Olefin epoxidation oxygen transfer process

During the last three decades, peroxo compounds of early transition metals (TMs) in their highest oxidation state, like TiIV, Vv, MoVI, WV1, and Revn, attracted much interest due to their activity in oxygen transfer processes which are important for many chemical and biological applications. Olefin epoxidation is of particular significance since epoxides are key starting compounds for a large variety of chemicals and polymers [1]. Yet, details of the mechanism of olefin epoxidation by TM peroxides are still under discussion. 

Coordination catalysis via alkyl hydroperoxides is well documented (4, 31). Selective oxidations of olefins to epoxides (Reaction 16), using especially Group IV, V, and VI transition-metal complexes, can occur possibly via oxygen-transfer processes of the type... 

Figure 8 Examples of oxygen transfer to different substrates using hydroperoxo or alkylperoxo species A, the epoxidation of olefins catalyzed by Mo (VI) complexes as in the Oxirane process B, the Baeyer-Villiger oxidation of ketones catalyzed by Pt(II) complexes C, the epoxidation of olefins catalyzed by Ti(IV) silicates D, the oxidation of organic sulfides catalyzed by V(V) complexes.

Using the olefin 80 and its deuterated analogue in the epoxidation by lb, the authors were able to measure the a- and / -secondary deuterium isotope effects elucidating the nature of the transition state of the oxygen transfer in these epoxidation processes <1996TL5991>. 

Although, as stated above, olefin epoxidation is commonly referred to as an electrophilic oxidation, recent theoretical calculations suggest that the electronic character of the oxygen transfer step needs to be considered to fully understand the mechanism [451]. The electronic character, that is, whether the oxidant acts as an electrophile or a nucleophile is studied by charge decomposition analysis (CDA) [452,453]. This analysis is a quantitative interpretation of the Dewar-Chatt-Dimcanson model and evaluates the relative importance of the orbital interactions between the olefin (donor) and the oxidant (acceptor) and vice versa [451]. For example, dimethyldioxirane (DMD) is described as a chameleon oxidant because in the oxidations of acrolein and acrylonitrile, it acts as a nucleophile [454]. In most cases though, epoxidation with peroxides occurs predominantly by electron donation from the 7t orbital of the olefin into the a orbital of the 0-0 bond in the transition state [455,456] (Fig. 1.10), so the oxidation is justifiably called an electrophilic process. 

On the basis of a detailed investigation on the effect of various additives on this process, a mechanistic scheme for the "oxo" forming reaction may be suggested. Also the subsequent oxygen transfer from the "oxo species to the olefin has been studied by kinetic experiments. This allows to establish the role played by the various species in the two consecutive oxygen transfer reactions. A comparison of the results thus obtained with those provided by the study of the overali epoxidation reaction, carried out by measuring the rates of epoxide formation, wiii be made. 

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. 

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... 

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