Rhenium, methyl

On warming to -20°C, methoxymethyl 57 reacts with the remaining CH3SO3F. The rhenium methyl product 55 forms, but oxonium salt or methylidene (58) intermediates are not detected. This is understandable if the methylidene 58 (or the oxonium salt) is reduced more rapidly than it is formed (Scheme 2, step d). Depending upon reaction conditions, 39 and/or... 

Now, one must wonder if there is any limit to the ability of metals to bond in a stable way to other o bonds, including those in non-reactive molecules like alkanes. In fact, evidence for an intermediate methane complex has been found at low temperature in the reductive elimination of methane from a cationic rhenium methyl hydride [34]. The ab initio theoretical study of the intermolecular process of oxidative addition of a methane C - H bond has led to the location of transition states where the bond is partially broken [35]. The same results have been fond for intramolecular oxidative additions which are related to agostic interactions. In fact, agostic interaction itself is a kind of non-oxidative coordination [15,36]. For unsaturated substrates like ethylene, the activation of a C - H bond seems to follow an intermolecular path, without any previous coordination of the double bond. A feasible explanation consists here of the fact that metal orbitals suitable for ethylene coordination are the same as those which are responsible for oxidative addition, thus making the processes competitive [37]. 

Cp(CO)2Re(THF) forms the complex 105 upon reaction with thiophene (89JA8753, 910M2436). Similar species are known for 2- and 3-methyl-, 2,5-dimethyl, and tetramethylthiophene (91IC1417). Thiophene in 105 is S-coordi-nated, and the sulfur atom is pyramidal. Treatment of 105 with Fc2(CO)9 produces 106, where the thiophene ligand is bridge-coordinated via the sulfur atom to rhenium and four carbon atoms of the dienic system with iron (the coordination mode). The pyramidal nature of the sulfur atom is preserved. The -coordination of thiophene separates the dienic and sulfur counterparts of the ligand and decreases the TT-electron delocalization, which leads to the enhanced basicity of the sulfur atom. 

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. 

The first example of a heterogeneous catalyst able to metathesize olefinic esters was recently reported (92). The combination of Re207/ (CH3)4Sn at an olefin/Re/Sn molar ratio of 219/6/1 converted methyl 4-pentenoate at 50°C in 51% conversion to ethylene and the corresponding dimethyl ester of 4-octene-l,8-dioic acid. This reaction exhibited a high degree of selectivity (>99%), and in the absence of (CH3)4Sn the rhenium catalyst was inactive (90). 

Particularly noteworthy was the rhenium catalysed cross-metathesis of trans-hex-3-ene with vinyl acetate or a,p-unsaturated esters [4]. For example, crossmetathesis of methyl frans-crotonate with frans-hex-3-ene gave the desired cross-coupled product without any self-metathesis of the crotonate (Eq. 2). 

Ma et al. also studied PVK-based PPLEDs using rhenium complex in [28,51], They have prepared the PPLED in configuration ITO/PVK bpyRe (27) DCM (28)/Al (DCM is 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4//-pyran), where the bpyRe... 

A control experiment with the unfunctionalized methyl bis(3,5-dimethylpyrazol-l-yl)acetate (51) showed no occupancy of the polymer sites and subsequently no A and A" signals but a typical IR spectrum of Merrifield polymer. Therefore, the results of our experiments prove a facial coordination of rhenium(I) by the monoanionic NJ), 0 tripod ligand as well as a solid phase fixation of the ligand and the resulting tricarbonyl complex (68). 

An efficient method has been developed by Pozdnyakov and Spivakov . In alkaline solution pertechnetate, in contrast to perrhenate, is reduced by hydrazine sulfate. After reduction technetiiun is no more extracted by methyl ethyl ketone. The distribution coefficient of technetium is by a factor up to 2500 smaller than that of rhenium. 

Procedure Hydrazin sulfate is added to a mixture of pertechnetate and perrhenate in 3-6 N NaON or KOH until its concentration is about 3 x 10 M. The solution is stirred and, after 10 min, rhenium is extracted by an equal voliune of methyl ethyl ketone. For complete separation of rhenium from technetium the extraction must be repeated 2-3 times. After a twofold extraction 99% of technetium and only 0.8 % of rhenium remain in the aqueous phase. 

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