Reaction with alkyl hydroperoxides

Cerium(IV) resembles Pb(IV) in its reactions with alkyl hydroperoxides, i.e.,... 

Oxidation of Co(II) or Mn(II) complexes by peracids [reaction (106) or (107)] is a facile reaction.124 Reaction (108) proceeds much more slowly, if at all. Thus, in contrast to the reaction with alkyl hydroperoxides (see preceding section), during the decomposition of peracetic acid the cobalt catalyst is present largely as Co(III) in both chlorobenzene and acetic acid solutions. Peracids are intermediates in the autoxidation of aldehydes, but the direct oxidation of aldehydes is a more favorable pathway for the regeneration of the reduced form of the catalyst (see Section II.B.3.e). The predominance of Co(III) supports the reaction (109) as the slower step in both solvents. 

In the autoxidation of neat hydrocarbons, catalyst deactivation is often due to the formation of insoluble salts of the catalyst with certain carboxylic acids that are formed as secondary products. For example, in the cobalt stearate-catalyzed oxidation of cyclohexane, an insoluble precipitate of cobalt adipate is formed. 18fl c Separation of the rates of oxidation into macroscopic stages is not usually observed in acetic acid, which is a better solvent for metal complexes. Furthermore, carboxylate ligands may be destroyed by oxidative decarboxylation or by reaction with alkyl hydroperoxides. The result is often a precipitation of the catalyst as insoluble hydroxides or oxides. The latter are neutralized by acetic acid and the reactions remain homogeneous. 

Another C-alkylation of purines is the free-radical reaction with alkyl hydroperoxides catalyzed by iron(II) ions. This reaction has been applied to guanine, hypoxanthine, and adenine, as well as their nucleosides. A typical example is the 8-methylation of guanine to give 5 other examples are listed in Table 42. 

Table 3.5. Alkene Epoxidation by Transition Metal Catalyzed Reaction with Alkyl Hydroperoxides SS...

Examination of the pH dependence of kiy for the reactions with alkyl hydroperoxides (Figure 1) shows that at low pH, kiy is independent of acidity as described by eq 4. Near neutrality it displays a "bell-shaped" dependence resulting from a superimposition of the pH log krate profiles of the reactions of eqs 5 and 6. At high pH, it displays a second "bell-... 

Nitro derivatives of a variety of heteroaromatic compounds enter the VNS reactions with alkyl hydroperoxide anions to produce the expected hydroxylation products [41, 137-139]. For instance, the VNS hydroxylation of 2-chloro-5-nitropyridine with ferf-butylhydroperoxide was shown to give 2-chloro-5-nitro-6-hydroxypytidine that exists in its tautomeric form of pyridone [41] (Scheme 44). It should be stressed that the SNAr of chlorine located in the highly activated position 2 was not competing with the VNS. 

Reactions 33 and 35 constitute the two principal reactions of alkyl hydroperoxides with metal complexes and are the most common pathway for catalysis of LPOs (2). Both manganese and cobalt are especially effective in these reactions. There is extensive evidence that the oxidation of intermediate ketones is enhanced by a manganese catalyst, probably through an enol mechanism (34,96,183—185). 

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

Unsymmetrical dialkyl peroxides are obtained by the reaction of alkyl hydroperoxides with a substrate, ie, R H, from which a hydrogen can be abstracted readily in the presence of certain cobalt, copper, or manganese salts (eq. 30). However, this process is not efficient since two moles of the hydroperoxide are consumed per mole of dialkyl peroxide produced. In addition, side reactions involving free radicals produce undesired by-products (44,66). 

The AE reaction catalyzed by titanium tartrate 1 and with alkyl hydroperoxide as terminal oxidant has been applied to a large variety of primary allylic alcohols containing all eight basic substitution patterns. A few examples are presented in Table 6.2. 

The phenomenon that early transition metals in combination with alkyl hydroperoxides could participate in olefin epoxidation was discovered in the early 1970s [30, 31]. While m-CPBA was known to oxidize more reactive isolated olefins, it was discovered that allylic alcohols were oxidized to the corresponding epoxides at the same rate or even faster than a simple double bond when Vv or MoVI catalysts were employed in the reaction [Eq. (2)] [30]. 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

In addition to this reaction, quinones and other alkyl radical acceptors retard polymer oxidation by the reaction with alkyl radicals (see earlier). As a result, effectiveness of these inhibitors increases with the formation of hydroperoxide groups in PP. In addition, the inhibiting capacity of these antioxidants grows with hydroperoxide accumulation. The results illustrating the efficiency of the antioxidants with cyclic chain termination mechanisms in PP containing hydroperoxide groups is presented in Table 19.12. The polyatomic phenols producing quinones also possess the ability to terminate several chains. 

Literature procedures for the synthesis of hydroperoxides include the preparation from hydrogen peroxide (via reaction with alkyl halides, -phosphites, -suEonates, alkenes. 

An alternative method for dialkyl peroxide synthesis is the nucleophilic addition of an alkyl hydroperoxide to an alkene under acid catalysis reported by Davies and coworkers (Scheme 31, path B) ". A similar reaction is the nucleophilic addition of alkylhy-droperoxides to vinyl ethers under acid catalysis, producing perketals. Perketals can be deprotected under mild conditions (THF/water/acetic acid) and this hydroperoxide protection-deprotection sequence has been used by Dussault and Porter as a means for the resolution of racemic hydroperoxides (see also Section II.A.2) . In this respect more detailed studies were carried out with the perketals 75, which were prepared via reaction of alkyl hydroperoxides with vinyl ethers (Scheme 33). Weissermel and Lederer reported that in the presence of teri-butyl hypochlorite, a-chlorodialkyl peroxides can be formed in yields between 12% and 45% (Scheme 31, path C)". a-Alkoxydialkyl peroxides and diperoxyacetals were prepared by Rieche and coworkers via acid catalyzed reaction of one or two equivalents of alkyl hydroperoxides with acetals, ketals or aldehydes (Scheme 31, path D)" or by methylation of the corresponding a-alkoxy hydroperoxides with diazomethane (yields 11%, 27%)" . The diperoxyacetals 76 were isolated in yields ranging from 39 to 77%. 

SCHEME 33. Perketal formation by reaction of alkyl hydroperoxides with vinyl ethers... 

TABLE 8. Results of the synthesis of dialkyl peroxides through reaction of alkyl hydroperoxides with epoxides... 

Sulfides are generally oxidized much faster than alkenes, and in the presence of excess oxidant further oxidation to the sulfone occurs. In the cases where the reaction is conducted in an asymmetric way, the chiral catalytic system may react faster with one enantiomeric sulfoxide to form the sulfone than with the other, so that kinetic resolution of the primarily formed sulfoxide may occur. In general, the reaction is carried out with alkyl hydroperoxides like TBHP in the presence of a metal catalyst like Mo, W, Ti or V complexes. In some cases the sulfoxidation with hydroperoxides can take place without the need of a metal catalyst. Both examples will be discussed in the following. 

As already mentioned above, sulfides are oxidized to the corresponding sulfoxides with alkyl hydroperoxides in the presence of various metal catalysts like Mo, W, Ti and V. In the presence of excess hydroperoxide further oxidation to the sulfone occurs. Sulfides are generally oxidized much faster than alkenes, which is reflected in the selective oxidation of unsaturated sulfides exclusively at the sulfur atom. During the last years many asymmetric versions of this reaction have been developed and can be mainly divided... 

In the presence of diisopropyl(ethyl)amine, tetrachlorosilane reacts with f-butyl hydroperoxide to give 1 1 adduct 9 (equation 16). Alkylperoxydiorganoalkoxysilanes are prepared from the reaction of chlorodiorganooxysilane with alkyl hydroperoxides in the presence of ammonia or organic base such as pyridine or triethylamine (equations 17 and 18). 

Bis(alkylperoxy)diorganosilanes 14 were prepared by the reaction of alkylperoxy diorganochlorosilane with alkyl hydroperoxides in the presence of ammonia or pyridine (equation 20). 

Bis(alkylperoxy)hydridosilanes are obtained from the reaction of dichlorohydridosilane with alkyl hydroperoxide in the presence of base B (equation 25). ... 

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