Catalyst precursor complex

To examine the effect on the catalysis of the anion of the metal complex, Brandts [96] immobilized the two different catalyst precursor complexes [Rh(COD)2]BE4 and [Rh(COD)Cl]2 on phosphotungstic acid-modified alumina to form 7-AI2O3/... 

Finally, a third means of ligand formation from an imidazolium cation, described by Dupont and co-workers, should be mentioned here [34]. They investigated the hydrodimerization/telomerization of 1,3-butadiene with palladium(II) compounds in [BMIM][BF4] and described the activation of the catalyst precursor complex [BMIM]2[PdCl4] by a palladium(lV) compound formed by oxidative addition of the imidazolium nitrogen atom and the alkyl group with cleavage of the C-N bond of the [BMIM] ion, resulting in bis(methyHmidazole) dichloropalladate (Scheme 5.2-5). However, this reaction was only observed in the presence of water. 

The catalyst precursor complex [ Rh(COD) Diop] BF4 has been used for the screening of five substrates containing prochiral C=C double bond (COD = 1,5-cyclooctadiene) [266]. These were methylacetamidoacrylate (SI), Z-a-methylacetamidocinnamate (S2), dimethylitaconate (S3), methone (S4) and rac-a-pinene (S5) (see Figure 4.56). Activated C=C bonds such as those in the two acetamido derivatives were more reactive. The most reactive molecule is the less sterically hindered substrate methylacetamidoacrylate. Reaction was less pronounced for unsubstituted and sterically hindered substrates such as methone. The reduction of C=0 bond in a-pinene is more difficult. These results are in agreement with the general trends reported for asymmetric hydrogenations. 

The use of organic supports (macroreticular/macro-porous polymers) as carriers for an organometallic complex, which by itself may serve as the catalyst precursor complex, has been widely studied. The polymers must be macroporous, providing a highly accessible internal surface area. 

Fig. 2. Structural diagram for the model Wilkinson s catalyst precursor complex Rh(PH3)3Cl...

The chapter mainly concerns homogeneous catalytic systems, but supported catalysts are also included in view of their advantageous separation and recycling. In fact, the immobilization of a catalyst or a catalyst precursor complex on a support is a common and suitable procedure that combines the advantages of homogeneous and heterogeneous catalyses. 

SCHEME 12.6 (See color insert.) Dissociation (A to B ) of labile N-atom of 0,N-ligand along with the optimized geometries for the lowest energy catalyst precursor complexes. The hydrogen atoms on the ligands are omitted for clarity and the unit of the indicated bond... 

The red tetrathiomolybdate ion appears to be a principal participant in the biological Cu—Mo antagonism and is reactive toward other transition-metal ions to produce a wide variety of heteronuclear transition-metal sulfide complexes and clusters (13,14). For example, tetrathiomolybdate serves as a bidentate ligand for Co, forming Co(MoSTetrathiomolybdates and their mixed metal complexes are of interest as catalyst precursors for the hydrotreating of petroleum (qv) (15) and the hydroHquefaction of coal (see Coal conversion processes) (16). The intermediate forms MoOS Mo02S 2> MoO S have also been prepared (17). 

Ionic liquids formed by treatment of a halide salt with a Lewis acid (such as chloro-aluminate or chlorostannate melts) generally act both as solvent and as co-catalyst in transition metal catalysis. The reason for this is that the Lewis acidity or basicity, which is always present (at least latently), results in strong interactions with the catalyst complex. In many cases, the Lewis acidity of an ionic liquid is used to convert the neutral catalyst precursor into the corresponding cationic active form. The activation of Cp2TiCl2 [26] and (ligand)2NiCl2 [27] in acidic chloroaluminate melts and the activation of (PR3)2PtCl2 in chlorostannate melts [28] are examples of this land of activation (Eqs. 5.2-1, 5.2-2, and 5.2-3). 

Recently, the possibility that complexes of unsaturated "C ligands other than alkylidenes might also serve as catalyst precursors in olefin metathesis has... 

The role of complexes 23-28 as catalyst precursors in the ring closing metathesis reactions was investigated. Three different diene substrates diethyldiallyl-malonate (29), diallyltosylamine (30). and dielhyldi(2-methylallyl)malonate (31) were added to the NMR tubes containing a solution of 5 mol% of catalyst precursor in an appropriate deuterated solvent. The NMR tubes were then kept at the temperatures reported in Table X. Product formation and diene disappearance were monitored by integrating the allylic methylene peaks in the H NMR spectra and the results are presented in Table X and the catalytic transformations are depicted in Scheme 3. 

Togni s [38] approach was therefore to test the ability of sparteine to act as an ancillary ligand in Pd(II)-allyl complexes—susceptible to nucleophilic attack by stabihzed anions such as Na[CH(COOMe)2]—which could be employed as catalyst precursors. In addition he speculated that the rather rigid and bulky sparteine would be able to induce significant differentiation between the two diastereotopic sites of 1,3-disubstituted allyl hgand, thus leading to enantioselection upon nucleophilic attack. 

The solids were used as catalysts in the benchmark cyclopropanation reaction between styrene and ethyl diazoacetate (Scheme 7). As far as the nature of the clay is concerned, laponite was foimd to be the best support for the catalytic complexes. The best enantioselectivity results (Table 7) were obtained with ligand 6b (69% ee in trans cyclopropanes and 64% ee in cis cyclopropanes) but the recovered solid showed a lower activity and enantioselectivity, which was attributed to partial loss of the chiral ligand from the support. In general, the use of the three chiral ligands led to enantioselectivity results that were intermediate between those obtained in homogeneous phase with CuCl2 and Cu(OTf)2 as catalyst precursors. This seemed to indicate that the sohd behaved as a counterion with an intermediate coordinating abihty to the copper centers. 

Some chiral mono-, acyl- and di-thioureas have been used as ligand for the Rh-catalysed asymmetric hydroformylation of styrene. Although thiourea ligands form inactive systems with [Rh(COD)Cl]2 as the catalyst precursor, in standard conditions (40 °C, 40 bar CO -l- H2 1/1), the cationic Rh complex [Rh(COD)2]Bp4 combined with monothioureas as the ligand showed moderate to good activity (Scheme 29) [114]. 

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