Ruthenium-Catalyzed Reactions for Organic Synthesis.

Publisher Summary The liquid phase oxidation of hydrocarbons by molecular oxygen forms the basis for a wide variety of petrochemical processes including the manufacture of phenol and acetone from cumene, adipic acid from cyclohexane, terephthalic acid from p -xylene, acetaldehyde and vinyl acetate from ethylene, and propylene oxide from propylene. The majority of these processes employ catalysis by transition metal complexes to attain maximum selectivity and efficiency. This chapter describes the subject of metal-catalyzed oxidations of organic compounds in the liquid phase, largely within a mechanistic framework. A better understanding of the catalytic action of metal complexes is essential from the point of view of increasing selectivity and efficiency. Metal-catalyzed oxidations may be conveniently divided into two types—homolytic and heterolytic. The homolytic catalysis involves soluble transition metal salts (homogeneous), such as the acetates or naphthenates of Co, Mn, and Cu, or the metal oxides (heterogeneous). Heterolytic catalysis involves reactions of organic substrates coordinated to transition metals.

Metal-Catalyzed Oxidations of Organic Compounds in the Liquid Phase: A Mechanistic Approach

Considerable evidence indicates that dissolved transition metal ions (TMI) are capable of catalyzing oxidations in atmospheric water droplets, at least in certain circumstances. Wide variations in the importance of TMI chemistry are expected in these systems because concentrations of transition metals in water droplets range over at least 4 orders of magnitude. In the present work we perform an extensive series of model calculations for TMI chemistry in raindrops. The specific TMI discussed are iron, manganese, and copper. The present treatment is restricted to homogeneous processes, that is, those involving dissolved molecules and ions. Results are presented for studies at pH 3 and pH 4, for both daytime and nighttime conditions. Among the results are the following: (1) At pH 3 and pH 4, Fe(III) is present largely as photosensitive hydroxide complexes. Our model results indicate that under atmospheric conditions the photolysis of these complexes is the primary daytime source of reactive free radicals within the droplets, even at quite low TMI concentrations. (2) TMI complex photolysis is not, of course, operative at night. At those times the presence of TMI continues to control the concentration of free radicals in raindrops through “Fenton type” reactions with hydrogen peroxide, for example, Fe(II) + H2O2 → Fe(III) + OH + OH−. (3) The oxidation of S(IV) to S(VI) by H2O2 is the most important daytime sulfite oxidation process in raindrops, but S(IV) oxidation catalyzed by Mn(II) can be significant under certain conditions, particularly at night or in the winter months. (4) Solution nitrogen chemistry is relatively unaffected by TMI. Its most important daytime chemical process is the (rather inefficient) photodissociation of the nitrate ion, a process which generates ozone (and hence HOx radicals). (5) Organic chemistry in atmospheric water droplets is very sensitive to the presence of hydroxyl radicals. Since OH· production is strongly influenced by catalysis involving iron complexes, the presence of soluble iron is a major stimulus for organic chemical processes (such as the conversion of alkyl aldehydes to carboxylic acids). (6) For cases where S(IV) concentrations exceed those of H2O2 the S(IV) effectively “titrates out” the H2O2. If the H2O2 concentration dominates, however, residual H2O2 remains to initiate a variety of solution oxidation chains, particularly those producing organic acids. The presence of H2O2 in the gas phase thus implies acid production in the aqueous phase, the form of the acid depending upon the particular oxidizable species available. (7) The chemical reactions and rate constants used in the calculations are relatively well determined, but our results are quite sensitive to the assumed concentrations of TMI and other species. Increased attention to measurement of species concentrations in fog, clouds, and rain is therefore indicated.

Kinetic model studies of atmospheric droplet chemistry: 2. Homogeneous transition metal chemistry in raindrops

Some aspects of the chemistry of manganese(III) in aqueous solution

Lipid oxidation has long been accepted as a classic free radical chain reaction mechanism with initiation, propagation, and termination stages. This view is not inaccurate, but it is incomplete. Initiation is generally treated as if it was spontaneous or random, but in reality, radical initiation is quite specific and varies with the source. Similarly, propagation is considered to involve only hydrogen abstraction where as internal cyclization, addition, scission, and disproportionation reactions are competitive or even dominant under some conditions, leading to variations in oxidation kinetics and, in particular, product mixes. Alternative reaction paths have been recognized for some time, but remain largely ignored in measurements of lipid oxidation. This chapter reviews initiation processes with a focus on differentiating rates, mechanisms, and specificity of radical formation; and it presents evidence for multiple propagation mechanisms that lead to different product mixes. Finally, a new integrated view of lipid oxidation is proposed to account for multiple propagation pathways. Practical application of this information should enable development of analyses that more accurately reflect the true extent of lipid oxidation and also antioxidant approaches that more effectively extend shelf life and preserve desirable food qualities.


Keywords:

lipid oxidation;
initiation;
propagation;
termination;
hydrogen abstraction;
cyclization reactions;
addition reactions;
scission reactions;
oxidation products

Lipid Oxidation: Theoretical Aspects

The autoxidation of benzaldehyde in glacial acetic acid catalyzed by cobalt salts has been studied by kinetic and analytical methods. In the initial phase of the reaction the oxygen reacts quantitatively with the aldehyde to form perbenzoic acid, but as the reaction proceeds the peracid concentration falls below that of the oxygen absorbed due to ( a ) the catalyzed decomposition of the perbenzoic acid, and ( b ) the reaction of the perbenzoic acid with benzaldehyde to give benzoic acid. The relative contributions of these reactions to the decay of the peracid concentration has been determined. The initiating reaction has been shown to be the interaction of the cobaltic ion with the aldehyde according to Co 3+ + C 6 H 5 CHO → Co 2+ + C 6 H 5 CO·. (1) Direct measurements of the rate of this reaction agreed with the value of the rate of initiation determined by an analysis of the kinetics of the rate of the inhibited oxidation. The overall rate of oxidation may be fully explained by the following kinetic scheme: C 6 H 5 CO· + O 2 → C 6 H 5 COOO· (2) C 6 H 5 COOO· + C 6 H 5 CHO → C 6 H 5 COOOH + C 6 H 5 CO· } (propagation), (3) 2C 6 H 5 COOO → inert products (termination). (4) The chain length and the activation energies of the elementary reactions have been determined. The oxidation was inhibited by hydroquinone, diphenylamine and β -napthol and retarded by benzoquinone. The rate of the retarded oxidations satisfied the relation rate of oxidation = k9 / k" + [benzoquinone]· By considering the reaction which is responsible for the retardation, C 6 H 5 COOO· + C 6 H 4 O 2 → inert products, (5) in conjunction with the normal termination reaction (4) it is shown that at retarder con­centrations below 10 -3 M radical-radical termination is the more important reaction, but at 10 -1 M retarder concentration radical-radical interaction contributes only 1% to the termination process.

The Cobalt-Salt-Catalyzed Autoxidation of Benzaldehyde

The oxidation of acetaldehyde in glacial acetic acid catalyzed by manganese, copper and cobalt acetate has been studied at room temperature. Analyses of the products have been carried out, and it is shown that the reaction proceeds in two well-defined stages: (a) the oxidation of the aldehyde to peracetic acid; (b) the reaction of peracetic acid with acetaldehyde to give a second peroxide, X. The metal-salt-catalyzed decomposition of both peroxides has been determined. Kinetic measurements show that the oxidation with each of the metal acetates follows a similar course and conforms to the general kinetic relationship dO$\_{2}$/dt = k [acetaldehyde]$^{\frac{3}{2}}$ [catalyst]$^{\frac{1}{2}}$. These measurements, together with other observations, indicate that the initiating process is an electron-transfer reaction between the aldehyde and the higher valence state of the ion of the metal catalyst: M$^{3+}$(M$^{2+}$) + RCHO \rightharpoondown M$^{2+}$ (M$^{+}$) + RCO$\cdot $+ H$^{+}$. This reaction leads to the establishment of the following chain process $ \matrix R\text{CO}\cdot +\text{O}\_{2}\rightharpoondown R\text{COOO}\cdot, \\ R\text{COOO}\cdot +R\text{CHO}\rightharpoondown R\text{COOOH}+R\text{CO}\cdot. \endmatrix $ The metal ion is maintained in the higher valence state by the reaction cycle $ \matrix R\text{COOOH}+M^{2+}(M^{+})\rightharpoondown R\text{COO}\cdot +M^{3+}(M^{2+})+\text{OH}^{-}, \\ R\text{COOOH}+M^{3+}(M^{2+})\rightharpoondown R\text{COOO}\cdot +M^{2+}(M^{+})+\text{H}^{+}. \endmatrix $

The Metal-Salt-Catalyzed Oxidation of Acetaldehyde

Polymers range in properties from almost completely amorphous to highly crystalline solids. Crystallinity is of great importance because it determines the physical and technological properties of the substance. Physical properties such as melting point or softening point and the mechanical properties of the polymer depend on chain orientation and crystallinity. The ability of polymers to crystallize depends, among other things, upon the regularity of the structure and the chemical nature of the chains. In the crystalline state the chains are arranged in an orderly and regular arrangement and any factor which decreases the regularity reduces the ability to crystallize. The recent discovery of stereospecific polymerization of the olefin hydrocarbons and its extension to other types of non-hydrocarbon monomers has led to many new classes of regular, generally crystalline polymers, some of which are already of great practical interest. The first stereoregular polymers obtained since the discovery of stereoregular catalysts in 1956 were the vinyl polymers with a hydrocarbon main chain such as the ones prepared from the α-olefins, conjugated dolefins and vinyl ethers. The major consequence of the regularity of steric structures of polymers from monomers CH2═CH X in which the X -groups all occupy the same spatial arrangement along the backbone chain is that the polymer molecules pack together in an ordered three-dimensional structure in which they display the properties of crystalline materials. The most striking property consequent upon crystallinity is the melting point—a property of fundamental interest, for instance, in the spinning of a fibre and in the manufacture of plastics of good high-temperature properties. Table 14 summarizes the melting points of a few of the isotactic polyolefins (Bawn & Ledwith 1962; Natta 1960) and these properties may be satisfactorily interpreted in terms of the geometrical shapes of polymeric molecules in the crystallites. While much of the earlier study on stereospecific polymers and that which clearly emphasized the relation between structure and physical properties was concerned with the α-olefins and other hydrocarbon polymers, more recent work has shown similar dependence among the more polar polymers (Bawn & Ledwith 1962), most of which in the crystalline state adopt the helical chain arrangement first observed for the poly-α-olefins. Table 15 summarizes some typical results.

Cecil Edwin Henry Bawn

Papers

The Cobalt-Salt-Catalyzed Autoxidation of Benzaldehyde

The Metal-Salt-Catalyzed Oxidation of Acetaldehyde

Recent Developments in High Polymers