Propene, molybdenum complex

Addition of hydride to [i7 -C5H5Mo(CO)(NO)-i7 -C3H5]PF6 provides the propene molybdenum analog (6) however, Faller and Rosan (57, 60) have found that hydride addition via cyanoborohydride is stereospecific and produces only one of the pairs of diastereoisomers. The asymmetry in charge induced by the difference in electronic demands of CO and NO appear to provide the driving force for this asymmetric induction. This is a particularly important observation because it represents an electronic control of induction, whereas the usual approach has been to utilize steric effects to promote asymmetric induction (88). The importance of the electronic asymmetry is dramatically demonstrated by perturbations of the endo-exo equilibrium in the molybdenum complexes (60). In this instance there are clearly disparate trans effects of the NO and CO. 

Table 5.3 Activities for propene metathesis of catalysts containing surface molybdenum complexes in different oxidation states (Yermakov 1975/76)...

Finally Zama et al ° investigated the nature of the mixed-valent dinuclear [Mo -Mo ] complex on mesoporous FSM-16 and silica by EPR. The complex had an intense signal at g = 1.926 flanked by an 11-line hyperfine pattern, owing to the natural abundance of Mo and Mo, so that the total nuclear spin of the M02 pair was J = f + P = 0, 5/2, 5. The complex was only stable at temperatures less than 623 K, as higher temperatures produced MoO phases as characterised by an asymmetric Mo + signal. This catalyst was found to be active in the propene metathesis reaction, and the role of the molybdenum complex in the reaction was directly evidenced by EPR. 

Tungsten hexachloride and molybdenum pentafluoride desulfurize 2-methylthiirane to propene (72DOK(207)899) and a ruthenium(II) complex desulfurizes thiirane (73JA4758). 

Figure 1.6 a SMPO process b the catalytic oxidation of propene with tert-butyl hydroperoxide in the presence of a molybdenum-oxo complex. 

Molybdenum(VI)-oxo complexes intervene as reactive species in the selective allylic oxidation of propene to acrolein in the gas phase over bismuth molybdate catalysts at high temperatures (>300 In industrial processes, selectivities in acrolein reaching 90% can be obtained... 

Apart Ifom the molybdenum carbene complexes already listed in Tables 2.1 and 2.2 Mo-based catalysts are of three main types (i) other Mo complexes, activated by a suitable cocatalyst (ii) M0CI5, also activated by a cocatalyst and (iii) supported oxides, generated in various ways. For the metathesis of terminal olefins higher than propene. Mo-based catalysts are generally more effective than the corresponding W-based catalysts. 

A method for the production of chiral oxirans from simple unfunctionalized alkenes employs the molybdenum peroxo-complex (51), which has a pentagonal-bipyramidal structure. The oxidations of propene, but-l-ene, and but-2-ene yield e.e. s of around 30% [all of (R) configuration] in PhN02 at 20°C. 

In the patent literature, numerous catalyst formulations for propene ammoxidation can be found, and the majority of these are formulated around molybdenum and/or antimony base oxides. The most effective catalysts for these reactions are complex metal oxide mixtures containing three to six... 

The generally accepted mechanism for olefin cross-metathesis is outlined for the case of propene in Mechanism 14.4. The catalyst belongs to a class of organometallics known as a metallocarbene, carbene complex, or alkylidene complex. Its structure is characterized by a carbon-metal double bond. In olefin metathesis the metal is typically ruthenium (Ru), tungsten (W), or molybdenum (Mo). Transition-metal carbene complexes were first prepared by Ernst O. Fischer (Munich) who shared the 1973 Nobel Prize in Chemistry with Geoffrey Wilkinson. 

Catalyst deactivation. The molybdenum-based catalysts deactivate faster than the rhenium-based ones. Studies concerning the stability of the catalyst during continuous metathesis of propene showed a loss of activity due to an intrinsic deactivation mechanism. Because of the high stability of both [Mo]=CH2 and [Mo]=CHCH3, the deactivation of the catalyst is assigned to isomerization of the intermediate metallacyclobutane complexes, leading to inactive 7i-complexes, in a way analogous to that depicted in Scheme 2. This hypothesis is supported by in situ UV/vis spectroscopic studies [67]. 

In the previous work [9] we have investigated ethene and propene metathesis reactions proceeding on molybdenaalkylidene centres. In the present work density functional study on ethene metathesis reaction proceeding on molybdenamethylidene centres of M0O3/AI2O3 catalyst is reported. Two variants of theoretical models of the active sites have been applied. In the first case, simple structures of the carbene and molybdacyclobutane complexes, in which hydroxyl groups replace the bonds between molybdenum and the carrier, are proposed. In the second case, molybdenum is attached to a small cluster of formula Al2(OH)6, which represents alumina. 

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