Palladium dioxygen complexes

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). 

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 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... 

One or two minor points may be added to Table 8 above the adducts of aldehydes and ketones to palladium and platinum complexes react with SO2, CO2 and NO2 with displacement of the organic moiety to give the products expected for addition of these molecules to a dioxygen complex diphenylacetylene displaces both the carbonyl and the dloxygen ligand Adducts of CS2 and thiourea have also been reported The products obtained with NO2 depend on the reaction conditions, especially the solvent reaction of NO2 with (r-BuNC)2M02 gives a trans dinitrate for M = Ni and a cis dinitrate for M = Pd ). 

The complexes of rhodium and iridium are generally less reactive than those of palladium and platinum no reaction was observed between Rh and Ir dioxygen complexes and CO2, CS2, aldehydes and ketones or between an iridium complex and CO, CO2 and The iridium complexes are not totally inert however, and reactions with... 

Bridged Complexes.—Superoxide ion, O2 , reacts with dinuclear chloride-bridged palladium(ii) complexes to form dioxygen products. The suggested mechanism involves bridge-splitting in two steps via nucleophilic displacement of chloride by superoxide. ... 

Dioxygen complexes of nickel, palladium and platinum undergo similar series of reactions with reactive substrates, equations (37), (38) and (39). All react with SO2, N2O4, NO, CO2, RNC, and PPhs, however, there are some differences in the products which are formed. For example, although Pd, Pt and Ni dioxygen complexes all react with N2O4, nickel forms a rmns-nitrato complex, equation (37 ) whereas Pd and Pt form cis-complexes, equation (38c) and (39c). 

Additional experiments with the same catalyst source showed further that addition of a base such as sodium acetate or sodium carbonate also promoted the syn-aminopalladation pathway. Obviously, under basic conditions, aminopalla-dation occurs through alkene insertion into a palladium amidato complex with a defined Pd-N bond [45]. Additional experiments by Stahl and White [46] employing an isolated palladium suUbnamidato complex and its intramolecular aminopallada-tion product suggested the alkene insertion reaction to be reversible in nature but to be turned over irreversibly in the presence of molecular dioxygen. Electronically enriched amidates favor the alkene insertion reaction. 

Palladium(O) complexes bearing mixed NHC-PR3 ligands, [(NHC)Pd(PR3)] complexes, readily reacted with O2 affording Pd" peroxo [(NHC)Pd(ii -02) PR3)] compounds, which were rather air stable both as solid and in solu-tion. Interestingly, no phosphine decoordination was observed under the reaction conditions. The coordinated dioxygen in [(NHC)Pd(ii -02)(PR3)] complexes displayed bond lengths of 1.430(2)-1.468(3) A for the 0-0 bond, which were comparable to that found in ris-[(IMes)2Pd(Ti -02)]. ... 

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. 

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. 

Efforts aimed at fully understanding the mechanism of these oxidation reactions are still needed but new insights regularly appear in the literature [203-205]. The isolation and characterization of a dioxygen-derived palladium(II)-hydroperoxide complex—species generally postulated as intermediates in this reaction—has been achieved for the first time by Stahl et al. [206] (Scheme 27). The capability of IMes ligands to undergo cis-trans isomerization has been pointed out as essential for the formation of this complex. 

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