Conventional hydroperoxidation

In the second step, a dilute H P-methanol solution is introduced in a fixed-bed epoxidation reactor. Make-up propene, recycled propene and HP from the product purification stage are fed into the reactor. The reaction is catalyzed by titanium silicalite, and takes place at 40-50 °C and 300 psi. HP per-pass conversion is initially 96% but drops down to 63% after 400 hours. PO selectivity is 95 mol.% propene per-pass conversion is 39.8%. This technology gives capital savings compared to conventional hydroperoxidation technologies however, it is likely that the operating costs of such a plant are higher than that of the latter. 

Irg 1076, AO-3 (CB), are used in combination with metal dithiolates, e.g., NiDEC, AO-30 (PD), due to the sensitized photoxidation of dithiolates by the oxidation products of phenols, particularly stilbenequinones (SQ, see reaction 9C) (Table 3). Hindered piperidines exhibit a complex behavior when present in combination with other antioxidants and stabilizers they have to be oxidized initially to the corresponding nitroxyl radical before becoming effective. Consequently, both CB-D and PD antioxidants, which remove alkyl peroxyl radicals and hydroperoxides, respectively, antagonise the UV stabilizing action of this class of compounds (e.g.. Table 3, NiDEC 4- Tin 770). However, since the hindered piperidines themselves are neither melt- nor heat-stabilizers for polymers, they have to be used with conventional antioxidants and stabilizers. 

Scott Oakes et al. (1999a, b) have shown how adoption of SC conditions can lead to a dramatic pressure-dependent enhancement of diastereoselectivity. In the case of sulphoxidation of cysteine derivatives with rert-butyl hydroperoxide, with cationic ion-exchange resin Amberlyst-15 as a catalyst, 95% de was realized at 40 °C and with SC CO2. By contrast, with conventional solvents no distereoselectivity was observed. Another example is the Diels-Alder reaction of acrylates with cyclopentadiene in SC CO2 at 50 °C, with scandium tris (trifluoromethanesulphonate) as a Lewis acid catalyst. The endoiexo ratio of the product was as high as 24 1, while in a solvent like toluene it was only 10 1. 

Scott Oakes et al. (1999a) have reported a dramatic pressure-dependent enhancement of diastereoselectivity for sulphoxidation of cysteine and methionine derivatives by using SC CO2 rather than conventional solvents. In the case of a derivative of cysteine, toluene/ methylene chloride gave a 50-50 mixture of stereoisomeric forms. With SC CO2 and tert-butyl hydroperoxide, however, 95% selectivity for just one stereoisomer was realized. 

Hydrocarbons oxidize to give a complex mixture of products which include hydroperoxides, alcohols, ketones, acids, esters, etc. (1). Polyolefins similarly can be oxidized by heat, radiation or mechano-initiated processes. The precise identification and quantification of these oxidation products are essential for the complete understanding and control of these destructive reactions. Conventional methods for the identification of oxidation products include iodome-... 

Consequently conventional antioxidant mechanisms must be expected to protect against photo-oxidation. Thus hydroperoxide decomposition to inert molecular products will reduce the rate of photoinitiation and scavenging of any of the free radical species will be beneficial, although the effectiveness of conventional antioxidants in photo-oxidations is limited by their own stability and the photo-sensitizing propensity of their products (3,). 

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. 

Results of a chemical activation induced by ultrasound have been reported by Nakamura et al. in the initiation of radical chain reactions with tin radicals [59]. When an aerated solution of R3SnH and an olefin is sonicated at low temperatures (0 to 10 °C), hydroxystannation of the double bond occurs and not the conventional hydrostannation achieved under silent conditions (Scheme 3.10). This point evidences the differences between radical sonochemistry and the classical free radical chemistry. The result was interpreted on the basis of the generation of tin and peroxy radicals in the region of hot cavities, which then undergo synthetic reactions in the bulk liquid phase. These findings also enable the sonochemical synthesis of alkyl hydroperoxides by aerobic reductive oxygenation of alkyl halides [60], and the aerobic catalytic conversion of alkyl halides into alcohols by trialkyltin halides [61]. 

They do not react with R02 radicals. The inhibited oxidation rate is proportional to the oxygen pressure and inversely proportional to the nitroxide concentration, but it is independent of the substrate concentration—i.e., rate oc [02]/[nitroxide]. The nitroxides react slowly with hydrocarbons and hydroperoxides and therefore have little tendency to initiate oxidation. However, they are of disappointingly little practical value compared with more conventional chain-breaking antioxidants because they must compete with molecular oxygen for the R radicals, and the... 

Conventional distillation is not a good method for removal of the cumene hydroperoxide, as can be seen by the results given in Table VI. The results show that a single distillation, even with a 30-plate column, does not remove all the inhibition. This is probably due to the decomposition of cumene hydroperoxide at the boiling point of cumene to give other inhibitors with a lower boiling point, which then pass through the column. We find that the inhibitors will be removed, however, if a vacuum distillation is made at room temperature. 

In order to prevent competing homoallylic asymmetric epoxidation (AE, which, it will be recalled, preferentially delivers the opposite enantiomer to that of the allylic alcohol AE), the primary alcohol in 12 was selectively blocked as a thexyldimethylsilyl ether. Conventional Sharpless AE7 with the oxidant derived from (—)-diethyl tartrate, titanium tetraisopropoxide, and f-butyl hydroperoxide next furnished the anticipated a, [3-epoxy alcohol 13 with excellent stereocontrol (for a more detailed discussion of the Sharpless AE see section 8.4). Selective O-desilylation was then effected with HF-triethylamine complex. The resulting diol was protected as a base-stable O-isopropylidene acetal using 2-methoxypropene and a catalytic quantity of p-toluenesulfonic acid in dimethylformamide (DMF). Note how this blocking protocol was fully compatible with the acid-labile epoxide. 

A number of transition metals are now known147-156 to form stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.157 158 Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. 

However, a recent kinetic study188 has shown unequivocally that chain initiation proceeds via the usual metal-catalyzed decomposition of the hydroperoxide. Thus, the rate of initiation of the autoxidation of cumene was, within experimental error, equal to the rate of production of radicals in the (Ph3P)4Pd-catalyzed decomposition of tert-butyl hydroperoxide in chlorobenzene at the same temperature and catalyst concentration. Moreover, long induction periods were observed (in the absence of added tert-butyl hydroperoxide), when the cumene was purified by passing it down a column of basic alumina immediately prior to use. Autoxidation of cumene purified by conventional procedures showed only short induction periods. These results further demonstrate the necessity of using highly purified substrates in kinetic studies. 

The first step of peroxidase catalysis involves binding of the peroxide, usually H2C>2, to the heme iron atom to produce a ferric hydroperoxide intermediate [Fe(IE)-OOH]. Kinetic data identify an intermediate prior to Compound I whose formation can be saturated at higher peroxide concentrations. This elusive intermediate, labeled Compound 0, was first observed by Back and Van Wart in the reaction of HRP with H2O2 [14]. They reported that it had absorption maxima at 330 and 410 nm and assigned these spectral properties to the ferric hydroperoxide species [Fe(III)-OOH]. They subsequently detected transient intermediates with similar spectra in the reactions of HRP with alkyl and acyl peroxides [15]. However, other studies questioned whether the species with a split Soret absorption detected by Back and Van Wart was actually the ferric hydroperoxide [16-18], Computational prediction of the spectrum expected for Compound 0 supported the structure proposed by Baek and Van Wart for their intermediate, whereas intermediates observed by others with a conventional, unsplit Soret band may be complexes of ferric HRP with undeprotonated H2O2, that is [Fe(III)-HOOH] [19]. Furthermore, computational analysis of the peroxidase catalytic sequence suggests that the formation of Compound 0 is preceded by formation of an intermediate in which the undeprotonated peroxide is coordinated to the heme iron [20], Indeed, formation of the [Fe(III)-HOOH] complex may be required to make the peroxide sufficiently acidic to be deprotonated by the distal histidine residue in the peroxidase active site [21],... 

Aryl-substituted enolizable keto compounds initiate the copolymerization of unsaturated polyesters with styrene. Gel times of the same order as those obtained with conventional peroxide initiators can be attained exotherms, however, are considerably lower, this latter effect being of technological interest—e.g., casting resins. Since a radical mechanism has been proved, it is postulated that radicals result from keto hydroperoxides which have been formed from the aryl-substituted enols via autoxidation. Steric effects and resonance may partly account for differences in the catalytic activity of some and for the inhibiting effect of other ketones and enols. NMR spectroscopy indicates further that cis-trans isomerism may influence the catalytic effectiveness of pure enols. 

At high cobalt concentrations, reaction (28) can compete with hydrogen abstraction (eq. (2)) [7]. It removes ROO radicals from the system in a manner reminiscent of the reaction with Mn (eq. (29)). However, instead of producing a hydroperoxide, the product of reaction (28) goes directly to MEK (eq. (29)). In contrast, in the conventional oxidation of n-butane, MEK appears to be an important intermediate but not the major one [10, 18]. Some or most of the MEK is subsequently oxidized, mostly to acetic acid, in either case. In the conventional oxidation, the susceptibility of MEK to oxidation is about the same as for butane. 

Besides MeAPO, several heterogeneous systems have been proposed for the oxidation of cydohexane vith various oxidants [2c], but almost all of them yield 01/ One as the main reaction products, with AA being only a minor product. Heterogeneous catalysts can be either oxides or metal cations and complexes incorporated on inorganic matrixes, such as active carbon, zeolites, aluminophosphates or conventional supports such as alumina and silica. The activity of these systems is greatly affected by the choice of solvent, which determines the polarity of the medium. In addition, the hydrophobicity of the support is important, since a hydrophobic environment rapidly expels the oxidized products from the reaction zone. When oxygen is used as the oxidant, these systems often need small amounts of hydroperoxides as co-catalysts. 

---