Phase transfer catalysis mechanism

M. R Halpem (ed.), Phase-Transfer Catalysis Mechanism and Syntheses (ACS Symposium Series 659), American Chemical Society, Washington, DC, 1997. 

Phase-Transfer Catalysis Mechanisms and Syntheses, Ed. M. Halpem, American Chemical Society,... 

M. E. Halpern, Phase-Transfer Catalysis. Mechanisms and Synthesis, Vol. 659,... 

Diez-Barra E, De la Hoz A, Merino S, Sanchez-Verdu P (1997) In Halpern ME (ed) Phase transfer catalysis, mechanism and syntheses. ACS Symposium Series 659, chap 14,... 

Maruoka, K. (2008) in Asymmetric Phase Transfer Catalysis, Wiley-VCH, Weinheim Halpem, M. E. (1997) in Phase-Transfer Catalysis. Mechanism and Synthesis, ACS Symposium Series, American Chemical Society, Washington. 

Phase transfer catalysis succeeds for two reasons First it provides a mechanism for introducing an anion into the medium that contains the reactive substrate More important the anion is introduced m a weakly solvated highly reactive state You ve already seen phase transfer catalysis m another form m Section 16 4 where the metal complexmg properties of crown ethers were described Crown ethers permit metal salts to dissolve m nonpolar solvents by surrounding the cation with a lipophilic cloak leav mg the anion free to react without the encumbrance of strong solvation forces... 

The reaction between acyl halides and alcohols or phenols is the best general method for the preparation of carboxylic esters. It is believed to proceed by a 8 2 mechanism. As with 10-8, the mechanism can be S l or tetrahedral. Pyridine catalyzes the reaction by the nucleophilic catalysis route (see 10-9). The reaction is of wide scope, and many functional groups do not interfere. A base is frequently added to combine with the HX formed. When aqueous alkali is used, this is called the Schotten-Baumann procedure, but pyridine is also frequently used. Both R and R may be primary, secondary, or tertiary alkyl or aryl. Enolic esters can also be prepared by this method, though C-acylation competes in these cases. In difficult cases, especially with hindered acids or tertiary R, the alkoxide can be used instead of the alcohol. Activated alumina has also been used as a catalyst, for tertiary R. Thallium salts of phenols give very high yields of phenolic esters. Phase-transfer catalysis has been used for hindered phenols. Zinc has been used to couple... 

The reaction with ammonia or amines, which undoubtedly proceeds by the SnAt mechanism, is catalyzed by copper and nickel salts, though these are normally used only with rather unreactive halides. This reaction, with phase-transfer catalysis, has been used to synthesize triarylamines. Copper ion catalysts (especially cuprous oxide or iodide) also permit the Gabriel synthesis (10-61) to be... 

The reaction of an a-halo sulfone with a base to give an alkene is called the Ramberg-Bdcklund reaction. The reaction is quite general for a-halo sulfones with an (x hydrogen, despite the unreactive nature of a-halo sulfones in normal 8 2 reactions (p. 437). Halogen reactivity is in the order I>Br>Cl. Phase-transfer catalysis has been used. In general, mixtures of cis and trans isomers are obtained, but usually the less stable cis isomer predominates. The mechanism involves formation of an episulfone, and then elimination of SO9. There is much evidence for... 

Figure 3.56 in Section 3.8 illustrates the mechanism of phase-transfer catalysis. Tables 4.4 and 4.5 give examples of industrial importance in agrochemicals, pharmaceuticals, fine chemicals, oleochemicals, etc. 

The effect of ultrasound on liquid-liquid interfaces between immiscible fluids is emulsification. This is one of the major industrial uses of ultrasound (74-76) and a variety of apparatus have been devised which will generate micrometer-sized emulsions (9). The mechanism of ultrasonic emulsification lies in the shearing stresses and deformations created by the sound field of larger droplets. When these stresses become greater than the interfacial surface tension, the droplet will burst (77,78). The chemical effects of emulsification lie principally in the greatly increased surface area of contact between the two immiscible liquids. Results not unlike phase transfer catalysis may be expected. 

Partitioning of carbocations between addition of nucleophiles and deprotonation, 35, 67 Perchloro-organic chemistry structure, spectroscopy and reaction pathways, 25, 267 Permutational isomerization of pentavalent phosphorus compounds, 9, 25 Phase-transfer catalysis by quaternary ammonium salts, 15, 267 Phosphate esters, mechanism and catalysis of nucleophilic substitution in, 25, 99 Phosphorus compounds, pentavalent, turnstile rearrangement and pseudoration in permutational isomerization, 9, 25... 

The mechanism of phase transfer catalysis is still a matter of discussion and remains a subject of some controversy. However, it will be roughly classified into two the extraction mechanism proposed by Starks121 and the interfadal mechanism by Makosza1111... 

M. Makosza, C. L Liotta, S. I. McCoy, A Look at Kinetics and Mechanism , Phases - The Sachem Phase Transfer Catalysis Review 1997, Issue 2, pp. 13-15. 

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