Cyclometallated palladium complexes

The reversible reaction of tri-n-butylstannylfuran with the cyclometallated palladium complex 24 yields the ti C) coordinated 2-furyl complex 25 (98JA11016). 

Many of the reactions outlined in Section 4 for Pd-C bonds occur in cyclometalated complexes. However, the existence of the Pd-C bond in a chelate ring imposes some kinetic stability on the bond. In general, mild acids and bases and oxygen are unreactive with cyclometalated palladium complexes. More vigorous reagents will lead to useful reactions, however. 

Ort/ o-cyclometallated palladium complexes discussed in Section 2.2.2 have found another very important application as chiral templates that promote Diels-Alder and other reactions. This chemistry has been developed by Leung and co-workers and has allowed the synthesis of an impressing number of mono- and diphosphines bearing P-stereogenic atoms with peculiar scaffolds... 

P-chiral dibenzophosphole oxide (52a) (Scheme 14) shows liquid crystalline behaviour [52], a property that is of interest in the area of electro-optical displays [53]. Chiral resolution of (52a) was achieved by column chromatographic separation of the diastereoisomers obtained following coordination of the o -benzophosphole (52b) to chiral cyclometallated palladium(II) complexes [52]. Notably, the presence of a stereogenic P-centre is sufficient to generate a chiral cholesteric phase. 

Cyclometallated iridium complexes, for OLEDs, 12, 145 Cyclometallated palladium(II) complexes from amines and pyridines, 8, 280 with C,C-chelating ligands, 8, 291 enantioselective synthesis, 8, 296 ferrocene-based palladacycles, 8, 292 four-membered palladacycles, 8, 297 imine- and oxime-based complexes, 8, 285 with N-N and N=N bonds, 8, 288 palladacycle catalysis, 8, 297... 

Other intermediates which can undergo similar cyclometalations include (2-methoxyphenyl)palladium [158, 159] and (l-naphthylmethyl)palladium complexes (Scheme 8.23) [160], Cyclometalation is usually promoted by high temperatures, and if these are necessary the yield of cross-coupling reactions with substrates prone to undergo cyclopalladation (for example 2-bromoanisole) can drop dramatically [43]. 

Many different palladium complexes have been used to form chelates via cyclometalation. Li2PdCLi is preferred over PdCb for solubility reasons. Also commonly used are (PhCN)2PdCl2, Pd(OAc)2, Pd(02CCp3)2, and Pd(acac)2. Where amines are the donor atom, dozens of different ligand types have reacted to form five-membered C,N-chelates. These include Al,Al-dimethylbenzylamines (23) and 3,2 -dimethylene-2-phenylpyridines (24). 

In some cases it is possible to control the site of palladation, choosing either an sp or sp C-H bond for insertion. When A -thiobenzoylpyrrolidine is treated with Pd in methanol, cyclometalation takes place at the aryl C-H, directed by one of the lone pairs on S (equation 73). When the same reaction is carried out in HMPA, an aUcyl C-H bond reacts. The reason for this change of reactivity with change of the solvent is not known. Certainly, different palladium complexes will be present in the two solvents. 

The formation of hydridopalladium complexes free of phosphine ligands (Figure 4) were observed when cyclometalated palladium chloride complexes were subjected to hydrogenolysis in DMF solutionsJ ... 

Solid-state thermolysis of 1 -alkyl-2,4-bipyridinium palladium complexes gave cyclometallated complexes [PdCl2(2,4 -R-bipy-H)] (R = H, Me, Bu, CsHu, CgHi, or C10H21) in quantitative yield. ... 

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. " ... 

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