Catalyst and hydroperoxide

A reaction of the metal catalyst and hydroperoxide, second order with regard to metal, has been previously postulated (15), and our experience suggests that this is not infrequent in hydrocarbons. In the presence of phenolic antioxidants Tl is correctly neglected for most practical purposes. With the metal chelates which we have been studying this would not be justifiable at relatively low concentrations such as M/20,000. 

An example of the synthesis of CCs from olefins in a single reactor has been reported by Srivastava [187, 188], by using titanosilicalite as catalyst and hydroperoxide as oxidant in the form of H202 or TBHP. The reaction was carried out in two steps, in which the olefin was first epoxidated at 233 K using H202 or TBHP. The C02 was then added in presence of N,N-dimethylamionopyridine as cocatalyst, to afford a 33% yield of the CC at 293 K. 

Esomeprazole (Nexium, 13.45), a proton-pump inhibitor, is marketed as a singleenantiomer drug under the name Nexium (Scheme 13.7).17 The diethyl ester of (+) -tartaric acid (13.43, R = ethyl) serves as a chiral ligand for the titanium catalyst, and hydroperoxide is the stoichiometric oxidant. Because of the chiral environment created by the (+)-tartrate ligand, the catalyst selectively adds an oxygen atom to just one of the lone pairs to form a new stereocenter at the sulfur atom. 

Hydroperoxide decomposition in the presence of transition metal compounds may be, in effect, more complicated than the above simple radical scheme suggests. It may involve, for example, the formation of intermediate complexes between the catalyst and hydroperoxide. The reaction may result in the formation of molecular products without the participation of free radicals or proceed itself by a radical chain mechanism [15]. 

Complexing between catalyst and hydroperoxide [330] probably occurs prior to equations (204) and (205) and this step may be retarded by introduction of compounds which are superior ligands. Thus reaction in alcohol-chlorobenzene mixtures are 100 X slower than in chlorobenzene alone [328]. 

The first practical method for asymmetric epoxidation of primary and secondary allylic alcohols was developed by K.B. Sharpless in 1980 (T. Katsuki, 1980 K.B. Sharpless, 1983 A, B, 1986 see also D. Hoppe, 1982). Tartaric esters, e.g., DET and DIPT" ( = diethyl and diisopropyl ( + )- or (— )-tartrates), are applied as chiral auxiliaries, titanium tetrakis(2-pro-panolate) as a catalyst and tert-butyl hydroperoxide (= TBHP, Bu OOH) as the oxidant. If the reaction mixture is kept absolutely dry, catalytic amounts of the dialkyl tartrate-titanium(IV) complex are suflicient, which largely facilitates work-up procedures (Y. Gao, 1987). Depending on the tartrate enantiomer used, either one of the 2,3-epoxy alcohols may be obtained with high enantioselectivity. The titanium probably binds to the diol grouping of one tartrate molecule and to the hydroxy groups of the bulky hydroperoxide and of the allylic alcohol... 

Limonene (15) can be isomerized to terpiaolene (39) usiag Hquid SO2 and a hydroperoxide catalyst (/-butyl hydroperoxide (TBHP)) (76). Another method uses a specially prepared orthotitanic acid catalyst with a buffer such as sodium acetate (77). A selectivity of about 70% is claimed at about 50% conversion when mn at 150°C for four hours. 

Homogeneous Systems Using Molybdenum and Tungsten Catalysts and Alkyl Hydroperoxides or Hydrogen Peroxide as the Terminal Oxidant... 

In real systems (hydrocarbon-02-catalyst), various oxidation products, such as alcohols, aldehydes, ketones, bifunctional compounds, are formed in the course of oxidation. Many of them readily react with ion-oxidants in oxidative reactions. Therefore, radicals are generated via several routes in the developed oxidative process, and the ratio of rates of these processes changes with the development of the process [5], The products of hydrocarbon oxidation interact with the catalyst and change the ligand sphere around the transition metal ion. This phenomenon was studied for the decomposition of sec-decyl hydroperoxide to free radicals catalyzed by cupric stearate in the presence of alcohol, ketone, and carbon acid [70-74], The addition of all these compounds was found to lower the effective rate constant of catalytic hydroperoxide decomposition. The experimental data are in agreement with the following scheme of the parallel equilibrium reactions with the formation of Cu-hydroperoxide complexes with a lower activity. 

Under the conditions where the chain oxidation process occurs, this reaction results in chain termination. In the presence of ROOH with which the ions react to form radicals, this reaction is disguised. However, in the systems where hydroperoxide is absent and the initiating function of the catalyst is not manifested, the latter has a retarding effect on the process. It was often observed that the introduction of cobalt, manganese, or copper salts into the initial hydrocarbon did not accelerate the process but on the contrary, resulted in the induction period and elongated it [4-6]. The induction period is caused by chain termination in the reaction of R02 with Mn"+, and cessation of retardation is due to the formation of ROOH, which interacts with the catalyst and thus transforms it from the inhibitor into the component of the initiating system. 

The catalyst can decompose hydroperoxide homolytically, as well as heterolytically (see Chapters 10 and 17). Special experiments on the oxidation of fuel T-6 were performed in the presence of MoS2 with combined initiation by the initiator (DCP) and hydroperoxide formed in T-6 in the presence and absence of MoS2 [10]. It was found that the rate of the free radical generation and the rate of the hydroperoxide decay proceeds by the equations... 

Some times even catalysts can include initiators to decompose into free radicals. In such type of reaction, an electron transfer mechanism is involved. Peroxides and hydroperoxides are decomposed in this way. For example, the decomposition of benzoyl peroxide by an aromatic tertiary amine at room temperature. 

Various transition metals have been used in redox processes. For example, tandem sequences of cyclization have been initiated from malonate enolates by electron-transfer-induced oxidation with ferricenium ion Cp2pe+ (51) followed by cyclization and either radical or cationic termination (Scheme 41). ° Titanium, in the form of Cp2TiPh, has been used to initiate reductive radical cyclizations to give y- and 5-cyano esters in a 5- or 6-exo manner, respectively (Scheme 42). The Ti(III) reagent coordinates both to the C=0 and CN groups and cyclization proceeds irreversibly without formation of iminyl radical intermediates.The oxidation of benzylic and allylic alcohols in a two-phase system in the presence of r-butyl hydroperoxide, a copper catalyst, and a phase-transfer catalyst has been examined. The reactions were shown to proceed via a heterolytic mechanism however, the oxidations of related active methylene compounds (without the alcohol functionality) were determined to be free-radical processes. 

The kinetics of the catalytic oxidation of cyclopentene to glutaraldehyde by aqueous hydrogen peroxide and tungstic acid have been studied and a compatible mechanism was proposed, which proceeds via cyclopentene oxide and /3-hydroxycyclopentenyl hydroperoxide. " Monosubstituted heteropolytungstate-catalysed oxidation of alkenes by t-butyl hydroperoxide, iodosobenzene, and dioxygen have been studied a radical mechanism was proved for the reaction of alkenes with t-BuOOH and O2, but alkene epoxidation by iodosobenzene proceeds via oxidant coordination to the catalyst and has a heterolytic mechanism. ... 

Since approximately 2.2 lb of /-butyl alcohol would be produced per 1 lb of propylene oxide, an alternative reactant in this method is ethylbenzene hydroperoxide. This eventually forms phenylmethylcarbinol along with the propylene oxide. The alcohol is dehydrated to styrene. This chemistry was covered in Chapter 9, Section 6 as one of the syntheses of styrene. Thus the side product can be varied depending on the demand for substances such as /-butyl alcohol or styrene. Research is being done on a direct oxidation of propylene with oxygen, analogous to that used in the manufacture of ethylene oxide from ethylene and oxygen (Chapter 9, Section 7). But the proper catalyst and conditions have not yet been found. The methyl group is very sensitive to oxidation conditions. 

In 1987, Chmielewski and coworkers reported for the first time on the preparation of enantiomerically pure hydroperoxides 63a and 63c derived from carbohydrates . The method employed consisted of the oxidation of 2,3-unsaturated glycosides (64a and 64b, see Scheme 29) with hydrogen peroxide in the presence of a M0O3 catalyst. The hydroperoxides 63a and 63c were isolated together with their S-anomers (63b and 63d). 

The CLD methods for HPLC using isoluminol (190) with microperoxidase catalysis, for determination of lipid hydroperoxides in clinical fluids, have been reviewed. Determination of phospholipids hydroperoxides by luminol (124) CL has been reviewed . A fast RP-HPLC method (retention times 1 to 2 min) for determination of hydroperoxides and other peroxide compounds includes UVD, which is not always effective, and CLD, attained on injection of luminol (124), the CL reagent (Scheme 3), hemin (75a), a catalyst, and NaOH to raise the pH of the solution. A FLD cell may act as CLD cell if the excitation source is turned off. The selectivity of CLD is of advantage over UVD in industrial analysis thus, for example, UVD of a sample from a phenol production line based on cumene oxidation (equation 13) shows peaks for cumyl hydroperoxide (27), unreacted cumene, cumyl alcohol and acetophenone, whereas CLD shows only the 27 peak. The... 

The oxidation of cyclohexane using Fe (PA)3 as catalyst and BTSP in pyridine results in the formation of cyclohexanone as the major product with a small amount of cyclohex-anol. The reaction is catalytic, but an increase of Fe (PA)3 from 0.1 mmol to 0.3 mmol did not influence very much the efficiency (51 to 67%). The addition of a small amount of water increased slightly the formation of the ketone and alcohol. A partial hydrolysis of BTSP to trimethylsilyl hydroperoxide could be the explanation for this effect (Scheme 10). 

In the earlier volume of this book, the chapter dedicated to transition metal peroxides, written by Mimoun , gave a detailed description of the features of the identified peroxo species and a survey of their reactivity toward hydrocarbons. Here we begin from the point where Mimoun ended, thus we shall analyze the achievements made in the field in the last 20 years. In the first part of our chapter we shall review the newest species identified and characterized as an example we shall discuss in detail an important breakthrough, made more than ten years ago by Herrmann and coworkers who identified mono- and di-peroxo derivatives of methyl-trioxorhenium. With this catalyst, as we shall see in detail later on in the chapter, several remarkable oxidative processes have been developed. Attention will be paid to peroxy and hydroperoxide derivatives, very nnconunon species in 1982. Interesting aspects of the speciation of peroxo and peroxy complexes in solntion, made with the aid of spectroscopic and spectrometric techniqnes, will be also considered. The mechanistic aspects of the metal catalyzed oxidations with peroxides will be only shortly reviewed, with particular attention to some achievements obtained mainly with theoretical calculations. Indeed, for quite a long time there was an active debate in the literature regarding the possible mechanisms operating in particular with nucleophilic substrates. This central theme has been already very well described and discussed, so interested readers are referred to published reviews and book chapters . 

In Studying asymmetric oxidation of methyl p-tolyl sulfide, employing Ti(OPr-/)4 as catalyst and optically active alkyl hydroperoxides as oxidants, Adam and coworkers collected experimental evidence on the occurrence of the coordination of the sulfoxide to the metal center. Therefore, also in this case the incursion of the nucleophilic oxygen transfer as a mechanism can be invoked. The authors also used thianthrene 5-oxide as a mechanistic probe to prove the nucleophilic character of the oxidant. 

Other sources of radical CF3, much less expensive than CF3I, have been discovered. These are the anodic oxidation of sodium trifluoroacetate (the decomposition being initated by a hydroperoxide or ruthenium catalyst) and trifluoromethyl bromide (CF3Br) using sodium dithionite as initiating agent. ... 

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