Dioxygen complexes, reactions with electrophiles

We concluded that dioxygen complexes of low-valent group VIII metals are nucleophilic in character. Thus, they undergo a 3 + 2 cycloaddition reaction with electrophilic olefins [20] as shown below. 

Mononuclear peroxo complexes, such as MoO(02)2(HMPT), show a more electrophilic character in their reactions. This particular complex may be used for the epoxidation of alkenes.176 Much interest has been shown in the reaction of small molecules with dioxygen complexes of this type with a view to catalytic oxidation. These reactions and their products are sumarized in Table 10. The... 

Iridium complexes appear to be less reactive towards carbonyl compounds, although they react with aldehydes " and hexafluoroacetone . This last reaction has been studied kinetically, and the authors find it to be first order in dioxygen complex and in hexafluoroacetone. They conclude the reaction takes place by a direct electrophilic attack on the coordinated dioxygen. This contrasts sharply with the results of Ugo et al. for (Ph3P)2Pt02 the slow substitution reactions of Ir(III) complexes and the absence of vacant coordination sites exclude the coordination of the substrate prior to the attack on coordinated dioxygen, and this may account for the much slower reactions of the iridium complexes. 

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]. 

The subsequent functionalization of an M-C bond present in the resulting organometallic intermediate is the next reaction step. Not much is known about the ways that would allow for an oxy-functionalization of transient monohydrocarbyl palladium(ll) and platinum(II) complexes. Both Pd -C and Pt -C bonds are relatively inert toward electrophiles. In particular, oxygen, the most abundant and one of the most practically attractive oxidants, is usually unreactive toward such species. As a result, dioxygen activation with organoplatinum(II) and organopalla-dium(II) complexes is of significant current interest [5-7]. 

Other electrophilic heterocumulenes (e.g., CS2, RNCO, RNCS, SO2) are also subject to insertion reactions. Accordingly, the insertion of CS2 into the M-N bond in metal dialkylamides produces dithiocarbamate derivatives and the reaction of SO2 with M-C bonds gives sulfinate complexes resulting from either 1,1-insertion [M-0S(0)R] or 1,2-insertion [M-S(R)02]. Rare examples of insertion of dioxygen into Pt-H bonds to generate remarkably stable hydroperoxide derivatives L Pt-OOH have also been reported. 

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