Rhenium complexes with olefins

The reactions of olefins with non-organometallic Tc(VII) compounds behaved similarly. In a recent study, [Tc03C1(AaA)] (86a) (in which AA stands for aromatic diamine derivatives) was shown to react quantitatively with olefins, and produce the corresponding Tc(V) diolato-complex [TcOC1(OaO)(AaA)] (87a). The process could not be run catalytically, as Tc(V) complexes tend to undergo disproportionation rather than reoxidation in the presence of water [97]. These alkene-glycol interconversions could not be performed with the analog Re(VII) compound. Rhenium displays completely contrary behaviour, in that alkenes can... 

When olefins are treated with certain catalysts (most often tungsten, molybdenum, or rhenium complexes), they are converted to other olefins in a reaction in which the alkylidene groups (R R2C=) have become interchanged by a process schematically illustrated by the equation ... 

The reaction of the rhenium alkylidyne complex 277 with diisopropyl-acetylene and with diethylacetylene [Eq. (196)] demonstrates the sensitivity of metathesis reactions toward steric factors (57). With diisopropylace-tylene an alkylidyne complex is obtained whereas the reaction with diethylacetylene gives a metallacyclobutadiene. In the metathesis reactions the alkyne with the bulkiest groups cleaves most easily from intermediate metallacyclobutadiene complexes. The rhenacyclobutadienes with the smallest substituents thus become sinks and slow down the effective rate of metathesis. The alkylidyne alkylidene rhenium complex 278 is an active olefin metathesis catalyst (52). Reaction with hexene transforms the neo-pentylidene group into a propylidene group as shown in Eq. (197). 

Fig. 17 Proposed reaction mechanism for the olefination of aldehydes catalyzed with (T -alkyne)methyl(dioxo)rhenium complexes. Reproduced with permission from [120]...

Evidence in favor of the [2+2] mechanism is circumstantial, but it does include several types of studies. This evidence includes nonlinear free energy relationships between the substituent parameters on vinylarenes and the rates of the dihydroxylation, and it includes temperature effects on selectivity. It also includes the results of studies on the cleavage of Cp Re(0)(diolate) complexes to Cp ReOj and free olefin. The electronic effects, enthalpy of activation versus the strain of the olefin, and secondary deuterium isotope effects obtained from these studies on the rhenium complex support a stepwise cleavage process that could occur by initial formation of an oxametallacyclobutane intermediate. In the end, however, the combination of computational data and isotope-effect measurements seem to have led the community to accept that osmium tetroxide reacts by the direct [3+2] pathway. The mechanism of the reaction of OjOsNR with olefins during aminohydroxylation presumably follows the same type of [3+2] pathway. 

Two distinct observations are made with respect to the hydrogenation of olefin 7i-complexes with catalysts such as raney nickel. When the olefin is firmly bound to a metal, complete catalytic hydrogenation is prohibited. Thus 7i-cyclopentadienylcobalt cyclooctatetraene (6-22), 7r-cyclopentadienyl tt-cyclopentadiene rhenium dicarbonyl (6-23), and Tr-cycloheptatriene iron tricarbonyl (6-24), each of which contains at least one uncomplexed double bond, readily undergo hydrogenation of the uncomplexed bonds only. The metal-olefin bond is preserved in each case. 

For the alkylation with olefins, a rhenium complex proved to be the best catalyst, which provided only monoalkylated products, even in the presence of a surplus of the reagent (see 444). With sterically hindered olefins, the para-alkylated compounds (see 446) turned out to be by far the main reaction product [152]. 

Schiff base complexes to aromatic silyl enol ethers/ " olefins/ sugars,Mn ° Schiff base complexes, " and rhenium(III) complexes " has been reported. Similar reactions are observed with the Mn (N)porphyrin complexes. The reaction products are dependant on the nature of the Schiff base ligand, both the yield and the enantiomeric excess being affected. " Salen nitridomanganese(V) complexes have been incorporated into Zeolite... 

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