Switchable Site-Selective Benzanilide C(sp2)-H Bromination via Promoter Regulation

Regioselective benzanilide bromination that generates either regioisomer from the same starting material is desirable. Herein, we develop switchable site-selective C(sp2)-H bromination by promoter regulation. This protocol leads to regiodivergent brominated benzanilide starting from the single substrate via selection of promoters. The protocol demonstrates excellent regioselectivity and good tolerance of functional groups with high yields. The utility effectiveness of this method has been well exemplified in the late-stage modification of biologically important molecules.


Introduction
Brominated benzanilides are an important class of structures essential for the function of almost all modern pharmaceuticals and agrochemicals [1][2][3][4].Because of the steric hindrance of -Br, the bromo group located in the ortho or para position of the benzanilide molecule produces different biological activities [5,6].Indeed, in anti-hypertensive compounds and anti-cancer agents, the bromo group is in the ortho position, while it is in the para position in anti-depressant drugs or anti-microbials agents (Figure 1).Therefore, the acquisition and rapid screening of small molecule libraries containing diverse brominated benzanilides is an essential aspect of hit compounds discovery.More importantly, based on the diversity-oriented synthesis (DOS) strategy, C-C or C-N bond formation via cross coupling starting from brominated benzanilides will efficiently generate structurally diverse compound libraries, such as benzidine, phenylenediamine, etc. [7][8][9][10][11][12].To date, various important small molecule modulators have been identified for "undruggable" targets from unbiased phenotypic screening of DOS-derived compound libraries [13][14][15][16].However, the classic approach to the generation of brominated benzanilides involves a two-step process: bromination of aniline followed by the benzoylation of the resulting brominated product, which suffers from limitations such as poor selectivity, low yields, complex purification method, etc. (Scheme 1a) [17][18][19].
Because of the challenge of selectivity and efficiency, the practical application of regioselective halogenation of C-H activation in a substrate containing two or more arenes is still uncommon [20,21].Over the past decade, direct functionalization of C(sp 2 )-H bonds has emerged as a powerful tool for generating new C Ar -halogen bonds [22][23][24][25][26].Among these, direct C-H cleavage, transition-metal-catalyzed functionalization of anilids [27][28][29][30], and hexafluoroisopropanol [31][32][33][34] (HFIP) promoted C-H activation of anilines are the most similar.Although these impressive approaches can be employed to construct halogenated anilids, to the best of our knowledge, directed C-H halogenation in terms of regioselectivity has not been previously described.For instance, knowledge about regioselective C(sp 2 )-H Molecules 2024, 29, 2861 2 of 16 bromination of N-methyl-benzanilides and its corresponding application in synthesis of biologically important chemicals is still surprisingly underdeveloped.Stimulated by the previous diversification of bioactive agents by a controlled catalyst or photo-induced construction of C-halogen bonds reported by Yu [35][36][37] and MacMillan [38][39][40][41], we envisioned that the regioselective C-H functionalization of benzanilides could be achieved via utility of different catalysts (Scheme 1b).Herein, we report the use of HFIP and Pd(II) as promoters for regioselective C-H bromination of benzanilides to provide a convenient route for synthesis of diverse brominated benzanilide derivatives.When Pd(II) is employed as promoter, bromination occurs on the ortho position of aniline, while a reversed regioselective product is obtained with an HFIP promoter.Preliminary studies of regioselective bromination on other acylanilide substrates are also described.
Molecules 2024, 29, x FOR PEER REVIEW 2 of 18 struct halogenated anilids, to the best of our knowledge, directed C-H halogenation in terms of regioselectivity has not been previously described.For instance, knowledge about regioselective C(sp 2 )-H bromination of N-methyl-benzanilides and its corresponding application in synthesis of biologically important chemicals is still surprisingly underdeveloped.Stimulated by the previous diversification of bioactive agents by a controlled catalyst or photo-induced construction of C-halogen bonds reported by Yu [35][36][37] and MacMillan [38][39][40][41], we envisioned that the regioselective C-H functionalization of benzanilides could be achieved via utility of different catalysts (Scheme 1b).Herein, we report the use of HFIP and Pd(II) as promoters for regioselective C-H bromination of benzanilides to provide a convenient route for synthesis of diverse brominated benzanilide derivatives.When Pd(II) is employed as promoter, bromination occurs on the ortho position of aniline, while a reversed regioselective product is obtained with an HFIP promoter.Preliminary studies of regioselective bromination on other acylanilide substrates are also described.struct halogenated anilids, to the best of our knowledge, directed C-H halogenation in terms of regioselectivity has not been previously described.For instance, knowledge about regioselective C(sp 2 )-H bromination of N-methyl-benzanilides and its corresponding application in synthesis of biologically important chemicals is still surprisingly underdeveloped.Stimulated by the previous diversification of bioactive agents by a controlled catalyst or photo-induced construction of C-halogen bonds reported by Yu [35][36][37] and MacMillan [38][39][40][41], we envisioned that the regioselective C-H functionalization of benzanilides could be achieved via utility of different catalysts (Scheme 1b).Herein, we report the use of HFIP and Pd(II) as promoters for regioselective C-H bromination of benzanilides to provide a convenient route for synthesis of diverse brominated benzanilide derivatives.When Pd(II) is employed as promoter, bromination occurs on the ortho position of aniline, while a reversed regioselective product is obtained with an HFIP promoter.Preliminary studies of regioselective bromination on other acylanilide substrates are also described.

Results and Discussion
Since different promoters generate various types of transition states based on the detailed mechanisms, regioselective bromination of benzanilides may occur with different promoters by activating distinct protons.To test our hypothesis, we conducted a model study using benzanilide (1) in the presence of NBS in the TFA/TFAA solvent system with Na 2 S 2 O 8 as the terminal oxidant (Table 1).After the screening of palladium catalysts, we discovered that only Pd(OAc) 2 and Pd(TFA) 2 could promote C-H activation, and bromination occurred on the ortho position of the amino group to provide the brominated product 2a (entry 3-4).In contrast, PdCl 2 and Pd/C failed to give the brominated benzanilides.Next, we turned our attention to other transition metal catalysts.However, cobalt, copper or nickel failed to promote the transformations (entry [7][8][10][11].Delightfully, after a comprehensive screening of additives, we found that brominated product 2b could also be readily prepared under the condition of hexafluoroisopropanol (HFIP), and the bromination reaction occurred on the para position of the amino group (entry 9).The reactions typically proceeded to completion within 3 h at 70 • C.This initial result showed that Pd(II)or HFIP-promoted regioselective bromination, with anilid as an effective directing group, could occur under optimized conditions (entry 6, 12).

Results and Discussion
Since different promoters generate various types of transition states based on the detailed mechanisms, regioselective bromination of benzanilides may occur with different promoters by activating distinct protons.To test our hypothesis, we conducted a model study using benzanilide (1) in the presence of NBS in the TFA/TFAA solvent system with Na2S2O8 as the terminal oxidant (Table 1).After the screening of palladium catalysts, we discovered that only Pd(OAc)2 and Pd(TFA)2 could promote C-H activation, and bromination occurred on the ortho position of the amino group to provide the brominated product 2a (entry 3-4).In contrast, PdCl2 and Pd/C failed to give the brominated benzanilides.Next, we turned our attention to other transition metal catalysts.However, cobalt, copper or nickel failed to promote the transformations (entry 7-8, 10-11).Delightfully, after a comprehensive screening of additives, we found that brominated product 2b could also be readily prepared under the condition of hexafluoroisopropanol (HFIP), and the bromination reaction occurred on the para position of the amino group (entry 9).The reactions typically proceeded to completion within 3 h at 70 °C.This initial result showed that Pd(II)-or HFIP-promoted regioselective bromination, with anilid as an effective directing group, could occur under optimized conditions (entry 6, 12). a Reaction conditions: benzanilide (0.1 mmol, 1.0 eq), NBS (1.2 eq), promoter (0.1 eq), Na 2 S 2 O 8 (1.2 eq), in TFAA (0.1 mL)/TFA (0.9 mL) RT, 10 h.b Conversion ratio.c Reaction conditions: benzanilide (0.1 mmol, 1.0 eq), NBS (1.2 eq), Na 2 S 2 O 8 (1.2 eq), in TFAA (0.1 mL)/TFA (0.9 mL), RT, 10 h.d Reaction conditions: benzanilide (0.1 mmol, 1.0 eq), NBS (1.2 eq), Na 2 S 2 O 8 (1.2 eq), in TFAA (0.1 mL)/TFA (0.9 mL), 70 • C, 1 h.e Reaction conditions: benzanilide (0.1 mmol, 1.0 eq), NBS (1.2 eq), TFAA (2.0 eq) in HFIP (1 mL), RT, 10 h.f Reaction conditions: benzanilide (0.1 mmol, 1.0 eq), NBS (1.2 eq), TFAA (2.0 eq) in HFIP (1 mL) 70 With these results of the optimized conditions of Pd(II)-or HFIP-promoted regioselectivity in hand, we set out to explore the scope and robustness of this new method.As illustrated in Figures 2 and 3, a variety of benzanilides were smoothly transformed into the corresponding brominated anilide products (2a-2j, 3a-3j) in excellent yields.The para-, meta-, and ortho-substituted benzoyl groups were well tolerated; only specific regioselective brominated benzanilides were observed.It is worth mentioning that different substrates use different reaction temperatures.Some substrates with electron-withdrawing groups (CF 3 -and NO 2 -) may require a higher reaction temperature because they have lower electron cloud density.Therefore, in these reactions, conditions should be tough.With all substrates, the corresponding regioselective bromination products could be consistently obtained.When Pd(OAc) 2 was used as a catalyst, the bromination reaction occurred on the ortho position of the amino group, and para bromobenzanilides were obtained when HFIP was involved.Notably, a special example was 2-chloro-N-methyl-N-phenylacetamide.We were delighted to observe that anilide, containing an aliphatic acyl group rather than the benzoyl motif, could also be employed successfully to provide regioselective bromination products (2j, 3j).This phenomenon expands the application scope of this novel method for regioselective modification of anilides and benefits the diversity of small molecule libraries available for further bioactivity screening.
substrates use different reaction temperatures.Some substrates with electron-withdrawing groups (CF3-and NO2-) may require a higher reaction temperature because they have lower electron cloud density.Therefore, in these reactions, conditions should be tough.With all substrates, the corresponding regioselective bromination products could be consistently obtained.When Pd(OAc)2 was used as a catalyst, the bromination reaction occurred on the ortho position of the amino group, and para bromobenzanilides were obtained when HFIP was involved.Notably, a special example was 2-chloro-N-methyl-N-phenylacetamide.We were delighted to observe that anilide, containing an aliphatic acyl group rather than the benzoyl motif, could also be employed successfully to provide regioselective bromination products (2j, 3j).This phenomenon expands the application scope of this novel method for regioselective modification of anilides and benefits the diversity of small molecule libraries available for further bioactivity screening.Since brominated iodobenzanilides are widely utilized in bioactive agents [42][43][44][45], to prove both the synthetic utility effectiveness of this method for large-scale synthesis, we prepared 2f and 3f on a gram scale under the optimized reaction conditions with only a 5 mol% catalyst (Scheme 2).Compared with conventional approaches requiring brominated alkylanilines, which are generally expensive, this method with operational simplicity is advantageous for rapid access to various kinds of ortho and para brominated benzanilides.We further showed the feasibility of this new reaction by performing the synthesis of biologically important molecules (Figures 2 and 3).With Pd(II) as a catalyst, 2-fluoro-N-methyl-N-phenylbenzamide was readily converted to brominated compound 2i as an RORγ inverse agonists analogue in a good yield [46]. RORγ is mainly expressed in immune cells, promoting the differentiation of Th17 cells and producing the key factor IL-17. RORγ plays the corresponding biological functions in a variety of important inflammatory pathways.Recent studies have found that an RORγ inverse agonist is a new therapy for the treatment of castration-resistant prostate cancer (CRPC).This type of brominated compounds can be conveniently synthesized by our method with excellent chemoselectivity, which provides an opportunity to generate novel anti-malignancy agents, such as a the DUBTAC ligand for stabilizing RORγ from degradation.Accordingly, HFIP-promoted bromination could conveniently transform 2-chloro-N-methyl-N-phenylacetamide into anti-microbicidal agent 3j [47], while its ortho-brominated analogue was readily synthesized via a Pd(II) catalyst with good regioselectivity.Since brominated iodobenzanilides are widely utilized in bioactive agents [42][43][44][45], to prove both the synthetic utility effectiveness of this method for large-scale synthesis, we prepared 2f and 3f on a gram scale under the optimized reaction conditions with only a 5 mol% catalyst (Scheme 2).Compared with conventional approaches requiring brominated alkylanilines, which are generally expensive, this method with operational simplicity is advantageous for rapid access to various kinds of ortho and para brominated benzanilides.We further showed the feasibility of this new reaction by performing the synthesis of biologically important molecules (Figures 2 and 3).With Pd(II) as a catalyst, 2-fluoro-N-methyl-N-phenylbenzamide was readily converted to brominated compound 2i as an RORγ inverse agonists analogue in a good yield [46]. RORγ is mainly expressed in immune cells, promoting the differentiation of Th17 cells and producing the key factor IL-17. RORγ plays the corresponding biological functions in a variety of important inflammatory pathways.Recent studies have found that an RORγ inverse agonist is a new therapy for the treatment of castration-resistant prostate cancer (CRPC).This type of brominated compounds can be conveniently synthesized by our method with excellent chemoselectivity, which provides an opportunity to generate novel anti-malignancy agents, such as a the DUBTAC ligand for stabilizing RORγ from degradation.Accordingly, HFIP-promoted bromination could conveniently transform 2-chloro-N-methyl-N-phenylacetamide into anti-microbicidal agent 3j [47], while its ortho-brominated analogue was readily synthesized via a Pd(II) catalyst with good regioselectivity.Further studies were performed to gain insight into the mechanisms, which are shown in Scheme 3. A consistent and significant kinetic isotopic effect value of 2.2 was observed with Pd(II) as a catalyst, indicating that C-H cleavage might be involved in the rate-limiting step of the ortho-bromination reaction.In parallel, with HFIP as the promoter, a KIE value of 1.6 was obtained, suggesting that the C-H activation step in para-bromination was partially turnover limiting.Taken together, during the whole transformation, other elementary steps (such as the ligand exchange step) either before or after C-H activation were also critical to the catalytic turnover rate.Further studies were performed to gain insight into the mechanisms, which are shown in Scheme 3. A consistent and significant kinetic isotopic effect value of 2.2 was observed with Pd(II) as a catalyst, indicating that C-H cleavage might be involved in the rate-limiting step of the ortho-bromination reaction.In parallel, with HFIP as the promoter, a KIE value of 1.6 was obtained, suggesting that the C-H activation step in para-bromination was partially turnover limiting.Taken together, during the whole transformation, other elementary steps (such as the ligand exchange step) either before or after C-H activation were also critical to the catalytic turnover rate.
observed with Pd(II) as a catalyst, indicating that C-H cleavage might be involved in the rate-limiting step of the ortho-bromination reaction.In parallel, with HFIP as the promoter, a KIE value of 1.6 was obtained, suggesting that the C-H activation step in para-bromination was partially turnover limiting.Taken together, during the whole transformation, other elementary steps (such as the ligand exchange step) either before or after C-H activation were also critical to the catalytic turnover rate.

Scheme 3. Mechanism study of the bromination reaction via kinetic isotope effect experiments.
Although detailed mechanisms remain to be ascertained, with all these results in hand, the proposed mechanisms of ortho and para bromination are shown in Schemes 4 and 5, respectively.With Pd(II) as the promoter, the catalytic cycle is proposed to involve four steps: (i) the ligand was transfered to form Pd(TFA)2, and palladacycle B was generated, (ii) oxidation by NBS to provide Pd(IV) intermediate C, (iii) a C-Br bond was formed via reductive elimination, (iv) Pd(II) was released to coordinate a new substrate.Although detailed mechanisms remain to be ascertained, with all these results in hand, the proposed mechanisms of ortho and para bromination are shown in ?? 4?? 5, respectively.With Pd(II) as the promoter, the catalytic cycle is proposed to involve four steps: (i) the ligand was transfered to form Pd(TFA) 2 , and palladacycle B was generated, (ii) oxidation by NBS to provide Pd(IV) intermediate C, (iii) a C-Br bond was formed via reductive elimination, (iv) Pd(II) was released to coordinate a new substrate.On the other hand, HFIP-promoted bromination also required an amide bond as the directing group.However, due to the steric repulsion from HFIP, intermediate D was more crowded, so the C-Br bond formation occurred on the para position of the amino group.

General Procedure for N-Methyl-Benzanilides Preparation
A mixture of corresponding benzoic acid (5 mmol), EDCI (5.5 mmol), HOBT (5.5 mmol), and DMAP (0.5 mmol) in DMF (30 mL) was stirred for 2 min at room temperature in a round bottom flask.Then, Et3N (15 mmol) was added.After 5 minutes, N-methylaniline (4.5 mmol) was slowly added.The reaction mixture was stirred at room temperature for 12 h.The reaction was quenched with water, and the mixture was washed once with saturated aqueous NaCl and extracted with EtOAc.The organic layer was dried with anhydrous Na 2 SO 4 and evaporated in vacuum to give the crude product, which was then purified by silica gel column chromatography to give N-methylbenzanilide.

General Procedure for Palladium-Catalyzed Bromination of Benzanilides
To a 15 mL sealed-tube were added benzanilide (0.10 mmol, 1.0 equiv), Na 2 S 2 O 8 (0.12 mmol, 1.2 equiv), NBS (1.2 mmol, 1.2 equiv), Pd(OAc) 2 (0.01 mmol, 0.1 equiv), and TFAA (0.1 mL).The reaction system was stirred at room temperature for 2 min.Then, TFA (0.9 mL) was added.The tube was sealed and heated.After completion of the reaction as determined by TLC (DCM:MeOH = 300:1), the reaction solution was allowed to cool to room temperature.Saturated aqueous NaHCO 3 was added to the reaction solution and washed with dichloromethane.Then the organic phase was separated and dried over anhydrous Na 2 SO 4 .After removing the solvent in vacuo, the residue was purified by silica gel column chromatography to give the desired product.

General Procedure for HFIP-Catalyzed Bromination of Benzanilides
To a 15 mL sealed-tube were added benzanilide (0.10 mmol, 1.0 equiv), NBS (0.12 mmol, 1.2 equiv), TFAA (0.12 mmol, 1.2 equiv), and HFIP (1 mL).The tube was sealed and heated.After completion of the reaction as determined by TLC (DCM:MeOH = 300:1), the reaction solution was allowed to cool to room temperature.Saturated aqueous NaHCO 3 was added to the reaction solution and washed with dichloromethane.Then the organic layer phase was separated and was dried over anhydrous Na 2 SO 4 .After removing the solvent in vacuo, the residue was purified by silica gel column chromatography to give the desired product.To a 100 mL round bottom flask, following the general procedure 3.3, 4-iodo-N-methyl-N-phenylbenzamide (1.0 g, 2.97 mmol), Na 2 S 2 O 8 (858 mg, 3.56 mmol), NBS (634 mg, 3.56 mmol), Pd(OAc) 2 (33 mg, 0.15 mmol), TFAA (3 mL), and TFA(27 mL) were added.The reaction mixture was stirred at 50 • C for 3 h.After completion of the reaction as determined by TLC, the residue was purified by silica gel column chromatography with a mixture DCM and MeOH as the eluent.(DCM:MeOH = 900:1).

Scheme 3 .
Scheme 3. Mechanism study of the bromination reaction via kinetic isotope effect experiments.

Table 1 .
Optimization of conditions for selective bromination of benzanilide a .

Table 1 .
Optimization of conditions for selective bromination of benzanilide a .