Palladium catalysis chlorinations

Substitution of chlorine atoms in 2.4-dichloroquinazoline with alkyl groups is readily effected under the influence of palladium catalysis with trialkylaluminum acting as donor of the alkyl group. Selectivity for position 4 in 2,4-dichloroquinazoline can be achieved with trimethyl- and triisobutylaluminum under the influence of tetrakis(triphenylphosphane)palladium(0) as catalyst. In the presence of another molar equivalent or more of trialkylaluminum, products 14 which carry the same or a different alkyl group in the 2-position are formed. ... 

No exact mechanistic proposal was formulated for the conversion of the a-alkyl palladium intermediate to the final carbon-chlorine bond. Transmetallation from palladium to copper, high-oxidation-state palladium catalysis, transient oxidation, or even simple reductive elimination from palladium] 11) may all be involved. 

Amination under palladium-catalysis is possible using hydroxylamines as electrophiles.Bromination and chlorination can be performed with N-halosuccinimides or PhICb, while iodine can be used directly. In case of PhICl2, the catalytic cycle involves palladium(III) intermediates.A diastereoselective method for the iodination of aromatic compounds was used by the Yu group (Scheme 5-196). 

Catalysis. The most important industrial use of a palladium catalyst is the Wacker process. The overall reaction, shown in equations 7—9, involves oxidation of ethylene to acetaldehyde by Pd(II) followed by Cu(II)-catalyzed reoxidation of the Pd(0) by oxygen (204). Regeneration of the catalyst can be carried out in situ or in a separate reactor after removing acetaldehyde. The acetaldehyde must be distilled to remove chlorinated by-products. 

The C—I bond is very unstable and more reactive than C—Br, C—Cl and C—F bonds. Iodine is the most expensive of the common halogens and is much less frequently used in synthesis than bromine, chlorine or fluorine. Organometallic reactions proceed with iodinated aliphatic or aromatic compounds more easily than with the other halogens. Noble metal catalysis with palladium complexes is most effective with iodinated compounds. A useful synthetic procedure is the facile reduction of iodinated derivatives under mild conditions. Replacement of iodine by hydrogen at an sp carbon is an exothermic reaction with A// = -25 kJ mol . ... 

A proposed mechanism [9] for the hydrosilylation of olefins catalyzed by platinum(II) complexes (chloroplatinic acid is thought to be reduced to a plati-num(II) species in the early stages of the catalytic reaction) is similar to that for the rhodium(I) complex-catalyzed hydrogenation of olefins, which was advanced mostly by Wilkinson and his co-workers [10]. Besides the Speier s catalyst, it has been shown that tertiary phosphine complexes of nickel [11], palladium [12], platinum [13], and rhodium [14] are also effective as catalysts, and homogeneous catalysis by these Group VIII transition metal complexes is our present concern. In addition, as we will see later, hydrosilanes with chlorine, alkyl or aryl substituents on silicon show their characteristic reactivities in the metal complex-catalyzed hydrosilylation. Therefore, it seems appropriate to summarize here briefly recent advances in elucidation of the catalysis by metal complexes, including activation of silicon-hydrogen bonds. 

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