Reactions significant

Biological Antioxidant Models. Tea extracts, tea polyphenol fractions, and purified catechins have all been shown to be effective antioxidants in biologically-based model systems. A balance between oxidants and antioxidants is critical for maintenance of homeostasis. Imbalances between free radicals and antioxidants may be caused by an increased production of free radicals or decreased effectiveness of the antioxidants within the reaction system. These imbalances can be caused by the radicals overwhelming the antioxidants within the system, or by an excess of antioxidants leading to a prooxidant functionaHty (105—118). When antioxidant defense systems are consistently overwhelmed by oxidative reactions, significant damage can... 

The above described reaction has been extended to the application of the AlMe-BINOL catalyst to reactions of acyclic nitrones. A series chiral AlMe-3,3 -diaryl-BINOL complexes llb-f was investigated as catalysts for the 1,3-dipolar cycloaddition reaction between the cyclic nitrone 14a and ethyl vinyl ether 8a [34], Surprisingly, these catalysts were not sufficiently selective for the reactions of cyclic nitrones with ethyl vinyl ether. Use of the tetramethoxy-substituted derivative llg as the catalyst for the reaction significantly improved the results (Scheme 6.14). In the presence of 10 mol% llg the reaction proceeded in a mixture of CH2CI2 and petroleum ether to give the product 15a in 79% isolated yield. The diastereoselectiv-ity was the same as in the acyclic case giving an excellent ratio of exo-15a and endo-15a of >95 <5, and exo-15a was obtained with up to 82% ee. 

These studies have indicated that the independent parameters controlling the postulated solid-phase reactions significantly affect the resulting acoustic admittance of the combustion zone, even though these reactions were assumed to be independent of the pressure in the combustion zone. In this combustion model, the pressure oscillations cause the flame zone to move with respect to the solid surface. This effect, in turn, causes oscillations in the rate of heat transfer from the gaseous-combustion zone back to the solid surface, and hence produces oscillations in the temperature of the solid surface. The solid-phase reactions respond to these temperature oscillations, producing significant contributions to the acoustical response of the combustion zone. 

COMMON NAME(S) SCIENTIFIC NAME USES ADVERSE REACTIONS SIGNIFICANT CONSIDERATIONS... 

In addition to these reactions, significant carbon deposition on the Pt surface was found to take place64 typically accounting for up to 20% of converted C2H4. 

Bulk or forced flow of the Hagan-Poiseuille type does not in general contribute significantly to the mass transport process in porous catalysts. For fast reactions where there is a change in the number of moles on reaction, significant pressure differentials can arise between the interior and the exterior of the catalyst pellets. This phenomenon occurs because there is insufficient driving force for effective mass transfer by forced flow. Molecular diffusion occurs much more rapidly than forced flow in most porous catalysts. 

As with the allylsilane cross-metathesis reactions, significant quantities of allyl stannane self-metathesis were not detected in any of the reactions and the trans isomer predominated in the cross-metathesis products. Identical reactions were carried out using allyltributyl stannane, in place of allyltriphenyl stannane, but the yields of the cross-metathesis products were consistently lower and in many cases dropped below 25%. 

Impurities with catalytic effects—Impurities that act as catalysts, reducing the activation energy of a process, may increase the rate of reaction significantly, even when present in small quantities. The presence of sulfuric acid, for example, increases the rate of decomposition and decreases the observed onset temperature of various isomers of ni-trobenzoic acid [28]. Also, other substances such as NaCl, FeCl3, platinum, vanadium chloride, and molybdenum chloride show catalytic effects. As a result, the decomposition temperature can be lowered as much as 100°C. Catalysts, such as rust, may also be present inadvertently. Some decomposition reactions are autocatalyzed, which means that one of more of the decomposition products will accelerate the decomposition rate of the original substance. 

Rh(CO)2l4]", which is beneficial for catalytic activity and also suppresses the WGS reaction significantly. 

The rhodium-catalyzed [m-i-n-i-1] reactions, as exemplified by the [2-t2-tl] may be accomplished with an array of substrates that have proven challenging for other metal complexes. Moreover, the ability to undertake the domino reactions significantly increases the molecular complexity and ultimately the synthetic utihty of this venerable reaction. Although the [i + 2 + 1] and [4-tl] reactions have not been as extensively studied, they will undoubtedly be the subjects of future synthetic efforts. 

Rhodium complexes were generally found to be more effective than iridium, but on the whole they show moderate activity in alkene oxygenation reactions. Significantly, epoxides, a typical product of the oxidation of olefins catalyzed by the middle transition metals, have rarely been evoked as products [18-22]. Although allylic alcohols [23] or ethers [24] have sometimes been described as products, the above cited rhodium and iridiiun complexes are characterized by an excellent selectivity in the oxygenation of terminal alkenes to methyl ketones. 

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