High-oxidation-state palladium complex

High-oxidation-state palladium complexes are frequently involved in oxidative C-H functionalization processes [43]. Generally, a Pd(II) intermediate that is formed via CH activation or oxidative addition to C-X is oxidized further to the corresponding high-oxidation-state Pd intermediate (see Figure 4.11). For C-H to C-C or C-X bond transformations, mononuclear Pd(IV) and also dinuclear Pd(III)-Pd(III) complexes have been proposed to be involved for substrates based on the 2-phenyl pyridine framework [44]. The irreversible C-C or C-X... 

Reactions that effect addition of two heteroatoms across an alkene are very powerful methods for the generation of heterocycles, and have significant potential synthetic utility. Several important advances in this area that involve the use of palladium catalysts have recently been reported [57]. Interestingly, many of these may involve high oxidation state Pd complexes as intermediates. 

The current high level of interest in binuclear metal complexes arises from the expectation that the metal centers in these complexes will exhibit reactivity patterns that differ from the well-established modes of reactivity of mononuclear metal complexes. The diphosphine, bis(diphenylphosphino)methane (dpm), has proved to be a versatile ligand for linking two metals while allowing for considerable flexibility in the distance between the two metal ions involved (1). This chapter presents an overview of the reaction chemistry and structural parameters of some palladium complexes of dpm that display the unique properties found in some binuclear complexes. Palladium complexes of dpm are known for three different oxidation states. Palladium(O) is present in Pd2(dpm)3 (2). Although the structure of this molecule is unknown, it exhibits a single P-31 NMR reso-... 

The t/ configuration is represented by a group of very stable oxidation states. Of those that favour four-coordinate planar coordination, platinum(II) stands out because the level of reactivity places its complexes within the range of rates conveniently studied by conventional, classical techniques and because the stability of the oxidation state is high. Oxidation state (II) is even more stable for palladium but the 10 fold greater reactivity tends to restrict the scope of any study. Complexes of gold(III) are generally more labile than those of Pt but can conveniently be studied by classical techniques however, they can be reduced to Au by many of the soft nucleophiles. Closer examination has shown that, as a rule, the reduction follows, rather than accompanies, the act of substitution " and much more should be done in this area. There are cases when the reduction is direct, e.g. ... 

Non-metathetical reactions catalysed by ruthenium-carbene complexes are multifaceted and cover a broad range of transformations, thanks in part to the large number of oxidation states and coordination geometries available for the metal centre, in sharp contrast with other elements such as rhodium, palladium and platinum, which reluctantly form compounds with high oxidation states and have a strong preference for the square planar geometry. Several... 

It seems that palladium complexes with high oxidative states and basic ligands can be involved in oxidative addition as weU as in o-bond metathesis. Cheng has demonstrated that phosphine-free Pd complexes together with alkenyl or aryl iodides are very efficient catalysts for 1,2-diboration of allenes. This reaction is completely regioselective and highly... 

To circumvent some of the above-mentioned drawbacks of sulfur-based mercury chemodosimeters, a system based on the alkyne oxymercuration of 58 has been developed (Fig. 22) [146]. 58 shows high selectivity, a limit of detection of ca. 8 ppm, resistance against strong oxidants, and a positive reaction even in the presence of cysteine, which is known to form stable mercury complexes and is used for the extraction of mercury from tissue samples. Another metal that is well-known for its catalytic ability is palladium, catalyzing different reactions depending on its oxidation state. Since this metal is toxic, assessment of the maximum allowable concentration of Pd in consumer products such as pharmaceuticals requires highly sensitive and selective detection schemes. For this purpose, indicator 60 was conceived to undergo allylic oxidative insertion to the fluorescein... 

Electronic ligand effects are highly predictable in oxidative addition reactions a-donors strongly promote the formation of high-valence states and thus oxidative additions, e.g. alkylphosphines. Likewise, complexation of halides to palladium(O) increases the electron density and facilitates oxidative addition [11], Phosphites and carbon monoxide, on the other hand, reduce the electron density on the metal and thus the oxidative addition is slower or may not occur at all, because the equilibrium shifts from the high to the low oxidation state. In section 2.5 more details will be disclosed. 

---