Heterolytic catalysis

Heterolytic catalysis is promoted by W6+, Ti4+, Cr3+, V5+, and many Mo6+ complexes. These complexes do not normally react with peroxides. However, in the presence of electron-rich molecules, such as alkenes, amines, sulfides, etc., oxygen insertion in the reactant occurs. For example,... 

Reactions with H2O2 may be divided into two classes arising from the homolytic vy. heterolytic cleavage of the 0-0 bond (173). In homolytic catalysis, the oxygen-centered radicals are intermediates the participation of concerted processes in heterolytic catalysis precludes paramagnetic intermediates. Product selectivity is usually higher in the latter class. Transition metal cations in low oxidation states, such as Cu, Ti, Cr, and Fe " ", catalyze the homolytic... 

On the basis of these redox potentials it seems likely that direct electron release to the benzenediazonium ion takes place only with iodide. This corresponds well with experience in organic synthesis iodo-de-diazoniations are possible without catalysts, light, or other special procedures (Sec. 10.6). For bromo- and chloro-de-di-azoniations, catalysis by cuprous salts (Sandmeyer reaction, Sec. 10.5) is necessary. For fluorination the Balz-Schiemann reaction of arenediazonium tetrafluoroborates in the solid state (thermolysis) or in special solvents must be chosen (see Sec. 10.4). With astatide (211At-), the heaviest of the halide ions, Meyer et al. (1979) found higher yields for astato-de-diazoniation than for iodo-de-diazoniation, a result consistent with the position of At in the Periodic System. It has to be emphasized, however, that in investigations based on measuring yields of final products (Ar-Hal), the possibility that part of the yield may be due to heterolytic dediazoniation is very difficult to quantify. 

Potassium peroxodisulphate (K2S2Og) also oxidizes sulphoxides to sulphones in high yield, either by catalysis with silver(I) or copper(II) salts at room temperature85 or in pH 8 buffer at 60-80 °c86-88. The latter conditions have been the subject of a kinetic study, and of the five mechanisms suggested, one has been shown to fit the experimental data best. Thus, the reaction involves the heterolytic cleavage of the peroxodisulphate to sulphur... 

The manifestation of noncovalent catalysis as a microsolvent effect is illustrated by cycloamylose-catalyzed decarboxylations of activated carboxylic acid anions. Anionic decarboxylations, as illustrated in scheme VII, are generally assumed to proceed by a rate-determining heterolytic... 

The question about the competition between the homolytic and heterolytic catalytic decompositions of ROOH is strongly associated with the products of this decomposition. This can be exemplified by cyclohexyl hydroperoxide, whose decomposition affords cyclo-hexanol and cyclohexanone [5,6]. When decomposition is catalyzed by cobalt salts, cyclohex-anol prevails among the products ([alcohol] [ketone] > 1) because only homolysis of ROOH occurs under the action of the cobalt ions to form RO and R02 the first of them are mainly transformed into alcohol (in the reactions with RH and Co2+), and the second radicals are transformed into alcohol and ketone (ratio 1 1) due to the disproportionation (see Chapter 2). Heterolytic decomposition predominates in catalysis by chromium stearate (see above), and ketone prevails among the decomposition products (ratio [ketone] [alcohol] = 6 in the catalytic oxidation of cyclohexane at 393 K [81]). These ions, which can exist in more than two different oxidation states (chromium, vanadium, molybdenum), are prone to the heterolytic decomposition of ROOH, and this seems to be mutually related. 

All schemes presented are similar and conventional to a great extent. It is characteristic that the epoxidation catalysis also results in the heterolytic decomposition of hydroperoxides (see Section 10.1.4) during which heterolysis of the O—O bond also occurs. Thus, there are no serious doubts that it occurs in the internal coordination sphere of the metal catalyst. However, its specific mechanism and the structure of the unstable catalyst complexes that formed are unclear. The activation energy of epoxidation is lower than that of the catalytic decomposition of hydroperoxides therefore, the yield of oxide per consumed hydroperoxide decreases with the increase in temperature. 

In the field of enzyme catalysis, heme-proteins such as cytochrome P450, for example, exhibit both types of 0-0 bond cleavages in organic hydroperoxides and peroxy acids (178). Heterolytic cleavage of HOOH/ROOH yields H20 or the corresponding alcohol, ROH and a ferryl-oxo intermediate (Scheme 4). Homolytic 0-0 bond cleavage results in the formation of a hydroxyl (HO ) or an alkoxyl (RO ) radical and an iron-bound hydroxyl radical. 

The next question which presents itself is whether we can explain why in some systems solvent co-catalysis occurs, whereas in others, apparently similar, it does not. Let it be said first that in fact there is very little experimental evidence on this point. From the thermochemical point of view one can say that alkyl halide co-catalysis is the more probable, the lower the heterolytic bond dissociation energy of the alkyl halide, the more stable the cation derived from the monomer, and the smaller the anion derived from the metal halide. It must, however, be remembered that the non-occurrence of alkyl halide co-catalysis may be due to a kinetic prohibition, i.e., an excessively high activation energy for a reaction which is thermodynamically possible. 

The term acid catalysis is often taken to mean proton catalysis ( specific acid catalysis ) in contrast to general acid catalysis. In this sense, acid-catalyzed hydrolysis begins with protonation of the carbonyl O-atom, which renders the carbonyl C-atom more susceptible to nucleophilic attack. The reaction continues as depicted in Fig. 7. l.a (Pathway a) with hydration of the car-bonium ion to form a tetrahedral intermediate. This is followed by acyl cleavage (heterolytic cleavage of the acyl-0 bond). Pathway b presents an mechanism that can be observed in the presence of concentrated inorganic acids, but which appears irrelevant to hydrolysis under physiological conditions. The same is true for another mechanism of alkyl cleavage not shown in Fig. 7.Fa. All mechanisms of proton-catalyzed ester hydrolysis are reversible. 

Another type of reaction was seen for dalvastatin (8.151), a prodrug that bears an unsaturated side chain. The hydrolysis of dalvastatin to the active acid competes with epimerization at C(6), the rate of the reaction being independent of pH above pH 2 [192], The mechanism is believed to be one of heterolytic cleavage of the C(6)-0 bond to generate a C-centered carbonium ion stabilized by the extended conjugated system characteristic of this compound. In the pH range 2 - 7, the rate of epimerization was found to be ca. 100 times faster than hydrolysis. Above pH 7, base catalysis accelerates hydrolysis, the rate of which increases ca. 100-fold between pH 7 and 9. These facts serve only to complicate the design of HMG-CoA reductase inhibitors and the interpretation of their pharmacokinetic behavior. 

It may be reasonable to argue that this further activation is achieved in several ways. The acid-catalysis required for Gal and de Bruin complex [Rh(/c -bpa)(cod)](PF6) to react with dioxygen can be used to protonate the peroxo compoimd (Scheme 10) to a hydroperoxo species. This is a way to achieve further activation of dioxygen, since it decreases the nucleophilic character of the peroxo hgand and makes interaction with the coordinated olefin easier. Recent works by Moro-oka [88,89] and Braun [90] (Scheme 15) have shown that peroxorhodium complexes can be protonated to hydroperoxo compounds. However, the addition of a second mole of acid leads to hydrogen peroxide ehmination rather than to the highly electrophilic oxo species (M = O) that could result from the heterolytic cleavage of the O - O bond with removal of water. 

Similar results were obtained for tert-butyl hydroperoxide and perchloric acid in 2-propanol. Thus, it is evident from the decomposition of hydrogen peroxide into free radicals that both heterolytic and homolytic reactions may be catalyzed by hydrogen ions. Further research is needed to investigate proton catalysis in certain homolytic reactions. 

Competition between Homolytic and Heterolytic Catalytic Decompositions of Hydroperoxides Reactions of Transition Metals with Free Radicals Reactions of Transition Metal Ions with Dioxygen Catalytic Oxidation of Ketones Cobalt Bromide Catalysis Oscillating Oxidation Reactions... 

Heterolytic scission of the -S-S- bond in which only electrophilic assistance is involved is the exception rather than the rule in reactions involving bond fission of this type. Kice et a/.165,166 have demonstrated that a variety of S-S bond cleavages involve concomitant electrophilic and nucleophilic catalysis including (a) the formation of aryl thiolsulfones from aryl thiolsulfinates and aryl sulfinic acids and (b) the hydrolysis (acetic acid—1% water and 60%... 

The homolytic and heterolytic nature of the oxidation is principally governed by the nature of the metal, but also by the nature of the ligands, the availability of coordination sites, the nature of the substrate and the temperature conditions. Consequently, the chemist has to overcome several difficulties, some inherent to homogeneous catalysis, some inherent to oxidation chemistry ... 

In relation to enzymic cytochrome P-450 oxidations, catalysis by iron porphyrins has inspired many recent studies.659 663 The use of C6F5IO as oxidant and Fe(TDCPP)Cl as catalyst has resulted in a major improvement in both the yields and the turnover numbers of the epoxidation of alkenes. 59 The Michaelis-Menten kinetic rate, the higher reactivity of alkyl-substituted alkenes compared to that of aryl-substituted alkenes, and the strong inhibition by norbornene in competitive epoxidations suggested that the mechanism shown in Scheme 13 is heterolytic and presumably involves the reversible formation of a four-mernbered Fev-oxametallacyclobutane intermediate.660 Picket-fence porphyrin (TPiVPP)FeCl-imidazole, 02 and [H2+colloidal Pt supported on polyvinylpyrrolidone)] act as an artificial P-450 system in the epoxidation of alkenes.663... 

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