Conditions for the Metallation

In this chapter, some procedures with simple aldimines and ketimines are described. They show representative conditions for the metallation of these nitrogen derivatives, their subsequent reactions with a number of electrophiles and the final hydrolytic cleavage of the C=N bond with formation of derivatives of the starting aldehydes or ketones. 

Eoders D, Eichenauer H, Baus U, Schubert H, Kremer KAM (1984) Tetrahedron 40 1345 

The most usual reagent for the metallation of aldimines and ketimines and the corresponding hydrazines is LDA. The lithiations are generally carried out in mixtures of THF and hexane. The formation of the lithium derivatives, which proceeds rather smoothly at temperatures in the region — 30 to + 10 °C, is visible by the appearance of a yellow colour. The use of BuLi cannot be recommended, since addition of this reagent across the C=N bond may seriously compete with the deprotonation. 

As may be concluded from the high yields of some alkylation reactions performed in liquid ammonia, the interaction between aldimines and sodamide or potassium amide gives rise to predominant or complete metallation. With lithium amide the ionization equilibrium is probably strongly on the side of the reactants. 

All reactions are carried out in an atmosphere of inert gas, except those in liquid ammonia. 

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Brissonneau L, Vahlas C (2000) Precursors and operating conditions for the metal-organic chemical vapor deposition of nickel films. Annales De Chimie-Science Des Materiaux 25(2), 81-90... 

L. Brissoimeau and C. Vahlas, Precursors and Operating Conditions for the Metal-Organic Chemical Vapor Deposition of Nickel Films, Annales de Chemie - Science des Materiaux, Vol.25, pp.81-90. 

The reaction conditions for the metallation of propadiene show that this compound is readily metallated. Introduction of alkyl substituents causes a marked decrease of the kinetic acidity of the allenic protons. This is reflected in the higher temperature in the lithiation of 1,1-dimethylallene. 

In Table 4 reaction conditions for the metallation of a number of olefinic compounds, as derived from our investigations, are summarized. The deprotonation of non-conjugated olefins proceeds rather sluggishly and satisfactory rates of conversion can be attained only when the substrate is used in a large excess. Under these conditions attack of the solvent (THF ) by the base is suppressed. 

The greater part of this chapter is devoted to the metallation of S,S-acetals and some related compounds and subsequent reactions of the organometallic intermediates with a number of electrophilic reagents. Representative conditions for the metallation of saturated sulfur compounds are given in Table 5. 

Table 5. Preparative reaction conditions for the metallation of saturated sulfur compounds...

The evaluation of a three-phase system where film and substrate phases are metals (e.g., deposition of a thin metal film on a metal substrate) requires yet another boundary condition for the metal/metal interface. Forstmann and Stenschke " postulated the continuity of the normal component of the energy current. This has been formulated as " ... 

The selection of a particular type of reduction depends on technical feasibiUty and the economics of the process as well as on physicochemical considerations. In particular, the reducing agent should be inexpensive relative to the value of the metal to be reduced. The product of the reaction, RX, should be easily separated from the metal, easily contained, and safely recycled or disposed of. Furthermore, the physical conditions for the reaction should be such that a suitable reactor can be designed and operated economically. 

Orthophosphate salts are generally prepared by the partial or total neutralization of orthophosphoric acid. Phase equiUbrium diagrams are particularly usehil in identifying conditions for the preparation of particular phosphate salts. The solution properties of orthophosphate salts of monovalent cations are distincdy different from those of the polyvalent cations, the latter exhibiting incongment solubiUty in most cases. The commercial phosphates include alkah metal, alkaline-earth, heavy metal, mixed metal, and ammonium salts of phosphoric acid. Sodium phosphates are the most important, followed by calcium, ammonium, and potassium salts. 

Chromic Acid Electrolysis. Alternatively, as shown in Figure 1, chromium metal may be produced electrolyticaUy or pyrometaUurgicaUy from chromic acid, CrO, obtained from sodium dichromate by any of several processes. Small amounts of an ionic catalyst, specifically sulfate, chloride, or fluoride, are essential to the electrolytic production of chromium. Fluoride and complex fluoride catalyzed baths have become especially important in recent years. The cell conditions for the chromic acid process are given in Table 7. 

Dianion formation from 2-methyl-2-propen-l-ol seems to be highly dependent on reaction conditions. Silylation of the dianion generated using a previously reported method was unsuccessful in our hands. The procedure described here for the metalation of the allylic alcohol is a modification of the one reported for formation of the dianion of 3-methyl-3-buten-l-ol The critical variant appears to be the polarity of the reaction medium. In solvents such as ether and hexane, substantial amounts (15-50%) of the vinyl-silane 3 are observed. Very poor yields of the desired product were obtained in dirnethoxyethane and hexamethylphosphoric triamide, presumably because of the decomposition of these solvents under these conditions. Empirically, the optimal solvent seems to be a mixture of ether and tetrahydrofuran in a ratio (v/v) varying from 1.4 to 2.2 in this case 3 becomes a very minor component. 

Currently, phthalic anhydride is mainly produced through catalyzed oxidation of o-xylene. A variety of metal oxides are used as catalysts. A typical one is V2O5 -1- Ti02/Sb203. Approximate conditions for the vapor-phase oxidation are 375-435°C and 0.7 atmosphere. The yield of phthalic anhydride is about 85% ... 

At) This statement should not necessarily discourage the use of the contact metal as a coating for the metal considered , provided that continuity is good under abrasive conditions, however, even a good coating may become discontinuous. 

Under certain conditions, it will be impossible for the metal and the melt to come to equilibrium and continuous corrosion will occur (case 2) this is often the case when metals are in contact with molten salts in practice. There are two main possibilities first, the redox potential of the melt may be prevented from falling, either because it is in contact with an external oxidising environment (such as an air atmosphere) or because the conditions cause the products of its reduction to be continually removed (e.g. distillation of metallic sodium and condensation on to a colder part of the system) second, the electrode potential of the metal may be prevented from rising (for instance, if the corrosion product of the metal is volatile). In addition, equilibrium may not be possible when there is a temperature gradient in the system or when alloys are involved, but these cases will be considered in detail later. Rates of corrosion under conditions where equilibrium cannot be reached are controlled by diffusion and interphase mass transfer of oxidising species and/or corrosion products geometry of the system will be a determining factor. 

Industrial atmospheres usually accelerate the corrosion of zinc. When heavy mists and dews occur in these areas, they are contaminated with considerable amounts of acid substances such as sulphur dioxide, and the film of moisture covering the metal can be quite acid and can have a pH as low as 3. Under these conditions the zinc is dissolved but, as the corrosion proceeds, the pH rises, and when it has reached a sufficiently high level basic salts are once more formed and provide further protection for the metal. These are usually the basic carbonate but may sometimes be a basic sulphate. As soon as the pH of the moisture film falls again, owing to the solution of acid gases, the protective film dissolves and renewed attack on the metal occurs. Hudson and Stanners conducted tests at various locations in order to determine the effect of atmospheric pollution on the rate of corrosion of steel and zinc. Their figures for zinc are given in Table 4.34 and clearly show the effect which industrial contamination has on the corrosion rate. 

Another application of the electrolysis of tantalum and niobium in fluoride melts is in the preparation of intermetalic compounds as a result of the interaction between the electrochemically precipitating metal and the cathode material. Based on an investigation of the electrochemical reduction of K2TaF7 or K2NbF7 in a LiF - NaF melt on nickel cathodes, Taxil and Qiao [565] determined the appropriate conditions for the formation of TaNi3 or NbNi3 in the form of stable phases in the bulk of the obtained layer. 

It must be emphasised that standard electrode potential values relate to an equilibrium condition between the metal electrode and the solution. Potentials determined under, or calculated for, such conditions are often referred to as reversible electrode potentials , and it must be remembered that the Nernst equation is only strictly applicable under such conditions. 

To obtain comparative values of the strengths of oxidising agents, it is necessary, as in the case of the electrode potentials of the metals, to measure under standard experimental conditions the potential difference between the platinum and the solution relative to a standard of reference. The primary standard is the standard or normal hydrogen electrode (Section 2.28) and its potential is taken as zero. The standard experimental conditions for the redox... 

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