Reagents accelerator

As described above, both MnC03 and CaC03 were transformed into y-Mn203 and CaO (via Ca(OH)2), respectively, at lower temperature in the superheated steam. In addition, die transformation of the reagents accelerates the solid state reaction, because the crystal lattice of each reagent was highly activated due to the transformation i.e. so-called the Hedvall effect [12]. 

In tanneries, sodium bisulfite is used to accelerate the unhairing action of lime. It is also used as a chemical reagent ia the synthesis of surfactants (qv). Addition to alpha-olefins under radical catalyzed conditions yields sodium alkylsulfonates (wetting agents). The addition of sodium bisulfite under base-catalyzed conditions to dialkyl maleates yields the sulfosucciaates. 

Detailed mechanistic studies by Fodor demonstrated the intermediacy of both imidoyl chlorides (6) and nitrilium salts (7) in Bischler-Napieralski reactions promoted by a variety of reagents such as PCI5, POCI3, and SOCh)/ For example, amide 1 reacts with POCI3 to afford imidoyl chloride 6. Upon heating, intermediate 6 is converted to nitrilium salt 7, which undergoes intramolecular electrophilic aromatic substitution to afford the dihydroisoquinoline 2. Fodor s studies showed that the imidoyl chloride and nitrilium salt intermediates could be generated under mild conditions and characterized spectroscopically. Fodor also found that the cyclization of the imidoyl chlorides is accelerated by the addition of Lewis acids (SnCU, ZnCh), which provides further evidence to support the intermediacy of nitrilium salts. ... 

When a positively charged substituent such as the trimethylam-monio group is anywhere on the ring, but most effectively when it is ortho to the leaving group, it can favorably affect the entropy of activation with anionic nucleophiles and accelerate reaction. A recent example of reagent-substituent interaction is the electrophilic substitution of 2-carboxybiphenyl, nitration (non-polar solvent) of which occurs only at the 2 -position and not the 4 -position and has been postulated to be due to the interaction of the nitronium ion with the carboxyl group. 

The possibility of a radical mechanism is supported by the observation of the accelerating effect of molecular oxygen on the cyclopropanation. Miyano et al. discovered that the addition of dioxygen accelerated the formation of the zinc carbenoid in the Furukawa procedure [24a, b]. The rate of this process was monitored by changes in the concentration of ethyl iodide, the by-product of reagent formation. Comparison of the reaction rate in the presence of oxygen with that in the... 

The landmark report by Winstein et al. (Scheme 3.6) on the powerful accelerating and directing effect of a proximal hydroxyl group would become one of the most critical in the development of the Simmons-Smith cyclopropanation reactions [11]. A clear syw directing effect is observed, implying coordination of the reagent to the alcohol before methylene transfer. This characteristic served as the basis of subsequent developments for stereocontrolled reactions with many classes of chiral allylic cycloalkenols and indirectly for chiral auxiliaries and catalysts. A full understanding of this phenomenon would not only be informative, but it would have practical applications in the rationalization of asymmetric catalytic reactions. 

Hie first effect has been exploited in numerous ligand-accelerated [30], enantioselective 1,2-additions of RyZn reagents to aldeliydes [26]. Dimetliylzinc, for example, has a linear structure and is not reactive towards aldehydes or ketones. Upon coordination of triazine, however, a tetraliedral configuration is produced at tlie zinc... 

Since the corrosion resistance of the high-silicon alloys depends upon the permanence and impermeability of a thin silica film on the surface of the metal, it is obvious that any reagent which can damage the film will cause accelerated corrosion of the metal. For this reason all solutions containing hydrofluoric acid must be regarded as incompatible with the alloys. 

In general, the reaction mechanism of elastomeric polymers with vulcanisation reagents is slow. Therefore, it is natural to add special accelerators to rubber compounds to speed the reaction. Accelerators are usually organic compounds such as amines, aldehyde-amines, thiazoles, thiurams or dithio-carbamates, either on their own or in various combinations. 

---