Rhenium alkylation

The mechanism of hydride abstraction from rhenium alkyl complexes of the type [Re(Cp)(NO)(PPh3)(R)] (50) has been examined using electrochemical techniques and provides the first rate data for metal alkyl/Ph3C reactions. Both the a-hydride abstraction in equation (17) and )8-hydride abstraction in equation (18) processes are shown to involve a preequilibrium electron-transfer step shown in equation (19), followed by rate-determining hydrogen-atom transfer between... 

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... 

Compounds of the formulas Re(CR]), ReO(CH3)4, Li2[Re2(CH3)g] [60975-25-9], Re02(CH3)3 [56090-011-8], and Re03CH3 [70197-13-6] have been prepared. The first two compounds were obtained from reaction of rhenium hahdes or oxyhahdes and methyllithium the last three were formed from the species by oxidation or reduction reactions. The use of these hydride and alkyl complexes as catalysts is under investigation. 

The catalytic system used in the Pacol process is either platinum or platinum/ rhenium-doped aluminum oxide which is partially poisoned with tin or sulfur and alkalinized with an alkali base. The latter modification of the catalyst system hinders the formation of large quantities of diolefins and aromatics. The activities of the UOP in the area of catalyst development led to the documentation of 29 patents between 1970 and 1987 (Table 6). Contact DeH-5, used between 1970 and 1982, already produced good results. The reaction product consisted of about 90% /z-monoolefins. On account of the not inconsiderable content of byproducts (4% diolefins and 3% aromatics) and the relatively short lifetime, the economics of the contact had to be improved. Each diolefin molecule binds in the alkylation two benzene molecules to form di-phenylalkanes or rearranges with the benzene to indane and tetralin derivatives the aromatics, formed during the dehydrogenation, also rearrange to form undesirable byproducts. 

The acetylacetonates are stable in air and readily soluble in organic solvents. From this standpoint, they have the advantage over the alkyls and other alkoxides, which, with the exception of the iron alkoxides, are not as easily soluble. They can be readily synthesized in the laboratory. Many are used extensively as catalysts and are readily available. They are also used in CVD in the deposition of metals such as iridium, scandium and rhenium and of compounds, such as the yttrium-barium-copper oxide complexes, used as superconductors. 1 1 PI Commercially available acetyl-acetonates are shown in Table 4.2. 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

Reaction of rhenium atoms with alkyl-substituted arenes forms dirhenium- l-arylidene compounds (2 2) (Figure 3). The products require insertion, presumably sequential, into two carbon-hydrogen bonds of the alkyl substituent. These reactions seem highly specific and require only the presence of an alkyl-substituted benzene that possesses a CH2 or CH3 substituent. Thus, co-condensation of rhenium atoms with ethylbenzene gives two isomers (see Figure 3) in which the products arise from insertion into the carbon-hydrogen bonds of the methylene or the methyl group. The product distribution in this reaction is in accord with statistical attack at all available sp3 C-H bonds. 

Several rhodium(I) complexes have also been employed as ATRP catalysts, including Wilkinson s catalyst, (177),391 421 422 ancj complex (178).423 However, polymerizations with both compounds are not as well-controlled as the examples discussed above. In conjunction with an alkyl iodide initiator, the rhenium(V) complex (179) has been used to polymerize styrene in a living manner (Mw/Mn< 1.2).389 At 100 °C this catalyst is significantly faster than (160), and remains active even at 30 °C. A rhenium(I) catalyst has also been reported (180) which polymerizes MM A and styrene at 50 °C in 1,2-dichloroethane.424... 

Structure determinations of secondary metal alkyl complexes are relatively rare, yet they provide an opportunity to assess interactions of the metal with the /3-atoms of the alkyl. The angles (excluding hydrogen) about C(24) all exceed 109°, ranging from 111.7° to 115.1°. There is no evidence for any Re 0 interaction (compare V), this distance exceeding 3 A. Both the /3-carbon, C(25), and its attached hydrogens are over 3 A from rhenium. The hydrogen on the a-carbon, C(24J, is 2.76 A from rhenium. 

Table 13. Enthalpy of formation, AHf (g) and disruption, AH (kj mol x) of alkyl and acyl derivatives of manganese and rhenium carbonyls...

Methylbis( r-peroxo)rhenium oxide hydrate, 0497 Triethyltin hydroperoxide, 2583 Trimethylsilyl hydroperoxide, 1330 Triphenyltin hydroperoxide, 3758 See also alkyl trialkyllead peroxides... 

The mechanism for allyl alcohol isomerisation has been studied and the presence of alkoxy and oxo groups in the metal catalyst seems to be essential [5], Not only vanadium, but also rhenium and molybdenum analogues are catalyst for this reaction. The mechanism is depicted in Figure 5.8. The substituents at oxygen can be alkyl groups or silyl groups. 

Numerous examples have been reported of transition metal alkyl complexes which can be converted into carbene complexes by a-hydride abstraction [429-431], This process can also proceed intramolecularly by oxidative insertion of the metal into the a-C-H bond. Figure 3.8. shows some illustrative examples of iron, rhenium, and... 

The rhenium perhydrocarbyl complex Re(=CBu )(= CHBu )(CH2Bu )2 reacts with the surface hydroxyl groups of a silica(7oo) (Scheme 2.29) to form a well-defined surface species, monografted on silica and containing one alkyl, one alkylidene and one alkylidyne ligands according to mass balance analysis IR, NMR, EXAFS [79-82] and calculations [83, 84]. 

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