Direct catalysis palladium catalysts

Direct nucleophilic displacement of halide and sulfonate groups from aromatic rings is difficult, although the reaction can be useful in specific cases. These reactions can occur by either addition-elimination (Section 11.2.2) or elimination-addition (Section 11.2.3). Recently, there has been rapid development of metal ion catalysis, and old methods involving copper salts have been greatly improved. Palladium catalysts for nucleophilic substitutions have been developed and have led to better procedures. These reactions are discussed in Section 11.3. 

A -Arylation of pyrroles can be achieved by conversion of 1-lithiopyrroles into the corresponding zinc compounds and then reaction with aryl bromides using palladium(O) catalysis or by direct reaction of the pyrrole with an aryl halide in the presence of base and the palladium catalyst. ... 

As carboxylic acid additives increased the efficiency of palladium catalysts in direct arylations through a cooperative deprotonation/metallation mechanism (see Chapter 11) [45], their application to ruthenium catalysis was tested. Thus, it was found that a ruthenium complex modified with carboxylic acid MesC02H (96) displayed a broad scope and allowed for the efficient directed arylation of triazoles, pyridines, pyrazoles or oxazolines [44, 46). With respect to the electrophile, aryl bromides, chlorides and tosylates, including ortho-substituted derivatives, were found to be viable substrates. It should be noted here that these direct arylations could be performed at a lower reaction temperatures of 80 °C (Scheme 9.34). 

Cobo, S., Heidkamp, J., Jacques, P. A., Fize, J., Fourmond, V., Guetaz, L., et al. (2012). A Janus cobalt-based catalytic material for electro-splitting of v/ater. Nature Materials, 11, 802—807. Consul, J. M. D., Peralta, C. A., Benvenutti, E. V., Ruiz, J. A. C., Pastore, H. O., Baibich, I. M. (2006). Direct decomposition of nitric oxide on alumina-modified amorphous and mesoporous silica-supported palladium catalysts. Journal of Molecular Catalysis A Chemical, 246, 33—38. 

Metal complex chemistry, homogeneous catalysis and phosphane chemistry have always been strongly connected, since phosphanes constitute one of the most important families of ligands. The catalytic addition of P(III)-H or P(IV)-H to unsaturated compounds (alkene, alkyne) offers an access to new phosphines with a good control of the regio- and stereoselectivity [98]. Hydrophosphination of terminal nonfunctional alkynes has already been reported with lanthanides [99, 100], or palladium and nickel catalysts [101]. Ruthenium catalysts have made possible the hydrophosphination of functional alkynes, thereby opening the way to the direct synthesis of bidentate ligands (Scheme 8.35) [102]. 

The copper and palladium transition metal catalysts noted in Table 18 proved to be superior to nickel, ruthenium and rhodium catalysts. The nature of the reacting species has not been unequivocally defined, but the following experimental observations may provide some insight (i) tetrahydrofuran solvent is essential for the palladium-mediated reactions, since complex reaction mixtures (presumably containing carbinols) were observed when the reactions were performed in either benzene or methylene chloride (ii) the reaction is truly catalytic with respect to palladium (2 mmol alkylaluminum, 0.05 mmol of Pd(PPh3)4), whereas the copper catdyst is stoichiometric and (iii) in the case where a direct comparison may be made (entries 1-8, Table 18), the copper-based system is superior to palladium catalysis with regard to overall yield. 

The results in the diazomethane reactions involving zinc(II) chloride catalysis have been explained by invoking a carbenoid intermediate. The properties of such a species will, of course, be sensitive to the nature of the metal and this might explain the different regioselectivity observed when diphenyldiazomethane is decomposed with rhodium and palladium salts in the presence of 5-methylenebicyclo[2.2.1]hept-2-ene (9). With rhodium(II) acetate as catalyst the exocyclic double bond is attacked exclusively, whereas palladium(II) chloride catalysis directs cyclopropanation to the endocyclic double bond. ... 

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