Epoxidation activity

Although these reagent combinations are not as generally useful as the peroxycar-boxylic acids, they serve to illustrate that epoxidizing activity is not unique to the peroxyacids. 

Metabolites No Yes 10, 11-epoxide (active) Yes 10-monohydroxy carbazepine (active) Yes (not active) No... 

Little used organic acceptors for these reactions, also [53,54], The use of a glycal as the radical source together with a functionalized enone as the radical acceptor is remarkable. Enones are swiftly reduced by Cp2TiCT [55] and thus epoxide activation must be considered as even more efficient. The product of the addition constitutes a valuable intermediate en route to derivatives of thyrsiferiol. 

The advantage of this mesoporous TS-1 over samples prepared by the conventional route is illustrated in Fig. 34. The two samples behave similarly for the oxidation of linear reactant oct-1-ene. But a marked difference was observed for the oxidation of bulkier cyclohexene. Because of the absence of diffusional constraints, the catalytic epoxidation activity in the mesoporous TS-1 enhanced by almost an order of magnitude for the oxidation of the bulkier cyclohexene. 

As long as the pore diameters are large enough for easy entry and exit of reactant and product molecules, the catalyst porosity features do not have a significant influence on the epoxidation activity. In a comparison between two epoxidation catalysts obtained by grafting Ti( o-PrO )4 on MCM-41 and an amorphous silica gel, respectively, the former showed a lower activity (189). 

A direct correlation between the concentration of the titanium oxo species and epoxidation activity was proposed by Lin and Frei (133). Loading TS-1/H202 with propene after evacuation, they observed by FTIR difference spectroscopy the loss of the bands characterizing propene (at 1646 cm-1) and TiOOH (at 837 and 3400 cm-1). Figure 48 is the infrared difference spectrum recorded immediately after loading the propene on TS-1/H202 Fig. 49 includes the spectra recorded 80 and 320 min later. 

With regard to epoxidation activity, the peroxo complex MoO(C>2)2-hinpt exhibits close similarity to peracids. Based on this observation, Sharpless et al. suggested [11] a concerted (direct) mechanism as an alternative to the insertion mechanism. That mechanism, which is assumed to proceed via a... 

Despite of the common reaction mechanism, peroxo complexes exhibit very different reactivities - as shown by the calculated activation energies -depending on the particular structure (nature of the metal center, peroxo or hydroperoxo functionalities, type and number of ligands). We proposed a model [72, 80] that is able to qualitatively rationalize differences in the epoxidation activities of a series of structurally similar TM peroxo compounds CH3Re(02)20-L with various Lewis base ligands L. In this model the calculated activation barriers of direct oxygen transfer from a peroxo group... 

To analyze the epoxidation activity of these peroxo complexes, we characterized for each complex TSs of oxygen transfer to the model olefin ethene. We compared the two mechanisms mostly discussed in the literature, namely insertion [2, 8, 9, 97] and a direct attack of the peroxo group on the olefin [11] (see Section 1). Direct oxygen transfer can be envisaged to occur... 

In summary, one can identify three factors that mainly affect the epoxidation activity of a TM peroxo complex (i) the strength of the M-0 and 0-0 interactions, (ii) the electrophilidfy of the peroxo oxygen centers and the olefin, and (iii) the interaction between the tt(C-C) HOMO of the olefin and the peroxo a (0-0) orbital in the LUMO group of the metal complex to which we will also refer as the relevant unoccupied MO, RUMO (Figure 5). 

We considered [71] the epoxidation activity of MoVI hydroperoxo intermediates formed via opening an ti2-02 peroxo group ... 

Interferences in the mussel homogenates may be responsible, in part at least, for the low measured epoxidation activity. Properties of endogenous inhibitors in several insect preparations have been reviewed by Wilkinson and Brattsten (29). 

Isolation and Epoxidation Activity of a Coordinatively Unsaturated Ru Complex at a SI02 Surface... 

In Nature, epoxide ring-opening is catalyzed by enzymes by employing the (double) hydrogen-bonding motif (Scheme 6.1) for epoxide activation toward... 

We associate the various additional fluorine peaks observed in the NMR spectra with principally the products of epoxide-active fluorine species reactions. The total fluorine in the form of these products is relatively constant from prepreg lot to lot (28.2 to 36.3%). 

The epoxidation activity of the titanium catalysts, as a function of the different solvents and R groups varied in the synthesis of the silsesquioxanes precursors, is reported in Fig. 9.2. Values are normalised to the activity of the complex obtained by reacting TifOPr1 with the pure cyclopentyl silsesquioxane 7fc3 in THF. The results show some general trends ... 

Fig. 9.4 Screening of the epoxidation activity of the titanium silsesquioxanes as a function of the solvent and of the trichlorosilanes used in the synthesis of the silsesquioxane precursors.

First, the hydrolytic condensation of cyclopentyltrichlorosilane in acetonitrile (Section 9.2.2) was repeated on a 125 ml scale (50 times up-scaling). Next, the silsesquioxane products were reacted with a titanium alkoxide, yielding a catalyst with the same epoxidation activity of the HTE lead and, therefore, confirming the applicability of HTE techniques to the synthesis of silsesquioxanes. The silsesquioxane products obtained prior to reaction with the titanium centre could be divided into two fractions one as a precipitate (A) and the other as solute in the reaction mixture (B). Fraction B was dried under reduced pressure and redissolved in tetrahydrofuran. 

Finally, the concept was broadened by supporting these titanium silsesquiox-anes on silica remarkably, the heterogeneous silica-supported catalysts displayed an epoxidation activity per mole of titanium (94% TBHP conversion after 3 h) similar to that of the homogeneous titanium-silsesquioxane a2b4 complexes, although with a lower selectivity towards 1,2-epoxyoctane (92%). These heterogeneous catalysts did not leach active species and proved to be recyclable [45]. 

---