Palladium complexes cyclometallation

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

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. 

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... 

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. ... 

Diphenylimidazole with palladium acetate forms the cyclometallated complex 80 (X = OAc) (97AOC491). The acetate group is replaced by chloride or bromide when 80 (X = OAc) reacts with sodium chloride or lithium bromide, respectively, to give 80 (X = C1, Br). Bromide with diethyl sulfide forms the mononuclear complex 81. Similar reactions are known for 1 -acetyl-2-phenylimidazole (96JOM(522)97). 1,5-Bis(A -methylimidazol-2-yl)pen-tane with palladium(II) acetate gives the cyclometallated complex 82 (OOJOM (607)194). 

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. 

A special type of reaction is observed with the platinum(IV) complex [PtI(Me)3] which cleaves the Af,N,Af, A -tetraphenyltetraaminoethylene under reduction to form the dimeric cyclometallated mono(NHC) complex of platinum(II) iodide [Eq. (31)]. Cyclometallation with the same ligand is also observed for ruthe-nium. Additional cyclometallations with various substituents of NHCs have been reported for ruthenium(II), rhodium(III), iridium(I), palladium(II), " and platinum(II). In the case of iridium, alkyl groups can be activated twice. In rare cases like for nickel(II) /x-bridging NHCs have been obtained. ... 

The choice of an ionic liquid was shown to be critical in experiments with [NBuJBr (TBAB, m.p. 110°C) as a catalyst carrier to isolate a cyclometallated complex homogeneous catalyst, tra .s-di(ri-acetato)-bis[o-(di-o-tolylphosphino) benzyl] dipalladium (II) (Scheme 26), which was used for the Heck reaction of styrene with aryl bromides and electron-deficient aryl chlorides. The [NBu4]Br displayed excellent stability for the reaction. The recycling of 1 mol% of palladium in [NBu4]Br after the reaction of bromobenzene with styrene was achieved by distillation of the reactants and products from the solvent and catalyst in vacuo. Sodium bromide, a stoichiometric salt byproduct, was left in the solvent-catalyst system. High catalytic activity was maintained even after the formation of visible palladium black after a fourth run and after the catalyst phase had turned more viscous after the sixth run. The decomposition of the catalyst and the formation of palladium... 

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... 

Complex compounds with halide bridges are prepared by immediate interaction of unsaturated hydrocarbons with metal salts (Sec. 2.2.4.1). Their examples are classic 7i-complexes of the type 123 which are characteristic for fi 8-metals [75]. Such complexes are also formed by the method of ligand exchange. The bridges of this type are widespread in products of cyclometallation reactions with the use of metal halides (Sec. 2.2.5.1) 371-374, 381, 382 [41,46,48]. An example of such a synthesis is the reaction of arylhydrazones of 2-oxopropionic aldehyde and benzoylformalde-hyde, as well as butadiene-2,3-dione 635, with palladium dichloride, leading (3.71) to dipalladium complexes 636 [88] ... 

Amongst some specific reactions used in regioselective syntheses, we note the cyclometallation processes [Sec. 2.2.5.1, reaction (2.8) Sec. 3.1.1.2, reactions (3.40)-(3.44) Sec. 3.3.2.3, reactions (3.226)-(3.228)]. In this respect, we note that C,N-donor ligands form, depending on the nature of Lewis acids, two types of complexes. In the case of immediate interaction (4.35) of azomethines 859 with titanium or tin tetrachlorides (MC14), the molecular complexes with M—N bond 860 [101] were obtained (route A), while the cyclometallation reaction with the use of palladium diacetate leads to binuclear chelates 861, in which the Pd — N, C metal-cycles are formed (4.35) [11,114-116] ... 

The tetrahedral [160-163] and cis-planar [100,134,164-167] structures are characteristic for chelates of type 874 with coordination units NiN4 and MN2S2, respectively, as well as chelates 868 discussed above. Original polyhedral forms were discovered by x-ray diffraction for nickel and palladium ICC of the discussed type 874. It is accepted that, in case of a nickel complex, the compound with a carbon-carbon bond 875 is formed [165,166] formation of palladium chelates is accompanied by the cyclometallation reaction leading to tetranuclear clusters 876, where the tridentate ligand behaves as C,N,S-donor [168]. 

C-H transformation is achieved by cyclometallation by use of a unique catalytic system which involves the in-situ formation of a palladacycle [1]. Our work in this field takes advantage of the stability toward /3-hydrogen elimination of as,exo-aryl-norbomylpalladium complexes formed by a sequence of oxidative addition of an aryl halide to palladium(O) and stereoselective insertion of norbornene into the... 

Because of the creative minds contributing to the field, the tools of C-H bond transformation available to synthetic chemists are actively expanding [1], Among these, coordination-directed C-H bond-activation has long preserved its appeal, because it enables selective functionalization of a particular C-H bond in the presence of other functional groups. This can be achieved by using a heteroatom (FG = functional group shown in Scheme 1) in the substrate structure to direct the metal complex to the proximity of the specific C-H bond. Even unactivated sp3-centered C-H bonds tend to react in a cyclometalation step with palladium, platinum [2], and ruthenium catalysts [3]. 

---