C-O oxidative addition

More recently, the first direct observation of C-O oxidative addition of an aromatic elher to a transition metal complex has been reported by Kakiuchi (Equation 4.76). This reaction occurs by initial chelate-assisted C-H bond cleavage, followed by isomerization to form the final Ru(II) aryloxide complex. 

The observation that the C-O cleavage reaction of ethyl aryl ethers occurs without the need to block ortho C-H activation suggests that the 1,2-dehydroaryloxylation is a more facile reaction than the ether C-O oxidative addition. Accordingly a competition experiment, in which a mixture of 4-methoxy-2,3,5,6-tetrafluorotoluene and 4-ethoxy-2,3,5,6-tetrafluorotoluene was added to (PCP)Ir(TBE), resulted in exclusive formation of (PCP)Ir(H)(OAr) and (PCP)Ir(ethylene) (the products obtained from C-O cleavage of the ethyl ether), and no evidence of any reaction of the methyl ether (Scheme 4.12). 

DFT calculations were employed to gain insight into the mechanism of the 1,2-dehydroaryloxylation. One possible pathway could involve initial direct C-O oxidative addition followed by j8-hydride elimination however, the earlier observations that direct C-O oxidative addition does not occur for methyl aryl ethers along with the observation that 4-ethoxy-2,3,5,6-tetrafluorotoluene reacts faster than 4-methoxy-2,3,5,6-tetrafluorotoluene (i.e., the substrate with the /3-C-H bond reacts faster than the substrate with the a-C-H bond) would argue against such a mechanism. Accordingly, the barriers to direct C-O addition for 4-ethoxy-2,3,5,6-tetrafluorotoluene and ethoxybenzene were calculated to be prohibitively high, 35.0 and 40.8 kcal/mol, respectively. In contrast, the barriers for addition of the fi-C-H bond followed by /3-aryloxy elimination and loss of ethylene (Fig. 4.4) were found to be considerably lower (Fig. 4.5). 

As with aryl ether substrates, we also investigated alkyl esters with alkyl groups higher than methyl. As with the higher alkyl ethers, such species undergo 1,2-H-O elimination instead of C-O oxidative addition. Ethyl acetate, for example. 

Methyl tosylate has also been found to react with (PCP)Ir to undergo C(sp )-0 oxidative addition. (PCP)Ir(TBV)(H) reacts rapidly at room temperature with methyl tosylate (1.1 equiv) to give the C-O oxidative addition product, (PCP)Ir(Me)(OTs), in quantitative yield (Scheme 4.18). KIE experiments involving competition between a 10-fold excess each of CH3OTS and CD3OTS yielded a KIE, = 2.4(2), that again indicates that C-H activation is involved during or before the... 

Electrophilic catalysis may play an important role in the case of the similar benzylic carbon, too. For an O-benzyl system, it was found in a 1997 experiment that palladium oxide is a much more effective catalyst than palladium metal when the catalyst has been prereduced with chemical reducing agents. This finding shows very clearly that the electrophilic character of the unreduced metal ions plays an important role in the hydrogenolysis of the benzyl C—O bonds. Additional support for this mechanism is the fact that a small amount of butylamine can inhibit the hydrogenolysis of the benzyl C—O bond. 

The isolation of the first NHC - M - H complexes obtained by oxidative addition of an imidazolium salt to a low valent group 10 metal was achieved by Cavell and coworkers in 2003 [156]. A NHC-Pt(O) complex with two monoalkene ligands reacted with an imidazolium salt to provide an isolable NHC-PtH complex (Scheme 41). Carbene metal hydrides of Ni and Pd were obtained one year later by C - H oxidative addition of the corresponding imidazolium salts to bis-NHC Ni(0) and Pd(0) complexes (Scheme 41) [157]. 

The need for an accurate balance of the reagents clearly emerges in the second variant of the alkylation-alkenylation process leading to o,o -unsynuneIilcaIly disubstituted alkenylarenes (Eq. 2 of Scheme 1). In this case it is necessary to start from an ortho-monoaUcylated aryl iodide and to increase the reaction tanperature to 55 C. After oxidative addition to palladium(O) and norbomene insertion, there is a strong tendency, also favored by the increased temperature, to close a benzocyclobutene ring because of the steric effect exerted by the ortho-alkyl substituent, and only a significant excess of the added alkyl halide can curtail this termination reaction. 

The microscopic reverse reaction, reductive elimination from [Tp Pt Me2(H)], was observed upon heating this compound in benzene at 110°C to form [TpMe2ptiv e(Ph)(H)]. Reductive elimination of a second equivalent of methane then leads to C-H oxidative addition of a second equivalent of benzene to yield [TpMe2ptiV(ph)2(H)]. Thermolysis of [Tp R MejfD)] at 60°C leads to scrambling into the methyl ligands, without loss of methane, in accord with a o-CH4 complex intermediate before reductive elimination of methane. ... 

TRIR (vCO) was used to examine pathway(s) by which 355 nm photolysis of raw5-Rh Cl(CO)(PMe3)2 led to the C-H oxidative addition product (Ph)(H)Rh Cl(CO)(PMe3)2. UV photolysis of CO2 interacting with Rh ( C O)2 species on an AI2O3 surface was followed by studying vCO bands.The IR spectrum of CO adsorbed on rhodium carbonyl clusters entrapped in FSM-16 and NaY zeolites shows that the only detectable species is Rh(CO)2.i ... 

A related 7C-o-rearrangement is the isomerization of coordinated alkenes to metal-alkenyls via C-X oxidative addition. The following transformations are representative, Eqs. 11.7 and 11.8 ... 

Several Pd(0) complexes are effective catalysts of a variety of reactions, and these catalytic reactions are particularly useful because they are catalytic without adding other oxidants and proceed with catalytic amounts of expensive Pd compounds. These reactions are treated in this chapter. Among many substrates used for the catalytic reactions, organic halides and allylic esters are two of the most widely used, and they undergo facile oxidative additions to Pd(0) to form complexes which have o-Pd—C bonds. These intermediate complexes undergo several different transformations. Regeneration of Pd(0) species in the final step makes the reaction catalytic. These reactions of organic halides except allylic halides are treated in Section 1 and the reactions of various allylic compounds are surveyed in Section 2. Catalytic reactions of dienes, alkynes. and alkenes are treated in other sections. These reactions offer unique methods for carbon-carbon bond formation, which are impossible by other means. 

Interesting formation of the fulvene 422 takes place by the reaction of the alkenyl bromide 421 with a disubstituted alkyne[288]. The indenone 425 is prepared by the reaction of o-iodobenzaldehyde (423) with internal alkyne. The intermediate 424 is formed by oxidative addition of the C—H bond of the aldehyde and its reductive elimination affords the enone 425(289,290]. 

The Pd-catalyzed elimination of the mesylate 909 at an anomeric center, although it is a saturated pseudo-halide, under mild conditions is explained by the facile oxidative addition to the mesylate C—O bond, followed by elimination of /3-hydrogen to give the enol ether 910[767],... 

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