Carbon-hydrogen coupling products

Ansatz (43) is the basis for many empirical findings, such as the observations that one-bond carbon-hydrogen couplings /( CH) are related to the hybridizations of the corresponding carbon atoms (61) or that carbon-carbon couplings y( 3C C) parallel the product sc-sc of the hybridizations of the carbon atoms (62) or that sometimes carbon-nitrogen couplings may be related to the hybridizations sc Sm (63). 

Dimerization.—The dimerization of organic substrates has obvious synthetic potential, and some further attention has been directed at carbonyl compounds in this context. aa-Disubstituted aldehydes, when treated with metal oxides, undergo oxidative coupling by hydrogen abstraction and subsequent carbon-carbon and carbon-oxygen bond formation (Scheme 97) the ratio of the two products varies with the metal oxide used, but further oxidation of the products is never observed. The isolation of carbon-oxygen coupled products offers an alternative explanation for the formation of a-hydroxyaldehydes by one-electron oxidants in aqueous acidic media. 

Finally, the change in selectivity for the methane/pentane couple for the two different substrates (18% for hexane, 56% for cyclohexane) can be explained as follows in the case of cyclohexane, the Ci to C5 products are formed through the second carbon-carbon bond cleavage via the hexyl surface intermediate D whereas in the case of hexane, the initial carbon-hydrogen bond activation step can lead to any of three alkyl surface intermediates (D, E, and F) before arriving at the key metallacychc intermediates... 

During the cross-couplings to form C—N, C—O, C—S, and C—P bonds, the arylpalladium halide complexes are converted to arylpalladium amide, alkoxide, thiolate, and phosphide complexes. Examples of each type of complex have now been isolated, and the reductive elimination of the organic products has been studied. Although the reductive elimination to form carbon-hydrogen and carbon-carbon bonds is common, reductive elimination to form carbon-heteroatom bonds has been studied only recently. This reductive elimination chemistry has been reviewed.23... 

The coupling product 177 is subjected to epoxidation to give epoxide compound 178 on which the C-l hydroxyl group will be generated via catalytic hydrogenation. After the dihydroxyl groups of the hydrogenation product 179 have been protected with cyclic carbonate, the C=C double bond between C-l2... 

Phosphorylation of the potassium salt of 2,2,6,6-tetramethylpiperi-3,4-dione (9) by chloro-phosphates (10) and (11) leads to the cyclic phosphate derivative (12) (Scheme 2) <85JOC209l>. This unusual product is formed due to intramolecular nitrogen participation and was characterized by 13C and 3IP NMR. There are no resonances in the 13C NMR spectra which would indicate the presence of an SPr group in the product and the 3IP NMR has a resonance at an unusual upfield shift indicating a cyclic phosphate. Structure (12) is consistent with these findings and with the extensive phosphorus-carbon and phosphorus-hydrogen coupling seen in its NMR spectra. 

As the photocatalytic carbon-carbon bond is formed, hydrogen evolves when the photocatalytic activation is done on colloidal ZnS [149, 150]. This dehydrodimerization also takes place with saturated ethers, with reactivity related to C H bond strength. Thus, 2,5-dihydrofuran (an allylic ether) is more easily activated than the isomeric 2,3-dihydrofuran (a vinyl ether). With the former substrate, all three dia-stereomeric coupling products are observed. Water is required for the reaction, and the primary photochemical product is thought to be a surface-bound hydroxyl radical. 

Organopalladium(II) halides add mainly to the electron-deficient carbon of an unsymmetrical alkene [46] to give 15, which readily isomerizes to 16 via a sequence of elimination and re-addition of the hydridopalladium(U) iodide. Finally, the elimination of iodoborane with the aid of triethylamine gives the head-to-tail-cross-coupling product. A deuterium labeling study proves the addition-elimination mechanism where a -hydrogen transfers to the terminal carbon (Scheme 2-17) [48]. 

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