Coupled catalytic systems

SCHEME 177. Coupled catalytic system for olefin dihydroxylation with H2O2 as terminal oxidant... 

Recently, 3,5-bis(trifluoromethyl)benzeneseleninic acid has been used in a tandem catalytic epoxidation." The concept of tandem catalysis has been applied to oxidation reactions by Backvall and co-workers for the direct dihydroxylation of olefins using a couple catalytic system and hydrogen peroxide as the terminal oxidant." In this context, the seleninic acid was used in combination with a trifluoromethyl oxaziridine catalyst (Scheme 17), using urea hydrogen peroxide as the terminal oxidant." This system showed... 

Also for the Sonogashira couplings, catalytic system 1 was used as ligand. The reactions proceeded well, with yields from 58-87%. The leaching of palladium did not exceed 0.2%. 

The coupled catalytic system of Scheme 8.7 was more recently immobilized in an ionic liquid, [bmimJPFfi [136]. After completion of the reaction, the product diol is extracted from the ionic liquid, and the osmium, NMO, and flavin stay in the ionic liquid. The immobilized catalytic system was reused 6 times without any loss of activity. In a subsequent study, ionic liquid [bmimjPFg was employed to immobilize a robust system where the flavin of the previous system had been replaced by VO(acac)2 or MeRe03 (MTO) [137]. A range ofalkenes were dfliydroxylated with this system, and it was demonstrated that for some of the alkenes the catalytic system can be recycled up to five times. 

The coupled catalytic system of Scheme 7.11 was recently immobilized in an ionic liquid [bmimJPFg [111]. After completion of the reaction the product diol is extracted from the ionic liquid and the osmium, NMO and flavin stays in the ionic liquid. The immobilized catalytic system was reused five times without any loss of activity. 

Scheme 5.10 Ruthenium-catalyzed aerobic oxidation of amines using a coupled catalytic system.

Several newer methods take advantage of the highly selective nature of organopaHadium reagents. A palladium acetate-triarylphosphine catalytic system has been employed to induce the coupling of bromobenzene with the desired acrylate ester (13). 

Cationic phosphine ligands containing guanidiniumphenyl moieties were originally developed in order to make use of their pronounced solubility in water [72, 73]. They were shown to form active catalytic systems in Pd-mediated C-C coupling reactions between aryl iodides and alkynes (Castro-Stephens-Sonogashira reaction) [72, 74] and Rh-catalyzed hydroformylation of olefins in aqueous two-phase systems [75]. 

The quantitative solution of the problem, i.e. simultaneous determination of both the sequence of surface chemical steps and the ratios of the rate constants of adsorption-desorption processes to the rate constants of surface reactions from experimental kinetic data, is extraordinarily difficult. The attempt made by Smith and Prater 82) in a study of cyclohexane-cyclohexene-benzene interconversion, using elegant mathematic procedures based on the previous theoretical treatment 28), has met with only partial success. Nevertheless, their work is an example of how a sophisticated approach to the quantitative solution of a coupled heterogeneous catalytic system should be employed if the system is studied as a whole. 

In this chapter we will discuss the results of the studies of the kinetics of some systems of consecutive, parallel or parallel-consecutive heterogeneous catalytic reactions performed in our laboratory. As the catalytic transformations of such types (and, in general, all the stoichiometrically not simple reactions) are frequently encountered in chemical practice, they were the subject of investigation from a variety of aspects. Many studies have not been aimed, however, at investigating the kinetics of these transformations at all, while a number of others present only the more or less accurately measured concentration-time or concentration-concentration curves, without any detailed analysis or quantitative kinetic interpretation. The major effort in the quantitative description of the kinetics of coupled catalytic reactions is associated with the pioneer work of Jungers and his school, based on their extensive experimental material 17-20, 87, 48, 59-61). At present, there are so many studies in the field of stoichiometrically not simple reactions that it is not possible, or even reasonable, to present their full account in this article. We will therefore mention only a limited number in order for the reader to obtain at least some brief information on the relevant literature. Some of these studies were already discussed in Section II from the point of view of the approach to kinetic analysis. Here we would like to present instead the types of reaction systems the kinetics of which were studied experimentally. 

A variety of such ternary catalytic systems has been developed for diastereoselective carbon-carbon bond formations (Table). A Cp-substituted vanadium catalyst is superior to the unsubstituted one,3 whereas a reduced species generated from VOCl3 and a co-reductant is an excellent catalyst for the reductive coupling of aromatic aldehydes.4 A trinuclear complex derived from Cp2TiCl2 and MgBr2 is similarly effective for /-selective pinacol coupling.5 The observed /-selectivity may be explained by minimization of steric effects through anti-orientation of the bulky substituents in the intermediate. 

Larhed et al. investigated enantioselective Heck reactions with 2,3-dihydrofuran as alkene [86]. In the coupling with phenyl triflate, conditions previously reported by Pfaltz [87] were attempted under microwave irradiation. Interestingly, the catalytic system Pd2(dba)3/(4S)-4-t-butyl-2-[2-(diphenylphosphanyl)phenyl]-4,5-dihydro-l,3-oxazole, identified by the Swiss team, was found suitable for high-temperature microwave-assisted enantioselective Heck reactions (Scheme 76). Using a proton sponge as a base and benzene as a solvent gave the best conversions (Scheme 76). At tempera-... 

A catalytic version of the coupling was also developed, by using 10 mol % of CuCl2 and 20 mol % of sparteine 1 (silver chloride was used as a stoichiometric oxidant to regenerate the copper (II) oxidant). This catalytic system was applied to the asymmetric cross-coupling leading to 101 in a 41% yield and 32% ee. 

Hydride species were also formed in the dehydrogenative coupling of hydrosilanes with DMF [45]. The catalytic system is applicable to tertiary silanes, which are known to be difficult to be converted into disiloxanes (Fig. 17). The catalytic reaction pathway involves the intermediacy of a hydrido(disilyl)iron complex... 

Kinetics are obtained directly for Hg(ir), Hg(I) and Tl(III). With Pd(II) a catalytic system was used " , depending on the continuous reoxidation of Pd(0) by a Cu(n)-02 couple. The inverse acidity dependence could result from a hydrolysis of the type... 

A tandem cyclisation/cross-coupling reaction between 6-halo-1-hexene 82 and Grignard reagents 83 is successfully catalysed by a NHC-Co catalytic system (Scheme 5.23) [23]. 

Another successful approach to catalyst immobilisation involves attachment of the carbene precursor to a peptide on solid support. Treatment with base generates the corresponding carbenes that undergo in situ complexation to Pd(ll) centres (Scheme 6.33). Again, the main drawback of this approach was the low reactivity of the catalytic system that only allowed the coupling of aryl iodides and bromides [116], The reasons for this outcome are in need of further studies. 

The use of imidazolium salts for in situ catalyst formation was shown to be optimal for the coupling of TMS-protected alkynes even with sterically demanding aryl bromides and avoids the formation of homocoupling-derived products. For this reaction, Nolan reported that the activation of chlorobenzene by this catalytic system was possible in moderate yield [125] (Scheme 6.41). 

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