Reaction, preferential

Iminothiobutyramide (30), containing four nucleophilic centers (only two of which might react with two electrophilic sites in phenacylbromide), undergoes the Hantzsch reaction preferentially, yielding the enamine (31) in dry dioxane or (4-phenylthiazol-2-yl)acetone (32) in isopropanol. Other enamines are obtainable from the ketone (32) by standard methods (626) (Scheme 15). 

Perfluoroalkyl groups adjacent to multiple bond systems lower the frontier molecular orbitals (FMOs) Therefore, cycloaddition reactions preferentially occur with electron-rich multiple-bond systems The preference of bis(trifluoromethyl)-substituted hetero-l,3-dienes for polar reacuons makes them excellent model compounds for developing new types of diene reactions deviating from the well documented Diels-Alder scheme (pathway 1) A systematic study of the reactions of diene (1 =2-3=4)-dienophile (5=6) combinations reveals new synthetic possibilities that have not yet been fully exploited as tools for preparative organic cherrustry (equation 25)... 

Carbanions (counterions M = K +, Na +, Li+ or R4N+) bearing acceptor-substituents A, c.g., — M- or — I-substitucnts, vinyl, or phenyl groups, add to carbonyl compounds in uncatalyzed reactions preferentially at the x-position. 

There are various ways in which CMEs can benefit analytical applications. These include acceleration of electron-transfer reactions, preferential accumulation, or selective membrane permeation. Such steps can impart higher selectivity, sensitivity, or stability to electrochemical devices. These analytical applications and improvements have been extensively reviewed (35-37). Many other important applications, including electrochromic display devices, controlled release of drugs, electrosynthesis, and corrosion protection, should also benefit from the rational design of electrode surfaces. 

The alkylation of phenol investigated over H-MCM-22, H-ITQ-2 and H-MCM-36 showed that the delamelation and pillaring did not improve the catalytic activity and this was explained on the secondary processes taking place during the preparation of the corresponding materials, and which strongly affect the total acidity and the acidity on the external surface. Also, the composition of the reaction products is not influenced to a considerable extent by product shape selectivity effects. This seems to show that the tert-butylation reaction preferentially proceed at (or close to) the external surface of the zeolite layers. 

A comprehensive treatment of nitro-compound photochemistry is not attempted here and would be beyond the scope of this volume. The discussion is thus restricted to aromatic and heteroaromatic nitro compounds. The reactions preferentially reviewed are those for which at least some experimental information has been given on efforts to elucidate the multiplicity of the reacting excited state, or those in which intermediate excited triplet states seem highly probable for various reasons. 

A corrosion-inhibiting admixture is a chemical compound which, when added in small concentrations to concrete or mortar, effectively checks or retards corrosion. These admixtures can be grouped into three broad classes, anodic, cathodic and mixed, depending on whether they interfere with the corrosion reaction preferentially at the anodic or cathodic sites or whether both are involved [48]. Six types of mechanisms, viz. anodic (oxidizing passivators), anodic (non-oxidizing passivators), cathodic, precipitation... 

Pyridines undergo radical substitution reactions preferentially at the 2-position. Yields and regioselectivity are generally higher if the reaction is carried out in an acid medium. The presence of a strongly electron-donating substituent (OH, OR, NR2) on the pyridine ring can alter the reactivity pattern of electrophilic and radical substitution. 

Nevertheless, two factors strongly influence the heat of sulfur chemisorption on metal surfaces relative coverage and crystallographic structure. Thus sulfur chemisorbs at high coordination sites and, as a result, a selective poisoning of structure-sensitive reactions, preferentially catalyzed by these sites, may occur. Such a simple geometrical model can be used to explain change in selectivities induced by sulfur adsorption. 

The acetone cyanohydrin is employed as a solvent this means it is present in great excess. Under thermodynamic control this reaction preferentially leads to the formation of a cyanohydrin with an axial nitrile group and an equatorial OH group, since a nitrile group is smaller than an OH group. 

Most anaerobically functioning mitochondria use endogenously produced fumarate as a terminal electron-acceptor (see before) and thus contain a FRD as the final respiratory chain complex (Behm 1991). The reduction of fumarate is the reversal of succinate oxidation, a Krebs cycle reaction catalysed by succinate dehydrogenase (SDH), also known as complex II of the electron-transport chain (Fig. 5.3). The interconversion of succinate and fumarate is readily reversible by FRD and SDH complexes in vitro. However, under standard conditions in the cell, oxidation and reduction reactions preferentially occur when electrons are transferred to an acceptor with a higher standard redox potential therefore, electrons derived from the oxidation of succinate to fumarate (E° = + 30 mV) are transferred by SDH to ubiquinone,... 

Molybdenum Sulphide Catalysts. Pure molybdenum sulphide is active for metathesis reactions. Preferential degenerate but-2-ene conversions of cis-io-cis and trans-Xo-trans indicate some stereoselectivity. Okuhara and Tanaka found that, over molybdenum sulphide, olefins having vinyl groups exchange vinyl hydrogens selectively and rapidly, but hydrogen scrambling in but-2-ene is extremely slow. 

A possible problem with the luminol system may be the nonuniform variation in band patterns as recently noted in Western blotting (Harper and Murphy, 1991). The time required to obtain maximal luminescence varies since shortly after the reaction strong bands are preferentially detected while blotting the membrane later after the reaction preferentially detects faint bands. This phenomenon has not yet been described for nucleic acid probes. 

For certain monomers such as vinyl acetate and ethylene, branching is much more significant. The free-radical (high-pressure) polymerization of low-density polyethylene (LDPE) includes a back-biting internal chain-transfer reaction that results in the formation of a short branch. It is this branching that results in an upper limit for the crystallinity of LDPE of about 60%-70% and a melt temperature of 110 C the backbiting reaction preferentially occurs with the formation of an intramolecular six-membered ring that results in preferential formation of a C4 short-chain branch as shown in Scheme 1.41. 

The majority of examples reported so far involve oxidative pathways. Here only one synthetic application is presented by J. Mattay. The oxidation of an enol silyl ether bearing an unsaturated side chain cyclisizes via the radical cation to a bicyclic ketone. It has been shown for 6-hexenyl systems that this radical cationic reaction preferentially proceeds via a 6-endo rather than a 5-exo mode as observed for free radical processes. 

The corrosion rate of metals may be controlled by either the anodic reaction or the cathodic reaction. In most cases of metallic corrosion, the cathodic hydrogen reaction controls the corrosion rate in acidic solution, while in neutral solution the cathodic oxygen reaction preferentially controls the corrosion rate of metals. Generally, the presence of corrosion precipitates, such as metal oxides and hydroxides, exerts a considerable influence on the corrosion of metals in neutral solutions. The effects of surface oxides on metallic corrosion will be discussed in the following section. 

Phenylcyclopenta[e][l,2,3,4]tetrazine (1) undergoes electrophilic substitution reactions preferentially at C7, as has been demonstrated by bromination with A-bromosuccinimide, formy-lation with phosphoryl chloride/dimethyl formamide and trifluoroacetylation with trifluo-roacetic anhydride/triethylamine.37 Addition of tetrafluoroboric acid to 1 results in the formation of the protonated salt 2.37... 

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