Investigating selective hydrogen isotope exchange on unsaturated moieties with active iridium complexes

There exist a wide range of reported iridium catalysts which can facilitate hydrogen isotope exchange (HIE) upon aromatic compounds. Within this array, several catalysts stand out for their ability to mediate the HIE under mild conditions, with high levels of isotope incoporation. One key drawback in the labeling protocol with each catalyst is the necessity of an aromatic group to accept the isotopic label, limiting the utility to compounds containing functionalised aromatic skeletons. Our first effort to overcome this limitation was to apply nonaromatic unsaturated compounds, with skeletons related to the aromatic moieties previously labelled. However, each catalyst currently capable of HIE under mild conditions is also capable of olefin hydrogenation, typically under similar reaction conditions. Pleasingly, we found that utilising our developed, commercially available, bulky NHC/phosphine complex (I), under optimised conditions, excellent isotope incorporation is delivered and the alkene reduction is negligible. Furthermore, isotope incorporation is observed exclusively β- to the donor group. We have proposed a mechanism for this mild and novel process, supported by experimental findings, which includes oxidative addition into the β-C-H bond, similar to aromatic HIE.