Acyl Amidines by Pd-Catalyzed Aminocarbonylation: One-Pot Cyclizations and 11C Labeling

A protocol for the carbonylative synthesis of acyl amidines from aryl halides, amidines, and carbon monoxide catalyzed by Pd(0) is reported herein. Notably, carbon monoxide is generated ex situ from a solid CO source, and several productive palladium ligands were identified with complementary benefits and substrate scope. Furthermore, sequential one-pot, two-step protocols for the synthesis of 1,2,4-triazoles and 1,2,4-oxadiazoles via acyl amidine intermediates are reported. In addition, this approach was extended to isotopic labeling using [11C]carbon monoxide to allow, for the first time, synthesis of 11C-labeled acyl amidines as well as a 11C-labeled 1,2,4-oxadiazole.


■ INTRODUCTION
Acyl amidines are useful intermediates in the synthesis of a number of heterocycles, such as 1,2,4-triazoles, 1,2 1,3,5triazines, 3 1,2-dihydro-3H-pyrrol-3-ones, 4 and 1,2,4-oxadiazoles. 5,6 They are also interesting motifs in drug discovery, and biologically active examples are found throughout the literature, including angiotensin II receptor ligands, 7 thrombin (prodrug), 8 β-secretase, 9 cathepsin D, 9 and renin inhibitors. 9 The most straightforward synthesis of acyl amidines is by acylation of amidines and was indeed reported by Pinner already in 1889 from an acid anhydride. 10 Alternative strategies include reaction of acylimidates with amines, 11,12 hydroalumination, 13 a copper-catalyzed multicomponent reaction, 14 and a rhodium-catalyzed synthesis from nitrosobenzene derivatives with N-sulfonyl-1,2,3-triazoles. 15 As part of our research program on the development of palladium(0)-catalyzed carbonylation reactions, we have previously investigated the use of amidine nucleophiles to afford acyl amidines. Initial attempts using molybdenum hexacarbonyl as an in situ solid CO source 16 were unsuccessful due to problematic purification of the product, and the project was halted. Since then, Staben and Blaquiere have published an elegant one-pot, two-step protocol in which they used aryl iodides (and one example of an aryl bromide), amidines, and carbon monoxide in a palladium(0)-catalyzed carbonylation to give acyl amidines, which were subsequently reacted with hydrazines to give the corresponding 1,2,4-triazoles (see Scheme 1). More recently, two-chamber systems such as COware developed by Skrydstrup et al. have enabled the use of ex situ carbon monoxide generated by a large array of convenient carbon monoxide sources. 17,18 With this progress in mind, we decided to re-evaluate the carbonylative synthesis of acyl amidines, this time taking advantage of a two-chamber system 19 for ex situ generation of carbon monoxide from Mo(CO) 6 . Noting that the acyl amidine was only isolated as one example by HPLC in the protocol by Staben and Blaquiere, we decided to focus on developing a method for the synthesis and isolation of acyl amidines using a safe and convenient solid source of carbon monoxide. 1 As secondary objectives, we noted that heterocycles other than 1,2,4triazoles should be accessible in a similar one-pot, two-step fashion and thus decided to pursue a protocol for the synthesis of 1,2,4-oxadiazoles, a structural motif present in many biologically active compounds as well as in approved drugs. 20 One of the major advantages of the carbonylation reaction, in comparison with other strategies to access carbonyl derivatives, is the ability to prepare 11 C-, 13 C-, or 14 C-labeled products using isotopically modified carbon monoxide. To demonstrate this versatility, the method was also translated into a radiochemical setting to produce 11 C-labeled acyl amidines and 1,2,4-oxadiazoles by employing [ 11 C]CO, thus enabling future positron emission tomography (PET) applications.

■ RESULTS AND DISCUSSION
The investigation started by screening solvents, catalysts, stoichiometry, time, and temperature to establish general reaction conditions for the reaction between aryl iodides and amidines, see Table 1. 4-Iodotoluene (1a) and benzamidine (2a) were chosen as model substrates, and the reaction was performed in a two-chamber setup (see SI) in which chamber 1 is the reaction chamber while chamber 2 serves as the COreleasing chamber. The CO-releasing system 21 was kept constant throughout the screening of reaction conditions, and chamber 2 thus contained 0.5 equiv of Mo(CO) 6 in 2.5 mL of 1,4-dioxane with 2.5 equiv of DBU as the base that promotes the release of CO. 22 Testing a number of suitable solvents with Pd(OAc) 2 as the sole component of the catalytic system revealed that DMF was most productive, giving 74% NMR yield (Table 1, entry 2), compared to 61% and 12% for DMA and DMSO, respectively (entries 1 and 3). Adding PPh 3 as a ligand (2:1 ratio to palladium) increased the yield to 92% (entry 4) with the use of Pd(PPh 3 ) 4 equally successful, giving 91% yield (entry 5). Increasing or decreasing the catalyst loading (10% or 2.5%) resulted in lower yields (70% and 12%, respectively, entries 7 and 6), and using 1a in excess provided no added advantage (entry 8, 76%). Decreasing the time and temperature to 2 h and 80°C gave a similar yield (entry 9, 87%, compare with entry 4). Thus, the reaction conditions were established using 5% Pd(OAc) 2 and 10% PPh 3 as the catalytic system in DMF, with the amidine nucleophile in excess toward the yielddetermining aryl iodide.
Next, an investigation of the scope of the reaction with regard to the (hetero)aryl iodide partner was performed, see Table 2. Excellent yields were achieved for 4-methyl-, 3methyl-, and 4-bromo-substituted iodobenzenes, furnishing 97%, 91%, and 90% of 3a, 3c, and 3d, respectively. The thiophene derivative 3e could be isolated in 63% yield from the corresponding iodide. Electron-poor 4-acetyl, 4-trifluoromethyl, and 3-nitro iodobenzenes gave varying yields of 83% (3b), 9% (3g), and 47% (3i) yields, respectively, whereas electronrich 4-iodoanisole (1f) gave 3f in 57% yield. Somewhat surprisingly, 2-methyl-substituted 3h was only isolated in 20% yield, while 1-iodonaphtalene (1k) gave 39% isolated yield of 3k. Unfortunately, pyridine derivative 3j was only formed in trace amounts. At this point, in an attempt to improve the outcome for the less productive (hetero)aryl iodides, other ligands (DPEphos, Xantphos, dppp, and dppf) were tested. The change of ligand for aryl iodides 1f−1k proved beneficial and resulted in improved yields for all but 3j, albeit with different ligands. The yield for 3f was improved from 57% to 87% by use of Xantphos, whereas the other ligands offered only slight improvements compared to PPh 3 . Xantphos also turned out to be beneficial in the synthesis of 3g and 3i, where the yields were drastically improved from 9% and 47% to 86% and 86%, respectively. For the sterically encumbered 3h, DPEphos was the best ligand, and the yield was raised to 70% compared to the 20% obtained with PPh 3 . However, this strategy was not successful in the case of 1-naphthyl derivative 3k, where the gain in yield was only modest with DPEphos. 2-Iodopyridine (1j) was not productive using DPEphos or Xantphos as ligand.
Given the good performance of aryl iodides, we also opted to investigate aryl bromides as aryl−palladium precursors in this reaction, see Table 4. Initial screening revealed that the reaction time and temperature needed to be increased, and the reactions were run for 4 h at 100°C. 4-Bromotoluene was productive with all ligands tested with good isolated yields using PPh 3 , Xantphos, and dppf, at 89%, 84%, and 72%, respectively. These three ligands were then used for investigation of electron-poor aryl bromide 4-bromoacetophenone and electron-rich aryl bromide 4-bromoanisole. Xantphos gave the best outcome for 4-bromoacetophenone with 78% yield compared with 19% yield with PPh 3 and 58% yield with dppf. Xantphos was also the ligand of choice for 4bromoanisole with 82% yield, while the performance of the other ligands was reversed in this case: PPh 3 gave 79% yield and dppf 29% yield. For the substrate 2-bromotoluene, DPEphos was included in the investigation due to the favorable results for the corresponding iodine derivative. Notably, Xantphos was unproductive, and dppf was found to be the most productive ligand with 32% isolated yield.
Having established viable conditions for the generation of the acyl amidines from aryl iodides/bromides and amidines, the investigation moved on to the direct use of the formed acyl amidine as an intermediate in the synthesis of 1,2,4-triazoles and 1,2,4-oxadiazoles. Pleasingly, an adaption of the protocol by Staben and Blaquiere 1 with the conditions developed herein to generate the acyl amidine intermediate gave triazole 5 in 65% yield over two steps (Scheme 2).
To demonstrate the utility of this protocol for the synthesis of biologically active compounds, we opted to exemplify this with the synthesis of a Nrf2 activator called DDO-7263 and a precursor to ataluren, a drug used for treatment of Duchenne muscular dystrophy (Scheme 3). DDO-7263 is an Nrf2 activator, recently suggested to act on Rpn6 to regulate the Nrf2 signaling pathway. 23−25 With our protocol, DDO-7263 could be synthesized in a one-pot, two-step fashion from commercially available starting materials. The yield of 37% is also higher than the overall yield of the first published literature procedure. 23 In addition, an ataluren precursor was prepared from 2-fluoroiodobenzene and 3-methyl-benzamidine in 52% yield, which upon subsequent benzylic oxidation can give the Duchenne muscular dystrophy drug ataluren. 6 ■ RADIOCHEMISTRY PET is a noninvasive imaging technique, widely used in cardiology, neurology, and oncology. 26−28 PET has also found applications in drug development owing to the possibility to study the pharmacokinectics and the pharmacodynamics of labeled drug candidates in vivo. 29−31 A radioisotope commonly incorporated in PET tracers is carbon-11 with a half-life of 20.4 min. With the possibility to produce carbon-11 in the form of [ 11 C]CO, we sought to investigate the possibility of synthesizing 11 C-labeled acyl amidines both for isolation and as a precursor for heterocycle synthesis.
To find conditions suitable for incorporation of carbon-11, a set of reactions was performed based on results from optimization of the Mo(CO) 6 reaction using 4-iodoanisole (1f) and benzamidine (2a) as model compounds (Table 5). Starting with Pd(OAc) 2 and Xantphos in DMF, the reaction was run at 120°C for 10 min (entry 1). This resulted in 98% of the gaseous [ 11 C]CO being trapped as nonvolatile 11 Clabeled products ([ 11 C]CO conversion, entry 1). The product selectivity, based on the crude HPLC chromatogram, was 49%, thereby giving a radiochemical yield (RCY, see SI for definitions and calculations) of 48% for 11 C-3f. To improve the product selectivity, the ligand and solvent were changed to Scheme 2. One-Pot, Two-Step Synthesis of 1,2,4-Triazole 5a and 1,2,4-Oxadiazole 6a−6d a a Isolated yield (>95% purity as determined by 1 H NMR). PPh 3 in 1,4-dioxane, which gave a 74% isolated yield of 3f using the conditions stated in Table 1. The product selectivity was improved, but a slight loss in [ 11 C]CO conversion resulted in a similar RCY of 49% (entry 2). A further improvement in product selectivity was obtained with Pd(PPh 3 ) 4 (67%). However, Pd(PPh 3 ) 4 imposed solubility issues, and to simplify the subsequent HPLC purification step, the amount of Pd(PPh 3 ) 4 was reduced to 0.1 equiv. Although there was a loss in [ 11 C]CO conversion, from 94% to 80%, the RCY was increased to 54% (entry 3). Running the reaction in DMF was very beneficial for the product selectivity (87%), but as the [ 11 C]CO conversion dropped to 54%, the RCY was not improved compared to entry 3.
Although the differences in the estimated RCY were small, the conditions from entry 3 were chosen for isolation of three 11 C-labeled acyl amidine derivatives ( Table 6). Electron-rich 11 C-3f and electron-poor 11 C-3b were gratifyingly isolated in 24% and 36% RCY, respectively. When shortening the reaction time to 5 min, the RCY dropped to 7% and 83 MBq 11 C-3f was isolated after 37 min starting from 3.2 GBq [ 11 C]CO. In comparison, with a 10 min reaction time, 190 MBq 11 C-3f was isolated after 41 min starting from 3.5 GBq [ 11 C]CO. Sterically hindered 11 C-3h, however, was not formed under the conditions employed. No formation was seen even when changing the solvent to DMF, raising the reaction temperature to 150°C, or changing the palladium source to Pd(OAc) 2 and DPEphos as ligand.
A principle of PET is the microdosing concept, i.e., only subpharmacological doses of the PET tracer should be injected. 32 The concept of molar activity is therefore an important parameter for estimation of the amount of 11 Clabeled tracer versus isotopically unmodified tracer in the isolated 11 C-labeled product fraction. The molar activity was calculated for 11 C-3b following two large irradiations. Starting from 14.3 and 15.4 GBq of [ 11 C]CO and isolating 1.7 and 2.1 GBq, the molar activities of 11 C-3b were at the end the purification, 488 and 650 GBq/μmol, respectively. The high molar activities are in line with previously reported results using [ 11 C]CO. 33,34 Building on the successful one-pot carbonylation/cyclization sequence developed using Mo(CO) 6 , 11 C-6a was synthesized from 1a and 2a (Scheme 4). The cyclization was tested with hydroxylamine hydrochloride and sodium hypochlorite, with only the former giving full consumption of the intermediate 11 C-3a (HPLC analysis). 5,6 Pleasingly, isotopically labeled 1,2,4-oxadiazole 11 C-6a could be isolated in a decay-corrected RCY of 25% and in 99% radiochemical purity. The RCY was based on the starting amount of [ 11 C]CO and the decaycorrected, isolated amount of 11 C-6a.
Step 2: Hydroxylamine hydrochloride (7 equiv) and 50% acetic acid (aq) were added to the reaction mixture. The reaction was heated at 150°C for another 5 min. phosgene. This is, however, to the best of our knowledge, the first time that [ 11 C]CO has been used in the synthesis of labeled acyl amidines (with 11 C) and the first example of the synthesis of a 11 C-labeled oxadiazole or a ring-atom-labeled heterocycle using [ 11 C]CO. The method presented herein therefore complements other 11 C-labeled precursors available for synthesis of 11 C-labeled heterocyclic derivatives, thus opening up a significant new area of 11 C chemical space.

■ CONCLUSION
We have developed a protocol for the palladium-catalyzed carbonylative synthesis of acyl amidines from (hetero)aryl iodides or aryl bromides and amidines using a bridged two-vial system to generate CO gas ex situ from Mo(CO) 6 . Excellent yields can be achieved when using an appropriate ligand, with PPh 3 generally working well for electron-rich and neutral (hetero)aryl iodides. DPEphos was shown to be a better choice for sterically hindered aryl iodides, whereas Xantphos was very productive for electron-poor aryl iodides. For less nucleophilic amidines, PPh 3 and dppf were the ligands of choice. These results highlight the influence of subtle differences in substrate/ligand properties on the reaction outcome and serve as a reminder that a "one-ligand-for-all-substrates approach" is not always possible. In total, the scope and limitations of the reaction were demonstrated in over 25 diverse examples including more challenging aryl bromides. A new strategy for the one-pot, two-step synthesis of 1,2,4oxadiazoles was also developed, allowing synthesis of unsymmetrically 3,5-substituted 1,2,4-oxadiazoles from (hetero)aryl iodides, amidines, CO, and hydroxylamine hydrochloride. These methods were also translated into a radiochemical setting and were successfully employed in a number of 11 Clabeling examples with good radiochemical yields. Finally, the synthesis of a 11 C-labeled 1,2,4-oxadiazole represents, to the best of our knowledge, the first incorporation of carbon-11 into a heterocyclic ring using [ 11 C]CO, opening up a significant scope for new 11 C chemistry development.
■ EXPERIMENTAL SECTION General Chemistry Information. All substrates, reagents, and solvents were commercially available and used without further purification. Heating was carried out using a 17.4 mm DrySyn reaction vial insert compatible with the two-chamber system used for carbonylation. Microwave heating was performed using a Biotage Initiator 2.5 equipped with an IR sensor that is used to determine the temperature. Analytical reversed phase HPLC-MS was performed on a Dionex Ultimate 3000 system using 0.05% HCOOH in water and 0.05% HCOOH in acetonitrile as mobile phase with MS detection, equipped with a C18 (Phenomenex Kinetex SB-C18 (4.8 × 50 mm)) column using a UV diode array detector. Purifications were performed on an automated Biotage Isolera Flash Chromatography System using 25 or 10 g prepacked Biotage SNAP KP-SIL columns. Carbon-11 was prepared by the 14 N(p,α) 11 C nuclear reaction using 17 MeV protons produced by a Scanditronix MC-17 Cyclotron at PET Centre, Uppsala University Hospital, and obtained as [ 11 C]carbon dioxide. The target gas used was nitrogen (AGA Nitrogen 6.0) containing 0.05% oxygen (AGA Oxygen 4.8). Preparative purification of [ 11 C] compounds was performed on a VWR La Prep Sigma system with a LP1200 pump, a 40D UV detector, a Bioscan flowcount radiodetector equipped with a Phenomenex Kinetex C18 (5 μm, 150 × 10.0 mm) column, and 0.1% trifluoroacetic acid (aq.) and acetonitrile as eluents. The identities, concentration, and radiochemical purities of the purified 11 C-labeled compounds were determined with either (A) a VWR Hitachi Elite LaChrom system (L-2130 pump, L-2200 autosampler, L-2300 column oven, L-2450 diode array detector in series with a Bioscan β + -flowcount radiodetector) equipped with a Merck Chromolith Performance RP-18e column (4.6 × 100 mm) and ammonium formate buffer (pH 3.5) and acetonitrile as eluents or (B) an Elite LaChrom VWR International (LaPrep P206 pump, an Elite LaChrom L-2400 UV detector in series with a Bioscan β+-flowcount detector) equipped with a Reprosil-Pur Basic C18 (5 μm 4.6 × 100 mm) with 8.1 mM ammonium carbonate (aq.) and acetonitrile mobile phase and using isotopically unmodified compounds as references. Accurate mass values were determined on a mass spectrometer equipped with an electrospray ion source and TOF detector. NMR spectra were recorded on a Bruker Avance III HD at 25°C and 400 MHz for 1 H, 101 MHz for 13  General Procedure for Synthesis of Acyl Amidines. The reaction was performed in a two-chamber system. Aryl halide (0.5 mmol) and amidine (1.5 equiv) were added to chamber 1 and dissolved in DMF (2 mL), followed by triethylamine (2.5 equiv) and Pd(OAc) 2 (5 mol %). The reaction was briefly stirred before addition of monodentate ligand (10 mol %) or bidentate ligand (5 mol %) and remaining DMF (0.5 mL) followed by capping. To chamber 2 Mo(CO) 6 (0.5 equiv) was added and dissolved in 1,4-dioxane (2.5 mL) followed by DBU (2.5 equiv) just before capping. The final concentration of the aryl halide in DMF was 0.2 M. Purification was done by direct injection on an automated Biotage Isolera Flash Chromatography System (silica gel, gradient elution using 0−100% EtOAc in isohexane, 22 CV).