Reactivities hydroperoxides

One further recent preliminary report describes the preparation of reactive hydroperoxide compounds [M(OEP)(Me)(OOH)] (M = P, As, Sb) and their oxygen atom transfer chemistry with triphenylphosphine, indicating that further interesting chemical applications of organoelement Group 15 porphyrins might be expected. 

More reactive hydroperoxides can be converted selectively to alcohols via the method of Bashkirov (Fig. 4.44), where a boric acid ester protects the product from further oxidation and thus increases the selectivity [121]. The method is used to convert C10-C20 paraffins to alcohols which are used as detergents and surfactants, for the oxidation of cyclohexane (see elsewhere) and cyclododecane to cyclododecanol (cyclododecanone) for the manufacture of nylon-12. 

Synthetic methods for preparation of 1,2,4,5-tetroxanes have been reviewed recently <2001COR601, 2002RMC113>. The most general method involves acid-catalyzed addition of hydrogen peroxide to carbonyl compounds and subsequent cyclization of the hydroperoxide intermediates. The direct synthesis is carried out normally in the presence of either sulfuric, perchloric, or methanesulfonic acids and affords symmetrically substituted tetroxanes (Equation 26). In many cases, for example, where the carbonyl compound is unsubstituted in the a-position, tetroxanes are contaminated with hexaoxonanes and open-chain hydroperoxides. Selective removal of the more reactive hydroperoxides can be achieved with dimethyl sulfide or potassium iodide. Recrystallization usually removes residual hexaoxonanes but, failing that, heating the mixture with perchloric acid in acetic acid can convert hexaoxonanes to tetroxanes or convert the thermodynamically less stable hexaoxonanes to more water-soluble lactones, which may facilitate the purification process <2002RMC113>. 

The rate of chain scission is increased in the presence of active hydrogen (e.g., water), probably due to reaction with carbonyl oxides to form reactive hydroperoxides. Crosslinking products may also be formed, especially with rubbers containing disubstituted double bonds (e.g., polybutadiene, BR, and styrene-butadiene rubber, SBR). 

As outlined in a simplified mechanisms in Fig. 4.1, degradation proceeds through a radical chain mechanism (2,3). Initiation typically occurs through exposure to heat generated during production. Trace metal impurities such as copper or iron accelerates radical formation. Reactive hydroperoxides are formed after reaction of the carbon-centered radical with oxygen. Thermally induced homolytic cleavage of hydroperoxides leads to additional reactive radical formation and subsequent polymer chain scission. 

Kinetics of the aralkyl hydroperoxides decomposition in the presence of tetraethylammonium bromide (Et NBr) has been investigated. Et NBr has been shown to reveal the catalytic properties in this reaction. The use of Et- NBr leads to the decrease up to 40 kJmol of the hydroperoxides decomposition activation energy. The complex formation between hydroperoxides and Et NBr has been shown by the kinetic and H NMR spectroscopy methods. Thermod5aiamic parameters of the complex formation and kinetic parameters of complex-bonded hydroperoxides have been estimated. The model of the reactive hydroperoxide - catalyst complex structure has been proposed. Complex formation is accompanied with hydroperoxide chemical activation. 

Molecular modeling of the Et NBr activated hydroperoxides decomposition. On the base of experimental facts mentioned above we eon-sider the salt anion and eation as well as acetonitrile (solvent) moleeule participation when model the possible stmcture of the reactive hydroperoxide - catalyst complex. Model of the substrate separated ion pair (Sub-SIP) is one of the possible realization of join action of the salt anion and eation in hydroperoxide moleeule activation. In this complex hydroperoxide molecule is located between cation and anion species. For the sym-metrie molecules such as benzoyl peroxide [16], lauroyl peroxide [17], and dihydroxydicyclohexyl peroxide [18] attack of salts ions is proposed to be along direction of the peroxide dipole moment and perpendicularly to the peroxide bond. Hydroperoxides are asymmetric systems, that is why different directions of ion s attack are possible. The solvent effect can be considered by means of direct inclusion of the solvent molecule to the complex stmcture. From the other hand methods of modem computer... 

Acryhc stmctural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acryhc adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated stmctural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, N,1S7-dimethyl- -toluidine, and saccharin, can be apphed to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

Oxidation begins with the breakdown of hydroperoxides and the formation of free radicals. These reactive peroxy radicals initiate a chain reaction that propagates the breakdown of hydroperoxides into aldehydes (qv), ketones (qv), alcohols, and hydrocarbons (qv). These breakdown products make an oxidized product organoleptically unacceptable. Antioxidants work by donating a hydrogen atom to the reactive peroxide radical, ending the chain reaction (17). 

Two secondary propagating reactions often accompany the initial peroxide decomposition radical-induced decompositions and -scission reactions. Both reactions affect the reactivity and efficiency of the initiation process. Peroxydicarbonates and hydroperoxides are particularly susceptible to radical-induced decompositions. In radical-induced decomposition, a radical in the system reacts with undecomposed peroxide, eg ... 

In the preparation of hydroperoxides from hydrogen peroxide, dialkyl peroxides usually form as by-products from the alkylation of the hydroperoxide in the reaction mixture. The reactivity of the substrate (olefin or RX) with hydrogen peroxide is the principal restriction in the process. If elevated temperatures or strongly acidic or strongly basic conditions are required, extensive decomposition of the hydrogen peroxide and the hydroperoxide can occur. 

The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides. 

Simplified nitrile mbber polymerization recipes are shown in Table 2 for "cold" and "hot" polymerization. Typically, cold polymerization is carried out at 5°C and hot at 30°C. The original technology for emulsion polymerization was similar to the 30°C recipe, and the redox initiator system that allowed polymerization at lower temperature was developed shortiy after World War II. The latter uses a reducing agent to activate the hydroperoxide initiator and soluble iron to reactivate the system by a reduction—oxidation mechanism as the iron cycles between its ferrous and ferric states. 

The parent indolo[2,3-fl]carbazole (1) has also been the subject of a study probing its reactivity toward oxidizing agents. One of the substrates involved, namely 85 (prepared from 1 and 2,5-dimethoxytetrahydrofuran in the presence of acid), was subjected to treatment with m-chloroperbenzoic acid, to give the dione 86 as the major product and a sensitive compound assigned the hydroxy structure 87. A cleaner reaction took place when 85 underwent oxidation with tert-butyl hydroperoxide assisted by VO(acac)2, to produce 86 exclusively in 86% yield. Likewise, A,N -dimethylindolo[2,3-fl]carbazole furnished the dione 88 on treatment with this combination of reagents (96J(X 413). 

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]. 

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