Homogeneous Epoxidation by Early Transition Metals Lewis Acid Mechanism

HOMOGENEOUS EPOXIDATION BY EARLY TRANSITION METALS (LEWIS ACID MECHANISM) 

Early transition metal ions in their highest oxidation states, such as Ti(IV), V(V), W (VI), and Mo(VI), tend to be stable toward changes in their oxidation states. Consequently, in epoxidation reactions with hydrogen peroxide or alkyl hydroperoxides they form adducts (M-OOH and M-OOR) that are the key intermediates in the 

In many ways, the behavior of Re(Vll) is similar to that of the other metals, but in reactions with hydrogen peroxide, it prefers to form peroxo complexes, as does in certain cases, Mo(Vl). Ti(rV) forms exclusively hydroperoxo intermediates. 

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. 

Epoxidation with hydroperoxides is the basis for the large-scale indirect production of propylene oxide by a process that has been called the Oxirane or Halcon processes. Early work was reported by Smith in a patent issued in 1956 [457], which described soluble heteropoly acids containing transition metals such as chromium, molybdenum, and tungsten that could be employed as homogeneous catalysts for the reaction of olefins with organic hydroperoxides and hydrogen peroxide. 

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