Epoxide reactions protonation

Catalytic turn-over [59,60] in McMurry couplings [61], Nozaki-Hiyama reactions [62,63], and pinacol couplings [64,65] has been reported by Fiirst-ner and by Hirao by in situ silylation of titanium, chromium and vanadium oxo species with McaSiCl. In the epoxide-opening reactions, protonation can be employed for mediating catalytic turn-over instead of silylation because the intermediate radicals are stable toward protic conditions. The amount of Cp2TiCl needed for achieving isolated yields similar to the stoichiometric process can be reduced to 1-10 mol% by using 2,4,6-collidine hydrochloride or 2,6-lutidine hydrochloride as the acid and Zn or Mn dust as the reduc-tant (Scheme 9) [66,67]. 

The computational results show that transition structures derived from hydroperoxo Re complexes lie slightly higher in energy than those obtained for the corresponding peroxo complexes, nevertheless their involvement in the epoxidation reaction cannot be excluded. However, for neither MoVI nor Revn evidence Get alone preference) for hydroperoxo reaction pathways is as clear as for TiIV complexes. Of course, more complex mechanisms involving intermolecular proton transfer and/or hydrogen bonded intermediates may change this picture to some extent. 

The epoxidation reaction was monitored by high performance liquid chromatograph (HPCL) on a reverse phase column. The reaction product was identified by IR, proton NMR and carbon-13 NMR spectroscopy. In the IR spectrum, absorption bands at 904 cm-1 and 842 cm-1 were attributed to the epoxy groups (12) peaks at 2.60, 2.78, and 3.15 ppm on the 1H-NMR spectrum were assigned to the three protons of the epoxide group (13) and in the 13C-NMR spectrum, the three glycidyl carbons were identified at 44.5, 50.6, and 70.6 ppm (14). 

Table 1. Kinetic parameters for non-catalyzed epoxide-anhydride-proton donor reactions"...

Epoxide Anhydride Proton donor Equa- tion Overall reaction order Wi X T rc) Ea kJ/mol Ref. 

The formation of these species, both of which probably coordinate a water molecule, is evident from the appearance of an intense yellow color of the solution. The bis-peroxorhenium complex is the most stable species. The protons of the coordinated water molecule are highly acidic, with consequences for the epoxidation reactions. 

Fluorinated alcohols, TFE and HFIP, have often been used as catalytic solvents for epoxidation reactions under mild conditions [34]. The merit of these fluorinated alcohols is, surely, their resistant nature toward oxidation. Another merit of their use in oxidation reactions has often been said to be their catalytic activities via protonation of an active oxygen species, such as H2O2. Experimental and computational studies indicate that the protonated hydrogen peroxide, H3C>2+, which is generated by the action of H2O2 with a strong acid, is a very powerful oxidant. In contrast, weak acids such as TFE appear to... 

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