Lewis acid mechanism molybdenum complexes

Alkenes can be transformed into epoxides by hydroperoxides and a catalyst, which often is a high-valent titanium or molybdenum complex acting as a Lewis acid. The mechanism is not clear in great detail in Figure 2.34 a suggested mechanism is given. Coordination of the alkene to the metal prior to attack of the electrophilic oxygen is not considered as a necessary step. 

Redox processes are fairly common in the formation of Z —CO— complexes of transition metals, and an example is given in Eq. (9). In this reaction, titanium is oxidized from the + 2 to the +3 state, thus becoming a better Lewis acid, and the molybdenum dimer is reductively cleaved, thus developing Z —CO— donor character (59). A characteristic low-frequency Z —CO— band is observed in the IR spectrum, and a crystal structure is available. A proposed mechanism for the redox process, based on CO mediated electron transfer, is discussed in Section IV,C. 

Legzdins and coworkers [118] reported an unprecedented transformation of NO bound to molybdenum or tungsten in mononitrosyl-diaryl complexes. A solid purple compound CpW(NO)(o-tolyl)2 when exposed to water vapor at ambient temperature gives CpW(0)(N-o-tolyl)(o-tolyl) with concomitant formation of the di-oxo aryl complex CpW(0)2(o-tolyl). The latter is exclusively formed when the reactant is exposed to anhydrous O2 (Scheme 2.5). The mechanism of this conversion is not clear. However, the characteristic chemistry of CpW(NO)(o-tolyl)2 resembles that estabhshed for other CpM(NO)R2 complexes such as CpW(NO)(CH2SiMe3)2, which indicates that it forms simple 1 1 adducts with both typical Lewis acids and bases. Water as the potential Lewis base is an exception and it is interesting to note that if CpW(NO)(o-tolyl)2 is treated with 0H2, the label is found to be incorporated only into the di-oxo aryl product... 

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