Cyclopalladation, palladacycles from

Cyclopalladated complexes, which are most conunonly prepared from tetrachloropaUa-date(II) and appropriate amines or imines, have drawn the attention of inorganic, coordination, and organometallic chemists since the first report in 1965 (for imines) and in 1968 (for amines) by Cope and co-workers. ° These palladacycle species can easily be transformed into chiral form by employing corresponding chiral amines or imines. Two representative examples of the chiral cyclopalladated species are illustrated in Figure 1. This section will describe three examples that represent applications of these chiral orthometallated palladium complexes in organic synthesis. 

In this section, only examples of Mizoroki-Heck reactions where a proper addition of the cr -aryl- or a -alkeny Ipalladium(II) complex to a double bond of an alkene or alkyne occurs are considered. As a consequence, an often-met deviation from the classic Mizoroki-Heck mechanism, the so-called cyclopalladation, will not be treated in further detail [12, 18]. However, as it is of some importance, especially in heterocycle formation and mainly because it will be encountered later during polycyclization cases, it shall be mentioned briefly below. Palladacycles are assumed to be intermediates in intramolecular Mizoroki-Heck reactions when j3-elimination of the formed intermediate cannot occur. These are frequently postulated as intermediates during intramolecular aryl-aryl Mizoroki-Heck reactions under dehydrohalogenation (Scheme 6.1). The reactivity of these palladacycles is strongly correlated to their size. Six-membered and larger palladacycles quickly undergo reductive elimination, whereas the five-membered species can, for example, lead to Mizoroki-Heck-type domino or cascade processes [18,19]. 

Dinuclear cyclometallated palladium(ii) complexes 81 have been obtained from Schiff base ligands. Cyclopalladated complexes of oximes 82 and 83 and their ligand-exchange reactions have been studied. Ketoximes form palladacycle dimers such as 84. " ... 

A-Heterocycles were synthesized via insertion of 1,1-dimethylallene into Pd-G bonds of cyclopalladated a-tetralone ketimines. Insertion reactions of alkynes into the Pd-G bond of cyclopalladated ferrocenylimines have also been described.Biomimetic hydrolysis of benzoates has been carried out with a water-soluble cyclopalladated aryl oxime, which was proposed as a potential green catalyst. Ghiral cyclopalladated liquid crystals 88 were obtained from amino acids and could possibly serve as enantioselective catalysts. " Palladacycle 89 has been derived from an anthracene-based Schiff base. 

Schiff bases derived from aminomethylferrocene were used to prepare a series of palladacycles 123 and 124. Four diastereoisomers of bis(cyclopalladated) [ PdCl[(77 -C5H5)Fe(77 -G5H3)GMe=N](PPh3) 2] in solution have been revealed by and P NMR spectra the meso form has been examined by X-ray crystallography. Gyclic... 

Ferrocenylimines derived from benzoylferrocene were synthesized and cyclopalladated by Bosque et The cr(GH) bonds in ferrocenyl- and bis(ferrocenyl)imines were activated in cyclopalladation reactions. Depending on the imine, five- and six-membered palladacycles were obtained. Bosque and Lopez also studied the cyclopalladation of mono- and bidentate primary ferrocenylimines. Mak / /. studied the cyclopalladation of ferrocenylimines in which the nitrogen of the imine is also substituted with a cyclic ether. Enantiopure bis(//-acetato)-bridged dimers were obtained. The insertion of diphenylacetylene was also investigated. 

Insertion of other unsaturated molecules such as GO or isonitriles in this type of palladacycle affords new types of <7,77 -enylpalladium complexes where the size of the cycle is increased in one carbon atom (Scheme 79). Cyclopalladated complexes derived from ferrocene also undergo double insertion of alkynes to give Equation (56) shows an example. It has been found for these systems that asymmetrically substituted internal alkynes insert in a head-to-tail manner. ... 

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