Oxidation reactions palladium chemistry

There is certain similarity in the order of reactivities between SnAt displacement reactions and oxidative additions in palladium chemistry. Therefore, the ease with which the oxidative addition occurs for these heteroaryl chlorides has a comparable trend. Even a- and y-chloroheterocycles are sufficiently activated for Pd-catalyzed reactions, whereas chlorobenzene requires sterically hindered, electron-rich phosphine ligands. 

Pyridine is a jt-electron-deficient heterocycle. Due to the electronegativity of the nitrogen atom, the a and y positions bear a partial positive charge, making the C(2), C(4), and C(6) positions prone to nucleophilic attacks. A similar trend occurs in the context of palladium chemistry. The a and y positions of halopyridines are more susceptible to the oxidative addition to Pd(0) relative to simple carbocyclic aryl halides. Even a- and y-chloropyridines are viable electrophilic substrates for Pd-catalyzed reactions under standard conditions. 

Thiazole is a jt-electron-excessive heterocycle. The electronegativity of the N-atom at the 3-position makes C(2) partially electropositive and therefore susceptible to nucleophilic attack. In contrast, electrophilic substitution of thiazoles preferentially takes place at the electron-rich C(5) position. More relevant to palladium chemistry, 2-halothiazoles and 2-halobenzothiazoles are prone to undergo oxidative addition to Pd(0) and the resulting o-heteroaryl palladium complexes participate in various coupling reactions. Even 2-chlorothiazole and 2-chlorobenzothiazole are viable substrates for Pd-catalyzed reactions. 

To summarize, both chloropyrazines and chloroquinoxalines are sufficiently activated to serve as viable substrates for palladium chemistry under standard conditions. In contrast to chlorobenzene, the inductive effect of the two nitrogen atoms polarizes the C—N bonds. Therefore, oxidative additions of both chloropyrazines and chloroquinoxalines to Pd(0) occur readily. One exception is 2-chloropyrazine A-oxide, which does not behave as a simple chloropyrazine. All Pd-catalyzed reactions with 2-chloropyrazine A-oxide failed, presumably because the nitrogen atom no longer possesses the electronegativity required for activation. 

This trend is also observed in palladium chemistry where the general order for oxidative addition often correlates with that of nucleophilic substitution. Not only are 2-, 4- and 6-chloropyrimidines viable substrates for Pd-catalyzed reactions, but 4- and 6-chloropyrimidines react more readily than 2-chloropyrimidines. 

In this chapter, we analyze the chemistry of dioxygen and benzoquinone in the context of palladium-catalyzed oxidation reactions. After a brief histor-... 

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 speed of the intramolecular p-hydride elimination means that the original substrate for the oxidative addition reaction must be chosen with care—the presence of hydrogen at an sp3 carbon in die p position must be avoided. Thus, substrates for oxidative addition reactions in palladium chemistry are frequently vinylic, allylic, or aromatic and never ethyl or n-propyl. 

Palladium hydride complexes are of considerable interest in the catalytic chemistry of palladium because of their postulated occurrence as intermediates in a number of reactions such as hydrogenations, isomerizations, and oxidation reactions. In contrast to Pt(II), which forms stable hydrides, most Pd(II) hydrides are unstable. Although there are earlier reports of unstable Pd(II) hydride complexes being formed, the first stable hydride was prepared by Brooks and Clocking by the following reaction (20, 21) ... 

Palladium chemistry dominates this area and the main problems are related to the way of reoxidizing Pd° efficiently. In general the reaction could be made catalytic in palladium by the use of an additional oxidant capable of reoxidizing the Pd to Pd . Typically, stoichiometric copper chloride, or catalytic amounts of copper chloride in the presence of air, have been used [28]. Other catalyst systems which have been described for bisalkoxycarbonylation of olefins to succinate derivatives are PdCl2 and butyl nitrite [29], Pd(OAc>2, O2 and benzoqui-none [30], and Pd(acac)2 and di-t-butyl peroxide [31]. So far, low TONs have delayed industrial applications. Because the reoxidation process is generating water, which causes side reactions, it is also necessary to add a water scavenger such as triethyl orthoformate in order to obtain good conversions and selectivities. 

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

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