Altered reactivity

Pierre and Handel have studied the effect of [2.1.1]-cryptate on the lithium aluminum hydride reduction of cyclohexanone in diglyme [16]. The [2.1.1]-cryptate strongly complexes lithium ion and if sufficient cryptate is used to sequester all of the lithium ion, no reduction occurs. Apparently, lithium ion is needed as an electrophilic catalyst for the reduction to occur (see Eq. 12.8). Consistent with this interpretation is the observation that even in the presence of cryptate, reduction will occur if an excess of lithium iodide is also present. The relatively low reactivity of tetrabutyl-ammonium borohydride in benzene solution may also reflect this property, at least in part [9]. Likewise, the jS-hydroxyethyl quaternary ammonium ions may be better catalysts than non-oxygenated quaternary ions because the hydroxyl may hydrogen bond to carbonyl and provide electrophilic catalysis [5]. Similar, though less dramatic results, have been observed in the reduction of aromatic aldehydes and ketones by lithium aluminum hydride in the presence of [2.1.1]-cryptate [17]. 

Herman, H. Newer Methods in Preparative Organic Chemistry 2, 213 (1963). 

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Contents Introduction and Principles. - The Reaction of Dichlorocarbene With Olefins. - Reactions of Dichlorocarbene With Non-Olefinic Substrates. -Dibromocarbene and Other Carbenes. - Synthesis of Ethers. - Synthesis of Esters. - Reactions of Cyanide Ion. - Reactions of Superoxide Ions. - Reactions of Other Nucleophiles. - Alkylation Reactions. - Oxidation Reactions. - Reduction Techniques. - Preparation and Reactions of Sulfur Containing Substrates. -Ylids. - Altered Reactivity. - Addendum Recent Developments in Phase Transfer Catalysis. 

Unlike conventional chemical reactions, the altered reactivity of chemical reactions undergoing ultrasonic irradiation is principally due to acoustic cavitation which essentially involves the free radical formation. The ultrasound produces highly reactive free radical species like H and OH radicals from the homolytic cleavage of water. Further they may react with any of other free radicals present or with neutral molecules like 02 and O3 to produce peroxy species, superoxide, hydrogen peroxide or hydrogen. When the aqueous solution is saturated with 02, extra... 

Immune response The development of specifically altered reactivity following exposure to an antigen. This may take several forms, e.g. antibody production, cell-mediated immunity, immunological tolerance. 

The notion of C02 activation by transition metal complexes is intimately related to C02 coordination chemistry, since it is through coordination that the electronic structure of the C02 molecule can be sufficiently perturbed to result in altered reactivity. Different modes of C02 coordination will manifest themselves in different types and degrees of altered reactivity. Whether these changes in the C02 electronic structure and consequent reactivity correspond to true activation must be examined on an individual reaction basis. In addition, there exists the larger question of whether coordination of C02 to a metal center is really a necessary step in promoting reaction of C02 with different substrates. Two obvious alternatives are (1) occurrence of direct substrate-C02 reaction in solution with the product... 

What is lacking at this point in theories relating lattice restraints and chemical reactivity is the identification of specific steric interactions which alter reactivity and an estimation of their magnitude. This requires an extensive database of structure-reactivity information for a series of closely related compounds. This we have from our studies on the solid state photochemistry and X-ray crystallography of a large number of variously substituted bicyclic dienones of general structure L (5). In this series, we recently observed a photorearrangement... 

Smith AT, Sanders SA, Thomeley RNF et al (1992) Characterisation of a haem active-site mutant of horseradish peroxidase, Phe41 Val, with altered reactivity towards hydrogen peroxide and reducing substrates. Eur J Biochem 207 507-519... 

The most prominent solvent effects are characteristic of ionic reactions (Parker, 1967, 1969) while rates of homolytic processes are rather independent of the nature of solvents (e.g., Pryor, 1966). In some radical reactions, however, the effect of solvent is significant. Complex formation between radical and solvent has been suggested sometimes to account for the altered reactivity (Mayo, 1953 Russell, 1958 Ingold, 1963b Andronov et al., 1967) in particular systems. 

Altered reactivity to touch Increase (f) or decrease ( ) Animal s flight reaction to downward finger pressure on the hindquarters is compared with controls... 

Lipoxygenase metabolites of arachidonic acid such as 12-hydroxyeicosa-tetraenoic acid (12-HETE) have been shown to be involved in the release of neurotiansmitters and other cellular functions (92-94). Basal generation of 12-HETE has been reported to be about 3.7-fold greater in SHR than in WKY platelets (95,96). Arachidonic acid has been shown to stimulate platelets by activating protein kinase C, by a prostaglandin-independent mechanism (97). It is possible that the lipoxygenase metabolites of arachidonic acid may also be involved in altered reactivity in hypertension. 

Francis, J. W., Henry, P. M. Oxidation of olefins by palladium(ll). Part XIV. Product distribution and kinetics of the oxidation of ethene by PdCl3(pyridine)- in aqueous solution in the presence and absence of CUCI2 a modified Wacker catalyst with altered reactivity. J. Mol. Catal. A Chemical 1995, 99, 77-86. 

Fulginiti VA, Eller JJ, Downie AW, Kempe CH. Altered reactivity to measles virus. Atypical measles in children previously immunized with inactivated measles virus vaccine. / Am Med Assoc. 1967 202 (12) 1075-1080. 

Actually, because of the altered reactivity of some substrates dissolved in RTILs and the unexpected catalytic activity and unforeseen by-products formation, the possible non-innocent nature of many ionic liquids has been hypothesized by some authors [37]. 

The mechanisms may be complex and give rise to a variety of symptoms, but the basis is a specific altered reactivity of the organism involving either humoral or cellular systems. The basis therefore is the recognition of a substance as an antigen. Antigens are ... 

Lewis acids have long been used in both polymerizations and copolymerizations to enhance the reactivities of monomers. The addition of ZnC, alkyl aluminum compounds, or boron halides has been shown to increase both the rate and degree of polymerization of monomers such as acrylonitrile and methyl methacrylate [8]. The use of Lewis acids to enhance electrophilicity of acrylate monomers has also been exploited to enhance alternation in copolymerizations with electron-rich alkenes such as isobutylene [9], Systems that would never produce alternating copolymers can be induced to do so with as little as 0.1 equivalents of an appropriate Lewis acid. This section focusses on efforts to utilize Lewis acids to both alter reactivity and control stereochemistry in copolymerizations. 

These molecules, propenal, methoxyethene, and etheneamine, show how we can apply VB theory and resonance to questions of reactivity. We looked at how structure and conjugation affect electron density and bond formation in both the reactant and the intermediate. When VB theory indicates that the particular disposition of function groups will change the electron distribution relative to an unsubstituted molecule, we can expect to see those differences reflected in altered reactivity. For propenal, the electron withdrawal by the formyl group causes decreased reactivity toward electrophiles and increased reactivity toward nucleophiles. For methoxyethene and ethenamine, the electron release of the substituents is reflected by increased reactivity toward electrophiles with strong selectivity for the P-carbon. 

The types of organosulfur compound present in petroleum feedstocks are alkyl and aryl thiols (RSH), thioethers (RSR ), disulfides (RSSR ), and thiophenic compounds (Fig. 1). The ease with which sulfur is abstracted depends very much on the nature of the sulfur-containing molecule aliphatic compounds (thiols, thioethers) are usually desulfurized much more easily than heteroaromatic (e.g. thiophenes, benzothiophenes, dibenzothiophenes). Among the latter, reactivity decreases in the order thiophene > benzothiophene > dibenzothiophene. The presence of aliphatic substituent groups can sometimes alter reactivity. The sterically hindered compound 4,6-dimethyldibenzothiophene is, for example, very difficult to desulfurize. 

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