Rhenium complexes olefin

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... 

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). 

The rhenium hydride (XIX) reacts with carbon monoxide under pressure giving the olefin complex (XXII) (107) an original formulation of this complex as a hydride was incorrect (82). 

Electrophilic attack by trityl cation on olefin complexes cani also occur to abstract a hydride from the allylic position. This process converts a neutral olefin complex into a cationic allyl complex and has been conducted with complexes that are less strongly backbonding than the complexes described in the preceding two paragraphs. One example of such an allylic hydride abstraction from a rhenium-propene complex is shown in Equation 12.69. An early example to form a homoconjugated system is shown in Equation 12.70. ... 

In the last decade, impressive advances have been made in the synthesis of alkylidene and alkylidyne complexes that contain metals from groups 4 [5] and 5 [5i,j,m, 6], especially Ti and V, but - except for the ROMP of norbornene by vanadium complexes - group 4 and 5 metals have not shown wide-ranging activity for olefin metathesis. Well-characterized rhenium(VII) complexes are known to be active for metathesis, and many rhenium(Vll) alkylidene and alkylidyne complexes have been isolated [2a], but little attention has been paid to the syntheses of rhenium alkylidene or alkylidyne complexes in the last decade. Theoretical calculations have been carried out on M(X)(CHR )(Y)(Z) complexes... 

Freeh CM, et al. Unprecedented ROMP activity of low-valent rhenium-nitrosyl complexes mechanistic evaluation of an electrophilic olefin metathesis system. Chemistry 2006 12(12) 3325-38. 

The only reported unconjugated olefin complexes of osmium are t clo-octa-1,5-diene complexes OsCl2PPh2Et [97] and [Os(C8Hi2)Cl2]x [98]. For rhenium a few olefin cottq>lexes are known such as the cyclopenta-diene derivative 7r-CsHsRe(CO)2CsH6 (p. 23) which may be compared to the 7r-CsHsMn(CX))2 olefiua analogues (p. 125). 

Although the number of applications of olefin metathesis to transition metal complexes is small compared to the number of applications in organic synthesis, this field is becoming increasingly important. Spectacular examples are the double RCM reactions of copper phenanthroline complexes as a synthetic route to catenanes [113] or a recently reported approach to steric shielding of rhenium complex terminated sp-carbon chains [114]. 

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 ... 

Gillespie, A.M. and White, D.P. (2001) Understanding the steric control of stereoselective olefin binding in cyclopentadienyl complexes of rhenium an application of de novo ligand design. Organometallics, 20, 5149. 

The use of organometallic rhenium complexes has found a very broad scope as oxidation catalysts as described in the previous section, making MTO the catalyst of choice for many oxidation reactions of olefins. Interestingly, MTO and related rhenium compounds have also found application in the reverse reaction, the deoxygenation of alcohols and diols. Especially in recent years, this reaction has attracted much attention due to the increased interest in the use of biomass as feedstock for the chemical industry. This section provides an overview of the use of rhenium-based catalysts in the deoxygenation reaction of renewables. 

Where rhenium-catalyzed deoxydehydration has attracted a lot of interest, only two reports concerning dehydration catalyzed by rhenium complexes are noteworthy in view of their application on biomass-derived substrates. The first was published in 1996 by Zhu and Espenson and uses MTO as catalyst for the dehydration reaction of various alcohols, either aliphatic or aromatic, to obtain the corresponding olefins. Using MTO in benzene or in the alcohol itself at room temperature after 3 days gives reasonable turnovers and, in the case of benzylic alcohols, good yields. In the same paper, MTO is used for the amination, etherification, and disproportionation of alcohols, which are all reactions interesting in the viewpoint of biomass transformation [123]. 

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