Choice of reactant

FIGURE 3.9 EPR of titanium citrate. The citrate complex of the Ti(III) ion at pH 9 is a general-purpose strong reductant of metalloproteins. This 3d system gives an S = 1/2 EPR spectrum with. -values just below ge. The axial signal below 3300 gauss is from Ni(I) in factor F-430. 

FIGURE 3.10 EPR of ferricyanide. Potassium ferricyanide is a general-purpose oxidant of metalloproteins. The low-spin 3c/5 Fe(III) of frozen dissolved K3Fe(CN)6 gives a broad 5=1/2 spectrum with main peak at g = 2.68 (v = 9407 MHz T = 15.5 K). The sharper feature around g = 2 is from an iron-sulfur cluster anaerobically oxidized in this experiment by ferricyanide to the [3Fe-4S]1+ form with S = 1/2. 

Some proteins may have redox centers with ° s significantly higher than +0.4 V (e.g., the multicopper protein laccase). No general approach is presently available to deal with the high end of the biological redox window, and, in fact, the reduction potentials of these systems (again, e.g., laccases) are frequently poorly defined. 

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Several methods for the synthesis of thiazole compounds are available, which can be classified into the partial structures illustrated in Scheme 1. The first of these structures (la) is by far the most useful and versatile of all the thiazole syntheses. By a judicious choice of reactants it allows... 

A wide variety of /3-lactams are available by these routes because of the range of substituents possible in either the ketene or its equivalent substituted acetic acid derivative. Considerable diversity in imine structure is also possible. In addition to simple Schiff bases, imino esters and thioethers, amidines, cyclic imines and conjugated imines such as cinnamy-lidineaniline have found wide application in the synthesis of functionalized /3-lactams. A-Acylhydrazones can be used, but phenylhydrazones and O-alkyloximes do not give /3-lactams. These /3-lactam forming reactions give both cis and /raMS-azetidin-2-ones some control over stereochemistry can, however, be exercised by choice of reactants and conditions. 

The superb elegance of this demonstration lies in the choice of reactants which permits no alternative mechani.sm. Cr" (d ) and Co" (d ) species are known to be substitutionally labile whereas Cr" (d ) and Co " (low-spin d ) are substitutionally inert, Only if electron transfer is preceded by the formation of a bridged internrediate can the inert cobalt reactant be persuaded to release a Cl ligand and so allow the quantitative formation of the (then inert) chromium product. Corroboration that electron transfer does not occur by an outer-sphere mechanism followed by los.s of CP from the chromium is provided by the fact that, if Cl is added to the solution, none of it finds its way into the chromium product. 

Reactions of hydrides are therefore similar to those of carbonyls, and by appropriate choice of reactants a product often can be made by either route, as is shown for the mercuration of trinuclear Ru complexes ... 

The mechanistic basis for the stereoselectivity of these conditions remains to be determined. The choice of reactant and conditions can be used to exert a substantial degree of control of the stereoselectivity. 

These observations were significant to our choice of reactants for probing CT at DNA-modified surfaces. In particular, an upright orientation of the DNA relative to the surface is required to probe DNA-mediated reactions otherwise a more direct reaction between an intercalating probe and the electrode might be possible. Consequently, reactants were selected such that a negative potential could be apphed, thereby initiating reduction of an intercalated redox probe distantly bound within DNA helix. Importantly, the... 

Crosslinking of polyurethanes proceeds in different ways depending on the stoichiometry and choice of reactants and reaction conditions. For example, an isocyanate-terminated trifunctional prepolymer is prepared by reaction of a polyol and... 

Another key factor is the choice of reactant proportion. We found that it is necessary to maintain a high metal-to-ligand ratio because a low ratio (such as Ln Cu gly Ac = 1 4 2 2) will lead to the final product being contaminated by a small number of light-blue crystals of CudglyutCICUh. 

However, in spite of the fact that good results were obtained by an appropriate choice of reactants and of the reaction conditions, a great deal of attention has been devoted during recent years to the search for successful comprehensive methods for 1,2-m-glycosylations. They are discussed in the next sections. 

When benzoylacetone reacts with N-methylethylenediamine, the other isomer is the major product (78%) [69JCS(C)1081], so that alternative regioselectivity is possible by suitable choice of reactant. 

In terms of mean absolute error, choice of reactant and transition-state geometry has very little effect on calculated relative activation energies. Nearly perfect agreement between calculated and experimental relative activation energies is found for 6-3IG calculations, irrespective of whether or not approximate geometries are employed. Somewhat larger discrepancies are found in the case of MP2/6-31G calculations, but overall the effects are small. 

A manufacturing precast for producing ortho-phthalate otters derived from alkyl acid ortho-phthalatos and olefins has boon developed and demonstrated on the pilot plant scale. Process variables Include choice of reactants, stoichiometry, reaction kinetics, recycle of recovered materials and the fate of the perchloric add catalyst. Seme physical properties of the ortho-phthalate esters have been determined and severed of the esters have been evaluated as plasticizers for polyvinyl chloride. The composite data show that the acid-olefin esterification process provides commercially acceptable plasticizers for polyvinyl chloride. 

All of these processes are of limited synthetic utility because of the requirement of the use of stoi chiometric amounts of palladium complexes. However, by judicious choice of reactants and condition the above-mentioned impediments to catalysis can be overcome. For example, an efficient palladium(II) catalyzed cyclization of o-allyl- and o-vinyl-anilines to indoles has been developed (equation 14).28 Be cause arylamines are -106 less basic than aliphatic amines, and because the cyclized product in thi system gave an enamine (indole) stabilized by aromatization, the problems of catalyst poisoning by sub strate or product were circumvented, and catalysis was successfully achieved. The system was quit tolerant of a variety of functional groups and was used to prepare indoloquinones in excellent yieli... 

By careful choice of reactants this technique, because of its simplicity, can lead to an understanding of some unsolved problems associated with graft copolymerizations. 

Reaction conditions (temperature, flow rate, choice of reactant) should be adjusted to give relatively low conversions (20% or less 2% or... 

In semi-batch operation, many elements determine the process safety. Among them we mention the temperature control strategy, the feed control strategy, and also the choice of reactant(s) to be initially charged and the reactant(s) to be fed. 

The reactivity-selectivity principle has long been part of the chemist s intuition. A qualitative statement of the principle is that highly reactive species are unselective in their choice of reactants compared to stable and therefore unreactive species. As a general proposition it is certainly incorrect for there are many clear exceptions. For example, highly reactive atomic oxygen reacts about one hundred times more rapidly with 2,3-dimethyl-2-butene than with... 

The great difference from the conventional meaning of these terms lies in the choice of reactants. Most conventional synthetic steps start from [nucleophile, electrophile] combinations, in the nomenclature of eqns (3) and (4), [R—E, E+] or [R—Nu, Nu-] pairs. Introducing an additional symbol, C=C, for double bonds (and this includes systems of conjugated double bonds) we also have the combinations [C=C(<5+), Nu-] and [C=C(5-), E+], depending upon the electrophilic or nucleophilic nature of C=C. 

An analysis of the thermodynamics of a CVD system, discussed further in Chapter 2, can provide valuable assistance in the choice of reactant concentrations, pressures and temperatures to use for a given chemical system. Such an analysis can also provide information on the composition of the deposited material as well as the maximum efficiency for use of reactants. However, a thermodynamical analysis only gives information on the theoretically-possible result, which may not actually be achievable. CVD systems are generally not operated at chemical equihbrium, although some systems, such as the deposition of silicon from chlorosilanes, come close. 

These stereoselectivity experiments appear to indicate that the reactants approach each other quite closely during the electron-transfer process. It should be possible by judicious choice of reactants to increase substantially the stereoselectivity observed in the above systems. This may in the future make a significant contribution to our understanding of the mechanisms of electron transfer reactions both in chemical systems and in biology, where electron-transfer reactions occur in a very asymmetric environment. 

This catalyzed reaction is 10 times as fast as the uncatalyzed one. We will consider the mechanism of carbonic anhydrase catalysis in Chapter 9. Enzymes are highly specific both in the reactions that they catalyze and in their choice of reactants, which are called substrates. An enzyme usually catalyzes a single chemical reaction or a set of closely related reactions. Side reactions leading to the wasteful formation of by-products are rare in enzyme-catalyzed reactions, in contrast with uncatalyzed ones. 

This relation shows that the rate divided by the stoichiometric coefficient is independent of the choice of reactant or product. The ratio between the reaction rate and the stoichiometric coefficient of anyone of the species can be expressed ... 

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