Catalyst combined ligand precursors

In 2007, Castanet reported an efficient carbonylative Suzuki reaction of pyridyl halides with different boronic acids using 50 bar of carbon monoxide. The catalyst used is a combination of Pd(OAc)2 and imidazolium salt precursors NHC-Cl. In all cases, carbonylated species 72 were obtained as the major products (Scheme 5.52). The best results were found with ligand precursor IMes-Cl, being also active in... 

The term catalyst is widely used. It is often applied to molecules that are not the catalyst, but a precursor for the catalyst. These should be termed pre-catalysts . This is particularly the case is industrial processes. A combination of metal salts, reagents, ligands and supports are combined in a reactor. Somehow, they combine in situ to generate the catalyst. Due to this combination process, some reactions may have an induction period. 

In a more recent publication the same group mentions that Ag(I) salts in combination with chiral phosphine ligands can catalyze the 1,3-dipolar cycloaddition involving the azomethine precursor 64b and methyl vinyl ketone (Scheme 6.43) [87]. The reaction, which presumably also required a stoichiometric amount of the catalyst, proceeds to give 65b in a good yield with 70% ee. 

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]. 

The MOP series of ligands59 (see Section 9.5.4.2) in conjunction with standard palladium precursors has been reported to catalyze the addition of HBcat to 1,3-enynes. With 1 mol.% catalyst produced by combination of Pd2(dba)3 and the monodentate ligand (Y)-MeO-MOP (22), axially chiral allenyl-boranes are formed (Equation (3)). Subsequent oxidation affords the corresponding alcohols with moderate ee values.60... 

Using the results of an earlier study concerning enantioselective copper-catalyzed intramolecular C—H insertion of metal carbenoids,109 an interesting system for optimizing the proper combination of ligand, transition metal, and solvent for the reaction of the diazo compound (75) was devised (see Scheme 19).110 The reaction parameters were varied systematically on a standard 96-well microtiter/filtration plate. A total of five different ligands, seven metal precursors, and four solvents were tested in an iterative optimization mode. Standard HPLC was used to monitor stereoselectivity following DDQ-induced oxidation. This type of catalyst search led to the... 

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