Catalysts molybdenum phosphine

The formation of rings that contain a thioether linkage does not appear to be catalyzed efficiently by Ru, even when terminal olefins are present. On the other hand, molybdenum appears to work relatively well, as shown in Eqs. 30 [207] and 31 [208]. Under some conditions polymerization (ADMET) to give poly-thioethers is a possible alternative [26]. Aryloxide tungsten catalysts have also been employed successfully to prepare thioether derivatives [107,166,169]. Apparently the mismatch between a hard earlier metal center and a soft sulfur donor is what allows thioethers to be tolerated by molybdenum and tungsten. Similar arguments could be used to explain why cyclometalated aryloxycarbene complexes of tungsten have been successfully employed to prepare a variety of cyclic olefins such as the phosphine shown in Eq. 32 [107,193]. 

Together with Schrock s molybdenum-imido compound 50 ° the ruthenium-phosphine complexes 51 and especially 52 developed by Grubbs " proved to be an outstanding achievement in the development of molecular catalysts for olefin metathesis reactions (Scheme 10). 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

The first reaction (346) consists of hydroperoxide formation by a typical autoxidation process, and the second represents selective epoxidation by the hydroperoxide. In the absence of the autoxidation catalyst, no reaction is observed under these conditions due to efficient removal of chain-initiating hydroperoxide molecules by reaction (347). Optimum selectivities obtain when the autoxidation catalyst is of low activity, which implies a low total activity of the catalytic system. The molybdenum complexes related to Mo02(oxine)2 are among the most effective catalysts for epoxidation.496 Although the autoxidation catalysts were limited to two types (phosphine complexes of noble metals and transition metal acetylacetonates), there is no reason, a priori, why other complexes such as naphthenates should not produce similar results. 

Besides zinc, copper and nickel, numerous other transition metals show high activity in epoxide carboxylaiion, excellent catalysts being chromium, manganese, iron or cobalt compounds in the presence of organic halides 1227). Halides of ruthenium, rhodium or cadmium were also used with phosphines to accelerate the reaction velocity (210). Kisch found catalyst systems which are active, eip-en at room temperature and under ordinary pressure [228]. Tlie most cftective catalyst was the combination of triphenylphosphine and molybdenum pcntachloridc propylene carbonate was formed in a 78% yield. 

The yields were found also to increase in the presence of phosphines, particularly trimethyl or tributyl phosphine. After all the improvements of the catalyst and reaction conditions the system became by far the most active of known non-biological catalytic systems for the reduction of dinitrogen at ambient temperature and pressure. The specific activity (the rate of N2 reduction per mole of the complex) reached and even exceeded that of nitrogenase. Up to 1000 turnovers relative to the molybdenum complex can be observed at atmospheric pressure and more than 10 000 turnovers at elevated N2 pressures. 

In early patents by Halcon, molybdenum carbonyls are claimed to be active catalysts in the presence of nickel and iodide [23]. Iridium complexes are also reported to be active in the carbonylation of olefins, in the presence of other halogen [24] or other promoting co-catalysts such as phosphines, arsines, and stibines [25]. The formation of diethyl ketone and polyketones is frequently observed. Iridium catalysts are in general less active than comparable rhodium systems. Since the water-gas shift reaction becomes dominant at higher temperatures, attempts to compensate for the lack of activity by increasing the reaction temperature have been unsuccessful. 

Compared to molybdenum- or tungsten-based Schrock catalysts, the reactivity of ruthenium-based systems is different. While reactivity slightly increases in the order Ielectron-withdrawing, groups [171]. The thermodynamics and in par-... 

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. 

The literature reports the synthesis of two types of catalyst based on tungsten and molybdenum chemically bonded with the polymer, and their use in the metathesis of oleflns with internal C=C bonds [264]. The catalysts are synthesized by bromination of polystyrene containing 2% divinylbenzene with subsequent treatment of the brominated polymer by n-BuLi in tetrahydrofuran. Lithium-polystyrene derivatives are thus formed. After reaction with a,a -dipyridyl or Ph2PCl, they are converted, respectively, to dipyridyl (A) or phosphine (B) derivatives of polystyrene that form active complexes after being heated with W(CO)5 or Mo(CO). ... 

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