC are the Starks extraction mechanism [4,5,49] and the M kosza interfacial mechanism [121,125]. However, the experimental results of PTC reactions indicate that there is a spectrum of mechanisms that fall within these two limiting mechanisms. Selected systems are discussed as follows. 

FIG. 1 Starks extraction mechanism for simple phase-transfer-catalyzed displacement reaction. 

The Starks extraction mechanism and the Brandstrom-Montanari modifieation can be described by the following reaction steps [19] ... 

In PT catalysis, the reaction mechanisms that have been proposed are the Starks extraction mechanism and Mqkosza s interfacial mechanism. These two mechanisms describe the zone where the organic reaction occurs or the phase where the rate-determining step is located. However, in reality, it is realized that many PTC reactions are conducted both at the interface and in the bulk solution, especially for a reaction controlled by the intrinsic organic reaction [3]. The distinction between these two mechanisms is recognized as the difference in the depth of the reaction zone penetrating the organic phase. 

Quaternary salts, crown ethers, cryptands, and polyethylene glycol (PEG) are the most common agents used for LLPTC. Over the last few decades, the two reaction mechanisms used to describe the phenomenon of a two-phase PTC reaction were the Starks extraction mechanism and M kosza interfacial mechanism. 

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. 

Figure 16.2 LLPTC mechanism, (a) Starks extraction mechanism and (b) BrandstrSm-Montanari moditication.

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