Olefin epoxidation radical process

Stereospecific epoxidation of olefins with radical 62, produced in a mixture of tetramethyltetrazene-ZnCl2 and oxygen, is a radical-catalyzed process. ... 

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]. 

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. 

In this way, the direct contact of O2 with the olefin is prevented and the radical process of addition across the double bond is avoided. Reactions (6.18) and (6.19) are slow and the selectivity towards the epoxide in reaction (6.18) strongly depends on the catalyst preparation, the nature of the metal, and the reaction temperature. Using propene, the formation of the epoxide is in concurrence with the formation of acetone and propionaldehyde. Moreover, depending on the preparation of the metal oxide, the same catalyst can push the reaction to the formation of acroleine or even to the total oxidation of propene to CO2 and water [117]. If, instead of the only olefin, a mixture of olefin and CO2 is admitted on the catalyst in its oxidized form, the carbonate is formed which can be recovered by condensation and the excess olefin recycled. 

Though the detailed mechanism of olefin epoxidation is still controversial, Scheme 8 depicts possible intermediates, metallacycle (a), K-cation radical (b), carbocation (c), carbon radical (d), and concerted oxygen insertion (e) [2, 216, 217]. As discussed above, the intermediacy of metallacycle has been questioned. One of the most attractive mechanism shown in Scheme 8 is the involvement of one electron transfer process to form the olefin 7C-cation radicals (b). Observation of rearranged products of alkenes, known to form through the intermediacy of the alkene cation radicals, in the course of oxidation catalyzed by iron porphyrin complexes is consistent with this mechanism [218, 219]. A -alkylation during the epoxidation of terminal olefins is also well explained by the transient formation of olefin cation radical [220]. A Hammett p value of -0.93 was reported in the epoxidation of substitute styrene by Fe (TPP)Cl/PhIO system, suggesting a polar transition state required for cation radical formation [221] Very recently, Mirafzal et al. have applied cation radical probes as shown in Scheme 9 to... 

Viewed in this way, reactions yielding cyclic ethers should be thought of as two-step processes, epoxides probably arising from peroxy radical additions to olefins followed by 1,3-displacements, and larger rings arising via intramolecular hydrogen abstraction by peroxy radicals followed by 1,4- or 1,5-displacements. In all such reactions it is probably the first step which is slow and which determines the yield of product observed. 

P-450 has been shown to catalyze epoxidation with retention of the olefin configuration (114). Ortiz de Montellano and co-woiicers have shown that heme N-alkylation accompanies epoxidation when terminal olefins are oxidized by P-450 (775). Further, the oxidation of 1,1,2-trichloroethylene is known to give trichloroacetaldehyde along with epoxide (776, 777). A mechanism that explains simultaneous epoxidation, heme alkylation, and halogen migration is depicted in Scheme XVI (777). In this process, initial electron transfer affords a transient rr-radical cation that can collapse with C-0 bond formation to give either radical or cation intermediates. 

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