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

Abstract

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 1,3,5-triazines,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 source¹⁶ 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 system¹⁹ 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 1,2,4-triazoles 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

Scheme 1. Previous Work by Staben and Blaquiere and the Work Presented Herein.

One of the major advantages of the carbonylation reaction, in comparison with other strategies to access carbonyl derivatives, is the ability to prepare ¹¹C-, ¹³C-, or ¹⁴C-labeled products using isotopically modified carbon monoxide. To demonstrate this versatility, the method was also translated into a radiochemical setting to produce ¹¹C-labeled acyl amidines and 1,2,4-oxadiazoles by employing [¹¹C]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 CO-releasing chamber. The CO-releasing system²¹ 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

Table 1. Optimization of Reaction Conditions for the Synthesis of Acyl Amidines from Aryl Iodides and Amidines^a.

entry	solvent	catalyst precursor	ligand	NMR yield (%)
1	DMA	5% Pd(OAc)2		61
2	DMF	5% Pd(OAc)2		74
3	DMSO	5% Pd(OAc)2		12
4	DMF	5% Pd(OAc)2	PPh3	92
5	DMF	5% Pd(PPh3)4		91
6	DMF	2.5% Pd(OAc)2	PPh3	12
7	DMF	10% Pd(OAc)2	PPh3	70
8	DMF	5% Pd(OAc)2	PPh3	76b
9	DMF	5% Pd(OAc)2	PPh3	87c

^aNMR yield calculated by addition of benzyl alcohol as internal standard. Reaction conditions: In chamber 1 1a (0.5 mmol), 2a (1.5 equiv), Pd(OAc)₂, ligand (2:1 ligand:Pd ratio), and Et₃N (2.5 equiv) were mixed in the specified solvent (2.5 mL), and in chamber 2 Mo(CO)₆ (0.5 equiv) and DBU (2.5 equiv) were mixed in 1,4-dioxane (2.5 mL). Both chambers were capped and heated at 100 °C for 4 h.

^bA 0.5 mmol amount of 2a and 1.5 equiv of 1a.

^cReaction run for 2 h at 80 °C.

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 PPh₃ as a ligand (2:1 ratio to palladium) increased the yield to 92% (entry 4) with the use of Pd(PPh₃)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% PPh₃ as the catalytic system in DMF, with the amidine nucleophile in excess toward the yield-determining 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-, 3-methyl-, 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 electron-rich 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 PPh₃. 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 PPh₃. 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.

Table 2. Investigation of the Aryl Iodide Scope for the Synthesis of Acyl Amidines^a.

^aIsolated yield (>95% purity as determined by ¹H NMR). Reaction conditions: In chamber 1 1a–k (0.5 mmol), 2a (1.5 equiv), Pd(OAc)₂ (5%), PPh₃ (10%), and Et₃N (2.5 equiv) were mixed in DMF (2.5 mL), and in chamber 2 Mo(CO)₆ (0.5 equiv) and DBU (2.5 equiv) were mixed in 1,4-dioxane (2.5 mL). Both chambers capped and heated at 80 °C for 2 h. Ligand screen: DPEphos (5%), Xantphos (5%), dppp (5%), dppf (5%).

^bOne millimole scale.

The scope of the reaction with regard to the amidine nucleophile was also investigated, see Table 3. DPEphos was used as the ligand for the investigation as we reasoned that the combination of its bidentate nature and increased flexibility would provide the greatest generality. Pleasingly, the ligand was in general productive, affording yields of 34–71% for products 3l–v, with aryl, heteroaryl, and alkyl amidines as nucleophiles. Specifically, electron-rich (hetero)aryl amidines worked well, giving a 71% isolated yield of 3l, 3m, and 3t, respectively. The 3-pyridine derivative 3r was isolated in 34% yield, whereas thiophene derivative 3s was isolated in 43% yield. Alkyl amidines furnished yields of the corresponding products 3n–3q in the range of 44–73%. Electron-poor aryl amidines afforded slightly lower yields, returning the chloro-substituted derivative 3u in 45% yield and the trifluoromethyl-substituted product 3v in 41% yield. Analogously to the less productive substrates in Table 2, 3u was synthesized using Xantphos, dppp, dppf, and PPh₃, thus affording 3u in 57%, 20%, 76%, and 78% yield, respectively. Of note was the poor performance by Xantphos, whereas dppf and PPh₃ returned the best yields and were also tested in the synthesis of 3v. Both ligands improved the yield in a similar extent as for 3v.

Table 3. Investigation of the Amidine Scope of the Synthesis of Acyl Amidines^a.

^aIsolated yield (>95% purity as determined by ¹H NMR unless otherwise stated). Reaction conditions: In chamber 1 1a (0.5 mmol), 2b–2j (1.5 equiv), Pd(OAc)₂ (5%), DPEphos (5%), and Et₃N (2.5 equiv) were mixed in DMF (2.5 mL), and in chamber 2 Mo(CO)₆ (0.5 equiv) and DBU (2.5 equiv) were mixed in 1,4-dioxane (2.5 mL). Both chambers were capped and heated at 80 °C for 2 h. Ligand screen: PPh₃ (10%), Xantphos (5%), dppp (5%), dppf (5%).

^b3o 85% pure.

^c3q > 90% purity as determined by ¹H NMR.

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 PPh₃, 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 PPh₃ and 58% yield with dppf. Xantphos was also the ligand of choice for 4-bromoanisole with 82% yield, while the performance of the other ligands was reversed in this case: PPh₃ 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.

Table 4. Investigation of the Aryl Bromide Scope for the Synthesis of Acyl Amidines^a.

^aIsolated yield (>95% purity as determined by ¹H). Reaction conditions: In chamber 1 1l–o (0.5 mmol), 2a (1.5 equiv), Pd(OAc)₂ (5%), ligand (X%), and Et₃N (2.5 equiv) were mixed in DMF (2.5 mL), and in chamber 2 Mo(CO)₆ (0.5 equiv) and DBU (2.5 equiv) were mixed in 1,4-dioxane (2.5 mL). Both chambers are capped and heated at 100 °C for 4 h. Ligand screen: PPh₃ (10%), DPEphos (5%), Xantphos (5%), dppp (5%), dppf (5%), (t-Bu)₃P BF₃K (10%).

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 Blaquiere¹ with the conditions developed herein to generate the acyl amidine intermediate gave triazole 5 in 65% yield over two steps (Scheme 2).

Scheme 2. One-Pot, Two-Step Synthesis of 1,2,4-Triazole 5a and 1,2,4-Oxadiazole 6a–6d.

Isolated yield (>95% purity as determined by ¹H NMR).

1,2,4-Oxadiazoles are accessible from acyl amidines by reaction with hydroxylamine hydrochloride or sodium hypochlorite.^5,6 After some experimentation, we discovered that the former reagent was suitable for the two-step one-pot preparation of 1,2,4-oxadiazoles. Specifically, we utilized this strategy to prepare unsymmetrical diaryl-1,2,4-oxadiazoles 6a–6d in 40–73% yield from the respective aryl iodide and 2a 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

Scheme 3. One-Pot, Two-Step Synthesis of Nrf2 Activator DDO-7263 and Ataluren Precursor (R = CH₃).

Isolated yield (>95% purity as determined by ¹H NMR). Reaction conditions as described in Scheme 2.

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 [¹¹C]CO, we sought to investigate the possibility of synthesizing ¹¹C-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 [¹¹C]CO being trapped as nonvolatile ¹¹C-labeled products ([¹¹C]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 ¹¹C-3f. To improve the product selectivity, the ligand and solvent were changed to PPh₃ 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 [¹¹C]CO conversion resulted in a similar RCY of 49% (entry 2). A further improvement in product selectivity was obtained with Pd(PPh₃)₄ (67%). However, Pd(PPh₃)₄ imposed solubility issues, and to simplify the subsequent HPLC purification step, the amount of Pd(PPh₃)₄ was reduced to 0.1 equiv. Although there was a loss in [¹¹C]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 [¹¹C]CO conversion dropped to 54%, the RCY was not improved compared to entry 3.

Table 5. Screening Conditions for ¹¹C Incorporation^a.

entry	catalyst precursor	ligand	solvent	[11 C]CO-conversionb (%)	product selectivity (%)	RCYc (%)	no. of exp
1	Pd(OAc)2	Xantphos	DMF	98 ± 0	49 ± 9	48 ± 9	3
2	Pd(OAc)2	PPh3	dioxane	91 ± 4	56 ± 7	49 ± 6	3
3	Pd(PPh3)4		dioxane	80 ± 5	67 ± 4	54 ± 6	3
4	Pd(PPh3)4		DMF	54 ± 4	87 ± 5	47 ± 6	3

^aReaction conditions: 1f (9.0 μmol), 2a (2.0 equiv), Pd(OAc)₂ (0.5 equiv), Xantphos (1.0 equiv), PPh₃ (1.0 equiv), Pd(PPh₃)₄ (0.1 equiv), Et₃N (4 equiv). All dry regents were dissolved in 400 μL of solvent before addition of Et₃N. See Supporting Information for definitions and calculations.

^bAmount of [¹¹C]CO converted to nonvolatile ¹¹C-labeled products. Decay corrected.

^cRadiochemical yield, RCY, estimated from the [¹¹C]CO conversion and HPLC analysis of the crude reaction mixture.

Although the differences in the estimated RCY were small, the conditions from entry 3 were chosen for isolation of three ¹¹C-labeled acyl amidine derivatives (Table 6). Electron-rich ¹¹C-3f and electron-poor ¹¹C-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 ¹¹C-3f was isolated after 37 min starting from 3.2 GBq [¹¹C]CO. In comparison, with a 10 min reaction time, 190 MBq ¹¹C-3f was isolated after 41 min starting from 3.5 GBq [¹¹C]CO. Sterically hindered ¹¹C-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.

Table 6. Synthesis of ¹¹C-Labeled Acyl Amidines^a.

^aReaction conditions as in entry 2, Table 5. Radiochemical purity was >95% in all experiments, determined using two different HPLC columns. The reported radiochemical yields were calculated from the amount of [¹¹C]CO collected in the reaction vial and the radioactivity of the isolated product (decay corrected). See Supporting Information for definitions and calculations.

^bFive minute reaction time.

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 ¹¹C-labeled tracer versus isotopically unmodified tracer in the isolated ¹¹C-labeled product fraction. The molar activity was calculated for ¹¹C-3b following two large irradiations. Starting from 14.3 and 15.4 GBq of [¹¹C]CO and isolating 1.7 and 2.1 GBq, the molar activities of ¹¹C-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 [¹¹C]CO.^33,34

Building on the successful one-pot carbonylation/cyclization sequence developed using Mo(CO)₆, ¹¹C-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 ¹¹C-3a (HPLC analysis).^5,6 Pleasingly, isotopically labeled 1,2,4-oxadiazole ¹¹C-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 [¹¹C]CO and the decay-corrected, isolated amount of ¹¹C-6a.

Scheme 4. One-Pot, Two-Step Synthesis of 1,2,4-Oxadiazole ¹¹C-6a.

Reaction conditions for step 1 as described in Table 4, entry 2. Note that a 5 min reaction time was used in step 1. 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.

Heterocyclic derivatives such as ¹¹C-indole,³⁵ substituted ¹¹C-quinoxaline-2,3-diones,³⁶ a substituted ¹¹C-quinazoline-2,4(1H,3H)-dione,³⁷ a substituted ¹¹C-1,2,4-thiadiazolidine-3,5-dione,³⁷ [carbonyl-¹¹C]7-methyl-8-phenyl-3-trimethylsilyl-pyrazolo[5.1-c][1,2,4]triazine-(1H)4-one,³⁸ and 2-[¹¹C]thymidine³⁹ have previously been synthesized from ¹¹C precursors such as [¹¹C]nitromethane, diethyl [¹¹C]oxalate, [¹¹C]carbon dioxide, [¹¹C]lithium trimethylsilyl ynolate, and [¹¹C]phosgene. This is, however, to the best of our knowledge, the first time that [¹¹C]CO has been used in the synthesis of labeled acyl amidines (with ¹¹C) and the first example of the synthesis of a ¹¹C-labeled oxadiazole or a ring-atom-labeled heterocycle using [¹¹C]CO. The method presented herein therefore complements other ¹¹C-labeled precursors available for synthesis of ¹¹C-labeled heterocyclic derivatives, thus opening up a significant new area of ¹¹C 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 PPh₃ 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, PPh₃ 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 1,2,4-oxadiazoles 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 ¹¹C-labeling examples with good radiochemical yields. Finally, the synthesis of a ¹¹C-labeled 1,2,4-oxadiazole represents, to the best of our knowledge, the first incorporation of carbon-11 into a heterocyclic ring using [¹¹C]CO, opening up a significant scope for new ¹¹C 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 ¹⁴N(p,α)¹¹C nuclear reaction using 17 MeV protons produced by a Scanditronix MC-17 Cyclotron at PET Centre, Uppsala University Hospital, and obtained as [¹¹C]carbon dioxide. The target gas used was nitrogen (AGA Nitrogen 6.0) containing 0.05% oxygen (AGA Oxygen 4.8). Preparative purification of [¹¹C] 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 ¹¹C-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 ¹H, 101 MHz for ¹³C, and ¹⁹F at 376.5 MHz using a SmartProbe BB/1H probe or on a Varian Mercury plus at 25 °C and 400 MHz for ¹H, 101 MHz for ¹³C, and ¹⁹F at 376.5 MHz. Chemical shifts (δ) are reported in ppm, indirectly referenced to tetrametylsilane (TMS) via the residual solvent signal (¹H: CHCl₃ δ 7.26, CD₂HOD δ 3.31, (CHD₂)(CD₃)SO δ 2.50, (CHD₂)(CD₃)CO δ 2.05. ¹³C: CDCl₃ δ 77.2, CD₃OD δ 49.0, (CHD₂)(CD₃)SO δ 39.5, (CD₃)₂CO δ 29.8, 206.3.)

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).

N-(Imino(phenyl)methyl)-4-methylbenzamide, 3a (CAS: 68167-55-5)

3a was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (115 mg, 97% (PPh₃) from 1a and 106 mg, 89% (PPh₃); 27 mg, 23% (DPEphos); 100 mg, 84% (Xantphos); 57 mg, 48% (Dppp); 86 mg, 72% (Dppf) and 51 mg, 43% ((t-Bu)₃P BF₃K) from 1l). ¹H NMR (400 MHz, CDCl₃) δ 8.27 (d, J = 8.2 Hz, 2H), 8.06–8.01 (m, 2H), 7.61–7.55 (m, 1H), 7.54–7.48 (m, 2H), 7.28–7.23 (m, 2H), 2.42 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 180.4 (carbonyl carbon, HMBC), 166.6, 142.8, 135.3, 135.1, 132.5 (2 × CH), 129.9 (2 × CH), 129.0 (4 × CH), 127.8 (2 × CH), 21.8. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₅N₂O239.1184; Found 239.1193.

4-Acetyl-N-(imino(phenyl)methyl)benzamide, 3b

3b was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (111 mg, 83% (PPh₃) from 1b and 25 mg, 19% (PPh₃); 104 mg, 78% (Xantphos); 77 mg, 58% (Dppf) from 1n). ¹H NMR (400 MHz, (CD₃)₂CO) δ 10.85 (s, 1H), 8.66 (s, 1H), 8.48–8.45 (m, 2H), 8.30–8.26 (m, 2H), 8.10–8.06 (m, 2H), 7.68–7.63 (m, 1H), 7.60–7.54 (m, 2H), 2.64 (s, 3H). ¹³C{¹H} NMR (101 MHz, (CD₃)₂CO) δ 197.9, 179.3, 168.3, 142.9, 140.4, 135.8, 133.2, 130.4, 129.5, 128.79, 128.75, 27.0. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₆H₁₅N₂O₂ 267.1134; Found 267.1135.

N-(Imino(phenyl)methyl)-3-methylbenzamide, 3c

3c was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (108 mg, 91% (PPh₃). ¹H NMR (400 MHz, CDCl₃) δ 10.74 (br s, 1H), 8.24–8.13 (m, 2H), 8.10–8.01 (m, 2H), 7.61–7.55 (m, 1H), 7.55–7.48 (m, 2H), 7.40–7.31 (m, 2H), 6.68 (br s, 1H), 2.44 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 180.8, 166.7, 137.73, 137.68, 135.2, 132.9, 132.4, 130.2, 128.9, 128.0, 127.4, 126.9, 21.5. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₅N₂O 239.1184; Found 239.1175.

4-Bromo-N-(imino(phenyl)methyl)benzamide, 3d (CAS: 68167-57-7)

3d was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (137 mg, 90% (PPh₃)). ¹H NMR (400 MHz, CDCl₃) δ 10.76 (br s, 1H), 8.27–8.21 (m, 2H), 8.06–8.00 (m, 2H), 7.63–7.56 (m, 3H), 7.56–7.49 (m, 2H), 6.68 (br s, 1H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 179.6, 167.1, 136.8, 135.0, 132.7, 131.43, 131.42, 129.0, 127.5, 127.1. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₄H₁₂BrN₂O 303.0133; Found 303.0141.

N-(Imino(phenyl)methyl)-5-methylthiophene-2-carboxamide, 3e

3e was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (76 mg, 63% (PPh₃)). ¹H NMR (400 MHz, CDCl₃) δ 10.50 (br s, 1H), 8.02–7.97 (m, 2H), 7.76 (d, J = 3.6 Hz, 1H), 7.60–7.54 (m, 1H), 7.53–7.47 (m, 2H), 6.81–6.77 (m, 1H), 6.57 (br s, 1H), 2.53 (d, J = 1.0 Hz, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 175.4, 166.2, 147.7, 141.4, 134.8, 132.5 (2 × CH), 128.9, 127.5, 126.7, 16.0. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₃H₁₃N₂OS 245.0749; Found 245.0760.

N-(Imino(phenyl)methyl)-4-methoxybenzamide, 3f (CAS: 1445133-92-5)

3f was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (72 mg, 57% (PPh₃); 77 mg, 61% (DPEphos); 111 mg, 87% (Xantphos); 83 mg, 65% (dppp); 79 mg, 62% (dppf) from 1f and 101 mg, 79% (PPh₃); 105 mg, 82% (Xantphos); 37 mg, 29% (Dppf) from 1n). 3f was synthesized according to the general procedure, scaled up to 1 mmol scale, and isolated as an off-white amorphous solid (188 mg, 74% (Xantphos)). ¹H NMR (400 MHz, CDCl₃) δ 8.38–8.33 (m, 2H), 8.05–8.00 (m, 2H), 7.56 (ddt, J = 8.3, 6.5, 1.4 Hz, 1H), 7.51–7.46 (m, 2H), 6.97–6.92 (m, 2H), 3.86 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 180.1, 166.3, 162.9, 135.4, 132.3, 131.9, 130.7, 128.9, 127.5, 113.4, 55.5. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₅N₂O₂ 255.1134; Found 255.1138.

N-(Imino(phenyl)methyl)-4-(trifluoromethyl)benzamide, 3g (CAS: 2052280-77-8)

3g was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (13 mg, 9% (PPh₃); 53 mg, 36% (DPEphos); 124 mg, 86% (Xantphos); 36 mg, 25% (dppp); 36 mg, 25% (dppf)). ¹H NMR (400 MHz, CDCl₃) δ 10.83 (br s, 1H), 8.48 (d, J = 8.0 Hz, 2H), 8.07–8.02 (m, 2H), 7.71 (d, J = 8.2 Hz, 2H), 7.65–7.60 (m, 1H), 7.57–7.51 (m, 2H), 6.79 (br s, 1H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 177.2 (carbonyl carbon, HMBC), 167.5, 140.9, 134.8, 133.3 (d, ²J_CF = 32.3 Hz), 132.9, 130.1, 129.1, 127.6, 125.2 (q, ³J_CF = 3.8 Hz), 122.8 (d, ¹J_CF = 272.3 Hz).¹⁹F NMR (376 MHz, CDCl₃) δ −62.8. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₂F₃N₂O 293.0902; Found 293.0898.

N-(Imino(phenyl)methyl)-2-methylbenzamide, 3h (CAS: 872266-80-3)

3h was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (24 mg, 20% (PPh₃); 83 mg, 70% (DPEphos); 69 mg, 58% (Xantphos); 50 mg, 42% (dppp); 69 mg, 58% (dppf) from 1h and 7 mg, 5% (PPh₃); 14 mg, 11% (DPEphos); 38 mg, 32% (Dppf) from 1o). ¹H NMR (400 MHz, CDCl₃) δ 8.24–8.12 (m, 1H), 8.00 (d, J = 7.9 Hz, 2H), 7.59–7.54 (m, 1H), 7.53–7.46 (m, 2H), 7.37–7.32 (m, 1H), 7.29–7.21 (m, 2H), 2.69 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 166.2 (HMBC), 139.0, 137.9, 135.3, 132.4, 131.6, 130.8 (2 × CH), 129.0, 127.5, 125.6, 22.0. Carbonyl carbon missing. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₅N₂O 239.1184; Found 239.1191.

N-(Imino(phenyl)methyl)-3-nitrobenzamide, 3i

3i was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (63 mg, 47% (PPh₃); 69 mg, 51% (DPEphos); 116 mg, 86% (Xantphos); 15 mg, 11% (dppp); 16 mg, 12% (dppf)). ¹H NMR (400 MHz, CDCl₃) δ 10.85 (s, 1H), 9.23–9.19 (m, 1H), 8.71–8.65 (m, 1H), 8.37 (ddd, J = 8.2, 2.4, 1.2 Hz, 1H), 8.10–8.03 (m, 2H), 7.68–7.60 (m, 2H), 7.60–7.52 (m, 2H), 6.81 (s, 1H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 177.9, 167.7, 148.3, 139.6, 135.4, 134.4, 132.9, 129.11, 129.05, 127.5, 126.3, 124.7. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₄H₁₂N₃O₃ 270.0879; Found 270.0884.

N-(Imino(phenyl)methyl)-1-naphthamide, 3k (CAS: 101716-52-3)

3k was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous liquid (54 mg, 39% (PPh₃); 60 mg, 44% (DPEphos)). ¹H NMR (400 MHz, (CD₃)₂CO δ 10.80 (br s, 1H), 9.20 (d, J = 8.5 Hz, 1H), 8.57 (d, J = 7.1 Hz, 1H), 8.27–8.23 (m, 2H), 8.05 (d, J = 8.1 Hz, 1H), 7.98–7.94 (m, 1H), 7.65–7.51 (m, 6H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 183.2 (carbonyl carbon, HMBC), 166.5, 135.6, 135.2, 134.1, 132.4, 132.1, 131.5, 130.0, 129.0, 128.5, 127.5, 127.1, 126.7, 125.9, 124.9. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₈H₁₅N₂O 275.1184; Found 275.1178.

N-(Imino(p-tolyl)methyl)-4-methylbenzamide, 3l

3l was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (90 mg, 71% (DPEphos)). ¹H NMR (400 MHz, CDCl₃) δ 10.69 (br s, 1H), 8.27 (d, J = 8.2 Hz, 2H), 7.93 (d, J = 8.2 Hz, 2H), 7.27 (dd, J = 15.4, 8.0 Hz, 4H), 6.67 (br s, 1H), 2.44–2.40 (m, 6H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 180.5, 166.5, 143.1, 142.6, 135.3, 132.4, 129.9, 129.6, 128.9, 127.5, 21.8, 21.7. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₆H₁₇N₂O 253.1341; Found 253.1344.

N-(Imino(4-methoxyphenyl)methyl)-4-methylbenzamide, 3m

3m was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (95 mg, 71% (DPEphos)). ¹H NMR (400 MHz, CDCl₃) δ 8.29–8.24 (m, 2H), 8.04–7.99 (m, 2H), 7.27–7.23 (m, 2H), 7.00–6.94 (m, 2H), 3.86 (s, 3H), 2.42 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃ and (CD₃)₂CO) δ 180.3, 166.1, 163.1, 142.5, 135.4, 129.8, 129.4, 128.9, 127.3, 114.2, 55.6, 21.8. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₆H₁₇N₂O₂ 269.1290; Found 269.1284.

N-(Adamantan-1-yl(imino)methyl)-4-methylbenzamide, 3n

3n was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (71 mg, 48% (DPEphos)). ¹H NMR (400 MHz, CDCl₃) δ 10.62 (br s, 1H), 8.18 (d, J = 8.1 Hz, 2H), 7.21 (d, J = 8.1 Hz, 2H), 6.26 (br s, 1H), 2.40 (s, 3H), 2.14–2.08 (m, 3H), 2.00–1.96 (m, 6H), 1.82–1.72 (m, 7H). ¹³C{¹H} NMR (101 MHz, (CD₃)₂CO) δ 180.2, 180.0, 142.4, 137.0, 130.3, 129.3, 41.1, 40.4, 37.3, 29.3, 21.5. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₉H₂₄N₂O 297.1967; Found 297.1973.

N-(Cyclopentyl(imino)methyl)-4-methylbenzamide, 3o

3o was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (59 mg, 51%, 85% pure (DPEphos)). ¹H NMR (400 MHz, CDCl₃) δ 10.33 (br s, 1H), 8.17–8.12 (m, 2H), 7.23–7.18 (m, 2H), 6.12 (br s, 1H), 2.67 (quint, J = 8.0 Hz, 1H), 2.39 (s, 3H), 2.06–1.78 (m, 6H), 1.72–1.61 (m, 2H). ¹³C{¹H} NMR (101 MHz, (CD₃)₂CO) δ 179.1, 165.3, 142.3, 129.8, 128.8, 127.8, 47.6, 31.6, 26.2. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₄H₁₉N₂O 231.1497; Found 231.1502.

N-(Cyclohexyl(imino)methyl)-4-methylbenzamide, 3p

3p was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (54 mg, 44% (DPEphos)). ¹H NMR (400 MHz, CDCl₃) δ 10.38 (br s, 1H), 8.15 (d, J = 8.1 Hz, 2H), 7.21 (d, J = 8.1 Hz, 2H), 6.16 (br s, 1H), 2.39 (s, 3H), 2.23 (tt, J = 11.8, 3.4 Hz, 1H), 2.03–1.96 (m, 2H), 1.89–1.81 (m, 2H), 1.77–1.70 (m, 1H), 1.58–1.46 (m, 2H), 1.40–1.19 (m, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 180.6, 176.5, 142.3, 135.3, 129.7, 128.8, 46.7, 30.7, 26.0, 25.9, 21.7. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₂₁N₂O245.1654; Found 245.1661.

N-(Cyclopropyl(imino)methyl)-4-methylbenzamide, 3q

3q was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (74 mg, 73%, >90% pure (DPEphos)). ¹H NMR (400 MHz, CDCl₃) δ 10.40 (br s, 1H), 8.12–8.06 (m, 2H), 7.22–7.16 (m, 2H), 6.49 (br s, 1H), 2.39 (s, 3H), 1.43–1.36 (m, 1H), 1.34–1.28 (m, 2H), 0.99–0.92 (m, 2H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 180.0, 174.7, 142.2, 135.4, 129.7, 128.8, 21.7, 17.1, 9.5. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₂H₁₅N₂O 203.1184; Found 203.1190.

N-(Imino(pyridin-3-yl)methyl)-4-methylbenzamide, 3r

3r was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (41 mg, 34% (DPEphos)). ¹H NMR (400 MHz, CDCl₃) δ 9.24 (s, 1H), 8.81–8.75 (m, 1H), 8.40–8.34 (m, 1H), 8.28–8.20 (m, 2H), 7.47–7.42 (m, 1H), 7.27–7.23 (m, 2H), 2.42 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 180.6, 164.4, 152.9, 148.7, 143.1, 135.5, 134.8, 131.3, 129.9, 129.1, 123.7, 21.8. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₄H₁₄N₃O 240.1137 Found; 240.1132.

N-(Imino(thiophen-2-yl)methyl)-4-methylbenzamide, 3s (CAS: 883041-84-7)

3s was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (52 mg, 43% (DPEphos)). ¹H NMR (400 MHz, CDCl₃) δ 8.25–8.21 (m, 2H), 7.65 (dd, J = 3.8, 1.1 Hz, 1H), 7.57 (dd, J = 5.1, 1.0 Hz, 1H), 7.27–7.23 (m, 2H), 7.13 (dd, J = 5.0, 3.8 Hz, 1H), 2.41 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 180.3, 160.9, 142.8, 140.3, 134.9, 132.2, 129.8, 129.0, 128.32, 128.25, 21.8. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₃H₁₃N₂OS 245.0749; Found 245.0759.

N-(Imino(2-methyl-1H-indol-3-yl)methyl)-4-methylbenzamide, 3t

3t was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (103 mg, 71% (DPEphos)). ¹H NMR (400 MHz, (CD₃)₂SO) δ 11.77 (s, 1H), 8.11 (d, J = 8.2 Hz, 2H), 8.01–7.97 (m, 1H), 7.40–7.37 (m, 1H), 7.27 (d, J = 7.9 Hz, 2H), 7.15–7.12 (m, 2H), 2.79 (s, 3H), 2.37 (s, 3H). ¹³C{¹H} NMR (101 MHz, (CD₃)₂SO) δ 177.9, 165.7, 141.5, 141.1, 136.2, 135.0, 128.9, 128.7, 126.4, 121.5, 120.5, 119.9, 111.2, 107.8, 21.1, 14.4. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₈H₁₈N₃O 292.1450; Found 292.1453.

N-((4-Chlorophenyl)(imino)methyl)-4-methylbenzamide, 3u

3u was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (61 mg, 45% (DPEphos); 78 mg, 57% (Xantphos); 27 mg, 20% (dppp); 103 mg, 76% (dppf); 106 mg, 78% (PPh₃)). ¹H NMR (400 MHz, CDCl₃) δ 8.26–8.21 (m, 2H), 8.00–7.95 (m, 2H), 7.48–7.43 (m, 2H), 7.28–7.23 (m, 2H), 2.42 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 180.6, 165.4, 142.9, 138.7, 135.0, 133.7, 129.9, 129.2, 129.01, 128.94, 21.8. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₄ClN₂O 273.0795; Found 273.0793.

N-(Imino(4-(trifluoromethyl)phenyl)methyl)-4-methylbenzamide, 3v

3v was synthesized according to the general procedure and isolated using automated flash chromatography (silica gel, gradient elution 0–100% EtOAc in isohexane over 22 CV) as an off-white amorphous solid (63 mg, 41% (DPEphos), 92 mg, 60% (dppf), 103 mg, 67% (PPh₃)). ¹H NMR (400 MHz, CDCl₃) δ 8.25 (d, J = 8.0 Hz, 2H), 8.13 (d, J = 8.2 Hz, 2H), 7.75 (d, J = 8.2 Hz, 2H), 7.26 (d, J = 8.0 Hz, 2H), 2.42 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 180.6, 165.0, 143.0, 138.7, 134.7, 133.6 (q, J = 32.6 Hz), 129.8, 128.9, 127.9, 125.8 (q, J = 3.7 Hz), 123.7 (q, J = 272.7 Hz), 21.7. ¹⁹F NMR (376 MHz, CDCl₃) δ −63.0. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₆H₁₄F₃N₂O 307.1058; Found 307.1051.

Synthesis of Compound 5

1-(4-Methoxyphenyl)-3-phenyl-5-(p-tolyl)-1H-1,2,4-triazole, 5

Step 1: The reaction was performed in a two-chamber system. 4-Iodotoluene (109 mg, 0.5 mmol) and benzamidine (90 mg, 0.75 mmol) were added to chamber 1 and dissolved in DMF (2 mL), followed by triethylamine (0.174 mL, 1.25 mmol) and Pd(OAc)₂ (5.6 mg, 0.025 mmol). The reaction was swiftly stirred before addition of triphenylphosphine (13.1 mg, 0.05 mmol) and DMF (0.5 mL) followed by capping. To chamber 2 Mo(CO)₆ (66 mg, 0.25 mmol) was added and dissolved in 1,4-dioxane (2.5 mL) followed by DBU (0.187 mL, 1.25 mmol) just before capping. The two-chamber system was heated at 80 °C for 2 h. After completion, the reaction was cooled and vented. Step 2: The reaction mixture was transferred to another reaction vial, and 4-methoxyphenylhydrazine (415 mg, 3 mmol) was added together with acetic acid (2 mL). The vial was sealed and heated at 80 °C for 1 h. After cooling to room temperature, the reaction mixture was diluted with 50 mL of ethyl acetate and washed with 5% NaOH (2 × 10 mL) and brine (10 mL). The organic layer was dried over MgSO₄ and concentrated on a rotatory evaporator. The residue was purified by flash chromatography (silica gel, 5–50% EtOAc in isohexane) to give the desired product as colorless solid.

It was isolated by flash chromatography (silica gel, 5–50% EtOAc in isohexane) as an off-white amorphous solid (111 mg, 65%). ¹H NMR (400 MHz, CDCl₃) δ 8.32–7.99 (m, 2H), 7.49–7.39 (m, 5H), 7.38–7.30 (m, 2H), 7.16 (d, J = 7.9 Hz, 2H), 7.01–6.90 (m, 2H), 3.86 (s, 3H), 2.36 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 161.6, 159.8, 154.8, 140.1, 131.5, 130.9, 129.3, 129.2, 128.8, 128.5, 127.0, 126.6, 125.2, 114.5, 55.6, 21.4. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₂₂H₂₀N₃O 342.1606; Found 342.1616.

General Procedure for the Synthesis of 1,2,4-Oxadiazoles

Step 1: Performed according to the general procedure for synthesis of aryl amidines. Step 2: The reaction mixture was transferred to another reaction vial. Hydroxylamine hydrochloride (5 mmol) and acetic acid (1 mL) were added to the reaction mixture. The reaction vial was sealed and heated in a microwave at 120 °C for 20 min. After cooling to room temperature, the reaction mixture was diluted with 50 mL of ethyl acetate and washed with 5% NaOH (2 × 10 mL) and brine (10 mL). The organic layer was dried over MgSO₄ and concentrated on a rotatory evaporator. The residue was purified by flash chromatography (silica gel, 0–30% EtOAc in isohexane) to give the desired product as colorless solid.

3-Phenyl-5-(p-tolyl)-1,2,4-oxadiazole,⁴⁰6a (CAS: 16112-24-6)

6a was synthesized according to the general procedure, with PPh₃ as ligand, and isolated by flash chromatography (silica gel, 0–30% EtOAc in isohexane) as an off-white amorphous solid (71 mg, 60%). ¹H NMR (400 MHz, CDCl₃) δ 8.21–8.15 (m, 2H), 8.14–8.05 (m, 2H), 7.51 (dd, J = 5.1, 2.0 Hz, 3H), 7.36 (d, J = 8.0 Hz, 2H), 2.46 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 175.9, 168.9, 143.5, 131.1, 129.8, 128.8, 128.2, 127.5, 127.1, 121.6, 21.8. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₃N₂O 237.1028; Found 237.1027.

3-Phenyl-5-(o-tolyl)-1,2,4-oxadiazole, 6b (CAS: 54494-15-4)

6b was synthesized according to the general procedure, with DPEphos as ligand, and isolated by flash chromatography (silica gel, 0–30% EtOAc in isohexane) as an off-white amorphous solid (85 mg, 73%). ¹H NMR (400 MHz, CDCl₃) δ 8.21–8.15 (m, 3H), 7.55–7.50 (m, 3H), 7.50–7.46 (m, 1H), 7.40–7.35 (m, 2H), 2.79 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 176.5, 168.7, 139.3, 132.3, 132.1, 131.3, 130.4, 129.0, 127.7, 127.3, 126.4, 123.6, 22.1. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₃N₂O 237.1028; Found 237.1034

5-(4-Methoxyphenyl)-3-phenyl-1,2,4-oxadiazole,⁴¹6c (CAS: 36364-17-7)

6c was synthesized according to the general procedure, with Xantphos as ligand, and isolated by flash chromatography (silica gel, 0–30% EtOAc in isohexane) as an off-white amorphous solid (88 mg, 70%). ¹H NMR (400 MHz, CDCl₃) δ 8.19–8.13 (m, 4H), 7.53–7.49 (m, 3H), 7.07–7.02 (m, 2H), 3.91 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 175.7, 169.0, 163.3, 131.2, 130.2, 129.0, 127.7, 127.3, 117.0, 114.7, 55.7. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₃N₂O₂ 253.0977; Found 253.0981.

3-Phenyl-5-(4-(trifluoromethyl)phenyl)-1,2,4-oxadiazole, 6d (CAS: 89804-66-0)

6d was synthesized according to the general procedure, with Xantphos as ligand, and isolated by flash chromatography (silica gel, 0–30% EtOAc in isohexane) as an off-white amorphous solid (58 mg, 40%). ¹H NMR (400 MHz, CDCl₃) δ 8.36 (d, J = 8.2 Hz, 2H), 8.21–8.16 (m, 2H), 7.84 (d, J = 8.2 Hz, 2H), 7.56–7.50 (m, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 174.5, 169.4, 134.4 (d, J = 33.2 Hz), 131.6, 129.1, 128.7, 127.7, 127.6, 126.7, 126.3 (q, J = 3.8 Hz), 125.0 (d, J = 271.6 Hz). ¹⁹F NMR (376 MHz, CDCl₃) δ −63.2. HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₀F₃N₂O 354.0830; Found 354.0841.

5-(3,4-Difluorophenyl)-3-(6-methylpyridin-3-yl)-1,2,4-oxadiazol,²³ DDO-7263

DDO-7263 was synthesized according to the general procedure, with Xantphos as ligand, and isolated by flash chromatography (silica gel, 0–30% EtOAc in isohexane) as an off-white amorphous solid (51 mg, 37%). ¹H NMR (400 MHz, CDCl₃) δ 9.24 (dd, J = 2.2, 0.9 Hz, 1H), 8.29 (dd, J = 8.1, 2.2 Hz, 1H), 8.10–7.97 (m, 2H), 7.36 (ddd, J = 9.7, 8.6, 7.7 Hz, 1H), 7.33–7.29 (m, 1H), 2.65 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 174.3–174.2 (m), 167.5, 161.9, 153.6 (dd, J_CF = 257.5, 12.7 Hz), 150.4 (dd, J_CF = 251.5, 13.1 Hz), 148.3, 135.1, 125.3 (dd, J_CF = 7.3, 3.9 Hz), 123.5, 121.2–121.1 (m), 120.2, 118.7 (d, J_CF = 18.2 Hz), 117.8 (dd, J_CF = 19.5, 1.6 Hz), 24.8. One carbon missing. ¹⁹F NMR (376 MHz, CDCl₃) δ −128.7 to −129.7 (m), −134.4 to −135.2 (m). HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₄H₁₀F₂N₃O 274.0792; Found 274.0791.

5-(2-Fluorophenyl)-3-(m-tolyl)-1,2,4-oxadiazole⁶ Ataluren Precursor

Ataluren precursor was synthesized according to the general procedure, with Xantphos as ligand, and isolated by flash chromatography (silica gel, 0–30% EtOAc in isohexane) as an off-white amorphous solid (66 mg, 52%). ¹H NMR (400 MHz, CDCl₃) δ 8.25–8.20 (m, 1H), 8.01 (s, 1H), 8.01–7.96 (m, 1H), 7.64–7.57 (m, 1H), 7.44–7.37 (m, 1H), 7.37–7.32 (m, 2H), 7.32–7.26 (m, 1H), 2.45 (s, 3H). ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 172.8 (d, J_CF = 4.5 Hz), 169.0, 160.9 (d, J_CF = 260.5 Hz), 138.8, 134.7 (d, J_CF = 8.6), 132.2, 131.1, 128.9, 128.3, 126.7, 124.9, 124.8 (d, J_CF = 3.7 Hz), 117.3 (d, J_CF = 21.0 Hz), 113.1 (d, J_CF = 11.4 Hz), 21.5. ¹⁹F NMR (376 MHz, CDCl₃) δ −108.3 to −108.4 (m). HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₂FN₂O 255.0934; Found 255.0938.

General Procedure for Synthesis of [Carbonyl-¹¹C]acyl Amidines

Aryl iodide (9 μmol), benzamidine (2 equiv), and Pd(PPh₃)₄ (0.1 equiv) were transferred to a 950 μL oven-dried conical vial and dissolved in 1,4-dioxane (400 μL). After capping the vial, the reaction mixture was sonicated before addition of triethylamine (4.0 equiv). Following purging with N₂, the vial was placed in the xenon system. The [¹¹C]CO₂ produced in the cyclotron was transferred to the xenon system in a stream of helium gas and concentrated on a CO₂ trap immersed in liquid nitrogen.^42,43 Heating of the trap released [¹¹C]CO₂, which was reduced to [¹¹C]CO over zinc heated to 400 °C. Residual [¹¹C]CO₂ was trapped on an Ascarite column, and [¹¹C]CO was concentrated on a CO trap immersed in liquid nitrogen. Before heating the trap, the carrier gas was changed from helium to xenon (>99.9%, 1.5 mL/min). The concentrated [¹¹C]CO was transferred to the capped reaction vial through a transfer needle.

After collection of [¹¹C]CO, the radioactivity was measured to determine the starting amount of [¹¹C]CO (A1). The reaction was heated at 120 °C for 10 min. Upon reaction completion, the radioactivity was measured a second time (A2) before the reaction vial was vented and purged with N₂ to remove unreacted [¹¹C]CO and any volatile ¹¹C-labeled compounds formed under the course of the reaction. A third radioactivity measurement (A3) was performed before semipreparative HPLC purification. Lastly, a final radioactivity measurement of the isolated ¹¹C product fraction was performed (A4).

The [¹¹C]CO conversion was based on the radioactivity measurements A2 and A3, with A3 decay corrected to the time of A2 measurement. After the isolation and final radioactivity measurement (A4), an aliquot was analyzed to determine the radiochemical purity and the identity of the ¹¹C-labeled product. The isotopically unmodified product was used as reference. The reported isolated radiochemical yields are decay corrected to time of A1 measurement and based on the ¹¹C-labeled product activity (A4) and the amount of [¹¹C]CO collected in the reaction vial (A1).

[Carbonyl-¹¹C]N-(imino(phenyl)methyl)-4-methoxybenzamide, ¹¹C-3f (CAS: 1445133-92-5)

¹¹C-3f was synthesized according to the general procedure (4 experiments). Purification method: 0–50% acetonitrile followed by 100% acetonitrile, total run time 25 min, flow 5 mL/min. Analytical method: (system A) 10–90% acetonitrile (10 min) followed by 100% acetonitrile, total run time 15 min, flow 2 mL/min (system B) 35% acetonitrile (10 min) followed by 100% acetonitrile, total run time 15 min, flow 2 mL/min. Exp 1: Starting from 5.0 GBq, 0.45 GBq was isolated at 45 min from EOB. Exp 2: Starting from 3.5 GBq, 0.19 GBq was isolated at 41 min from EOB. Exp 3: Starting from 7.4 GBq, 0.40 GBq was isolated at 44 min from EOB. Exp 4: 5 min reaction time. Starting from 3.2 GBq, 0.083 GBq was isolated at 37 min from EOB. Analytical HPLC R_t = 3.5 min (system A); R_t = 8.2 min (system B).

[Carbonyl-¹¹C]4-acetyl-N-(imino(phenyl)methyl)benzamide, ¹¹C-3b

¹¹C-3b was synthesized according to the general procedure (4 experiments). Purification method: 5–50% acetonitrile followed by 100% acetonitrile, total run time 25 min, flow 5 mL/min. Analytical method: same as above. Exp 1: Starting from 5.0 GBq, 0.56 GBq was isolated at 38 min from EOB. Exp 2: Starting from 4.3 GBq, 0.38 GBq was isolated at 40 min from EOB. Exp 3: Starting from 15.4 GBq, 2.1 GBq was isolated at 38 min from EOB. Exp 4: Starting from 14.3 GBq, 1.7 GBq was isolated at 38 min from EOB. Analytical HPLC R_t = 4.4 min (system A); R_t = 4.6 min (system B).

Molar Activity Determination of ¹¹C-3b

The molar activity was determined in experiments 3 and 4. From an aliquot of the isolated ¹¹C-product fraction, 50 μL was injected and analyzed with system B at 254 nm. The molar activity was calculated with the equation derived from the calibration curve (see Supporting Information).

Synthesis of [Carbonyl-¹¹C]3-phenyl-5-(p-tolyl)-1,2,4-oxadiazole, ¹¹C-6a

Hydroxylamine hydrochloride (7.0 equiv) was dissolved in 150 μL of 50% acetic acid (aq). Intermediate [carbonyl-¹¹C]N-(imino(phenyl)methyl)-4-methylbenzamide was synthesized as above except for a 5 min reaction time. The hydroxylamine hydrochloride solution was added to the reaction mixture and heated for another 5 min at 150 °C. Purification method: 60% acetonitrile followed by 100% acetonitrile, total run time 25 min, flow 5 mL/min. Analytical method: (system A) 10–90% acetonitrile (10 min) followed by 100% acetonitrile, total run time 15 min, flow 2 mL/min (system B) 10–90% acetonitrile (10 min) followed by 100% acetonitrile, total run time 15 min, flow 2 mL/min. Exp 1: Starting from 5.1 GBq, 0.34 GBq was isolated at 43 min from EOB. Exp 2: Starting from 3.6 GBq, 0.32 GBq was isolated at 39 min from EOB. Analytical HPLC R_t = 9.7 min (system A); R_t = 8.9 min (system B).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.joc.2c02115.

• Calculations and definitions regarding the ¹¹C chemistry, NMR spectra (¹H NMR, ¹³C{¹H} NMR) of compounds 3a–v, and HPLC chromatograms of ¹¹C-labeled compounds 3b, 3f, and 6a (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.