FULL PAPER DOI: 10.1002/adsc.201((will be filled in by the editorial staff)) The Synthesis of Highly Active Iridium(I) Complexes and their Application in Catalytic Hydrogen Isotope Exchange Jack A. Brown,a Alison R. Cochrane,a Stephanie Irvine,a William J. Kerr,a* Bhaskar Mondal,a John A. Parkinson,a Laura C. Paterson,a Marc Reid,a Tell Tuttle,a Shalini Andersson,b and Göran N. Nilssonb a Department of Pure and Applied Chemistry, WestCHEM, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, United Kingdom Fax: (+44)-141-548-4822; tel: (+44)-141-548-2959; e-mail: [email protected] b Medicinal Chemistry, AstraZeneca, R&D Mölndal, SE-431 83 Mölndal, Sweden Received: ((will be filled in by the editorial staff)) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please delete if not appropriate)) Abstract. : A series of robust iridium(I) complexes bearing a Furthermore, a number of industrially-relevant drug sterically encumbered N-heterocyclic carbene ligand, molecules have also been labelled, including the alongside a phosphine ligand, has been synthesised and sulfonamide containing drug, Celecoxib. Alongside detailed investigated in hydrogen isotope exchange processes. These NMR experiments, initial mechanistic investigations have complexes have allowed isotope incorporation over a range also been performed, providing insight into both substrate of substrates with the use of practically convenient binding energies, and, more importantly, relative energies deuterium and tritium gas. Moreover, these active catalysts of key steps in the mechanistic cycle as part of the overall are capable of isotope incorporation to particularly high exchange process. levels, whilst employing low catalyst loadings and in short reaction times. In addition to this, these new catalyst species Keywords: hydrogen isotope exchange; iridium; catalysis; have shown flexible levels of chemoselectivity, which can be deuterium; tritium altered by simple manipulation of preparative approaches. Introduction Despite colossal financial commitment to drug discovery, the pharmaceutical industry is burdened by unsustainable attrition rates associated with new chemical entities. Accordingly, positioning Scheme 1. Ideal hydrogen isotope exchange process. metabolism and pharmacokinetic studies earlier in such discovery programmes is crucial in assessing the overall properties of a drug candidate and as aligned activity in the labelling of a range of substrates, to reducing the present levels of attrition of over however, it is often found that (super-)stoichiometric 90%.[1] One key methodology applied extensively quantities of the catalyst and lengthy reaction times within this area is isotopic labelling, which allows the are required to deliver the observed levels of isotope biological fate of a potential drug molecule to be incorporation.[8] In addition, tritiation reactions carefully monitored.[2] In particular, transition metal- promoted by Crabtree’s catalyst frequently produce catalysed hydrogen isotope exchange (HIE) offers a considerable quantities of radioactive waste, which is simple and direct technique, which potentially allows a clear drawback with regards to both cost and the regioselective labelling of fully functionalised environmental concerns.[2c,9] In view of the issues molecules (Scheme 1), eliminating the need for any associated with Crabtree’s catalyst, it is evident that additional synthetic processes or prolonged alternative metal-based complexes for use in preparative pathways associated with the installation hydrogen isotope exchange would be beneficial to of an isotopic label. pharmaceutical partners with, in particular, the Whilst various transition metal species have desired catalysts being capable of labelling an demonstrated activity within the field of HIE,[3-6] for a assortment of substrates to high levels of number of years Crabtree’s catalyst, incorporation under mild reaction conditions and with [Ir(COD)(PCy )(py)]PF ,[7] was regarded as the lowered catalyst loadings. Following the work of 3 6 industry standard to facilitate such a process. This Nolan[10] and Buriak[11] in the field of olefin iridium-based complex demonstrates appreciable hydrogenation, we felt that in order to enhance 1 catalytic activity in HIE processes, relative to Under the standard reaction conditions developed Crabtree’s catalyst, greater steric demand around the within our laboratory, employing a catalyst loading of metal centre, coupled with balanced electronic 5 mol% in DCM under 1 atmosphere of D gas for 16 2 parameters, was required.[12] In this regard, we have h, complexes 5a-e facilitated high levels of H/D reported on the preparation and application of a series exchange across a wide range of substrates (6a-i) of new iridium complexes bearing a bulky N- including ketones, amides, and heterocyclic heterocyclic carbene (NHC), 1,3-bis(2,4,6- functionalities (Table 2). In general, compounds 7a-g, trimethylphenyl)imidazol-2-ylidene (IMes), alongside which are labelled via a 5-membered metallocyclic an appreciably encumbered phosphine, and which intermediate (5-mmi), were delivered with high have appreciable potential as highly active catalysts deuterium incorporations in a regioselective and for HIE with deuterium.[13] Additionally, we have reproducible manner. Furthermore, the isotopic shown that these robust and practically accessible labelling of weakly coordinating nitrobenzene, 6g, iridium species can be applied within the selective proceeded without incident and with no reduction of reduction of alkenes (and alkynes)[14] and in the Z- the nitro moiety, as has been detected with previously selective dimerization of terminal alkynes.[15] employed catalysts.[5f,8b] It is important to highlight Herein, we report the activity and selectivity that the levels of isotope exchange in benzamide, 6c, spectrum of this class of iridium complex in the area were somewhat more variable, ranging from 32% in of deuterium and, importantly, tritium labelling, as the presence of complex 5a, to 90% in reactions well as our initial investigations into the overall catalysed by complex 5d. Indeed, this substrate is mechanism of the hydrogen isotope exchange process notoriously difficult to label effectively, with 110 with these emerging catalyst species. mol% of Crabtree’s catalyst being previously Table 2. HIE studies with complexes 5a-e.a),b) Results and Discussion First of all, and importantly in a practical sense, a robust and readily utilisable route to our novel Ir(I) complexes bearing a sterically encumbered NHC in conjunction with a bulky tertiary phosphine ligand was achieved according to a variation of a procedure delineated by Herrmann.[16] The use of sodium ethoxide in the relatively weakly coordinating solvent, benzene, was crucial to allow the in situ generation and introduction of the NHC ligand, avoiding the necessity to pre-form and isolate the free carbene species under glovebox conditions. Following this modified synthetic route, five novel Ir(I) complexes were delivered in good yields (Table 1). These bright red catalysts are air- and moisture-stable crystalline solids, and have been characterised by NMR and mass spectral techniques. In addition, the structures of complex 5a-c have been confirmed by X-ray crystallography.[13a] Table 1. Preparation of iridium complexes.a) Entry PR Complex Yieldb) 3 1 PPh 5a 62 3 2 PBn 5b 59 3 3 PMe Ph 5c 71 2 4 PMePh 5d 69 2 5 P(OiPr) 5e 75 a) 5 mol% of Ir catalyst employed over 16 h. b) Average 3 a) Reaction conditions: (a) NaOEt, PhH, r.t., 10 min. (b) 4, incorporation into the positions shown over two separate PhH, r.t., 5 h. (c) AgPF , THF, r.t., 30 min. (d) PR , THF, reaction runs; the percentage given refers to the level of D 6 3 r.t., 2 h. b) Isolated yields. incorporation over the total number of positions shown, e.g. 97% for the two possible positions in 7a indicates 1.94 D incorporation. 2 required to deliver the isotopically-enriched product which is capable of labelling through both a 5- and a with only a moderate 65% D incorporation.[8b] The 6-mmi at three possible sites in the molecule. At a 5 examination of additional substrates revealed the mol% catalyst loading, complex 5c showed excellent ability of our novel Ir(I) complexes to also facilitate levels of incorporation into positions a and b (both isotope exchange in C-H bonds positioned five bonds via a 5-mmi), while position c (via a 6-mmi) showed away from the required coordinating functionality i.e. more moderate D uptake. Pleasingly, when the via a 6-membered metallocyclic intermediate (6- catalyst loading was reduced to 0.5 mol%, not only mmi). This process is believed to be energetically did we again observe exclusive labelling via the 5- more demanding and, as such, often leads to lower mmi over the 6-mmi, there was also selectivity noted levels of deuteration. In our hands, the simplest between positions a and b. The preferential labelling substrate of this class, acetanilide, 6h, was labelled up at position a suggests more effective iridium metal to an excellent 95% D loading. Furthermore, centre coordination with the more available oxygen deuterium incorporation in benzanilide, 6i, occurred lone pairs within the amide carbonyl group over the with high degrees of exchange observed in positions oxygen atoms of the nitro unit. labelled via both a 5-mmi and a 6-mmi. To further explore the capabilities of our complexes, a series of rate and activity studies were performed. A loading study revealed that excellent deuterium incorporation is maintained in the presence of catalyst quantities as low as 0.5 mol% (Scheme 2). Furthermore, whereas catalyst 5a mediated the 97% deuteration of 6a within 120 minutes, more electron- rich catalysts, 5b and 5c, delivered more rapid labelling (60 and 90 minutes, respectively). Presumably, the more electron-rich catalysts better facilitate the C-H activation process central to the H/D exchange reaction.[17] Scheme 3. Labelling selectivity for a 5-mmi over a 6-mmi. Based on these findings, it was envisaged that our overall protocols could be manipulated to achieve selective labeling into a position labelled via a 6-mmi (Scheme 4). This was accomplished by employing the heavily labelled compound 7j (a: 96%; b; 93%) as Scheme 2. Catalyst loading and labelling time study. substrate with H to allow H exchange at position a, 2 leaving the deuterium in place at position b. As shown in Scheme 4, employing 5 mol% of complex The excellent levels of efficiency displayed by 5c under an atmosphere of hydrogen resulted in the these complexes prompted us to examine their selective removal of deuterium via the 5-mmi, while catalytic activity further. As demonstrated in the the level of D at position b remained high. labelling of 6i (Table 2), benzanilide substrates offer Accordingly, this section of our study has shown that two potential sites of labelling, through either a 5- or it is now possible to access both classes of selectively 6-mmi. It was therefore proposed that the emerging labelled compounds, which illustrates a further catalysts had the potential to exert regioselective distinct practical advantage delivered by these novel deuteration in substrates of this type. Such iridium catalyst systems. To our knowledge, this level discrimination of H/D exchange in one position over of labelling selectivity by this direct approach is another would be of particular benefit to unprecedented in the literature. pharmaceutical partners, allowing specific drug metabolites to be traced preferentially during in vivo distribution studies. In an attempt to establish such selective HIE, the labelling of benzanilide 6j was studied. As illustrated in Scheme 3, the use of 5 mol% of complex 5c delivered appreciably high levels of isotope incorporation in both positions a and b. In contrast, reducing the amount of catalyst present in the reaction system to 0.5 mol% resulted in a Scheme 4. Selective removal of the D Label via the 5-mmi. significant decline in the degree of labelling observed in position b, via a 6-mmi, while the elevated level of exchange in position a, proceeding via the more To further illustrate the capabilities of these favourable 5-mmi, was maintained. A similar result catalysts, a number of available drug molecules were was obtained with the benzanilide derivative 6k, applied in hydrogen isotope exchange reactions. The 3 ability to label fully functionalised drug scaffolds is Table 4. Catalyst turnover. central to the application of HIE catalysis, especially as aligned to the endeavours of pharmaceutical partners. Pleasingly, complexes 5a-c performed very well with a series of drug substrates at relatively low catalyst loadings and over short reaction times (7l-o; Table 3). Of particular note is the high level of D incorporation in the Pfizer COX-2 inhibitor, Celecoxib, 7m. In addition to the high level of D uptake obtained in position b, the D incorporation at Entry L1/L2 % D TONa) TOFb) position a represents the first example of deuterium (catalyst) labelling adjacent to a primary sulfonamide moiety as 1 IMes/PBn 88 1056 176 facilitated by complexes of this type,[18] albeit at (5b) 3 relatively moderate levels. Furthermore, Sanofi- 2 Py/Ph P 29 348 58 3 Aventis’ anti-androgen Nilutamide, 7n, provides a (Crabtree’s) further example of preferential labelling via a 5-mmi a) Measured as no. moles of substrate converted/no. moles over a 6-mmi. of catalyst employed. b) Measured as no. moles of substrate converted/no. moles of catalyst employed/hours. Table 3. HIE studies with marketed drug molecule substrates. limited evidence to support the perceived sequence of events.[5o] Therefore, we were interested in embarking upon focused mechanistic investigations to increase the general comprehension of such catalysed hydrogen isotope exchange process. Further, by embedding our emerging iridium complex class within this study, we envisaged that any enhanced understanding could provide beneficial foundations to be used in the design of future catalyst systems. Initially and to this end, an exchange reaction was carried out in an NMR tube, using deuterated dichloromethane, to allow us to monitor two key features in the proposed pathway: the initial removal of the cyclooctadiene (COD) unit from the iridium complex, and the resultant geometry of the major ligands around the metal centre. Since the formation of iridium hydrides is thought to be central to the removal of the COD ligand, the reverse exchange reaction (Scheme 5) was carried out by employing fully deuterated acetophenone 9 and exchanging with a) 5 mol% of Ir catalyst employed over 16 h. b) 10 mol% of hydrogen gas, in an attempt to observe any potential Ir catalyst employed over 1 h. c) 2.5 mol% of Ir catalyst Ir-hydride species. Complex 5a was chosen as this is employed over 1 h. d) 5 mol% of Ir catalyst employed over the slowest acting of this series of catalysts, with 10 1 h. mol% being employed in order to fully observe intermediate complexes and reaction progression by NMR analysis (see Supporting Information for full Following the exploration of substrate scope, we details). sought to gain further insight regarding the effectiveness of our novel system via the analysis of catalyst turnover. As shown in Table 4, the deuteration of acetophenone using catalyst 5b delivered very good turnover within a 6 h reaction period, using extremely low levels of catalyst. Under the same conditions, Crabtree’s catalyst showed a vastly reduced efficiency; presumably due to its Scheme 5. Reverse exchange reaction monitored in an lower relative activity and instability over time.[12] NMR tube. Mechanistic Investigations Prior to the monitoring of the reaction, it was necessary to gather data on the parent complex, to Although the mechanism of iridium-catalysed HIE allow the detection of any changes that occur as the was proposed by Heys in 1996,[5d] there exists only overall process progresses. Accordingly, 1H and 31P 4 NMR spectra of complex 5a were obtained in CD Cl . by the disappearance of the multiplet peaks at 4.39- 2 2 The most relevant 1H spectrum peaks at the beginning 4.36 and 3.35-3.33 ppm; importantly, following this of the reaction were the multiplets at 4.39-4.36 and further analysis the peaks corresponding to the 3.35-3.33 ppm, as shown in Figure 1. These peaks iridium hydrides had also disappeared, suggesting correspond to the olefinic protons of the COD ligand, that these hydrides play a key role in the removal of and are expected to lose intensity and disappear at the the COD ligand. Furthermore, in the absence of the beginning of the reaction as the COD is reduced. COD ligand, 31P NMR analysis showed that the With regards to the 31P NMR, the peak present at original phosphine signal (16.35 ppm) had been 16.35 ppm corresponds to the phosphine ligand replaced with a new peak at 20.80 ppm, indicating bound to the metal centre. that the parent complex had been fully converted to an alternative phosphorus-containing iridium species 6883.47083.47573.43073.46263.4 3153.33443.37833.33533.33823.3 wit hin the reaction manifold. 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 ppm 5743.61 4237.531-4611.041-7005.441-9588.841-7472.351- Figure 2. 2D 31P/13C HMQC experiment of the parent complex. 20 0 -20 -40 -60 -80 -100 -120 -140 ppm To further investigate the new COD-free iridium Figure 1. 1H (top) and 31P (bottom) NMR spectra of intermediate, a second 2D HMQC 13C/31P correlation complex 5a in CD Cl 2 2. spectrum was obtained. As illustrated in Figure 3, the carbene carbon signal at 177 ppm had been replaced by a new signal at ~160 ppm. This change in Figure 2 shows a 2D 31P/13C HMQC experiment chemical shift was accompanied by a correlation to of the same parent complex 5a. This allowed the 2J PC the phosphine signal at 20.80 ppm, and with an coupling constant between the bound phosphorus appreciably elevated 2J coupling constant of 118 Hz. atom and the carbon of the carbene ligand to be PC Indeed, such large 2J coupling constants in similar established. It was envisaged that by carrying out this PC iridium complexes, bearing phosphine and NHC correlation experiment as the reaction proceeds, the ligands, have some literature precedent. For example, observed magnitude of this 2J coupling constant PC iridium(I) complex 11, depicted in Figure 4, has been would provide some indication of the geometry of the shown to have the triphenylphosphine and bis-benzyl two major ligands. As presented in Figure 2, a NHC ligands in a relative trans-arrangement.[19a] The distinct correlation was observed between the bound data for this compound reveal the carbene carbon of phosphine at 16.35 ppm and the carbene carbon at the NHC to have a chemical shift of 177.6 ppm and a 177 ppm. The 2J coupling constant was measured at PC 2J coupling constant of 116 Hz. In an analogous 8.6 Hz, in accord with these two ligands adopting a PC manner, iridium(III) complex 12 has the tri-iso- cis-arrangement in the parent complex. In turn, we propylphosphine and IMes ligands in a trans- were confident that the geometry of the ligands in any configuration, with a similar 2J coupling constant reaction intermediates could then be deduced by PC value of 108 Hz.[19b] However, the chemical shift of comparing the coupling constants obtained the carbene carbon in this latter complex is throughout the reaction. Consequently, the reaction significantly more upfield than that of complex 11, (Scheme 5) was performed in an NMR tube using d - 5 with a value of 165 ppm. From the evidence obtained acetophenone 9 and hydrogen gas, with CD Cl as the 2 2 regarding the activated catalytic species in our HIE reaction solvent. Following the initial addition of reaction, and from comparison with available hydrogen gas, several peaks emerged in the 1H NMR literature data,[19] it is proposed that the intermediate spectrum in the region of -13-0 ppm, indicating the within this manifold has the phosphine and NHC presence of the expected iridium hydrides. The ligands in a trans geometry; further, the 13C NMR reaction was then further monitored to assess the chemical shift of the carbene carbon is consistent reduction and removal of the COD ligand, as shown 5 with this new catalytic intermediate possessing an three isomers shown in Figure 6. In addition to the iridium(III) oxidation state. trans-isomer 13, two cis-isomers were also optimised to local minima on the potential energy surface (14 and 15, Figure 6). The relative energies of the three isomers indicate that 14 and 15 are destabilised by ppm 8.83 and 10.12 kcal/mol, respectively, relative to 13. 130 These findings support the experimental evidence that the phosphine and NHC adopt a trans-orientation 140 within the activated Ir species. 150 160 170 180 21.5 21.0 20.5 20.0 19.5 ppm Figure 3. 2D 31P/13C HMQC spectrum of intermediate Ir complex. Figure 6. Optimised structures of 13 and its isomers at DFT level. Our DFT calculations were further expanded to focus on the possible reaction pathway by which hydrogen isotope exchange proceeds. These more detailed investigations allowed the construction of the potential energy surface (PES) associated with the proposed catalytic cycle. Using that suggested by Figure 4. Known Ir complexes with phosphine/NHC Heys as a basis,[5d] a mechanistic pathway employing ligands in a trans-arrangement. our Ir(I) complexes in conjunction with acetophenone, 6a, was determined (Scheme 6). It is proposed that, following exposure to deuterium gas, complex 5a To further support the above experimental loses the COD ligand as d -cyclooctane. The evidence that the phosphine and NHC ligands align 4 resulting coordinately unsaturated Ir species is themselves in a trans-configuration, density stabilised by coordination of the substrate, e.g. 6a, functional theory (DFT) studies (see Supporting delivering 16. Following oxidative addition to give Information for full details on the level of theory 13, fluxionality of the dihydrogen hydride (13 → 17) used) were employed to calculate the relative allows isotopic hydrogen to orient in the position cis energies of the proposed cis- and trans-isomers of the to the Ir-C bond, resulting, ultimately, in reductive intermediate iridium species. Using Heys’ mechanism as a guide,[5d] the most sterically hindered elimination (17 → 18) and isotope incorporation into the ortho-position of the substrate aryl ring, 7a’. In species along the catalytic cycle is envisaged to be relation to modifications to Heys’ original that formed subsequent to oxidative insertion with the mechanistic scheme, we now believe that the initial substrate, e.g. 13 with acetophenone (Figure 5). activated iridium species is stabilised by association Therefore, the energetic ordering of the possible of the substrate forming complex 16 in which the isomers of 13 was investigated to determine the ortho aryl C-H is agostically coordinated to the probable active configuration within the HIE iridium centre. Furthermore and in a more general manifold. sense, a series of 31P NMR magnetization transfer experiments, analogous to the work of Grubbs’ within the field of ruthenium-catalysed olefin metathesis,[20] indicate that the phosphine ligand does not dissociate from the iridium metal centre throughout the catalytic cycle (see Supporting Information for experimental details). Figure 5. Oxidative addition intermediate in the catalytic All of the intermediates and transition states cycle. involved in our proposed catalytic cycle, with the incorporation of deuterium, have been fully optimised. Therefore, the enthalpy changes during the reaction Potential orientations of the D , hydride, and progress have been monitored. Relative enthalpies of 2 substrate ligands were investigated; however, all the stationary points on the PES and the barriers to alternative configurations converged to one of the oxidative addition, H/D exchange, and reductive 6 region of 0.02 Å (see Supporting Information for details). Comparing the calculated energetic parameters detected along the reaction path for both PPh and PMe (Table 5), the axial phosphine 3 3 substituent has a detectable impact on the energy of the oxidative addition step, with a difference of 1.43 kcal/mol calculated for TS(16-13) between the species bearing PPh and the smaller PMe . 3 3 With regards to the key steps in our proposed catalytic cycle and considering the PPh system, the 3 oxidative addition of the iridium into the aryl C-H bond, requires a moderate activation enthalpy of 16.70 kcal/mol (Table 5, Figure 7). Further, the conversion of 16 to 13 is endothermic by 3.66 kcal/mol with respect to 16. The subsequent H/D exchange step (13 → 17) is calculated to be rapid, with an activation barrier of 2.25 kcal/mol. In contrast to oxidative insertion, reductive elimination is favoured enthalpically, presumably due to the decrease in steric strain within the vicinity of the metal centre. Accordingly and based on this calculated PES, we can conclude that the rate- limiting step within such H/D exchange processes appears to involve the oxidative addition or reductive elimination of the ortho-C-H bonds. In a complementary experimental study, we strengthened this argument by observing a primary kinetic isotope effect of approximately 3.7 (see Supporting Information for full details). This suggests that C-H activation[21] is, indeed, rate-limiting. Having established the relative reaction energetics in HIE reactions employing acetophenone, 6a, attention turned to the reactivity of our catalysts Scheme 6. Proposed HIE catalytic cycle. with respect to different target molecules. Such studies also offered the potential to explain the selective labelling of substrates via a 5-mmi as elimination have also been calculated for each species opposed to a 6-mmi. Three substrates were selected shown in Scheme 6. In addition to the use of PPh3 for investigation: acetophenone, 6a, 2-phenylpyridine, and in order to probe the possibility of performing 6e, and acetanilide, 6h (Figure 8). computational studies of more minimised cost, PMe3 In view of the similar energy values calculated was also investigated as the axial phosphine ligand for complexes bearing either PPh or PMe as the 3 3 (Table 5). The decreased ligand size in the PMe3 axial ligand, the remaining theoretical studies were systems does not alter the mechanism of the reaction, performed with the smaller tertiary phosphine ligand nor does it strongly affect the geometry of the ligands in place. As our previous explorations had supported around the equatorial plane, i.e. there is no large C-H activation as being rate determining, the effects rearrangement of the ligands and the changes in bond of the alternative substrates were examined within lengths between the full (PPh3) systems and the this portion of the catalytic process (Table 6). As trimmed (PMe ) version are calculated to be in the 3 Table 5. Relative energies (∆E), enthalpies (∆H), and Gibbs free energies (∆G) for the catalytic cycle calculated at DFT level (all are in kcal/mol). ∆Ea) ∆Hb) ∆Gb) Species PPh PMe PPh PMe PPh PMe 3 3 3 3 3 3 16 0.00 0.00 0.00 0.00 0.00 0.00 TS(16-13) 17.21 18.14 16.70 18.13 18.65 18.35 13 3.33 0.93 3.66 1.20 2.88 0.27 TS(13-17) 5.86 3.17 5.91 3.66 5.05 2.10 17 3.43 0.93 3.75 1.19 3.00 0.30 TS(17-18) 17.39 18.39 16.94 18.35 18.50 18.62 18 -0.42 -0.44 -0.45 -0.48 -0.41 -0.42 a) Energies are zero-point corrected. b) All enthalpies and Gibbs free energies are at 298.15K. 7 Figure 7. The reaction PES employing Ph P as the phosphine ligand. 3 resulting from the complexation of the substrate to the iridium metal, forming the species akin to 16, is calculated as the difference in enthalpy between the enthalpy of the optimised complex, 16, and the sum of the enthalpies of the optimised substrate and the optimised [(D )Ir(PMe )(NHC)]+ complex. As 2 3 Figure 8. Substrates used in the DFT calculations. illustrated by the values in Table 7, all complexes have a stabilising interaction enthalpy with the substrates. It should be noted that the values revealed by the activation enthalpy values the presented represent binding enthalpies, as opposed to substrates labelling via a 5-mmi, acetophenone, 6a, free energies, and, as such, entropic effects have not and 2-phenylpyridine, 6e, were found to be most been accounted for. In all cases, the entropic active. The identification of acetanilide, 6h, as the contribution will negatively affect the stability of the least active substrate further supports our more complex relative to the separated reactants. general observations that compounds which are As shown in Table 7, substrate 6e binds most labelled via a 6-mmi are less reactive in our catalysed strongly to the iridium centre compared to other HIE reactions (vide supra).[5d] substrates, presumably due to the better binding ability of the more basic N atom, relative to a Table 6. Enthalpy changes for the oxidative addition step carbonyl O atom. In addition, substrate 6h binds to with three different substrates. the iridium centre with a similar strength as displayed ∆Ha),b) by substrate 6a. In both cases the coordination Species 6a 6e 6h proceeds via interaction of the oxygen atom lone pair of electrons to the metal core. Hence, it can be 16 0.00 0.00 0.00 deduced that the activation enthalpy, rather than the TS(16-13) 18.13 17.53 23.04 binding enthalpy, dictates the more effective labelling 13 1.20 2.73 5.11 via a 5- over a 6-mmi. a) All enthalpy changes are in kcal/mol at 298.15 K. b) Calculations are based on the catalyst bearing the smaller Table 7. Calculated binding energies (PR = PMe ). PMe ligand. 3 3 3 Substrate 6a 6e 6h ∆H a) -23.48 -31.42 -23.94 bind Further theoretical calculations were performed to a) All relative enthalpies are in kcal/mol at 298.15 K. determine the binding energies of the three substrates under investigation (Table 7), since these had previously also been shown to influence labelling efficiencies.[22] The stabilisation energy, ∆H , bind 8 Incorporation of Tritium was the extremely successful tritiation of the drug molecule Celecoxib, 19m; further and in contrast Following the success of our emerging iridium with the analogous deuterium experiment, only the phosphine/carbene species for H/D exchange, and positions ortho to the pyrazole directing group with an improved understanding of the catalytic displayed tritium incorporation. With respect to all mechanism, investigations were undertaken to examples shown in Table 8, we were pleased to note ascertain whether these Ir species could also facilitate that the process was remarkably clean, with no hydrogen-tritium exchange reactions. Since significant tritiated by-products being observed. radionuclides can be detected more sensitively than Indeed, in previous studies we have revealed an stable isotopes, it is common practice within appreciably different reaction profile, possessing an pharmaceutical laboratories to prepare both the stable array of T-possessing by-products, when Crabtree’s isotopically-labelled molecule and the radio-labelled catalyst is employed.[9] compound.[2c,9] Therefore, it was important to determine the activity of the developed catalysts in the labelling of substrates with tritium. Due to the Conclusions more challenging hydrogen-tritium exchange process, based on the increased tritium-tritium bond strength, By embedding a bulky N-heterocyclic carbene ligand 5 mol% catalyst loading was required. Nonetheless, in combination with different phosphine ligands, a over a relatively short reaction time of only 2 hours, a series of highly active iridium-based catalysts have successful tritiation protocol was achieved (Table 8). been developed for hydrogen isotope exchange. Such Table 8. Tritiation studies with catalysts 5a-c.a),b) catalysts have been shown to produce high incorporations of deuterium and tritium adjacent to a good range of relevant functional units within a series of compounds, including pharmaceutically-active agents. Additionally, combined experimental and computational studies have led to a deeper understanding of the nature of such catalyst systems and the active species generated, as well the overall reaction mechanism. These investigations have indicated that the C-H activation process is the rate- limiting step within this HIE manifold. Further, the C-H activation event also constitutes the regiochemistry-determining step when both 5- and 6- metallocyclic intermediates are possible, leading to appreciable selectivities within the established exchange processes. With regards the pivotal C-H activation, we believe that the electron-rich ligand set employed as part of the emerging catalyst systems is central to the facilitation of the requisite oxidative addition step within the catalytic cycle. Further, the sterically crowded ligand sphere will, in turn, aid reductive elimination. In relation to this latter point, when sterically less demanding phosphine/NHC combinations were applied within our laboratory, the a) 5 mol% of Ir catalyst employed over 2 hours. b) T resulting complexes displayed appreciably lowered x activity within HIE processes. It should also be noted (where x = 0, 1, 2, 3, 4) refers to the number of tritium that the bulky NHC and phosphine ligands used atoms incorporated into the molecule, e.g. for 19b, T (23) 1 within our series of iridium complexes could provide indicates that 23% of the sample contained one tritium atom. See Supporting Information for further details. the active catalytic species, formed within the reaction manifold, with enhanced levels of stability by preventing the existence of inactive iridium Tritiation via a 5-mmi proved relatively facile with clusters[12] or driving the presence of monomeric high levels of T uptake being observed (19b, 19e, active iridium species.[11b] 19p). Perhaps as anticipated, hydrogen-tritium Overall, the novel catalysts developed for HIE exchange through a 6-mmi was less efficient, with reactions within this programme have the clear only 7% incorporation achieved with acetanilide, 19h. potential to replace Crabtree’s catalyst as the industry As anticipated, only the position activated through standard in this area. Moreover, these complexes are the 5-mmi in 19j was labelled regioselectively and stable in air and at room temperature, retaining their with good levels of effectiveness. Of particular note activity over prolonged storage periods. The 9 experimental and computational methods employed room temperature for the allotted reaction time. After this time, excess deuterium gas was removed from the system here, aligned with the associated mechanistic and under vacuum. The reaction mixture was concentrated theoretical insights delivered through this study, are under reduced pressure and the catalyst complex was now being employed within our laboratory in precipitated using diethyl ether (~10 mL) and removed by endeavours to develop catalyst systems of further filtration through a plug of silica. Concentration of the filtrate under reduced pressure yielded the enhanced activity and more widespread substrate product/substrate mixture, which was analysed by 1H applicability. NMR spectroscopy. The level of isotope incorporation into the substrate was determined by 1H NMR analysis of the reaction products. As such, the residual proton signal from the site of incorporation was compared against that of a Computational Methods site where incorporation was not expected or occurred. Density functional theory (DFT) was employed to Full details of all experimental procedures, analyses, and calculate the electronic structures and energies for all DFT calculations (including optimised Cartesian coordinates) can be found in the Supporting Information. species involved in H/D exchange reactions. A hybrid meta-GGA exchange correlation functional M06[23] was used in conjunction with the 6-31G(d,p)[24] basis Acknowledgements set for main group non-metal atoms and the Stuttgart RSC[25] effective core potential along with the We thank the University of Strathclyde (J.A.B.), AstraZeneca, associated basis set for Ir. Harmonic vibrational R&D Mölndal (S.I. and A.R.C.), and the Carnegie Trust (M.R.) frequencies are calculated (with the incorporation of for postgraduate studentship funding. Mass spectrometry data deuterium wherever needed) at the same level of were acquired at the EPSRC UK National Mass Spectrometry Facility at Swansea University. T.T. thanks the Glasgow Centre theory to characterise respective minima (reactants, for Physical Organic Chemistry for funding. intermediates, and products with no imaginary frequency) and first order saddle points (TSs with one imaginary frequency). All calculations have been References performed with the GAUSSIAN 09 quantum chemistry programme package. More detailed [1] a) R. Mahajan, K. Gupta, J. Pharm. Bioallied Sci. 2010, discussion of the computational methods, with full 2, 307-313; b) J. Arrowsmith, P. Miller, Nature references, can be found in the Supporting Reviews Drug Discovery 2013, 12, 569-569. Information. [2] a) W. J. S. Lockley, J. Label. Compd. Radiopharm. 2007, 50, 779-788; b) J. Atzrodt, V. Derdau, T. Fey, J. Experimental Section Zimmermann, Angew. Chem. Int. Ed. 2007, 46, 7744- 7765; c) E. M. Isin, C. S. Elmore, G. N. Nilsson, R. A. General Procedure for the Synthesis of Complexes 5a- Thompson, L. Weidolf, Chem. Res. Toxicol. 2012, 25, 5e: 532-542; d) W. J. S. Lockley, A. McEwen, R. Cooke, J. Label. Compd. Radiopharm. 2012, 55, 235-257. A flame dried and nitrogen cooled Schlenk tube was charged with 3 and dry benzene [3] For examples using platinum catalysis, see: J. L. (10 mL). The solution was treated with freshly prepared 1 Garnett, R. J. Hodges, J. Am. Chem. Soc. 1967, 89, M sodium ethoxide solution and stirred for 10 minutes. After this time, 4 was added and the mixture stirred for 5 h 4546-4547. at r.t. The solvent was removed under high vacuum and the [4] For examples using rhodium catalysis, see: a) M. R. residue triturated with dry ether prior to filtration through celite under N . After solvent exchange to dry THF (15 Blake, J. L. Garnett, I. K. Gregor, W. Hannan, K. Hoa, 2 mL), AgPF6 was added and the resultant slurry stirred for M. A. Long, J. Chem. Soc., Chem. Commun. 1975, 30 minutes at r.t. After filtration through celite under N , 2 930-932; b) W. J. S. Lockley, Tetrahedron Lett. 1982, the solution was treated with phosphine and the resultant ruby red solution was stirred for 2 h. Purification by 23, 3819-3822; c) W. J. S. Lockley, J. Label. Compd. recrystallisation yielded the desired complex. Radiopharm. 1984, 21, 45-57; d) W. J. S. Lockley, J. Label. Compd. Radiopharm. 1985, 22, 623-630; e) D. Standard Hydrogen Isotope Exchange Procedure: Hesk, J. R. Jones, W. J. S. Lockley, J. Pharm. Sci. 1991, 80, 887-890. A flame dried and nitrogen cooled 250 mL 3-neck round bottomed flask, equipped with two stopcock valves and a [5] For examples using iridium catalysis, see: a) R. H. suba seal, was charged with the iridium complex (5 mol%) Crabtree, E. M. Holt, M. Lavin, S. M. Morehouse, and dry DCM (2.5 mL), followed by the substrate (0.215 mmol). The suba seal was replaced with a greased glass Inorg. Chem. 1985, 24, 1986-1992; b) R. Heys, J. stopper and the reaction vessel was cooled to -78 C in a Chem. Soc., Chem. Commun. 1992, 680-681; c) J. R. dry ice/acetone bath, prior to being purged twice with Heys, A. Y. L. Shu, S. G. Senderoff, N. M. Phillips, J. nitrogen. The flask was then evacuated and filled with Label. Compd. Radiopharm. 1993, 33, 431-438; d) A. deuterium gas via balloon. The flask was removed from the slurry and allowed to warm to room temperature. Y. L. Shu, W. Chen, J. R. Heys, J. Organomet. Chem. (NOTE: the glass stopper must be physically restrained as 1996, 524, 87-93; e) L. P. Kingston, W. J. S. Lockley, the reaction mixture warms to room temperature; further A. N. Mather, E. Spink, S. P. Thompson, D. J. standard caution should also be observed ensuring the Wilkinson, Tetrahedron Lett. 2000, 41, 2705-2708; f) J. robust nature of the glassware used and the employment of less vigorous stirring at this stage in the process). The G. Ellames, S. J. Gibson, J. M. Herbert, W. J. Kerr, A. reaction mixture was then allowed to stir vigorously at H. McNeill, Tetrahedron Lett. 2001, 42, 6413-6416; g) 10