Tertiary hydroperoxides, structural

Teeth whiteners, percarbamide, 623 Temperature, reaction rates, 903-12 Terminal olefins, selenide-catalyzed epoxidation, 384-5 a-Terpinene, peroxide synthesis, 706 a-Terpineol, preparation, 790 Terrorists, dialkyl peroxide explosives, 708 Tertiary amines, dioxirane oxidation, 1152 Tertiary hydroperoxides, structural characterization, 690-1... 

The yield of the formed hydroperoxide depends on the structure of the oxidized hydrocarbon. The tertiary hydroperoxides appeared to be the most stable. Hence they can be received by hydrocarbon oxidation in high yield (see Table 1.3). 

The important role of reaction enthalpy in the free radical abstraction reactions is well known and was discussed in Chapters 6 and 7. The BDE of the O—H bonds of alkyl hydroperoxides depends slightly on the structure of the alkyl radical D0 H = 365.5 kJ mol 1 for all primary and secondary hydroperoxides and P0—h = 358.6 kJ mol 1 for tertiary hydroperoxides (see Chapter 2). Therefore, the enthalpy of the reaction RjOO + RjH depends on the BDE of the attacked C—H bond of the hydrocarbon. But a different situation is encountered during oxidation and co-oxidation of aldehydes. As proved earlier, the BDE of peracids formed from acylperoxyl radicals is much higher than the BDE of the O—H bond of alkyl hydroperoxides and depends on the structure of the acyl substituent. Therefore, the BDEs of both the attacked C—H and O—H of the formed peracid are important factors that influence the chain propagation reaction. This is demonstrated in Table 8.9 where the calculated values of the enthalpy of the reaction RjCV + RjH for different RjHs including aldehydes and different peroxyl radicals are presented. One can see that the value A//( R02 + RH) is much lower in the reactions of the same compound with acylperoxyl radicals. 

When the alkyl hydroperoxide has been fully formed, only one half of the oxidizing power of the oxygen has been utilized. Alkyl hydroperoxides are therefore unstable, the stability being dependent upon the structure. Tertiary hydroperoxides are the most, and primary hydroperoxides the least, stable. The degradation reaction, which is essentially a second stage in the oxidation, may be either inter- or intramolecular the degradation may be either bi- or monomolecular. The rate of degradation is a function of the temperature and is easily subject to catalysis. 

Novel functionalized peroxides which may be used as UPR curing agents as well as initiators for polymerization reactions and as monomers for polymerizations to form peroxy-containing polymers were elaborated [162]. Initiators may be prepared by reacting hydroxy-containing tertiary hydroperoxides with diacid halides, dichloroformates, phosgene, diisocyanates, acid anhydrides and lactones to form the functionalized peroxides. These reaction products may be further reacted, if desired, with dialcohols, diamines, aminoalcohols, epoxides, epoxy alcohols, epoxy amines, diacid halides, dichloroformates and diisocyanates to form additional fimctionalized peroxides. The use of monoperoxyoxalates of the structure (Scheme 24) as initiators... 

In 1996, Li and Ramamurthy reported the use of zeoHtes to enhance the regioselectivity in the O2 ene-reaction. In contrast to the reaction in solution, in which the cis effect applies, a remarkable twin selectivity is observed in zeolites. This was mechanistically rationalized in terms of coordination of the double bond to the Na+ ion in the zeoHte (cation-ti interaction), which polarizes the double bond for Markovnikov selectivity and induces conformational constraint in the allylic cis substituents (Figure 8.8). The latter effect disfavors stericaUy the formation of the cis perepoxide-like structure or prevents a perpendicular orientation of the ds-hydrogens for coordination and abstraction. Complications may arise from the formation of tertiary hydroperoxides from /one-hydrogen abstraction, which may selectively decompose on prolonged irradiation this falsifies the regioselectivity. ... 

Organic peroxide-aromatic tertiary amine system is a well-known organic redox system 1]. The typical examples are benzoyl peroxide(BPO)-N,N-dimethylani-line(DMA) and BPO-DMT(N,N-dimethyl-p-toluidine) systems. The binary initiation system has been used in vinyl polymerization in dental acrylic resins and composite resins [2] and in bone cement [3]. Many papers have reported the initiation reaction of these systems for several decades, but the initiation mechanism is still not unified and in controversy [4,5]. Another kind of organic redox system consists of organic hydroperoxide and an aromatic tertiary amine system such as cumene hydroperoxide(CHP)-DMT is used in anaerobic adhesives [6]. Much less attention has been paid to this redox system and its initiation mechanism. A water-soluble peroxide such as persulfate and amine systems have been used in industrial aqueous solution and emulsion polymerization [7-10], yet the initiation mechanism has not been proposed in detail until recently [5]. In order to clarify the structural effect of peroxides and amines including functional monomers containing an amino group, a polymerizable amine, on the redox-initiated polymerization of vinyl monomers and its initiation mechanism, a series of studies have been carried out in our laboratory. 

C(C=0)C1 group to the precise structure (primary, secondary or tertiary) of the alkyl groups to which it is linked. However, our subsequent work with NO showed that its products are also sensitive to the alkyl structure yet in addition NO reacts with oxidized polymers to give distinctly different products from alcohol and hydroperoxide groups (see below). Consequently the COCl2 products were not explored further. 

Haring and Schreier have modified the active site of subtilisin cross-linked enzyme crystals by introducing selenium into it and thereby converting the enzyme into a peroxidase [36], The rigid CLC matrix allowed them to chemically modify subtilisin without loss of the tertiary structure. The kinetic resolution of racemic 2-hydroxy- 1-phenylethyl hydroperoxide was demonstrated using the semisynthetic CLC (Fig. 12). The reaction time was 25-30 min with an ee of 97%. The authors demonstrated the stability of these semisynthetic CLCs by cycling their enzyme 10 times. 

Abbreviations AD, asymmetric dihydroxylation BPY, 2,2 -bipyridine DMTACN, 1,4-dimethyl-l,4,7-triazacyclonane EBHP, ethylbenzene hydroperoxide ee, enantiomeric excess HAP, hydroxyapatite LDH, layered double hydroxide or hydrotalcite-type structure mCPBA, meta-chloroperbenzoic acid MTO, methyltrioxorhenium NMO, A-methylmorpholine-A-oxide OMS, octahedral molecular sieve Pc, phthalocyanine phen, 1,10-phenantroline PILC, pillared clay PBI, polybenzimidazole PI, polyimide Por, porphyrin PPNO, 4-phenylpyridine-A-oxide PS, polystyrene PVP, polyvinylpyridine SLPC, supported liquid-phase catalysis f-BuOOH, tertiary butylhydroperoxide TEMPO, 2,2,6,6-tetramethyl-l-piperdinyloxy TEOS, tetraethoxysilane TS-1, titanium silicalite 1 XPS, X-ray photoelectron spectroscopy. 

The abstraction of a hydrogen atom occurs preferentially at the tertiary carbon of the structure, leading to a polystyryl radical. This radical adds to oxygen to form a peroxy radical. By abstraction of another hydrogen atom, the peroxy radical leads to a hydroperoxide. Hydroperoxides have an IR absorption at 3450 cm-1. The decomposition of the hydroperoxide either by photolysis or by thermolysis gives an alkoxy radical that may react in several ways ... 

The evolution of HBr in the bromination reactions and the uptake of one bromine atom per ring indicate substitution at a secondary allylic carbon atom. The ease of oxidation and cross linking of the polymers and the presence of hydroxyl groups imply the intermediate formation of hydroperoxides on allylic carbon atoms. Treatment of the hydroxyl-containing polymer with benzoyl chloride indicates that the bulk of the hydroxyl groups are on secondary carbon atoms, since tertiary hydroxyl groups would tend to be replaced by chlorine. Although these results do not permit the elimination of structure B, it appears that the bulk of the structural units in the polycyclopentadiene corresponds to 1,2- addition (A). 

Previously, we have examined the formation of amino acid hydroperoxides following exposure to different radical species [100]. We observed that valine was most easily oxidised, but leucine and lysine are also prone to this modification in free solution. Scheme 12 illustrates the mechanism for formation of valine hydroperoxide. However, tertiary structure becomes an important predictor in proteins, where the hydrophobic residues are protected from bulk aqueous radicals, and lysine hydroperoxides are most readily oxidised. Hydroperoxide yield is poor from Fenton-derived oxidants as they are rapidly broken down in the presence of metal ions [101]. Like methionine sulphoxide, hydroperoxides are also subject to repair, in this case via glutathione peroxidase. They can also be effectively reduced to hydroxides, a reaction supported by the addition of hydroxyl radical in the presence of oxygen. Extensive characterisation of the three isomeric forms of valine and leucine hydroxides has been undertaken by Fu et al. [102,103], and therefore will not be discussed further here. 

As it was mentioned above, polypropylenes are more prone to oxidation, hence, requiring significantly higher amounts of antioxidants and UV stabilizers compared to PE. It was shown that oxygen intake is much faster in polypropylene compared to that in PE [10], The primary reason is in the microbranched chemical structure of PP (see above), containing tertiary hydrogens that makes formation of hydroperoxides in PP much easier compared to that in polyethylenes. Overall, the mechanisms of oxidation (both photo- and thermooxidation) in PP and PE are quite different. For example, the termination reaction rates for oxidation in PE are 100-1000 times faster compared to PP [11]. 

This polymer possesses a reactive tertiary carbon on the isopropyl group which may be selectively oxidized to form a hydroperoxide with the foUow-ing structure ... 

The structure and behavior of methyl radicals on surfaces was compared with that in aqueous solution. Methyl radicals were produced and characterized by EPR in a fast-mixing flow system of Ti(III) with tertiary butyl hydroperoxide.(133) As a side investigation, the reaction of methyl radicals with oxygen dissolved in water was monitored not only by EPR but by chemiluminescence using an electronic image intensifier.(134)... 

MTBE methyl tertiary-butyl ether PO propylene oxide SAPO silico-aluminophosphate SBU secondary building unit SDA structure-directing agent TBAOH tetrabutylammonium hydroxide TBHP a -butyl hydroperoxide TBOT tetrabutyl orthotitanate TEA tetraethylammonium... 

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