Olefin epoxidation H2O2 oxidation

An overview of the pubhshed known results of tungsten-catalyzed epoxidation of unfunctionahzed olefins with H2O2 as oxidant is given in Table 23. 

Herrmann and coworkers observed short reaction times (0.02-14 h, generally 1 h). Conversions (89 to >99%) as well as epoxide selectivities (90 to >99%) were very high. Bipyridine has been employed by Rudler and coworkers under biphasic conditions with good results. Later, in 1998, Nakajima and coworkers presented their results on this topic in which they proposed that it is not the bipyridine but the bipyridine-A,A -dioxide, which is formed during the reaction, that is responsible for the suppression of the acidity of the MTO/H2O2 system . With bipyridine-A, A -dioxide as additive, a variety of olefins could be oxidized to the corresponding epoxides at room temperature with yields ranging from 80 to 96%. 

SCHEME 96. MTO-catalyzed epoxidation of various olefins using H2O2 as oxidant in the presence of different cocatalysts... 

A major concern in H2O2 oxidation is the alcohol/olefin chemoselectivity (Rao, 1991). This biphasic oxidation was initially developed for olefin epoxidations but with an (aminomethyl)phosphonic acid additive for high selectivity (Rudolph et al., 1997). Now, the removal of this additive has been found to significantly increase the rate and selectivity of alcohol oxidation. 

Already in the first reports on olefin oxidation with the MTO/H2O2 system [3], it was noted that the formation of diols from the desired epoxides, caused by the Br0nsted acidity of the system, is a major drawback of this system. The solution for this problem was found in the same report by the addition of a nitrogen base. This method has been explored extensively since and has become an important factor in the MTO-catalyzed olefin epoxidation. 

The effect of zeolite porosity on the reaction rate was also well demonstrated in liquid-phase oxidation over titanium-containing molecular sieves. Indeed, the remarkable activity in many oxidations with aqueous H2O2 of titanium silicalite (TS-1) discovered by Enichem is claimed to be due to isolation of Ti(IV) active sites in the hydrophobic micropores of silicalite.[42,47,68 69] The hydrophobicity of this molecular sieve allows for the simultaneous adsorption within the micropores of both the hydrophobic substrate and the hydrophilic oxidant. The positive role of hydrophobicity in these oxidations, first demonstrated with titanium microporous glasses,[70] has been confirmed later with a series of titanium silicalites differing by their titanium content or their synthesis procedure.[71] The hydrophobicity index determined by the competitive adsorption of water and n-octane was shown to decrease linearly with the titanium content of the molecular sieve, hence with the content in polar Si-O-Ti bridges in the framework for Si/Al > 40.[71] This index can be correlated with the activity of the TS-1 samples in phenol hydroxylation with aqueous H2C>2.[71] The specific activity of Ti sites of Ti/Al-MOR[72] and BEA[73] molecular sieves in arene hydroxylation and olefin epoxidation, respectively, was also found to increase significantly with the Si/Al ratio and hence with the hydrophobicity of the framework. 

Solid acids or bases often play a catalytic role in the activation of an oxidant such as H2O2. Although this topic is not discussed in detail here, we nevertheless illustrate it with two examples. First, LDHs can function as solid bases to catalyze the conversion of aromatic nitriles to peroxy-imidic acids the latter are excellent stoichiometric oxidants for olefin epoxidation (393) ... 

By contrast the epoxidation of olefins by H2O2 (and organic peroxides) which is catalyzed by TS-1 and TS-2 [80-83] is also catalyzed by other Ti containing solids including Ti-Al-Beta [84,85], Ti-MCM-41 [86] and mixed Ti02-Si02 oxides such as the ones described in [87,88]. 

OH), or Mn (Me2EBC)(O)2, and is consistent with the earlier conclusion that the Mn(IV) complex is not capable of direct epoxidation of norbomylene. However, a distinct difference exists between the two terminal oxidants for these systems, in that f-BuOOH reacts predominantly via a radical pathway whereas such a reaction path is very clearly minor in the H2O2 oxidation of olefins in the same catalyst system. 

The in situ generated peroxocomplexes were tested for the catalytic epoxidation of various olefins, such as allyhc alcohols, homoallylic alcohols and non-functionalized olefins. The results of these H2O2 oxidations in an alcohol-water system are summarized in Table 2 for the hydrophilic catalyst A, and in Table 3 for the lipophihc material C. Especially for the more reactive alkenes, the turnover number comes close to the maximum of 300. The epoxide selectivity generally exceeds 90%, with minimal solvolysis. With catalyst A, some substrates gave a lower selectivity. For instance, the product distribution for cyclohexene is 65% epoxide, 27% of allylic oxidation products and only 4% of the diol. The epoxycyclohexane selectivity increases to 91% with the hydrophobic material C. 

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