Rhenium-catalyzed reactions

As noted previously, the rhenium-catalyzed reactions dealt with to this point can be carried out in vessels open to the atmosphere. This is advantageous because of the convenient working procedures it allows. On the other hand, it means that molecular oxygen is not available as the stoichiometric oxidizing reagent (53-55). Three rhenium(V) compounds, 24-26, have been prepared that do activate 02. 

Oxidation of thiophene or thiophene derivatives with various oxidants in the presence of dienophile iV-phenylmaleimide always leads to the Diels-Alder cycloaddition products. Typical oxidants H2O2/CF3CO2H [41], wt-CPBA [22, 42], m-CPBA/BF3 Et20 complex [13], and TPHP (terf-butyl hydroperoxide) [43] are shown in Schemes 27, 28, 29, and 30. The rhenium-catalyzed reactions are of... 

Kuninobu and Takai developed a rhenium-catalyzed reaction of ketoester 43 with an alkyne, leading to cyclooctenone 44, in which an alkyne is formally inserted into a C-C=0 bond (Eq. (6.10)) [22, 23]. This ring-expansion reaction is applicable to the synthesis of up to 10-membered carbocycles. The C-C bond cleavage is proposed to proceed via retro-Aldol fragmentation of intermediate 45 or 46. 

Rhenium-catalyzed [3 + 2]-cycloaddition reactions between imines and nitriles affording indene derivatives were reported by Kuninobu et al. these authors demonstrated that only 3 mol.% of [ReBr(CO)3(THF)]2 allowed the transformation to proceed in high yield (Scheme 69).311... 

Transition-metal catalysts are, in general, more active than the MPVO catalysts in the reduction of ketones via hydrogen transfer. Especially, upon the introduction of a small amount of base into the reaction mixture, TOFs of transition-metal catalysts are typically five- to 10-fold higher than those of MPVO catalysts (see Table 20.7, MPVO catalysts entries 1-20, transition-metal catalysts entries 21-53). The transition-metal catalysts are less sensitive to moisture than MPVO catalysts. Transition metal-catalyzed reactions are frequently carried out in 2-propanol/water mixtures. Successful transition-metal catalysts for transfer hydrogenations are based not only on iridium, rhodium or ruthenium ions but also on nickel [93], rhenium [94] and osmium [95]. It has been reported that... 

An pFl-dependent coordination isomerism has been observed for the tumor targeting rhenium(V) 0x0 complex [ReO(DMSA)2] (DMSA = 2,3-mercaptosuccinate). In solution the crystallo-graphically characterized syn,endo-isomQx (87a) slowly isomerizes into the anti- (87b) and the iyn,exo-isomers (87c) which has consequences for the biodistribution of the potential pharmaceutical (Scheme 10). The conversion rate decreases with increasing pH suggesting an acid catalyzed reaction. The syn,exo-comy> ex is favored in alkaline solutions (pH > 8.4) which can be understood in terms of the repulsions between the deprotonated carboxylic groups and the 0x0 ligand. 

Rhenium-catalyzed deoxygenation reactions, on the other hand, can be divided in two topics deoxydehydration and dehydration. These reactions have mostly been applied on biomass-derived substrates such as sugar alcohols and polyols. This procedure lowers the high 0 C ratio present in biomass and as such is of particular interest for the production of both fuels and chemicals from biomass. 

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]. 

Scheme 21 Rhenium-catalyzed condensation reaction of furfural with glycerol to obtain the corresponding 1,3-dioxolane and 1,3-dioxane...

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... 

Thiiranes such as 2-methylthiirane and epithiocyclohexane, catalyzed by rhenium complexes, are useful reagents for sulfurization of phosphorus(iii) compounds (Equation (32) Table 3) <1997TL7701>. Mild reaction conditions and a short reaction time make this method of sulfurization of phosphorus(ni) compounds most interesting. Possible mechanisms for the rhenium-catalyzed sulfurization of phosphorus(lll) compounds are outlined in Scheme 38. 

In the case of the rhenium-catalyzed oxidation of methoxy- and hydroxy-substituted substrates, there is some complementary work concerning the general mechanism of the arene oxidation [10b, 11]. Since the major products in the oxidation of such arenes or phenols are the quinones, the formation of intermediary epoxides seems to be a predominant reaction step. When p-substituted phenols such as 2,6-di( -butyl)-4-methylphenol are treated with the MTO/H2O2 oxidant and acetic acid as solvent, the formation of hydroxydienones is observed. This is also reported for the oxidation using dimethyldioxirane as oxidant [20]. Since an arene oxide intermediate was postulated for the dioxirane oxidation, a similar mechanism is plausible here [11], e. g., for the oxidation of l,2,3-trimethoxy-5-methylbenzene (Scheme 3) or 2,6-di(f-butyl)-4-methyl-phenol. 

Another TM-catalyzed reaction that has been studied at the DFT and ab initio levels of theory is the epoxidation of ethylene with rhenium peroxo complexes. Table 17 shows calculated reaction energies at the MP2, B3LYP, and CCSD(T) levels of theory using basis set II (84). 

Li CZ, Chen L, Garland M (2008) Synthetic applications of synergism using catalytic binuclear elimination reactions. Further examples of rhodium-manganese and rhodium-rhenium-catalyzed hydroformylations. Adv Synth Catal 350 679-690... 

It was found during studies of ammonia synthesis on iron that the incorporation of a condenser downstream of the sample valve in the external circulation loop of the HPLP apparatus (Fig. 7), enabled the system to be run as a flow rather than a batch reactor. This is true for any reaction system where the reactants are more volatile than the products, since the condenser temperature can be adjusted to trap the products almost exclusively, allowing a nearly pure stream of reactants to impinge on the catalyst. In the case of ammonia synthesis, (where, next to the product, nitrogen at a partial pressure of 5 atm was the most condensable species) a slurry of isopentane (— 159.9 °C) was found to be the ideal condenser medium. During a study of rhenium-catalyzed ammonia synthesis the isopentane condenser was switched in periodically to reduce the ammonia partial pressure to below that at which it appeared to poison the catalyst. In this way, the rhenium was able to produce ammonia in excess of the amount usually leading to poisoning. 

In 2012, a rhenium-catalyzed synthesis of pyridines from y0-enamino ketones and alkynes was reported by Takai, Kuninobu, and coworkers [112]. In the presence of Re2(CO)io, with Af-acetyl / -enamino ketones and alkynes as the substrates, multisubstituted pyridines were produced in good yields and regioselectivity (Scheme 3.54). For the reaction mechanism, the reaction proceeded via insertion of alkynes into a carbon-carbon single bond of /3-enamino ketones, intramolecular nucleophilic cyclization, and elimination of acetic acid. And the presences of A-acetyl moieties are important for the reaction selectivity. 

Recently, Goopen has reported a new protocol that draws on easily available ruthenium chloride trihydrate (RUCI3 3H2O) as a catalyst precursor instead of the expensive [Ru(cod)(methallyl)2] [189]. In this new protocol, the catalyst is generated in situ, affording comparable yields. Furthermore, in 2007 Kuninobu and Takai reported the synthesis of ( )-enamide by rhenium-catalyzed hydroamination of unactivated terminal alkynes [190]. However, this method proceeds with less efficiency and lower reaction scope with respect to the mthenium one. 

Takai et al. reported rhenium-catalyzed formal [2- -1- -2- -1] cycloaddition of a-substituted- 3-ketoesters 121, electron-rich alkynes 122, and electron-deficient alkynes 124, leading to densely substituted benzenes 125 (Scheme 6.34) [47b,48], They proposed that this reaction proceed via retro-aldol reaction (C—C singlebond cleavage) to form 2-pyranones followed by aromatization with the electron-deficient alkyne (Scheme 6.34). The use of in situ-generated arynes 128 in place of electron-deficient alkynes 124 afforded naphthalene and anthracene derivatives 129 (Scheme 6.35) [47b]. 

The use of various heterocyclic additives in the MTO-catalyzed epoxidation has been demonstrated to be of great importance for substrate conversion, as well as for the product selectivity. With regard to selectivity, the role of the additive is obviously to protect the product epoxides from deleterious, acid-catalyzed (Brons-ted or Lewis acid) ring-opening reactions. This can be achieved by direct coordination of the heterocyclic additive to the rhenium metal, thereby significantly decreasing its Lewis acidity. In addition, the basic nature of the additives will increase the pH of the reaction media. 

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