C—O coupling

The spectrum is in substantial agreement with the data reported by Wenkert et. al, (6). However, on the basis the proton coupled carbon 13 NMR spectra, the assignments for 6 and 7ot are reversed [Brambilla (16)] from those previously reported. The new assignments are based on long range H-C-O-C couplings. 

Only one of three possible diastereomers is obtained additionally, 7% O-C coupling product is found. 

On the other hand, 4-dimethylaminop3ridine or 2-methoxypyridine favors O—C coupling. This reaction provides a regiospeciflc route to xanthones (equation II). ... 

Activation ofquinones. Quinones under ordinary conditions react with phenol ethers to give products of C—C and O—C coupling. Acid catalysis favors C—C coupling (equation I). ... 

FIG. 9. Four-quadrant plots of DFT results for an ethyl /J-D-glucopyranoside mimic of a disaccharide, showing phase shifting of 3/Ccoc Karplus curves for C-C-O-C coupling pathways bearing an internal electronegative substituent. The mimic has deoxy functions at C-3, C-4, and C-6. 

As in PTBTt, the esr spectrum of the radical-anion PTBT displays a-and o- C coupling constants halved with respect to those of PTM- mono-radieals (14.7 and 5.3 G) (Ballester and Pascual, 1985) i.e. a rapid spin-charge exchange [142] is again indicated. 

Heck, Suzuki, allylation, and other O-C coupling reactions 870... 

A generalized oxidative coupling reaction of phenolic monomers is shown in Scheme 1. Two kinds of couplings are possible, (a) C-C coupling to form phenylene units, and (b) C-O-C coupling to form oxyphenylene units. 

The possible dimers from the radical coupling of phenols is shown in Scheme 2. Among the five possibilities, three dimers are as a result of C-C coupling (I-III) while the other two are from C-O-C coupling (IV-V). To get a detailed analysis of the weak resonances, a vertically expanded H NMR spectrum recoreded at the intermediate stage of the reaction is shown in Figure 3. 

The substituents in the 2- and 6-positions must not exceed a certain geometrical size. Otherwise, instead of regular -O-C- coupling leading to the poly (phenylene ether)s, there is simply a -C-C- coupling of the monomers to form diphenylquinones. This reaction is favored by higher temperatures. The pale-yellow coloration of poly(-2,6-dimethyl-l,4-phenylene ether) may be caused by the presence of quinones. 

The reduction of CO2 at metallic cathodes has been studied with almost every element in the periodic table °. This reaction can be driven electrochemi-cally or photochemically " and semiconductors have been used as cathodic materials in electrochemical or photoelectrochemical cells . The aim of these studies has been to find cathodes that discriminate against the reduction of H2O to H2 and favor the reduction of CO2 and also to find a cathode selective for one product in the reduction of CO2. A fundamental requirement is that the latter process occurs at a lower overpotential on such electrodes. However the purposes mentioned before in metallic cathodes depends on a series of factors such a solvent, support electrolyte, temperature, pressure, applied overpotential, current density, etc. (we will see the same factors again in macrocyclic electro-catalysis). For instance when protons are not readily available from the solvent (e.g., A,A -dimethylformamide), the electrochemical reduction involves three competing pathways-oxalate association through self-coupling of COj anion radicals, production of CO via O-C coupling between and COj and CO2, and formate generation by interaction of C02 with residual or added water. ... 

The radical anion CO2 has a life as short as microseconds, as discussed in Chap. 1. It can be stabilized by interaction with the solvent or radical traps. It can attack a second CO2 molecule via a hetero O-C coupling and generate the dianion [A], in Scheme 8.3 which can be the source of CO and carbonate, as already seen. However, by protonation, the following couple of products can be obtained either CO and hydrogencarbonate HCO3 or CO2 and formate HC02. ... 

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