Epoxidation radical process

Other metals can also be used as a catalytic species. For example, Feringa and coworkers <96TET3521> have reported on the epoxidation of unfunctionalized alkenes using dinuclear nickel(II) catalysts (i.e., 16). These slightly distorted square planar complexes show activity in biphasic systems with either sodium hypochlorite or t-butyl hydroperoxide as a terminal oxidant. No enantioselectivity is observed under these conditions, supporting the idea that radical processes are operative. In the case of hypochlorite, Feringa proposed the intermediacy of hypochlorite radical as the active species, which is generated in a catalytic cycle (Scheme 1). 

The formation of epoxides from oxygen appears to be a free radical process. 

Electrochemical oxidation of epoxides in absence of nucleophiles, catalyses a rearrangement to the carbonyl compound. The electrolyte for this process is dichlo-romethane with tetrabutylammonium perchlorate. Reaction, illustrated in Scheme 8.7, involves the initial formation of a radical-cation, then rearrangement to the ketone radical-cation, which oxidises a molecule of the substrate epoxide. The process is catalytic and requires only a small charge of electricity [73]. 

An intermolecular iron-catalyzed ring expansion reaction involving epoxides and alkenes provided tetrahydrofurans via radical processes <07CEJ4312>. Cp2TiCl is able to promote cyclization of 2,3-epoxy alcohols containing a p-(alkoxy)acrylate moiety to form tetrahydrofurans <07TL6389>. As shown in the following example, an intramolecular addition of carbon radicals to aldehydes was reported to afford tetrahydrofuran-3-ols... 

Simple amines in the presence of Oxone oxidize alkenes to oxiranes. For example, Oxone, pyridine, and a 2-pyrrolidine derivative in a medium of aqueous acetonitrile selectively converts the triene in Equation (72) to a single epoxide. This process also proceeds using noncyclic alkenes. The mechanism is believed to proceed via a single-electron transfer (SET) process involving radical cation intermediates <2000JA8317>. 

Cp2TiCl2 and other substituted bis-Cp titanium derivatives, including enantiomerically pure chiral complexes, have been used as excellent catalysts for the reductive ring opening of epoxides. This process involves Ti(m) species and subsequent radical reactions.1175,1180-1184... 

Product distributions from various olefins clearly show that at least two mechanisms are involved. Cyclohexene provided a higher yield of oxide than that of cyclohexene-1-one. The formation of cyclohexene-1-one suggests that the ROO radical serves as a reactive intermediate in this system, but a radical process, operating alone, would produce mostly the ketone and alcohol products with very little epoxide. Most significantly, czs-stilbene gave czs-stilbene oxide as the dominant product with only a minor yield of the frazzs-stilbene oxide, whereas a radical process would give mostly trzzzzs-product [81]. 

The results obtained allow us to propose the reaction mechanism comprising the following elementary steps of the chain radical process leading to epoxide and isobutyric acid formation ... 

The oxidation of cyclohexene using hydrogen peroxide was chosen as a test reaction for the catalytic evaluation of the titanium modified hexagonal NaY sanq)les. Scheme I illustrates some of the typical products of cyclohexene oxidation. The epoxide and the diol which is a hydrolysis product of the epoxide, generally reflect a concerted process. In contrast the allylic alcohol and ketone are often ascribed to an autoxidation or radical process. We anticipated that some homolytic decomposition of the peroxide may be observed with these acidic zeolites. In fact, there was -74% conversion of H2O2 over calcined hexagonal NaY after heating at 55 C for 24 hours. This resulted in only a 1% conversion of... 

Interestingly, arene iron salts 62 have also proven to be effective catalysts for the free radical polymerization of epoxides, acrylates, etc. [3,36]. Similar to cationic polymerization, radical processes are also photo chemically initiated reactions but are initiated in the presence of radical precursor additives (halogenat-ed solvents, bis(p-iV,i -dimethylaminobenzylide)cyclopentanone, etc.). 

Light-induced polymerisation is one of the most efficient methods to rapidly cure monomers that are inactive towards radical species, like vinyl ethers [49,50] or epoxides [51-54], The subject has been recently covered extensively by Crivello and Dietlieker in an excellent textbook [55]. This technology has not yet achieved the commercial significance of corresponding free-radical processes, mainly because of the more limited choice of monomers available and their lower reactivity. 

There is no effective epoxidation of cholest-4-ene-3-one, which has a carbonyl conjugated with the olefin bond, but ketalization of the conjugated carbonyl shifts the double bond to the 5,6-position and epoxidation occurs as described above ". The non-conjugated cholest-5-ene-3-one yields a mixture of epimeric 6-hydroxy-4-ene-3-ones, where the C=C bond has been shifted, and a 4-ene-3,5-dione this reaction was insensitive to the addition of a radical inhibitor, indicating a non-radical process. Ru(TMP)CO also catalyzes equally well this same reaction, but the true catalyst was again the trans-dioxo species formed from the carbonyl via reaction with a 6-hydroperoxy-4-ene-3-one (cf. Fig. 5), formed by radical-initiated, incipient autoxidation of the cholest-5-ene-3-one. 

Work in this laboratory has shown also that the Ru(poip)(0)2 complexes (porp = TMP, TDCPP, and TDCPP-Clg) are practically inactive for thermal 02-oxygenation of saturated hydrocarbons . Some activity data for 0.2 mM Ru solutions in benzene under air at 25°C for optimum substrates such as adamantane and triphenylmethane at 6 mM did show selective formation of 1-adamantol and trityl alcohol, respectively, but with turnover numbers of only -0.2 per day the maximum turnover realized was -15 after 40 days for the TDCPP system Nevertheless, this was a non-radical catalytic processes there was < 10% decomposition of the Ru(TDCPP)(0)2, and a genuine O-atom transfer process was envisaged . Quite remarkably (and as mentioned briefly in Section 3.3), at the much lower concentration of 0.05 mM, Ru(TDCPP-Clg)(0)2 in neat cyclooctene gave effective oxidation. For example, at 90°C under 1 atm O2, an essentially linear oxidation rate over 55 h gave about -70% conversion of the olefin with - 80% selectivity to the epoxide however, the system was completely bleached after - 20 h and, as the activity was completely inhibited by addition of the radical inhibitor BHT, the catalysis is operating by a radical process, but in any case the conversion corresponds to a turnover of 110,000 As in related Fe(porp) systems (Section 3.3, ref. 121), the Ru(porp) species are considered to be very effective catalysts for the decomposition of hydroperoxides (eqs. 

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