Catalysis/catalysts halides, organic synthesis

On the basis of these redox potentials it seems likely that direct electron release to the benzenediazonium ion takes place only with iodide. This corresponds well with experience in organic synthesis iodo-de-diazoniations are possible without catalysts, light, or other special procedures (Sec. 10.6). For bromo- and chloro-de-di-azoniations, catalysis by cuprous salts (Sandmeyer reaction, Sec. 10.5) is necessary. For fluorination the Balz-Schiemann reaction of arenediazonium tetrafluoroborates in the solid state (thermolysis) or in special solvents must be chosen (see Sec. 10.4). With astatide (211At-), the heaviest of the halide ions, Meyer et al. (1979) found higher yields for astato-de-diazoniation than for iodo-de-diazoniation, a result consistent with the position of At in the Periodic System. It has to be emphasized, however, that in investigations based on measuring yields of final products (Ar-Hal), the possibility that part of the yield may be due to heterolytic dediazoniation is very difficult to quantify. 

Among common carbon-carbon bond formation reactions involving carbanionic species, the nucleophilic substitution of alkyl halides with active methylene compounds in the presence of a base, e. g., malonic and acetoacetic ester syntheses, is one of the most well documented important methods in organic synthesis. Ketone enolates and protected ones such as vinyl silyl ethers are also versatile nucleophiles for the reaction with various electrophiles including alkyl halides. On the other hand, for the reaction of aryl halides with such nucleophiles to proceed, photostimulation or addition of transition metal catalysts or promoters is usually required, unless the halides are activated by strong electron-withdrawing substituents [7]. Of the metal species, palladium has proved to be especially useful, while copper may also be used in some reactions [81. Thus, aryl halides can react with a variety of substrates having acidic C-H bonds under palladium catalysis. 

The fundamental theory of phase transfer catalysis (PTC) has been reviewed extensively. Rather than attempt to find a mutual solvent for all of the reactive species, an appropriate catalyst is identified which modifies the solubility characteristics of one of the reactive species relative to the phase in which it is poorly solubilized. The literature on the use of PTC in the preparation of nitriles, halides, ether, and dihalocarbenes is extensive. Although PTC in the synthesis of C- and 0-alkylated organic compounds has been studied, the use of PTC in polymer synthesis or polymer modification is not as well studied. A general review of PTC in polymer synthesis was published by Mathias. FrecheE described the use of PTC in the modification of halogenated polymers such as poly(vinyl bromide), and Nishikubo and co-workers disclosed the reaction of poly(chloromethylstyrene) with nucleophiles under PTC conditions. Liotta and co-workers reported the 0-alkylation of bituminous coal with either 1-bromoheptane or 1-bromooctadecane. Poor 0-alkylation efficiencies were reported with alkali metal hydroxides but excellent reactivity and efficiencies were found with the use of quaternary ammonium hydroxides, especially tetrabutyl- and tetrahexylammonium hydroxides. These results are indeed noteworthy because coal is a mineral and is not thought of as a reactive and swellable polymer. Clearly if coal can be efficiently 0-alkylated under PTC conditions, then efficient 0-alkylation of cellulose ethers should also be possible. 

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