Molybdenum-based catalyst systems

Having traversed some of the key events in the history of olefin metathesis, it is now appropriate to discuss some of the resultant fruits of that early labor in the form of practical applications in organic synthesis. Since the general reaction was bom in the industrial sector, we felt it appropriate to commence with some examples of commercial processes. Among several of the profitable industrial procedures that benefit from olefin metathesis, one of the oldest is the Phillips triolefin process (Scheme 7a) which utilizes a molybdenum-based catalyst system to convert propene (17) into a mixture of 2-butene (18) and ethene (19). These products are then used as monomers for polymer synthesis as well as for general use in petroleum-related applications. The reverse reaction can also be employed to prepare propene for alternative uses. 

Fiirstner A, Mathes C, Lehmann CW. Alkyne metathesis development of a novel molybdenum-based catalyst system... 

Temperature Programmed Reaction. Examination of another redox system, propylene oxidation on M0O3, provides further insight. It is well accepted that propylene oxidation on molybdenum-based catalysts proceeds through formation of allylic intermediates. From isotopic studies it has been demonstrated that formation of the allylic intermediate is rate-determining (H/D effect), and that a symmetric allylic species is formed ( C labelling). 

A molybdenum oxychloride-based catalyst system, MoOCl4- -Bu4Sn-EtOH, is more active than Mods ones. " In the polymerization of 1-chloro-l-octyne by the oxychloride-based catalyst, propagation rate is improved to be faster and MWD of the formed polymer is smaller. This ternary catalyst also induces living polymerization of... 

Molybdenum-based catalysts are highly active initiators, however, monomers with functionalities with acid hydrogen, such as alcohols, acids, or thiols jeopardize the activity. In contrast, ruthenium-based systems exhibit a higher stability towards these functionalities (19). An example for a molybdenum-based catalyst is (20) MoOCl2(t-BuO)2, where t-BuO is the tert-butyl oxide radical. The complex can be prepared by reacting M0OCI4 with potassium tert-butoxide, i.e., the potassium salt of terf-butanol. 

The cyclization of 8c to 9c created a trisubsti-tuted double bond, thus preparing new ground for the application of RCM in complex systems. Actually the substrate failed to cyclize with the ruthenium-based Grubb s catalyst, but 20 mol% of a molybdenum-based catalyst described by Schrock led to the cyclized product 9c in 86 % yield (benzene, 55 "C), unfortunately again with a 1.0 1.0 ratio of Z to isomers [11]. 

The NHC-coordinated catalysts 2 and 5 also exhibit dramatically improved substrate scope relative to bis(phosphine) catalysts. For example, whereas catalyst 1 is unreactive toward sterically congested substrates and cannot form tetra-substituted RCM products, catalysts 2 and 5 readily form tetra-substituted olefins in five- and six-membered rings systems (Eq. 4.17 E = C02Et) [98,100]. They also mediate CM between terminal olefins and 2,2-disubstituted olefins to form new trisubstituted double bonds [102]. Previously, these transformations could only be accomplished using molybdenum-based catalysts. 

Antimonate-Based Catalysts. In addition to the bismuth-molybdenum oxide catalyst system, several other mixed metal oxides have been identified as effective catalysts for propylene ammoxidation to acrylonitrile. Several were used commercially at various times. In particular, the iron-antimony oxide catalyst is currently used commercially by Nitto Chemical (now Dia-Nitrix Co. Ltd., Japan) and its licensees around the world, although the catalyst was originally discovered and patented by SOHIO (20,21) and by UCB (22). Nitto Chemical improved the basic iron-antimony oxide catalyst with the addition of several elements that promote activity and selectivity to acrylonitrile. Key among these additives are tellurium, copper, molybdenum, vanadium, and tvmgsten (23-25). 

Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl2 [758], Me4Sn/Al203 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. 

Numerous catalyst systems have been developed. Most common catalysts are based on tungsten of molybdenum. Transition metals ranging from group IV to group VIII have been found to be suitable. The catalysts are commonly classified as given in Table 1.4. 

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