Mixed ligand catalysts

On the basis of these data a series of mixed ligand catalysts has been prepared by this procedure. It is also apparent from this work that care must be taken to determine how best to treat the oxidized complexes to permit the incorporation of the added ligand. If this problem is solved with each of the oxidized species, a broad spectrum of catalytically active materials will become available. 

Chiral MOFs have to cope with two kinds of challenges when they are used as catalysts. First, in those cases where chirality is induced via sp carbon centers, the incorporation of such centers may confer a degree of flexibility to the framework that eventually impairs the stability after desolvation. Such problems can be circumvented, for instance, using mixed ligand systems where a rigid nonchiral backbone ensures the stability of the structure [82], MOFs with large and flexible chiral ligands... 

Subsequently, Lowenthal and co-workers,3 la,9 Evans et al.,31b and Muller et al.98 reported chiral bis(oxazoline) ligands 185, 186, and 83 as shown in Figure 5-12. The gem-dimethyl [(bis)oxazoline] 83-coordinated copper catalyst is the most widely used ligand. The catalyst is prepared in situ by mixing ligand 83 with an equal molar amount of CuOTf. Asymmetric cyclopropanation of isobutylene with ethyl diazoacetate (EDA) gives ethyl 2,3-dimethylcyclopro-pane carboxylate with >99% ee. 

The complexes [lr(COD)(NCMe)(PR3)]BF4 (PR3 = P Pr3, PMe3), which are closely related to Crabtree s catalyst, also contain mixed ligands, very basic phosphines, P Pr3 or PMe3, and a nitrogen donor ligand, acetonitrile. These mixed-ligand pair combinations have been shown to be very useful for spectroscopic observations and have provided detailed information on the mechanism of reactions performed by those complexes [9, 10]. 

These oxidized species, 2, 3, and 4, were used to prepare series of mixed ligand complexes and thus made available catalyst systems in which reactivity and selectivity can be carefully controlled. 

The reaction involves the catalyst-substrate complex (158). Molecular models show that in the mixed ligand complex (158), the N—O- group is in a position to attack the acetyl group of (157). The zinc complex (156) is also an excellent catalyst for the hydrolysis of p-nitrophenyl acetate in fact it is comparable in reactivity to hydroxide ion, although its p.Ka is only 6.5, (Table 29). 

Fig. 16 Structures of the redox catalyst tris(l,10-phenanthroline-5,6-dione) ruthenium(II) perchlorate 1 and the mixed ligand system consisting of l,10-phenanthroline-5,6-dione and one N,N, A-tris(aminoethyl)amine ligand 2...

The scheme reduces to its most simple form when carbon monoxide is the only ligand present in the system, because equilibria of mixed ligand/carbon monoxide complexes do not occur. The kinetics of the hydroformylation reaction using hydrido rhodium carbonyl as the catalyst was studied by Marko [20]. For 1-pen-tene the rate expression found is ... 

It is not exactly understood how the mixed ligand Rh/dppb/PPh3 catalyst system functions. Matsumoto proposed that the arm-on, arm-off equilibrium shown in Scheme 12 is operational. A species such as (5) would function much like a normal HRh(CO)(PPh3)2 catalyst, but the ability to reform the chelate to form a slightly more electron-rich complex (6) would tend to inhibit alkene isomerization and/or degradation reactions which require 16e unsaturated species. P NMR studies of Rh/chelating phosphine complexes indicate that a variety of species can form, the most dominant of which are... 

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