Marc Reid

C–H functionalization for hydrogen isotope exchange

The various applications of hydrogen isotopes (deuterium, D, and tritium, T) in the physical and life sciences demand a range of methods for their installation in an array of molecular architectures. In this Review, we describe recent advances in synthetic C-H functionalisation for hydrogen isotope exchange.

Application of neutral iridium(I) N‐heterocyclic carbene complexes in ortho‐directed hydrogen isotope exchange

Bench-stable complexes of the type [Ir(COD)(NHC)Cl] (NHC = N-heterocyclic carbene) have been investigated within the field of hydrogen isotope exchange. By employing a sterically encumbered NHC within such complexes and catalyst loadings of only 5 mol%, moderate to high deuterium incorporations were achieved across a range of aromatic ketones and nitrogen-based heterocycles. The simple and synthetically accessible catalysts reported herein present alternatives to phosphine-based species and increase the available labelling systems with respect to established iridium-based isotope exchange methodologies.

Applications of hydrogen isotopes in the life sciences

Hydrogen isotopes are unique tools for identifying and understanding biological and chemical processes. Hydrogen isotope labelling allows for the traceless and direct incorporation of an additional mass or radioactive tag into an organic molecule with almost no changes in its chemical structure, physical properties, or biological activity. Using deuterium-labelled isotopologues to study the unique mass-spectrometric patterns generated from mixtures of biologically relevant molecules drastically simplifies analysis. Such methods are now providing unprecedented levels of insight in a wide and continuously growing range of applications in the life sciences and beyond. Tritium (3 H), in particular, has seen an increase in utilization, especially in pharmaceutical drug discovery. The efforts and costs associated with the synthesis of labelled compounds are more than compensated for by the enhanced molecular sensitivity during analysis and the high reliability of the data obtained. In this Review, advances in the application of hydrogen isotopes in the life sciences are described.

Iridium-catalyzed formyl-selective deuteration of aldehydes

We report the first direct catalytic method for formyl-selective deuterium labeling of aromatic aldehydes under mild conditions, using an iridium-based catalyst designed to favor formyl over aromatic C-H activation. A good range of aromatic aldehydes is selectively labeled, and a one-pot labeling/olefination method is also described. Computational studies support kinetic product control over competing aromatic labeling and decarbonylation pathways.

Iridium-Catalysed ortho-Directed Deuterium Labelling of Aromatic Esters—An Experimental and Theoretical Study on Directing Group Chemoselectivity

Herein we report a combined experimental and theoretical study on the deuterium labelling of benzoate ester derivatives, utilizing our developed iridium N-heterocyclic carbene/phosphine catalysts. A range of benzoate esters were screened, including derivatives with electron-donating and -withdrawing groups in the para- position. The substrate scope, in terms of the alkoxy group, was studied and the nature of the catalyst counter-ion was shown to have a profound effect on the efficiency of isotope exchange. Finally, the observed chemoselectivity was rationalized by rate studies and theoretical calculations, and this insight was applied to the selective labelling of benzoate esters bearing a second directing group.

Base-catalyzed Aryl-B(OH)2 Protodeboronation Revisited: from Concerted Proton-Transfer to Liberation of a Transient Arylanion

Pioneering studies by Kuivila, published more than 50 years ago, suggested ipso protonation of the boronate as the mechanism for base-catalyzed protodeboronation of arylboronic acids. However, the study was limited to UV spectrophotometric analysis under acidic conditions, and the aqueous association constants (Ka) were estimated. By means of NMR, stopped-flow IR, and quenched-flow techniques, the kinetics of base-catalyzed protodeboronation of 30 different arylboronic acids has now been determined at pH > 13 in aqueous dioxane at 70 °C. Included in the study are all 20 isomers of C6HnF(5-n)B(OH)2 with half-lives spanning 9 orders of magnitude: <3 ms to 6.5 months. In combination with pH-rate profiles, pKa and ΔS⧧ values, kinetic isotope effects (2H, 10B, 13C), linear free-energy relationships, and density functional theory calculations, we have identified a mechanistic regime involving unimolecular heterolysis of the boronate competing with concerted ipso protonation/C-B cleavage. The relative Lewis acidities of arylboronic acids do not correlate with their protodeboronation rates, especially when ortho substituents are present. Notably, 3,5-dinitrophenylboronic acid is orders of magnitude more stable than tetra- and pentafluorophenylboronic acids but has a similar pKa.

Hydrogen isotope exchange with highly active iridium(I) NHC/phosphine complexes: a comparative counterion study.

Herein, we present a range of substrates that undergo hydrogen isotope exchange with an iridium(I) N-heterocyclic carbene/phosphine complex bearing the less coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]borate counterion and compare these with labelling using the equivalent, more established hexafluorophosphate complex. The changes in reactivity and selectivity of these complexes in a series of solvents are examined. Copyright

Iridium-catalysed ortho-deuteration of primary sulfonamides: an experimental and computational study

On 4 December 1913, the journal Nature published a letter by Frederick Soddy, of the University of Glasgow, in which the term isotope was publicly used for the first time. Soddy had been at Glasgow since 1904 and realized the chemical identity of ‘mesothorium’ (228Ra—which Soddy separated from thorium minerals) and Marie Curie’s radium (226Ra—from uranium minerals). Soddy was awarded the Nobel Prize for Chemistry in 1921, largely for this work. A number of historical radioactive samples (including Soddy samples) survive at Glasgow, as well as some of his equipment, and these provide a rare glimpse into the birth of radiochemistry. High precision gamma-ray spectrometry has been carried out on them, and together with archival research, this provides new insights into their preparation and history. Since Soddy’s breakthrough, the ratio of known isotopes to elements has grown from 1 to nearly 27!Isotopic labelling with heavy hydrogen isotopes (D2 and T2) is widely used as a means to monitor the biological fate of a potential drug molecule and represents a particularly industry-facing example of chemoselective organometallic catalysis. Consequently, preliminary studies from our laboratory have allowed expedient access to a series of novel iridium complexes, such as 2, that are able to catalyse the ortho-deuteration of various coordinating functionalities and pharmacophores, such as ketones, amides and nitro compounds 2 (Scheme 1). As part of our latest studies, we recently reported an efficient protocol for ortho-deuteration using more readily accessible Ir(I)chloro-carbene complexes. Turning to more challenging substrate classes, the utility of bench-stable catalysts such as 5 has now evolved to deliver the first highly effective strategy for the ortho-deuteration of primary sulfonamides at room temperature (Scheme 2). Additionally, we have used experimental and computational methods in parallel to explain the origins of observed chemoselectivity in labelling multi-functional drug molecules like 7, highlighting the importance of substrate–complex interactions during complexation. The details of all such studies will be delineated in this lecture.