Rhenium reactions with alkanes

Aspects of the apparatus for the synthesis using metal atoms are described. The reactions of the atoms of rhenium, tungsten, and osmium with hydrocarbons including alkanes are described. It is shown that metal atom reactions with alkanes can give isolable organometallic compounds including l-alkylidene compounds. 

The platinum-catalyzed reaction of alkanes with chlorine leads to alkyl chlorides and alcohols (Table 6, entry 46) with modest rates and conversions [50], Cydooctane can be easily dehydrogenated (Table 6, entry 47) in the presence of a stabilized vinylalkane by use of the neutral rhenium compound ReH7(PR3)2 [51]. By employing an iridium-based catalyst, the photochemical dehydrogenation of methylcydohexane to methylenecyclohexane is performed at room temperature... 

Manganese and Rhenium.- Reactions of Mn and Re vapours with benzene in conjunction with PMe / cyclohexene, or an alkane, have afforded a variety of new complexes including (n-PhH)Mn(PMe )2H,... 

In 1949, the development of a catalyst based on a combination of platinum and an acidic component (e.g. A1203, A1C13) allowed the use of lower reaction temperatures than with the early catalysts.6 However, problems were still encountered with chlorine corrosion. In the 1960s, Universal Oil discovered that the addition of rhenium to a bifunctional Pt/Al203 catalyst resulted in slower deactivation by carbon deposition, and other dopants have since been found to modify the catalyst acidity and resistance to poisons, e.g. Cl, Sn, Ir. More recently, catalysts based on zeolites and noble metals have been shown to be more resistant to nitrogen and sulphur compounds, while giving a high activity and selectivity to branched alkanes. 

Thus we examined the reactions of borane-Lcwis base adducts with rhenium polyhydride complexes to synthesize highly fluxional polyhydride(borane) complexes. However, treatment of several boranes with the rhenium complexes in deuterated solvents resulted in an unexpected reaction H-D exchange between the boranes and solvents. This is the first example of H-D exchange between sp2 carbon and sp3 boron, and closely associated with metal-catalyzed deuteration of alkanes.4... 

The objective of the process is to convert saturated hydrocarbons (alkanes and cycloalkanes) in petroleum naphtha fractions to aromatic hydrocarbons as selectively as possible, since the latter have excellent antiknock ratings (1,2). Naphtha fractions are composed of hydrocarbons with boiling points in the approximate range of 50-200°C. Reaction temperatures of 425-525°C and pressures of 10-35 atm are employed in the process. Reforming catalysts commonly contain platinum (3-5) or a combination of platinum and a second metallic element such as rhenium (6) or iridium (2,7). 

The attractive features of platinum-rhenium and platinum-iridium catalysts can be combined in a reforming operation. The data for the reactions of selected hydrocarbons considered earlier for platinum-rhenium and platinum-iridium catalysts indicate that the former catalyst is more selective for the conversion of cycloalkanes to aromatics, while the latter is more selective for the dehydrocyclization of alkanes. Since cycloalkane conversion occurs primarily in the initial part of a reforming system while dehydrocyclization is the predominant reaction after the cycloalkanes have reacted, it is reasonable to use a platinum-rhenium catalyst in the front of the system and to follow it with a platinum-iridium catalyst (32). 

Reaction of Cp Re(CO)2(Bpin)2 (16), prepared from Cp Re(CO)j (15) and puiaBa, led to the regiospecific formation of 1-borylpentane in quantitative yield under irradiation of light in pentane. Thus, the catalytic cycle involves oxidative addition of pin2B2 to Cp Re(CO)j with photochemical dissociation of CO, oxidative addition of C-H bond to Cp Re(CO)2(Bpin)2 (16) giving a rhenium(V) intermediate (17), and finally reductive elimination of an alkylboronate with association of CO (Scheme 2.4) [51]. The interaction required for C-H activation of alkane with 16 is not known but higher reactivity of primary over secondary C-H bonds has been reported in both oxidative addition (17) and bond metathesis (18) processes [52]. Isomerization of a sec-alkyl group in Cp Re(H)(R)(CO)(Bpin)2 (17) to an n-alkyl isomer before reductive elimination of pinB-R is another probable process that has been reported in metal-catalyzed hydroboration of internal alkenes [15c]. 

These peculiarities of the rhodium system (greater selectivity and easier reductive elimination of its hydrido alkyl complexes) are in agreement with a lower exothermicity of its CH insertion reaction. Irradiation of the rhenium complex CpLjRe also gave hydrido alkyl compounds CpL2Re(R)H (L = PMe3 R = methyl, -hexyl, cyclopropyl) with the corresponding alkane cyclohexane can be used as an inert solvent [24]. 

Baudry, Ephritikhine and Felkin found the first soluble organometallic system capable of functionalizing alkanes selectively and catalytically under mild conditions. The bis(phosphine)rhenium heptahydrides L2ReH7 14 react with cycloalkanes C H2 n = 6, 7, 8) in the presence of dimethylbutene to give the corresponding cycloolefins (reaction 16) [32, 33] these are not further dehydrogenated to dienes or benzene. The... 

These dehydrogenation reactions are essentially hydrogen transfer reactions between the alkane and dimethylbutene. In the case of the rhenium system, the species 17, L2ReH3, was invoked to account for the products of the reactions of 14 with various organic substrates [38]. ITie same species L2ReH3 can also be responsible for the activation of... 

The reaction of [Re(Me)(NO)(PPh3)Cp] with trifluoromethanesulfonic acid (triflic acid) in dichloromethane at 0 °C results in cleavage of the Re-Me bond and formation of [Re(0S02CF3)(N0)(PPh3)Cp] by formal ion exchange of methyl anion for the triflate anion. 3,64 Methane was not detected in this reaction. However, in other protonation reactions of rhenium alkyls, [Re(R)(NO)(PPh3)Cp], formation of alkane, RH, has been confirmed.63... 

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