Molybdenum-palladium catalysts

Catalysts. The methanation of CO and C02 is catalyzed by metals of Group VIII, by molybdenum (Group VI), and by silver (Group I). These catalysts were identified by Fischer, Tropsch, and Dilthey (18) who studied the methanation properties of various metals at temperatures up to 800°C. They found that methanation activity varied with the metal as follows ruthenium > iridium > rhodium > nickel > cobalt > osmium > platinum > iron > molybdenum > palladium > silver. 

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

Cyclooctadiene) (pyridine) (tri-cyclohexylphosphine)iridium(I) hexa-fluorophosphate, 88 Palladium catalysts, 230 Potassium-Graphite, 252 Sodium borohydride-Molybdenum(VI) oxide, 279... 

Asymmetric allylboration, characteristics, 9, 197 Asymmetric allylic alkylation, allylic alcohols with copper, 11, 99 with iridium, 11, 105 with molybdenum, 11, 109 with nickel, 11, 102 with non-palladium catalysts, 11, 98 with platinum, 11, 103 reaction systems, 11, 112 with rhodium, 11, 104 with ruthenium, 11, 108 with tungsten, 11, 111... 

AUylic alkylations. This complex in combination with 2,2 -bipyridyl (bpy) catalyzes nucleophilic alkylation of allylic acetates and carbonates, but is less active than molybdenum or palladium catalysts. The displacement occurs with retention of configuration, as with Mo and Pd catalysts. However, alkylation occurs almost entirely at the more substituted end of the allylic group, regardless of the nucleophile. 

Hydroalkoxylation of alkenols catalyzed by acids <05HCA3055 05OL4117>, silverfl) triflate <05OL4553>, palladium catalysts <05SL1609> and molybdenum catalysts <05CL790> has also been utilized in the formation of tetrahydrofurans. As can be seen... 

Metathesis with alkynes is also quite useful in synthesis,particularly for internal alkynes although terminal alkynes are not good partners in this reaction.358 internal metathesis reactions with alkynes are known,including the conversion of 439 to 440 (in 73% yield) in FUrstner s synthesis of prostaglandin E2-1,15-lactone.360 Note the use of a molybdenum metathesis catalyst for this reaction. Diynes also react with alkynes in an intermolecular reaction to form aromatic rings. An example is the conversion of 441 to a 6 1 mixture of 442/443, in 82% yield.36la a similar, palladium-catalyzed cycloaromatization is also known.362 The metathesis disconnections are... 

A vast majority of the allylic substitution reactions have been reported with palladium catalysts. However, complexes of other metals also catalyze allylic substitution reactions. In particular, complexes of molybdenum,tungsten, ruthenium, rhodium, and iridium " have been shown to catalyze the reactions of a variety of carbon nucleo-pliiles. In addition, complexes of ruthenium, rhodium, and iridium catalyze the reactions of phenoxides, alkoxides, amines, and amine derivatives. " The regioselectivity of the allylic substitution process witli these metals can often complement the regioselectivity of the reactions catalyzed by palladium complexes. The regioselectivity... 

Many diastereoselective allylations form a new stereocenter at one of the allylic carbons and at the nucleophilic carbon. For example, an iridium complex containing a phosphite ligand catalyzes enantioselective and diastereoselective formation of products containing two stereocenters, one at the original nucleophile and one at the original allyl electrophile (Equation 20.58). In another example shown in Equation 20.59, Trost s palladium catalyst leads to the reaction of allylic esters with chiral azlactone pronucleophiles with high diastereomeric and enantiomeric excess, as does the related molybdenum catalyst. In these cases, the metal appears to control the new stereocenter at the allyl group, as well as the relationship between this stereocenter and the new stereocenter formed at the nucleophile. 

HDPE resias are produced ia industry with several classes of catalysts, ie, catalysts based on chromium oxides (Phillips), catalysts utilising organochromium compounds, catalysts based on titanium or vanadium compounds (Ziegler), and metallocene catalysts (33—35). A large number of additional catalysts have been developed by utilising transition metals such as scandium, cobalt, nickel, niobium, molybdenum, tungsten, palladium, rhodium, mthenium, lanthanides, and actinides (33—35) none of these, however, are commercially significant. 

The catalyst for the second stage is also a bifimctional catalyst containing hydrogenating and acidic components. Metals such as nickel, molybdenum, tungsten, or palladium are used in various combinations and dispersed on sofid acidic supports such as synthetic amorphous or crystalline sihca—alumina, eg, zeofites. These supports contain strongly acidic sites and sometimes are enhanced by the incorporation of a small amount of fluorine. 

The predominant process for manufacture of aniline is the catalytic reduction of nitroben2ene [98-95-3] ixh. hydrogen. The reduction is carried out in the vapor phase (50—55) or Hquid phase (56—60). A fixed-bed reactor is commonly used for the vapor-phase process and the reactor is operated under pressure. A number of catalysts have been cited and include copper, copper on siHca, copper oxide, sulfides of nickel, molybdenum, tungsten, and palladium—vanadium on alumina or Htbium—aluminum spinels. Catalysts cited for the Hquid-phase processes include nickel, copper or cobalt supported on a suitable inert carrier, and palladium or platinum or their mixtures supported on carbon. 

In past years, metals in dilute sulfuric acid were used to produce the nascent hydrogen reductant (42). Today, the reducing agent is hydrogen in the presence of a catalyst. Nickel, preferably Raney nickel (34), chromium or molybdenum promoted nickel (43), or supported precious metals such as platinum or palladium (35,44) on activated carbon, or the oxides of these metals (36,45), are used as catalysts. Other catalysts have been suggested such as molybdenum and platinum sulfide (46,47), or a platinum—nithenium mixture (48). 

Some data have been obtained on the activity of the catalyst in a reduced state [for nickel (141,143,144), palladium (144°), and molybdenum (145, 145a). In the case of nickel catalysts the formation of nickel in the zero oxidation state takes place during the reduction of the surface organometallic compound by H2. The infrared spectrum shows the total restoration of the concentration of Si—OH groups (139), so the reduction proceeds according to the scheme ... 

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