Accelerative reaction

Abstract Development and scope of conventionally difficult molecular transformation on site-selective acylation of carbohydrates and polyol compounds are described. A salient feature is that the site-selectivity can be controlled independently from the intrinsic reactivity of the substrate, i.e., catalyst-controlled selectivity. Therefore, some substrates undergo acylation with reversal of their intrinsic reactivity. The mechanistic aspects of catalyst-controlled site-selective acylation are discussed with the emphasis on the strategy relying on the accelerative reaction rather than the decelerative one. An unconventional retrosynthetic route based on catalyst-controlled site-selective acylation is proposed toward extremely short-step total synthesis of natural glycosides of an ellagitannin family. Application to the late-stage functionalization of the complex natural products of biological interest is also described. 

Keywords Accelerative reaction Acylation Catalyst-controUed reaction Molecular recognition Organocatalyst Retrosynthesis Site-selectivity... 

Most of the LFRP research ia the 1990s is focused on the use of nitroxides as the stable free radical. The main problems associated with nitroxide-mediated styrene polymerizations are slow polymerization rate and the iaability to make high molecular weight narrow-polydispersity PS. This iaability is likely to be the result of side reactions of the living end lea ding to termination rather than propagation (183). The polymerization rate can be accelerated by the addition of acids to the process (184). The mechanism of the accelerative effect of the acid is not certain. 

Bunnett and co-workers have shown that an or Ao-carboxy-late anion decreases the rate of reaction of 4-nitrochlorobenzene with methoxide ion but rather strongly increases the reaction rate with piperidine. This effect arises from an accelerative increase in the... 

Reactions of iV -alkylated or arylated azinium compounds with nucleophiles proceed more readily than those of the parent, uncation-ized azines, and the ring tends to open. The iV -substituent may bring into play an accelerative effect from the London forces of attraction. Increased displaceability of the substituent in iV -alkyl-azinium compounds has been noted for 2-halopyridinium (87) 1-haloisoquinolinium, 4-halopyrimidinium, 4-methoxypyrid-inium (88), 4-phenoxy- and 4-acetamido-quinazolinium (89), 3-methylthiopyridazinium, and 2-car boxymethylthiopyrimidi-nium salts (90). The latter was prepared in situ from the iV -alkyl-pyrimidine-2-thione. The activation can be effectively transmitted to... 

These subsystems profoundly affect the fuel cell system performance. As an example, the inherently slow air (oxygen) electrode reaction must be acceler-... 

Abstract. Auto-accelerated polymerization is known to occur in viscous reaction media ("gel-effect") and also when the polymer precipitates as it forms. It is generally assumed that the cause of auto-acceleration is the arising of non-steady-state kinetics created by a diffusion controlled termination step. Recent work has shown that the polymerization of acrylic acid in bulk and in solution proceeds under steady or auto-accelered conditions irrespective of the precipitation of the polymer. On the other hand, a close correlation is established between auto-acceleration and the type of H-bonded molecular association involving acrylic acid in the system. On the basis of numerous data it is concluded that auto-acceleration is determined by the formation of an oriented monomer-polymer association complex which favors an ultra-fast propagation process. Similar conclusions are derived for the polymerization of methacrylic acid and acrylonitrile based on studies of polymerization kinetics in bulk and in solution and on evidence of molecular associations. In the case of acrylonitrile a dipole-dipole complex involving the nitrile groups is assumed to be responsible for the observed auto-acceleration. 

A similar explanation of accelerative effects observed in grafting with other solvents such as the higher straight chain alcohols, DMF, DMSO, acetone, chloroform and cyclohexane (Table VI) has been advanced (6). It appears that there is a relationship between G(H) value of solvent and the extent to which the solvent participates in accelerated grafting. The radiolysis pathway thus contributes, but not exclusively, to the mechanism of the overall copolymerisation reaction. 

The rearrangement of unsymmetrical allylamines 9 was investigated to exclude any competing 1,3-rearrangement during the course of the reaction. Allyl vinyl amines 10 were generated via condensation starting from allylamine 9 and isobutyraldehyde 2. The substrates 10 were subjected to the acid-accel-... 

The rate constants and k represent rate constants for a surface reaction and have units m mol s and s respectively. The accelerative effects are about 10 -10 fold. They indicate that both reactants are bound at the surface layer of the micelle (surfactant-water interface) and the enhanced rates are caused by enhanced reactant concentration here and there are no other significant effects. Similar behavior is observed in an inverse micelle, where the water phase is now dispersed as micro-droplets in the organic phase. With this arrangement, it is possible to study anion interchange in the tetrahedral complexes C0CI4 or CoCl2(SCN)2 by temperature-jump. A dissociative mechanism is favored, but the interpretation is complicated by uncertainty in the nature of the species present in the water-surfactant boundary, a general problem in this medium. 

Structure-Reactivity Relationship of Olefins. The relative reactivity of a series of olefins toward the potent oxidizing species, X, formed by the interaction of TPP Mn(II) with 02, was investigated by means of a competitive reaction technique. As shown in Table VII, the relative reactivity of an olefin, as followed by gas-liquid chromatographic determination, increases on introduction of an alkyl substituent onto the olefinic carbon atom other than the reacting carbon atom. However, the introduction of an alkyl substituent onto the reacting carbon atom reduces (or compensates) the accelerative electronic effect, as seen in the comparison between cyclohexene and n-hexene. This situation becomes clearer if one compares the two dialkyl ethylenes, cyclohexene and methylenecyclohexane, where the former has a single substituent on the reacting carbon and the other has none the observed relative reactivity is 1 27.2. 

The deacylation reaction shows an accelerative deviation from the first-order kinetics. This peculiar behavior is most apparent for the samples 20% or more acylated, and explained best by intramolecular imidazole catalysis. Jencks and C riudo reported that imidazole catalyzed the hydrolysis of acetyl imidazc (61). A similar mechanism was proposed for the deacylation of pofyvinylimidazcde and the accelerative deacylation behavior was attributed to the increaang local imidazole concentration along the polymer chain. 

M + NHj MNH, + 1/2Hj, Catalysts such as spongy Pt or Fe oxides greatly acceler ate the reaction. These amides may serve for theprepnof the very explosive silver ami des... 

---