Chromium catalysts olefin epoxidation

Although salen complexes of chromium, nickel, iron, ruthenium, cobalt, and manganese ions are known to serve as catalysts for epoxidation of simple olefins, the cationic Mn-salen complex is the most efficient. 

Metallosalen complex [salen = N, A-ethylenebis(salicyldeneaminato)] has a structure similar to metalloporphyrin, and these two complexes catalyze the epoxidation of olefins. For example, Kochi et al. have found that metallosalen complexes such as (salen )manganese(III) [25] and (salen)chromium(IIl) complexes [26] (hereafter referred to as Mn- and Cr-salen complexes, respectively) serve as catalysts for the epoxidation of unfunctionalized olefins by using iodosylbenzene [25] or sodium hypochlorite [27], In particular, cationic Mn-salen complex is a good catalyst for epoxidation of unfunctionalized olefins, which proceeds through an oxo(salen)manganese(V) species (Scheme 6B.14) [25,28], The presence of oxo-Mn(V)-salen... 

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. 

Oxo-transfer from metal complexes to olefins results in a net two-electron reduction at the metal center. As a result, only metals capable of shuttling between oxidation states can be effective oxo-transfer catalysts. Iron, manganese, ruthenium, and chromium have proven effective for catalytic epoxidation via oxo-transfer [8,9], and in synthetic systems studied thus far for enantioselective catalysis, these metals are most commonly coordinated by tetradentate porphyrin (1) and salen (2) ligand frameworks (Fig. 1). 

Imido and 0x0 compounds are intermediates in many of the transfers of oxygen atoms and nitrene units to olefins to form epoxides and aziridines, and they are intermediates in many of the insertions of oxygen atoms and nitrene units into the C-H bonds of hydrocarbons to form alcohols and amine derivatives. The enantioselective epoxidation of allylic alcohols (Scheme 13.22) " is the most widely used epoxida-tion process, and the discovery and development of this process was one of the sets of chemistry that led K. Barry Sharpless to receive the Nobel Prize in Chemistry in 2001. The mechanism of this process is not well established, despite the long time since its discovery and development. Nevertheless, most people accept that transfer of the oxygen atom occurs from a titanium-peroxo complex - rather than from an 0x0 complex. Jacobsen s and Katsuki s - manganese-salen catalysts for the enantioselective epoxidations of unfunctionalized olefins, which were based on Kochi s achiral chromium- and manganese-salen complexes, are a second set of... 

In order to observe rapid rates and high epoxide selectivity, the conditions under which reaction (226) is run must be within fairly restricted limits. In most instances, an excess of olefin over hydroperoxide will result in more efficient use of hydroperoxide and thus in greater selectivity [370]. In general, the lower the temperature, the less radical decomposition of hydroperoxide and the higher the selectivity. The maximum temperature at which each metal complex may be run without a large amount of radical decomposition varies with the metal center. For molybdenum catalysts epoxide selectivities of 98% can be achieved at 100 °C but fall to 75-80% at 130°C. For vanadium complexes the maximum temperature for selective operation is 80 °C and for chromium it is below 60 [370]. 

Reactions of hydroperoxides with oleflns in the presence of a variety of other metal centers have also been investigated. Hydrogen peroxide epoxidizes olefins as well in the presence of oxy compounds of W, Mo, V, Os, Ti, Zr, Th, Nb, Ta, Cr and Ru [411-422]. Although CrOa-oxidation of oleflns has been shown to give epoxides [423-425], chromium complexes such as [Cr(acac)a] are not particularly effective epoxidation catalysts at elevated temperatures [426]. It has recently been shown [427] that OSO4 is an effective catalyst for the hydroxylation of oleflns by tert-butyl hydroperoxide in base equation (268). 

---