C-O oxidation

Sodium dichromate, Na C O Oxidizes primary alcohols to yield carboxylic acids and secondary alcohols to yield ketones (Sections 17.7 and 19.2). 

A nitrogen-bridged bis(iron-phthalocyanine) catalyst, previously used for C-H to C-O oxidations, has now been used for C-C bond formation it catalyses hydroacylation of alkenes, adding acetaldehyde across a variety of double bonds - linear, cyclic, enone,... 

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... 

Fig. 4.15 Examples of dimeric ellagitannins composed of both C-glucosidic and glucopuranosic moieties and derived from C-O oxidative coupling processes their intemnit linkage is of the valoneoyl type (Okuda s fft-DOG type).

Table 7.2.6. Hexahydroxydiphenoyl ester derivatives, Group 2B The structure of some dimeric and trimeric C-O oxidatively coupled metabolites...

Guo et al. developed a chemoenzymatic strategy for the synthesis of bastadins-2, -3, and -6 (232). The requisite dimeric dityrosine and isodityrosine were successfully prepared by C-C and C-O oxidative phenolic coupling of mono- and dihalogenated derivatives of tyrosine and tyramine using horseradish (233) and soybean peroxidases. 

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