The application of iridium(I) NHC/phosphine catalysts has delivered highly selective deuteration of indole, azaindole, and pyrrole N-heterocycles, which represent an important and relatively underexplored class of labeling substrates. Common N-protecting groups have been used to selectively direct C−H activation and can be removed under mild conditions with retention of the deuterium label. The method is exemplified by the labeling of the drug molecule sumatriptan. Complementary DFT studies have been conducted to facilitate the rationalization of the very good selectivity offered by the mild and convenient labeling process.
NaNO is used in oxidative Pd-catalyzed processes as a complementary co-catalyst to common oxidants, e.g., Cu salts, in C-H bond activation and Wacker oxidation processes. NaNO and NaNO (with air or O) assist the sp-C-H bond acetoxylation of substrates bearing an N-directing group. It has been proposed previously that a redox couple is operative. The role played by NO anions is examined in this investigation. Evidence for an NO anion interaction at Pd is presented. Palladacyclic complexes containing NO anions are competent catalysts for acetoxylation of 8-methylquinoline, with and without exogenous NaNO. The oxidation of 8-methylquinoline to the corresponding carboxylic acid has also been noted at Pd. O-Labeling studies indicate that oxygen derived from nitrate appears in the acetoxylation product, the transfer of which can only occur by interaction ofO at Pd with a coordinating-acetate ligand. Nitrated organic intermediates are formed under catalytic conditions, which are converted to acetoxylation products, a process that occurs with (50 °C) and without Pd (110 °C). A catalytically competent palladacyclic dimer intermediate has been identified. Head-space analysis measurements show that NO and NO gases are formed within minutes on heating catalytic mixtures to 110 °C from room temperature. Measurements by in situ infrared spectroscopy show that NO is formed in sp-C-H acetoxylation reactions at 80 °C. Studies confirm that cyclopalladated NO complexes are rapidly oxidized to the corresponding NO adducts on exposure to NO(g). The investigation shows that NO anions act as participating ligands at Pd in aerobic sp-C-H bond acetoxylation processes and are involved in redox processes.
An iridium(I) N-heterocyclic carbene/phosphine complex has been applied to the C-H activation and hydrogen isotope exchange of quinoline N-oxides. The isotope labelling proceeds under exceptionally low catalyst loadings of 0.25 mol%, and delivers products with high levels of deuterium incorporation selectively at the C8 position. A broad substrate scope is demonstrated, with the method tolerant of electron-poor and -rich substrates, and of substitution adjacent to the site of C-H activation. The isotope label is fully retained under standard deoxygenation conditions to give the corresponding labelled quinoline, and the labelling and deoxygenation can be combined in a one-pot procedure.
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