Formation of dialkyl peroxides

Dialkyl peroxides ROOR are often obtained as the principal product or as by-product of autoxidation of organic compounds, but almost always as a result of secondary reactions of alkyl hydroperoxides. Direct union of relatively long-lived radicals with oxygen is of minor importance and occurs only in exceptional cases, e.g., in the formation of bis(triphenylmethyl) peroxide from triphenylmethyl 343 

In most cases dialkyl peroxides arise from the alkyl hydroperoxides formed as primary products this is so in autoxidation of isochroman,333 phthalan,334 2-methyl-l,3-dioxolane,344 and tetrahydroacenaphthacene.345 Formation of dialkyl peroxides from alkyl hydroperoxides can occur under the influence of acids, and in this ionic dimerization 333,334 hydrogen peroxide is also formed  

An alternative method of obtaining dialkyl peroxides is a radical reaction under the influence of heavy-metal catalysts.346,347 

As an example of ionic dimerization the preparation of bis-(l, 1-dimethylphthalan) 3,3 -peroxide from 1,1-dimethylphthalan 3-hydroperoxide is described 334 

1 -Dimethylphthalan 3-hydroperoxide (500 mg) is dissolved in cold 2N-sodium hydroxide (10 ml) and treated first with methanol (5 ml) and then with 2N-sulfuric acid (20 ml). An oil separates it crystallizes in the refrigerator (yield, 390mg, 86%). Recrystallization from benzene/light petroleum gives the peroxide, m.p. 114-115°. 

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Reaction conditions depend on the reactants and usually involve acid or base catalysis. Examples of X include sulfate, acid sulfate, alkane- or arenesulfonate, chloride, bromide, hydroxyl, alkoxide, perchlorate, etc. RX can also be an alkyl orthoformate or alkyl carboxylate. The reaction of cycHc alkylating agents, eg, epoxides and a2iridines, with sodium or potassium salts of alkyl hydroperoxides also promotes formation of dialkyl peroxides (44,66). Olefinic alkylating agents include acycHc and cycHc olefinic hydrocarbons, vinyl and isopropenyl ethers, enamines, A[-vinylamides, vinyl sulfonates, divinyl sulfone, and a, P-unsaturated compounds, eg, methyl acrylate, mesityl oxide, acrylamide, and acrylonitrile (44,66). 

At 0°C, in a DMF solution containing 0.2M BuBr and saturated with a 5% oxygen-nitrogen gas mixture, the second reduction step was essentially non-electrochemical. The method is general for the formation of dialkyl peroxides diethyl peroxide can be conveniently prepared by using Et2S04 as both the aprotic solvent and the reactant. From the point of view of ptactical electrosynthesis, the system 02/BuBr/Bu4NBr/ DMF is convenient, as the anode reaction,... 

Support for this hypothesis comes from the fact that the product distributions observed depended on the nature of the catalyst (Table 1). Indeed the proportion of dialkyl peroxide formed diminished as the ligands on the iron center became more electron donating. The increased electron density at the metal center should shift the redox potential of the metal center to more negative values and disfavor the reduction of the metal catalyst in either the initiation or termination steps of the dialkyl peroxide formation mechanism. 

C—O Bond Formation Trapping of ArLi intermediates can be achieved with trimethyl borate or triisopropyl borate [41]. The intermediate hthium borate complexes, when treated with water, afford oxidizable boronates. With fluorodimethoxyborane, spontaneous elimination of flnoride leads directly to boronates, which are precursors of phenols by oxidation. Chemoselective metal-heteroatom exchange or deprotonation generates carbanions in the presence of dialkyl peroxides, and intramolecular trapping provides an efficient route to dihydrobenzofuranes and dihydrobenzopyrans [42]. 

The NOBS system undergoes an additional reaction that forms a diacyl peroxide as a result of the nucleophilic attack of the peracid anion on the NOBS precursor as shown in equation 21. This undesirable side reaction can be minimized by the use of an excess molar quantity of hydrogen peroxide (91,96) or by the use of shorter dialkyl chain acid derivatives. However, the use of these acid derivatives also appears to result in less efficient bleaching. The dependence of the acid group on the side product formation is apparentiy the result of the proximity of the newly formed peracid to unreacted NOBS in the micellar environment (91). A variety of other peracid precursor stmctures can be found (97—118). 

There are much fewer data for the dialkyl peroxides. The gas phase enthalpy of formation difference between the diethyl and dibutyl peroxides of about 40 kJ moH per methylene group is about twice that of the normal methylene increment of ca 21.6 kJmoH. The 219 kJmoH enthalpy of formation difference between the di-fert-butyl and di-fert-amyl peroxide is so large as to be incredible. 

FIGURE 1. Enthalpies of formation of alkyl hydroperoxides and dialkyl peroxides Vi. number of carbon atoms (Iq, kJ mol )... 

Of the five dialkyl peroxides with enthalpy of formation data, only those of unquestioned accuracy, dimethyl, diethyl and ferf-butyl peroxide, are included in the analysis. The enthalpies of the formal hydrogenolysis reaction 6 are remarkably consistent for the methyl, primary and tertiary compounds -335.0 2.9 kJmoR for the liquid (using the estimated enthalpy of vaporization for dimethyl peroxide) and -279.5 3.5 kJmoR for... 

As was the case for the alkyl hydroperoxides in reaction 4, the enthalpies of the oxy-gen/hydrocarbon double exchange reaction 8 for dialkyl peroxides are different depending on the classification of the carbon bonded to oxygen. For R = Me, Et and f-Bu, the liquid phase values are —4, 24.6 and 52.7 kJmoR, respectively, and the gas phase values are 0.1, 25.7 and 56.5 kJmoR, respectively. For the formal deoxygenation reaction 9, the enthalpies of reaction are virtually the same for dimethyl and diethyl peroxide in the gas phase, —58.5 0.6 kJ moR. This value is the same as the enthalpy of reaction of diethyl peroxide in the liquid phase, —56.0 kJ moR (there is no directly determined liquid phase enthalpy of formation of dimethyl ether). Because of steric strain in the di-ferf-butyl ether, the enthalpy of reaction is much less negative, but still exothermic, —17.7 kJmol (Iq) and —19.6 kJmol (g). 

The enthalpies of reaction 16 for solid and gaseous dibenzoyl peroxide are —45.8 and —47.3 kJmoU, respectively. These values are much smaller than those calculated for the liquid dialkyl peroxides ca —56 kJmoU ), the acyl peresters ca —70 kJmoU ) or the non-aromatic diacyl peroxides (—89 or —59 kJmol ). However, we have no reason not to accept the result. It would be futile to use this result for further calculations concerning the solid phase enthalpies of formation of bis(o-toluyl) peroxide, bis(p-toluyl) peroxide and dicinnamoyl peroxide because all the peroxide and the anhydride product enthalpy of formation data are from the same suspect source . 

About 30 years ago, an enthalpy of formation was reported for 3,3,4,4-tetramethyl-l,2-dioxetane . Both by direct microcalorimetric combustion measurements of the neat solid and by reaction calorimetry (of the solid itself, and in acetone solution to form acetone), a consensus value was derived. Now, is the enthalpy of formation plausible , notwithstanding the very large error bars Consider reaction 6 for the dioxetane that produces 2,3-dimethyl-2,3-butanediol . The liquid phase enthalpy of reaction is —329 kJmoU. It is remarkable that this value is compatible with that for the dialkyl peroxides, ca —335 kJmoU, despite the ring strain that might be expected. 

The group value of 0-(0)(C) calculated from the dialkyl peroxides gives a value of —57.1 kcal. per mole for the heat of formation of tert-BuOOH, compared with the measured value (11) of —52.3 kcal. per mole. Since the calculated value is more negative, one cannot account for the difference by the otherwise reasonable assumption that the hydroperoxide decomposed a little prior to combustion. An alternative would be that group additivity did not apply to the hydroperoxides, but Benson s... 

Olefins react with manganese(III) acetate to give 7-lactones.824 The mechanism is probably free-radical, involving addition of CH2COOH to the double bond. Lactone formation has also been accomplished by treatment of olefins with lead tetraacetate,825 with a-bromo carboxylic acids in the presence of benzoyl peroxide as catalyst,826 and with dialkyl malonates and iron(III) perchlorate Fe(C104)3-9H20.827 Olefins can also be converted to 7-lactones by indirect routes.828 OS VII, 400. 

The reaction of Pt02(PPh3)2 with alkyl halides results in the formation of alkylperoxo complexes, presumably via an SN2 nucleophilic attack of the terminal oxygen on the carbon atom, and occurs with inversion of configuration.44 Addition of excess alkyl halide to the alkylperoxo complex results in the formation of the dialkyl peroxide (equation 4S).44... 

Oxidation reactions of hydrocarbons have a typical course. From the low rates, the reaction accelerates successively due to the consecutive formation of another source of free radicals which increases the rate of the primary initiation reaction. The amplification of the number of reactive free radicals is caused mainly by the decomposition of alkyl hydroperoxides, dialkyl and diacyl peroxides and peracids which are formed as intermediates in the oxidation reaction. 

This chapter deals with the formation and behavior of peroxides in which the 0—0 group forms part of a ring. The most important of these heterocycles are peroxides of carbonyl compounds, which may also contain two or three peroxy groups in the same ring ozonides, which are also peroxides of carbonyl compounds, i.e., peroxidic acetals and endoperoxides, as cyclic dialkyl peroxides. 

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