Selective Deuteration of Heterocycle N-Oxides via Iridium-Catalysed Hydrogen Isotope Exchange

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 CH 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.