Dioxygen, complexes Direct’ synthesis

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 use of directed reductions has proven to be a versatile and reliable tool in complex molecule synthesis, in particular in the synthesis of macrolide natural products. An example that highlights this strategy is the synthesis of the antifungal polyene macrolide roxaticin (137), which incorporates both 1,3-syn and 1,3-anti dioxygenated structural motifs (Scheme 2.16). In their impressive total syntheses, Mori [83] and Evans [84] took advantage of both syn- and anti-selective reductions. In all cases, these proceeded in excellent yield and diastereoselectivity. 

Aromatics for oxidative polymerization usually show high oxidation potentials. The direct oxidation of these monomers by 02 does not proceed because of the large potential gap between the monomers and 02. A novel catalysis by vanadyl complexes is applicable to the synthesis of PPS by the 02-oxidative polymerization of diphenyl disulfide (Eq. 10) [93-103], In the presence of VO(acac)2, the polymerization is accompanied by a quantitative 02 uptake. Dioxygen is essential for the VO-catalyzed oxidative polymerization of diphenyl disulfide. 

Recently, the synthesis and X-ray crystal structure of [Cu(dcmpz)2(py)2], 229, were reported (40). Complex 229 was obtained either by reacting [Cu(dcmpz)], 12, (27) with dioxygen in pyridine or by the direct reaction of CuCl2 with Hdcmpz in the presence of pyridine. Complexes having two... 

From the viewpoints of reaction mechanism and efficiency in organic synthesis, oxidation of phenols with dioxygen catalyzed by cobalt-, manganese- and related metal-amine complexes has been studied . In particular, much effort has been directed toward constructing new efficient catalysts by a combination of metals with... 

Analogously, in the presence of silica-supported palladium catalysts, benzene is oxidized under ambient conditions to give phenol, benzoquinone, hydroquinone and catechol [37b]. Palladium chloride, used for the catalyst preparation, is believed to be converted into metallic palladium. The synthesis of phenol from benzene and molecular oxygen via direct activation of a C-H bond by the catalytic system Pd(OAc)2-phenanthroline in the presence of carbon monoxide has been described [38]. The proposed mechanism includes the electrophilic attack of benzene by an active palladium-containing species to to produce a a-phenyl complex of palladium(ll). Subsequent activation of dioxygen by the Pd-phen-CO complex to form a Pd-OPh complex and its reaction with acetic acid yields phenol. The oxidation of propenoidic phenols by molecular oxygen is catalyzed by [A,A"-bis(salicylidene)ethane-l,2-diaminato]cobalt(ll)[Co(salen)] [39]. 

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