Efficient Synthesis of 1H-Benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-one Derivatives Using Ag2CO3/TFA-Catalyzed 6-endo-dig Cyclization: Reaction Scope and Mechanistic Study

A small library of 1H-benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-one derivatives was prepared in good to excellent yields, involving a Ag2CO3/TFA-catalyzed intramolecular oxacyclization of N-Boc-2-alkynylbenzimidazole substrates. In all experiments, the 6-endo-dig cyclization was exclusively achieved since the possible 5-exo-dig heterocycle was not observed, indicating the high regioselectivity of this process. The scope and limitations of the silver catalyzed 6-endo-dig cyclization of N-Boc-2-alkynylbenzimidazoles as substrates, bearing various substituents, were investigated. While ZnCl2 has shown limits for alkynes with an aromatic substituent, Ag2CO3/TFA demonstrated its effectiveness and compatibility regardless of the nature of the starting alkyne (aliphatic, aromatic or heteroaromatic), providing a practical regioselective access to structurally diverse 1H-benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-ones in good yields. Moreover, the rationalization of oxacyclization selectivity in favor of 6-endo-dig over 5-exo-dig was explained by a complementary computational study.


Introduction
Fused heteropolycycles containing nitrogen and oxygen in the skeleton constitute an important class of heterocyclic compounds that can be found in numerous biologically active compounds [1-3] and natural products [4,5]. Among them, the structural motif 1H-benzo [4,5]imidazo[1,2-c][1,3]oxazin-1-one is present in pharmacologically interesting molecules, which display fungicidal activity (compound A, Figure 1) [6]. In addition, compound B (Figure 1) has been known as a potent recognition site for the detection of the highly toxic phosgene gas [7].

Results and Discussion
Our synthesis begins with the generation of 2-brominated benzimidazole 2, starting from 2-mercaptobenzimidazole 1, following the known reported procedure [18]. Selective Molecules 2023, 28, 2403 3 of 17 bromination of 1 was performed with bromine and hydrogen bromide in acetic acid at room temperature according to a literature procedure [19], providing 85% yield. Compound 2 was subsequently protected by a tert-butoxycarbonyl group using (Boc) 2 O as the reagent in the presence of triethylamine in a mixture of MeCN/DMF (1:1) at room temperature leading to compound 3 in 76% yield (Scheme 1). The N-Boc-2-bromobenzimidazole 3 served as a building block to introduce substituted alkynes at the C-2 position via the Sonogashira cross-coupling reaction.
applications. Considering the environmental impact, the Ag2CO3/TFA is an environmentally benign system catalyst.

Results and Discussion
Our synthesis begins with the generation of 2-brominated benzimidazole 2, starting from 2-mercaptobenzimidazole 1, following the known reported procedure [18]. Selective bromination of 1 was performed with bromine and hydrogen bromide in acetic acid at room temperature according to a literature procedure [19], providing 85% yield. Compound 2 was subsequently protected by a tert-butoxycarbonyl group using (Boc)2O as the reagent in the presence of triethylamine in a mixture of MeCN/DMF (1:1) at room temperature leading to compound 3 in 76% yield (Scheme 1). The N-Boc-2-bromobenzimidazole 3 served as a building block to introduce substituted alkynes at the C-2 position via the Sonogashira cross-coupling reaction. In order to prepare a series of N-Boc-2-alkynylbenzimidazoles 4 as substrates which could undergo intramolecular cyclization, compound 3 was engaged in Sonogashira cross-coupling with several alkynes.
After screening the various conditions, the use of phenylacetylene (1.5 equiv.), Pd(OAc)2 (10 mol%), PPh3 (20 mol%) and CuI (15 mol%) in triethylamine at room temperature proved to be the most appropriate choice conditions for obtaining alkynylated product 4b in excellent yield (Scheme 2). In order to prepare a series of N-Boc-2-alkynylbenzimidazoles 4 as substrates which could undergo intramolecular cyclization, compound 3 was engaged in Sonogashira crosscoupling with several alkynes.
After screening the various conditions, the use of phenylacetylene (1.5 equiv.), Pd(OAc) 2 (10 mol%), PPh 3 (20 mol%) and CuI (15 mol%) in triethylamine at room temperature proved to be the most appropriate choice conditions for obtaining alkynylated product 4b in excellent yield (Scheme 2). The scope and limitation of the Sonogashira cross-coupling were investigated starting from N-Boc-2-bromobenzimidazole 3 with various terminal alkynes (Scheme 2). As illustrated in Scheme 2, it was found that the nature of terminal alkynes (aliphatic, aromatic or heteroaromatic) did not dramatically affect the efficiency of this The scope and limitation of the Sonogashira cross-coupling were investigated starting from N-Boc-2-bromobenzimidazole 3 with various terminal alkynes (Scheme 2). As illustrated in Scheme 2, it was found that the nature of terminal alkynes (aliphatic, aromatic or heteroaromatic) did not dramatically affect the efficiency of this cross-coupling, since, in all cases, the expected compounds were obtained in moderate to good yields. Thus, this procedure is compatible with several substituents (electron-donating or electronwithdrawing groups) on the aryl rings (methoxy, methyl, chlorine, nitro, ester and fluorine groups). Moreover, under the same conditions, alkynes bearing an heteroaryl groups, such as 2-thienyl or 2-pyridyl, were also able to be introduced in satisfactory yields [4k (54%) and 4l (63%)].
Initially, the treatment of N-Boc-2-hexynylbenzimidazole 4a with ZnCl 2 (1.5 equiv.) in dichloromethane at 40 • C gave successfully and exclusively the tricyclic core 5a in 80% isolated yield. However, the reaction starting from N-Boc-2-phenylethynylbenzimidazole 4b required a longer time and gave a mixture of the expected product 5b along with the starting material 4b in a 60/40 ratio, respectively (Table 1, entry 2). Increasing the temperature to 60 • C slightly improved the 4b/5b ratio from (60:40) to (50:50) (Table 1, entry 3). These results clearly indicate the inefficiency of ZnCl 2 to promote intramolecular cyclization from N-Boc-2-alkynylbenzimidazole derivatives when the substituent of alkyne is an aromatic ring, such as phenyl.  a The ratio of mixture (4/5) was calculated from the crude 1H NMR spectrum. b Compound 5a was isolated in 80% yield after purification by silica-gel column chromatography. C Compound 5b was isolated in 90% yield after purification by silica-gel column chromatography.
To our delight, the combination of a catalytic amount of Ag2CO3 (0.1 equiv.) and TFA (2 equiv.) significantly improved the conversion of the starting material to 85% while the reaction time decreased to 24 h (Table 1, entry 4). Performing the reaction with dichloroethane as a solvent, instead of dichloromethane, resulted in a complete conversion of the starting material 4b to the target heterocycle 5b, which was isolated in an excellent yield of 90% after purification by silica-gel column chromatography (Table 1, entry 5). Otherwise, dichloroethane has a greater impact on the conversion rate of the reaction, which could be related to its higher boiling point compared to dichloromethane. A suitable solvent is crucial to this reaction. Under the same conditions, decreasing the temperature to 40 °C significantly affected the formation of the desired product 5b, since the conversion of substrate 4b remained incomplete, despite a longer reaction time of 24 h, proving the importance of heating at 60 °C to obtain a full conversion (Table 1, entry 6).
The best results in terms of the time and yields were obtained with Ag2CO3/TFA as a catalytic system. Having established the required conditions for efficient annulation, various N-Boc-2-alkynyl(arylethynyl)benzimidazoles 4a-o, which are suitable substrates to undergo intramolecular cyclization, were subjected to these optimized reaction conditions in order to study the scope and limitations of our process (Scheme 3). All reaction mixtures were stirred at 60 °C until the starting material was completely consumed, monitored by thin-layer chromatography (TLC) using a mixture of petroleum ether/ethyl a The ratio of mixture (4/5) was calculated from the crude 1H NMR spectrum. b Compound 5a was isolated in 80% yield after purification by silica-gel column chromatography. c Compound 5b was isolated in 90% yield after purification by silica-gel column chromatography.
To our delight, the combination of a catalytic amount of Ag 2 CO 3 (0.1 equiv.) and TFA (2 equiv.) significantly improved the conversion of the starting material to 85% while the reaction time decreased to 24 h (Table 1, entry 4). Performing the reaction with dichloroethane as a solvent, instead of dichloromethane, resulted in a complete conversion of the starting material 4b to the target heterocycle 5b, which was isolated in an excellent yield of 90% after purification by silica-gel column chromatography (Table 1, entry 5). Otherwise, dichloroethane has a greater impact on the conversion rate of the reaction, which could be related to its higher boiling point compared to dichloromethane. A suitable solvent is crucial to this reaction. Under the same conditions, decreasing the temperature to 40 • C significantly affected the formation of the desired product 5b, since the conversion of substrate 4b remained incomplete, despite a longer reaction time of 24 h, proving the importance of heating at 60 • C to obtain a full conversion (Table 1, entry 6).
The best results in terms of the time and yields were obtained with Ag 2 CO 3 /TFA as a catalytic system. Having established the required conditions for efficient annulation, various N-Boc-2-alkynyl(arylethynyl)benzimidazoles 4a-o, which are suitable substrates to undergo intramolecular cyclization, were subjected to these optimized reaction conditions in order to study the scope and limitations of our process (Scheme 3). All reaction mixtures were stirred at 60 • C until the starting material was completely consumed, monitored by thin-layer chromatography (TLC) using a mixture of petroleum ether/ethyl acetate (v/v = 8/2) as the eluent. The oxacyclization conditions were found to be compatible with a variety of R groups in starting materials 4a-o, such as alkyl, cycloalkyl, aryl and heteroaryl, bearing electron-withdrawing or -donating substituents.

Scheme 3. Ag2CO3/TFA-mediated annulation of N-Boc-2-alkynyl-benzim
The presence of a strong electron-withdrawing group, such oxy function on the phenylethynyl group, promote the 6-endo-dig The nature of the substituents on the phenyl ring slightly affected the outcome of the cyclization. As given in Scheme 3, the reactions with the substrates having electrondonating groups, such as methoxy and methyl, were performed efficiently, affording the expected compounds 5c-e in good yields. Benzimidazoles containing chlorine atom as an electron-withdrawing group at the ortho, meta or para position 4f-h were successfully cyclized, giving access to the desired products 5f (51%), 5g (80%) and 5h (92%). As observed, the steric hindrance of the ortho-position seems to substantially influence the cyclization efficiency.
The presence of a strong electron-withdrawing group, such as nitro or carbomethoxy function on the phenylethynyl group, promote the 6-endo-dig cyclization, leading to new fused benzimidazoles in excellent yields [5i (84%) and 5j (95%)]. Encouraged by the good results obtained with different aryl R groups, benzimidazoles bearing ethynylheteroaryl were exposed under the same silver-catalyzed oxacyclization conditions. Interestingly, substrates having an heteroaromatic ring on the triple bond, such as 3-thienyl and 2-pyridyl, were found to also be compatible and exclusively provided the corresponding heterocycles 5k and 5l in 78% and 71% yields, respectively. The intramolecular cyclization of substrates bearing a fluorine atom on the phenyl ring, R = 2-fluorophenyl and R = 4-fluorophenyl, works well, giving the desired compounds in good yields [5m (88%) and 5n (68%)]. When the alkyne substituent is a bulky cyclohexyl group, the cyclization proceeded smoothly and provided the desired heterocycle 5o in excellent 90% yield. It is noteworthy that among the two possible oxacyclization products 5 and 6, only the 6-endo-dig cyclization heterocycle 5 was formed in all experiments, with variations mainly in the isolated yields.

Theoretical Calculation (Computational Studies)
To investigate the reaction pathway, and in particular, to understand the mechanism and stereoselectivity of the annulation reaction catalyzed by Ag 2 CO 3 and TFA, a series of computational experiments were performed by density functional theory (DFT) calculations. The proposed mechanism for the competing intramolecular cyclization pathways of the target benzimidazoxazinone 5b is outlined in Scheme 4.  In order to justify the expected 6-endo annulation, the intermediates and the transition states (TS) of both cyclization pathways [6-endo-dig (path a, blue) or 5-exo-dig (path b, red)] were computed. All the structures were optimized at the B3LYP level in the gas phase and then in DCE (see the Supplementary Materials for details). The energy profiles of different reaction pathways are depicted in Figure 3. In order to justify the expected 6-endo annulation, the intermediates and the transition states (TS) of both cyclization pathways [6-endo-dig (path a, blue) or 5-exo-dig (path b, red)] were computed. All the structures were optimized at the B3LYP level in the gas phase and then in DCE (see the Supplementary Materials for details). The energy profiles of different reaction pathways are depicted in Figure 3.

ΔE (kcal/mol)
In order to justify the expected 6-endo annulation, the intermediates and t tion states (TS) of both cyclization pathways [6-endo-dig (path a, blue) or 5-exo-di red)] were computed. All the structures were optimized at the B3LYP level i phase and then in DCE (see the Supplementary Materials for details). The energ of different reaction pathways are depicted in Figure 3. As shown in Figure 3, the energy barriers of the transition states TSIIa endo -TSII'a endo , the intermediate Ia endo and the product 5b for 6-endo-dig oxacyclization (path a, blue) are much lower than those of 5-exo-dig (path b, red). This suggests that the preferential 6-endo-dig lactonization of compound 4b is favored both kinetically and thermodynamically.
To confirm our mechanistic hypothesis, we extended the earlier studies to the calculated natural population analysis (NPA) of each carbon in the alkyne group for all starting materials 4a-o ( Table 2).
The calculated NPA revealed that the positive charge is located on the carbon atom denoted β leading to the 6-endo-dig cyclization, while the Cα leading to the undesired annulation (5-exo-dig) product 6b is negatively charged, which is in excellent agreement with the experiment, regardless of the nature of the alkyne substituent. the intermediate Iaendo and the product 5b for 6-endo-dig oxacyclization (path a, blue) are much lower than those of 5-exo-dig (path b, red). This suggests that the preferential 6-endo-dig lactonization of compound 4b is favored both kinetically and thermodynamically.
To confirm our mechanistic hypothesis, we extended the earlier studies to the calculated natural population analysis (NPA) of each carbon in the alkyne group for all starting materials 4a-o (Table 2).  The calculated NPA revealed that the positive charge is located on the carbon atom denoted β leading to the 6-endo-dig cyclization, while the Cα leading to the undesired annulation (5-exo-dig) product 6b is negatively charged, which is in excellent agreement with the experiment, regardless of the nature of the alkyne substituent.

General Information
All reactions were performed under an inert atmosphere of argon in oven-dried glassware equipped with a magnetic stir bar. Solvents for reactions were obtained from Thermo Fisher Scientific in extra dry quality and stored under argon over activated 3 Å sieves. All reagents were purchased from Fluorochem and used as received without additional purification. Reactions were monitored by thin-layer chromatography (TLC) analysis using silica gel 60 F254 plates. All products were visualized by exposure to UV light (longwave at 365 nm or shortwave at 254 nm). Column chromatography was performed using silica gel 60 (230-400.13 mesh, 0.040-0.063 mm). Eluents were distilled by the standard methods before each use. All new compounds were characterized by NMR spectroscopy (1H, 19F and 13C), high-resolution mass spectroscopy (HRMS) and melting point (if solids). NMR spectra were recorded at 300 MHz for 1 H, 282 MHz for 19 F, and 75 MHz for 13 C with a Bruker ® 300 MHz NMR spectrometer. Proton and carbon magnetic resonance spectra ( 1 H NMR and 13 C NMR) were recorded using tetramethylsilane (TMS)

General Information
All reactions were performed under an inert atmosphere of argon in oven-dried glassware equipped with a magnetic stir bar. Solvents for reactions were obtained from Thermo Fisher Scientific in extra dry quality and stored under argon over activated 3 Å sieves. All reagents were purchased from Fluorochem and used as received without additional purification. Reactions were monitored by thin-layer chromatography (TLC) analysis using silica gel 60 F254 plates. All products were visualized by exposure to UV light (longwave at 365 nm or shortwave at 254 nm). Column chromatography was performed using silica gel 60 (230-400.13 mesh, 0.040-0.063 mm). Eluents were distilled by the standard methods before each use. All new compounds were characterized by NMR spectroscopy (1H, 19F and 13C), high-resolution mass spectroscopy (HRMS) and melting point (if solids). NMR spectra were recorded at 300 MHz for 1 H, 282 MHz for 19 F, and 75 MHz for 13 C with a Bruker ® 300 MHz NMR spectrometer. Proton and carbon magnetic resonance spectra ( 1 H NMR and 13 C NMR) were recorded using tetramethylsilane (TMS) as an external standard and CDCl 3 (7.28 ppm for 1 H NMR and 77.04 ppm for 13 C NMR) or DMSO-d6 (2.50 ppm for 1 H NMR and 40.0 ppm for 13 C NMR) as internal standards. 19 F spectra were unreferenced. Data for NMR are reported as follows: chemical shift (δ ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, sep = septet, m = multiplet and br = broad resonance) and coupling constants J are reported in Hertz (Hz). All NMR spectra were processed in MestReNova. HRMS experiments were performed on a hybrid tandem The experimental data are in accordance with the previously reported data [44,45].

General Procedure for the Synthesis of Tert-butyl 2-alkynyl-1H-benzimidazole-1-carboxylate (4a-o)
An oven dried 25 mL Schlenk tube equipped with a magnetic stir bar was charged with tert-butyl 2-bromobenzimidazole-1-carboxylate 3 (300 mg, 1.01 mmol), PPh 3 (53 mg, 0.2 mmol), Pd(OAc) 2 (23 mg, 0.1 mmol), CuI (29 mg, 0.15 mmol) and triethylamine (8 mL). The vial was sealed with a septum-lined pierceable cap, evacuated and backfilled with argon (×3). Then, a solution of alkyne (1.5 mmol, 1.5 equiv.) in 2 mL of Et 3 N was added dropwise to the reaction mixture via a syringe. The reaction mixture was stirred at room temperature for 20 h under an argon atmosphere. After completion of the reaction (monitored by TLC), the solution was filtered through a plug of celite eluting with ethyl acetate (25 mL) and the combined filtrate was dried with MgSO 4 and concentrated under vacuum. The crude product was directly purified by column chromatography using a mixture petroleum ether/EtOAc as an eluent to give the pure desired products 4a-o.

Details of DFT Calculations
The structure of each studied species was optimized by using the Turbomole 7.4 program package [48]. Before their visualization using TmoleX (version 4.5.3), the structure of each individual species was optimized in the gas phase, with a convergence criterion of 10 −8 Hartree, using the hybrid functional B3LYP and the triplet-ζ basis set 6-311 + G* [49] to collect its more stable 3D conformer. The stability of each structure was then investigated during further DFT calculations. During this step, the energy of each species was then minimized again using DFT calculations combining the Resolution of Identity (RI) approximation [50,51], within the Turbomole 7.4 program package using the B3LYP function with the def2-TZVP basis set [52][53][54]. All minimum energy structures were obtained with full optimization, without constraints. Corrections for long-range non-bonding interactions were given using the Grimme D3 dispersion model [54,55]. An implicit solvent model was additionally undertaken, using the COSMO Model implemented in Turbomole to determine thermodynamic and charge population properties in DCE. Analytical frequencies were then run on each structure at 1 atm and 298.15 K to finally calculate each energy.

Conclusions
In conclusions, we have reported an efficient and general access to 1H-benzo [4,5] imidazo[1,2-c][1,3]oxazin-1-ones, involving an intramolecular deprotective heterocyclization sequence catalyzed by a combination between Ag 2 CO 3 and TFA in dichloroethane at 60 • C. While this procedure is compatible with a wide range of aliphatic, aromatic and heteroaromatic alkynes, ZnCl 2 -mediated heterocyclization showed a limitation when the starting alkyne was aromatic. In all experiments, no trace of 5-exo-dig heterocycles was observed, since only the 6-endo-dig products were obtained exclusively in good to excellent yields, proving the high selectivity of this silver-catalyzed oxacyclization. In addition, a computational study was performed in order to rationalize the mechanism of 6-endo-dig oxacyclization and it was found that experimental results are in a good agreement with the theoretical calculation. The synthetic potential of our catalytic system (Ag 2 CO 3 /TFA) to promote intramolecular 6-endo-dig cyclization showed that N-Boc-2-alkynyl-imidazole and N-Boc-2-alkynyl-benzimidazole substrates are interesting for the synthesis of other new polycyclic heterocycles.