Palladium organometallic complexes

In this paragraph, the stoichiometric palladium organometallic complexes are discussed. As mentioned earlier, we will not give a comprehensive overview—over 64000 entities with a palladium-carbon bond are known— but we rather focus on the most common classes and complexes. For an overview describing some recent complexes important to catalysis, see ref Due to the facile 3-hydride elimination of palladium compounds with alkyl chains, they are mostly involved as intermediates in catalytic processes. 

Figure 3.21 Structure of the dimeric palladium(I) complex [(dppp)Pd]2(CF3 S03)2. (Reprinted with permission from Organometallics, 1992, 11, 23. Copyright (1992) American Chemical Society.)...

Organometallic complexes of copper, nickel, and palladium have been used in indole syntheses from arenes. Most of the reactions proceed under relatively mild conditions and in some cases give rise to formation of the less common 2-substituted compounds.68 Good yields of such 2-substituted derivatives are formed in reactions of o-iodoarylamines with cuprous acetylides in dimethylformamide (Scheme 41 ).69 The efficiency of this type of... 

Han, L.-B., Choi, N., and Tanaka, M., Oxidaative addition of HP(0)Ph2 to platinum(O) and palladium(O) complexes and palladium-catalyzed regio- and stereoselective hydrophosphinylation of alkynes, Organometallics, 15, 3259,... 

A review on the organometallic complexes of palladium(O) has been published recently in a companion volume13 and here we restrict ourselves to considering areas of particular interest to coordination chemists. 

The inclusion of the nitrogen donor atom in an aromatic heterocycle allows the possibility of -bonding with the metal centre, giving these ligands some similarities to tertiary phosphines. Thus pyridyl and bipyridyl ligands stabilize organometallic complexes of palladium (see companion volume).2 ... 

Organom etallic Compounds. Organometallic complexes of platinum are usually more stable than palladium complexes. Carbon monoxide complexes of platinum are formed more readily than with palladium. Mononuclear and polynuclear complexes in oxidation states 0 to +2 exist such as... 

Electron-rich, unsaturated hydrocarbons, which are normally resistant to nucleophilic attack, become generally reactive towards nucleophiles upon complexation to an electrophilic transition metal such as palladium(II), platinum(II) or iron(II). Complexation also directs the regio- and stereo-chemistry of the nucleophilic attack, the result of which is a new organometallic complex, which can often be used to promote additional functionalization of the original substrate. Synthetically useful examples of such processes are presented in the following sections. 

Three methods are commonly employed for the in situ preparation of organopalladium derivatives (i) direct metallation of an arene or heterocyclic compound with a palladium(II) salt (ii) exchange of the organic group from a main group organometallic to a palladium(II) compound and (iii) oxidative addition of an organic halide, triflate or aryldiazonium salt to palladium(O) or a palladium(O) complex. 

For a more complete description of the principles of transition metal catalysis, a number of the excellent reviews published recently should be consulted. These include, in addition to those already cited, several reviews of rhodium (89) and palladium (90-93) chemistry, addition (94, 95) and insertion (96) reactions, homogeneous catalytic hydrogenation (97), hydride complex chemistry (98), nitrogen fixation (99), and organometallic complexes as potential synthetic reagents (100). 

Bianchini C, Meli A, Oberhauser W, Zuideveld MA, Freixa Z, Kamer PCJ, Spek AL, Gusev OV, Kalsin AM (2003) Methoxycarbonylation of ethene by palladium(II) complexes with l,l -bis(diphenylphosphino)ferrocene (dppf) and l,l -bis(diphenylphosphino)octamethyl-ferrocene (dppomf). Organometallics 22(12) 2409-2421. doi 10.1021/om021049b... 

Aside from full catalytic cycles, or reaction steps within them, as discussed above, structural issues about organometallic complexes with catalytic properties have also been analyzed with QM/MM methods. For instance, Helmchen and coworkers carried out a conformational analysis of two (w-l,3-dimethylallyl)(phosphinooxazoline)Pd complexes [128], and Magistrato et al investigated the role of n-n stacking interactions in square planar palladium complexes [129]. 

Ruiz J, Lorenzo J, Sanglas L, Cutillas N, Vicente C, Villa MD, Aviles FX, Lopez G, Moreno V, Perez J, Bautista D (2006) Palladium(II) and platinum(II) organometallic complexes with the model nucleobase anions of thymine, uracil, and cytosine antitumor activity and interactions with DNA of the platinum compounds. Inorg Chem 45 6347-6360... 

Ruiz J, Villa MD, Cutillas N, Lopez G, de Haro C, Bautista D, Moreno V, Valencia L (2008) Palladium(II) and Platinum(II) Organometallic Complexes with 4, 7-dihydro-5-methyl-7-oxo[l, 2, 4]triazolo[l, 5-a]pyrimidine. Antitumor activity of the platinum compounds. Inorg Chem 47 4490 1505... 

Shultz LH, Brookhart M, Measurement of the Barrier to -Hydride Elimination in a -Agostic Palladium-Ethyl Complex A Model for the Energetics of Chain-Walking in (-Diimine)PdR+ Olefin Polymerization Catalysts, Organometallics, 20, 3975-3982 (2001)... 

In reactions requiring palladium 0), formation of the active complex may be achieved more conveniently by reduction of a palladium(ll) complex, for example, Pd 0Ac)2- Any phdsphine may then be used in the reaction, without the need to synthesize and isolate the corresponding palladium 0)-phosphine complex. Only 2-3 equivalents of phosphine may be needed, making the palladium(O) complex coordinatively unsaturated and therefore very reactive. The reduction of palladium li)to palladium(o) can be achieved with amines, phosphines, alkenes, and organometailics such as DIBAL-H, butyl lithium, or trialkyl aluminium. The mechanisms are worth giving as they illustrate the basic steps of organometallic chemistry. 

Waltmann AW, Grubbs RH (2004) A new class of chelating A-heterocyclic carbene hgands and their complexes with palladium. Organometallics 23 3105-3107... 

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