Metal-oxo systems

By contrast to the metaphosphimates, the metaphosphate analogues in which two chalcogens are replaced by imido groups form dimeric, dianionic ligands (24). There is only one example of a metal complex of the oxo system (24, E = O). An N,0-chelated bis(dimethylalumimun) complex is prepared by the reaction of two equivalents of trimethylaliuninum with ds-[ BuNH(0)P(//-N Bu)2P-(0)NH Bu] [20]. The structure of this complex is comparable to that of (15). 

The results prompted Jefford and coworkers to re-examine the iron(II) degradation of artemisinin in aqueous acetonitrile with iron(II) chloride (Scheme 10), a system they suggested was closer to the physiological conditions than iron(II) bromide in THF. They reported that iron(II) chloride catalysed isomerization of artemisinin to afford the same products identified by Posner (13 and 21), except that deoxyartemisinin 3 was not observed. When the reaction was carried out in the presence of cyclohexene, none of the expected epoxide was produced, which suggested (in sharp contrast to Posner s results) that a high-valent metal oxo species was not involved. 

The importance of this reaction also lies in the fact that asymmetric epoxidation of alkenes other than allylic alcohols is possible with this catalytic system (see Section 9.3.4). The third reaction relates to catalysts developed by Unilever for improved detergent action in the presence of hydrogen peroxide. The important point to note is that catalytic intermediates with metal-oxo groups play a pivotal role in all these reactions. 

Apart from potential industrial applications, homogeneous catalytic systems with metal-oxo intermediates have direct relevance to certain biological oxidation reactions. The biological reactions involve metalloenzymes such as Cyt P450, which has an iron-porphyrin complex in its active site. The enzyme catalyzes hydroxylation of a hydrocarbon by oxygen. This hydroxylation does not proceed through a radical-chain mechanism. There is sufficient evidence to indicate that a catalytic intermediate with an Fe=0 group is responsible for the hydroxylation reaction. 

Oxidation of organic compounds in general and alkenes in particular is an enormous area, and transition-metal-based catalysts have played a pivotal role. Several books have summarized the state of the art.1 Historically, this has been an area in which empirical development of catalytic oxidation systems has outpaced mechanistic understanding. However, with the recent advent of new synthetic techniques, the number of well-characterized metal oxo compounds has expanded considerably in the past 15 years,2 and the prospect of understanding the behavior of the M=0 bond in reactions which create C—O bonds has drawn within reach. 

First successful use of a chiral metal oxo-based system for catalytic asymmetric epoxidation... 

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