C-O reductive elimination

C-O Reductive Elimination from High Valent Pt and Pd Centers... 

Keywords C-O reductive elimination Mechanism Palladium Platinum Contents... 

The C-O reductive elimination chemistry of high valent metal species discovered and matured over the last four decades is being actively explored now. The goals of this Chapter are to overview the C-O bond elimination reactivity of organoplatinum and organopalladium complexes with the metal in an oxidation state greater than - -2 and to discuss the C-O reductive elimination mechanisms involving these complexes. 

The reactivity of Pt and Pd complexes in a C-O reductive elimination reaction depends on a number of factors, including structural and solvent effects. One of the factors is the tendency of a particular high valent metal center to be reduced to... 

For instance, a cationic aqua complex/<2c-[PtMe3(OH2)3]" is indefinitely stable in aqueous solutions at elevated temperatures, whereas diphosphine - supported trimethyl platinum(lV) complexes such as/<2c-(dppbz)Pt Me3(OR) and/ac-(dppe) Pt MesCOR) shown in Fig. 5 undergo concurrent C-C and C-O reductive elimination at 120°C [21, 23, 24]. 

The phenyl complex 12 of a similar structure is kinetically more resistant to C-O reductive elimination than the methyl analog 9 as different reductive elimination mechanisms are expected to be operational in these two cases, concerted three-center in the case of the aryl derivative and an Sn2 mechanism in the case of the alkyl complex. In particular, the symmetric phenyl complex (dpms)Pt Ph(OH)2, 12, in Fig. 6 is completely inert in acidic aqueous solutions at 100°C [25], whereas its methyl analog 9, (dpms)Pt Me(OH)2, eliminates methanol via an Sis/2 mechanism at room temperature [9]. 

Improving the quality of the leaving group, Z - ArO , in complex 8 by introducing electron-withdrawing substituents to the para-position of an aryloxide ArO leads to a faster C-O reductive elimination kinetics (p-CN > P-CF3 > p-Cl > H > p-Me p-OMe). It was suggested that the energy of the transition state TSd (Fig. 9) and the rate of dissociation of an aryloxide from the Pt center are more... 

ElectrophUicity of an alkyl metal(IV) complex is an important factor affecting its reactivity toward external nucleophiles (4). Protonation of symmetric (dpms) PtMe (OH)2 complex 9 (Fig. 6) in water was shown to enhance its susceptibility to nucleophilic attacks [9, 30]. In the absence of strong acid additives, the C-O reductive elimination is sluggish even at 90°C [9], whereas in the presence of 1 equivalent of HBF4 the reaction proceeds at a noticeable rate at room temperature. 

In the case of the C sp )-0 elimination reaction shown in Fig. 7, it was established that a dissociation of the carboxylate ligand trans- to the aryl from the starting Pd complexes 13 does occur at temperatures much lower than those required for the C-O reductive elimination reaction. This observation suggests that the reaction might proceed via a cationic intermediate 6 and follow a reaction path shown in Fig. 11 with a solid line. If so, the transition state TSd which corresponds to the carboxylate dissociation is of a noticeably lower energy compared to the TSc corresponding to the R-Z elimination step. A Hammett analysis of the kinetics of the reaction in Fig. 7 showed that increased nucleophilicity of the benzoate /5-XC6H4COO in complexes 13 leads to a faster kinetics (p-OMe > p-Me > H > p-OPh > p-F > p-Cl > p-Ac > P-CF3 > p-CN > P-NO2), a trend opposite to that observed for trimethyl Pt complexes 8 involved in an S iv2-type reaction shown in Fig. 5. As the electronic requirements to the nucleophilicity of Z for the... 

Interestingly, dinuclear Pt systems have long been known to allow for catalytic aerobic oxidation of olefins leading to the corresponding olefin oxides and carbonyl compounds [38], Formation of epoxides, in particular, was thought to be a result of an intramolecular C(sp )-0 attack similar to the one in Fig. 13 where the oxygen atom of a p-hydroxoaUcyl intermediate attacks the metal(III)-bound carbon atom (Fig. 14). No studies of C-O reductive elimination have been performed for these systems. 

The C-O reductive elimination reaction shown in Fig. 14 can be viewed as a intramolecular version of nucleophilic elimination reactions with one of the Pt atoms as a leaving group Z (Fig. 9). As in the case of S iv2 processes discussed above, one can consider a mechanism involving an attack of the nucleophile upon a six coordinate Pt atom (Fig. 9, dashed line) or a dissociation - elimination mechanism involving a formally five coordinate Pt transient (Fig. 9, solid line) that forms upon a heterolytic cleavage of the Pt -Pt bond. 

Williams BS, Holland AW, Goldberg KI (1999) Direct observation of C-O reductive elimination from Pt(IV). J Am Chem Soc 121 252-253... 

Palladium-catalyzed aromatic C-H acetoxylation was first reported in 1966 [100, 101]. In 1971, Henry proposed Pd(IV) intermediates in the Pd-catalyzed acetoxylation of benzene with K2Cr207 in AcOH [102], Subsequent reports by Stock [103] and Crabtree [104] also discussed the possible intermediacy of Pd(IV) complexes in the acetoxylation of benzene (Fig. 27a). In 2004, Sanford reported the regioselec-tive ort/to-acetoxylation of 2-arylpyridines and proposed a reaction mechanism involving aromatic C-H metallation at Pd(II), oxidation of the resulting aryl Pd(II) intermediate to a Pd(IV) complex, and product-forming C-O reductive elimination (Fig. 27b) [105-108]. 

In 2009, we suggested that Pd-catalyzed aromatic C-H acetoxylation may proceed via dinuclear Pd(III) complexes instead of via mononuclear Pd(IV) intermediates [109]. On the basis of dinuclear Pd(II) complex 36, the product of cyclometallation of 2-phenylpyridine (48) with Pd(OAc)2 [110], a synthesis cycle based on dinuclear Pd(III) complexes was established (Fig. 28). Oxidation of 36 with PhI(OAc)2, a common oxidant in Pd-catalyzed aromatic acetoxylation, afforded dinuclear Pd(III) complex 50. Complex 50 was observed to undergo C-O reductive elimination under pseudocatalytic conditions to generate 49 in 91% yield. The critical dinuclear Pd(III) intermediate (50) was crystallographically characterized the Pd-Pd distance in 50 was measured to be 2.555 A (compared with 2.872 A for 36 [111]), consistent with the formation of a Pd-Pd single bond. Dinuclear Pd(III) complex 50 was found to be a kinetically competent catalyst in the acetoxylation of 2-phenylpyridine with PhI(OAc)2. 

The experimentally derived rate law for chlorination is consistent with dinuclear Pd(III) complex 54 being the immediate product of oxidation during catalysis. Complex 54 has one apical chloride ligand and one apical acetate ligand and thus, upon thermolysis, could undergo either C-Cl reductive elimination, to generate 52, or C-O reductive elimination, to generate 55 (Fig. 32). We evaluated and confirmed the kinetic and chemical competence of 54 as an... 

In this context, we should also mention that, very recently, a series of coumestan analogs 75 were prepared by intramolecular cydization of related 4-(2-hydroxyphenyl)coimiarins 74 (Scheme 33) [130]. The C-O cydization reaction was in this case promoted by copper(II) acetate in the presence of catalytic amounts of Zn(OTf)2, which facilitates the initial electrophilic metalation of Ore C-3 position of the coxunarin by the copper(II) ion with concomitant alcohol-ate coordination. Final C-O reductive elimination gives the coupled tetracyclic products 75. 

In 2012, Chen and coworkers developed the palladium-catalyzed intramolecular amination of C(sp )-H at y and 5 positions to synthesize a series of nitrogen-containing heterocycles [15], including azetidine (Scheme 2.11), pyrrolidines, and indolines. With the optimal conditions, using catalytic Pd(OAc)2, an oxidant (PhI(OAC)2, 2.5 equiv.) and an acid additive (AcOH) in toluene at llO C, Chen tested them on other picolinamide substrates bearing primary y-C(sp )-H bonds surprisingly, the seemingly unfavorable four-membered azetidine was obtained as the major product. It is possible that a Pd(IV) intermediate was formed via PhI(OAC)2 oxidation of the palladacycle intermediate because the subsequent C-N and C-O reductive elimination pathways would lead to the formation of the cyclized and acetoxylated product. It was also noteworthy that no P H elimination product was detected under the aforementioned reaction conditions. 

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