Palladium-dioxygen

An interesting stoichiometric oxidative cleavage reaction of an oxime with a palladium dioxygen complex has been observed [160]. The palladium dioxygen complex, [Pd(PPh3)2(02)] has been shown to rapidly deoximate a variety of ketoximes in benzene at 25 °C. The yield of ketone formed was 98%. A l -dipolar cycloaddition of the dioxygen complex to the ketoxime was proposed, equation (119). 

The oxidation of ethylene to acetaldehyde by dioxygen catalyzed by palladium and cupric salts found important technological application. The systematic study of this process was started by Smidt [245] and Moiseev [246]. The process includes the following stoichiometric stages [247,248] ... 

The same group reported a palladium-mediated oxidation of methane to a methanol derivative employing a CuCl2 and Pd/C-based catalyst system and dioxygen in a trifluoroacetic acid/water mixture.18 A system was also described, which mediated the oxidation of ethane (Equation (10)). 

Reaction between carbon monoxide and dioxygen. The steady-state formation of C02 was measured on palladium particles vapor... 

Again we see that an alkene isomerisation reaction has taken place. Another important, useful reagent applied in this field is also pictured in Figure 15.7, viz. the use of benzoquinone as the re-oxidant for palladium. Quinone takes the role of dioxygen as oxidising agent. It is very efficient and both quinone and hydroquinone are inert towards many substrates. Furthermore, no water is formed, as is the case when dioxygen is used. 

The transformation of alcohols to the corresponding carbonyl compounds or carboxylic acids is one of the few examples in which a heterogeneous (solid) catalyst is used in a selective, liquid phase oxidation (7,2). The process, which is usually carried out in an aqueous slurry, with supported platinum or palladium catalysts and with dioxygen as oxidant, has limited industrial application due to deactivation problems. 

Keywords Palladium Oxidation Dioxygen Benzoquinone Catalysis... 

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

Historical Perspective on the Use of Benzoquinone and Dioxygen in Palladium-Catalyzed Oxidation Reactions... 

Chart 2 Catalytic systems developed for direct dioxygen-coupled palladium-catalyzed alcohol oxidation... 

Fundamental Studies of Palladium(O) Oxidation by Benzoquinone and Dioxygen... 

A study of Pd(PPhs)4,4, reported recently by Roth and coworkers, reveals yet another mechanistic pathway for Pd° oxygenation [122]. Complete dissociation of one PPha ligand from Pd occurs in solution to produce a three-coordinate palladium(O) species, 26. Kinetic studies reveal that 26 reacts with dioxygen via parallel associative and dissociative pathways (Scheme 5). The latter dissociative pathway results in the formation of the two-coordinate complex 27, which undergoes very rapid reaction with dioxygen in a manner directly analogous to that of the well-defined two-coordinate NHC complexes 23 and 24. 

Relationship Between Reactions of Dioxygen and Alkenes (Including Benzoquinone) with Palladium(O)... 

The results outlined above highlight numerous similarities between dioxygen-and BQ-mediated oxidation of palladium(O). The key steps in the respective pathways appear virtually identical (Scheme 10). Recent studies of the reactivity of dioxygen and electron-deficient alkenes with Pd°-alkene complexes reinforce these similarities. 

Further studies revealed that electron-deficient alkenes are capable of displacing O2 from a peroxopalladium(II) complex. The reaction of nitrostyrene derivatives with (bc)Pd(02) results in quantitative displacement of dioxygen and formation of the (bc)Pd(ns ) complex (i.e., the reverse reaction in Eq. 21) [138]. Moreover, preliminary results reveal that dioxygen and BQ undergo reversible exchange at a bathocuproine-coordinated Pd center (Eq. 22) (Popp BV, Stahl SS, unpublished results). This observation is the most direct experimental result to date that establishes the similar reactivity of dioxygen and BQ with palladium. 

Palladium-catalyzed, Wacker-type oxidative cycHzation of alkenes represents an attractive strategy for the synthesis of heterocycles [139]. Early examples of these reactions typically employed stoichiometric Pd and, later, cocat-alytic palladium/copper [140-142]. In the late 1970s, Hegedus and coworkers demonstrated that Pd-catalyzed methods could be used to prepare nitrogen heterocyles from unprotected 2-allylanilines and tosyl-protected amino olefins with BQ as the terminal oxidant (Eqs. 23-24) [143,144]. Concurrently, Hosokawa and Murahashi reported that the cyclization of allylphenol substrates can be accomplished by using a palladium catalyst with dioxygen as the sole stoichiometric reoxidant (Eq. 25) [145]. 

Palladium(0)-catalyzed cross-coupling of aryl halides and alkenes (i.e., the Heck reaction) is widely used in organic chemistry. Oxidative Heck reactions can be achieved by forming the Pd -aryl intermediate via direct palladation of an arene C - H bond. Intramolecular reactions of this type were described in Sect. 4.1.2, but considerable effort has also been directed toward the development of intermolecular reactions. Early examples by Fu-jiwara and others used organic peroxides and related oxidants to promote catalytic turnover [182-184]. This section will highlight several recent examples that use BQ or dioxygen as the stoichiometric oxidant. 

But not only palladium(O) complexes can activate CO or O2, also palla-dium(II) complexes have been reported to be active in the presence of carbon monoxide or dioxygen as it was shown in the direct synthesis of polycarbonate from CO and phenol or bisphenol A [79,80]. The authors could confirm the positive influence of the NHC ligand comparing the activity and reactivity of the palladium-carbene complex with the corresponding PdBr2 catalyst. The molecular weights and yields of the polycarbonates improved with increasing steric hindrance of the substituents in the l,T-position of the car-bene complex. 

The proposed mechanism is given in Scheme 15. Initially the dissociation of water, maybe trapped by the molecular sieve, initiates the catalytic cycle. The substrate binds to the palladium followed by intramolecular deprotonation of the alcohol. The alkoxide then reacts by /i-hydride elimination and sets the carbonyl product free. Reductive elimination of HOAc from the hydride species followed by reoxidation of the intermediate with dioxygen reforms the catalytically active species. The structure of 13 could be confirmed by a solid-state structure [90]. A similar system was used in the cyclization reaction of suitable phenols to dihydrobenzofuranes [92]. The mechanism of the aerobic alcohol oxidation with palladium catalyst systems was also studied theoretically [93-96]. 

The oxidation to methyl ketones without cleavage of the double bond was reported recently for a palladium NHC complex [108]. When the authors used the previously described catalyst 13 in THF with dioxygen for the oxidation of styrene they found that together with the phenylmethylketone a significant amount of y-butyrolactone was formed. Analysis of the mechanism led to the conclusion that THF is oxidized to a hydroperoxide species which is the real oxidant. They therefore tried tert-butylhydroperoxide (TBHP) and found immediate conversion without any induction period. Optimized conditions include 0.75 mol % of the previously described dimeric complex... 

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