Use of Triphenylphosphine-Bromotrichloromethane (Pph3-Brccl3)

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Proceedings Use of Triphenylphosphine-Bromotrichloromethane (PPh3-BrCCl3) in the Preparation of Acylhydrazines, N-Methylamides, Anilides and N-Arylmaleimides From Carboxylic Acids †

Abdullah Al-Hemyari, Areej Hashim, Muna Bufaroosha and Thies Thiemann *

Department of Chemistry, College of Science, United Arab Emirates University, PO Box 15551 Al Ain, UAE; [email protected] (A.A.-H.); [email protected] (A.H.); [email protected] (M.B.) * Correspondence: [email protected]. † Presented at the 23rd International Electronic Conference on Synthetic Organic Chemistry, 15 November 2019–15 December 2019; Available online: https://ecsoc-23.sciforum.net/.

Published: 14 November 2019

Abstract: In certain countries, many of the reagents used to transform carboxylic acids to acyl halides such as phosphorus trichloride, phosphorus tribromide, phosphorus pentachloride, phos- phoryl chloride, thionyl chloride and sulfuryl chloride are difficult to come by. Against this background, the authors developed the reaction system triphenylphosphine (PPh3)–bromotrichloromethane (BrCCl3) to prepare acyl halides in situ. In the following, the use of this reagent combination is joined with the reaction of the in situ prepared acyl halides with nitrogen nucleophiles, specifically with hydrazines, methylamine and anilines. The reaction is also used in an intramolecular variant by the reaction of maleanilic acids to N-arylmaleimides.

Keywords: appel-type reaction; N-arylmaleimides; acylhydrazines; anilides

1. Introduction Although there are a number of ways to transform carboxylic acids to acyl halides, the reagents needed such as phosphorus trichloride, phosphorus tribromide, phosphorus pentachloride, phos- phoryl chloride, thionyl chloride and sulfuryl chloride are not always available in certain countries. Against this background, the authors developed the reaction system triphenylphosphine (PPh3)– bromotrichloromethane (BrCCl3) to prepare acyl halides in situ [1,2]. This reaction system is a varia- tion of the Appel reagent [3], where tetrachloromethane (CCl4) is replaced by the environmentally less hazardous BrCCl3. It was noted that the in situ prepared acyl halides could be reacted with alcohols, and primary and secondary amines to esters and amides, respectively [4]. In the following, the treatment of carboxylic acids is followed by their reaction with hydrazines, methylamine and anilines. Also, further examples [5] will be given to transform maleianilic acids to N-arylmaleimides with the action of PPh3-BrCCl3 in the presence of triethylamine as base.

2. Materials and Methods

2.1. General Remarks Melting points were measured on a Stuart SMP 10 melting point apparatus and are uncorrected. Infrared spectra were measured with a Thermo/Nicolet Nexus 470 FT-IR ESP spectrometer and a

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Perkin Elmer Spectrum Two spectrometer. 1H and 13C NMR spectra were recorded with a Varian 400 NMR spectrometer (1H at 395.7 MHz, 13C at 100.5 MHz). The assignments of the carbon signals were aided by DEPT 90 and DEPT 135 experiments (DEPT = Distortionless Enhancement by Polarisation Transfer). The chemical shifts are relative to TMS (solvent CDCl3, unless otherwise noted). Mass spectra were measured with a JMS-01-SG-2 spectrometer, and with an Agilent QTOF 6540 UHD. Column chromatography, where necessary, was performed on recycled silica gel (S, 0.063 mm–0.1 mm, Riedel de Haen and Merck grade 9385).

2.2. Starting Materials Benzoic acid (1a), 4-nitrobenzoic acid (1g), 2,4-dichlorophenylacetic acid (6c) (Wako), methylammonium chloride (Merck-Schuchardt), cinnamic acid (4a) (Merck-Schuchardt), 4- methoxycinnamic acid (4b) (Fluka), 4-hydroxybenzoic acid (BDH Chemicals Ltd.), aniline (2d) (Aldrich), p-toluidine (2b) (4-methylaniline, Merck-Schuchardt), ammonium hydroxide (aq. solution of ammonia, 25 wt %, Sigma Aldrich), and 1-methyl-1-phenylhydrazine (Fluka) were acquired commercially. 3-Chlorobenzoic acid (1b), 4-bromobenzoic acid (1f), 3-bromobenzoic acid (1d), 2- bromobenzoic acid (1c), 2,3-dimethoxybenzoic acid (1h) and 4-methoxybenzoic acid (1e, anisidic acid) were prepared from the respective benzaldehydes by oxidation with AgO in basic medium. 4- Alkoxybenzoic acids were prepared in three steps from 4-hydroxybenzoic acid (transformation to the methyl ester with CH3OH/H2SO4, alkylation of the phenolic function with KOH, Alkyl-I, DMSO [6] and hydrolysis with aq. NaOH in CH3OH). 4-Ethoxy- and 4-propoxyaniline, 18a and 18b, were prepared from 4-hydroxyacetanilide by alkylation to the corresponding 4-alkoxyacetanilides 17a/17b (KOH, Alkyl-I, DMSO [6]) with their subsequent hydrolysis (NaOH, dioxane-MeOH (5.5:1 v/v) [7]. 4-Triethylamine was distilled over solid KOH and subse quently stored over solid KOH. 2,4- Dinitrophenylhydrazine was dried before use in an oven at 37 °C for 48 h.

2.3. General Procedures

N-Methyl 3-chlorobenzamide (9a).—To a suspension of triphenylphosphine (PPh3, 980 mg, 3.74 mol) in dry CH2Cl2 (12 mL), BrCCl3 (760 mg, 3.83 mmol) is added dropwise. The solution which turns dark yellow is stirred at rt for 20 min. Then, 3-chlorobenzoic acid (1b, 485 mg, 3.10 mmol) is added, and the resulting mixture is heated at reflux for 25 min. Thereafter, methylammonium chloride (8, 360 mg, 5.41 mmol) is added, and subsequently dry triethylamine (940 mg, 9.30 mmol). The mixture is stirred at reflux for 5 h. Thereafter, the cooled reaction mixture is subjected directly to column chromatography on silica gel (CH2Cl2-ether: 10:1) to give 9a as a colorless solid (436 mg, 83%) (mp. 68 °C; Lit. 69–70 °C); IR (KBr/cm−1) ν 3338, 3300, 3065, 1638, 1553, 1471, 1407, 1325, 743, 734, 678; 1H NMR (400 MHz, DMSO-d6) δ 2.75 (3H, s, NCH3), 7.46 (1H, dd, 3J = 6.8 Hz, 3J = 6.4 Hz), 7.54 (1H, d, 3J = 6.8 Hz), 7.74 (1H, d, 3J = 6.4 Hz), 7.81 (1H, s), 8.58 (1H, bs, NH); 13C NMR (100.5 MHz, DMSO-d6) δ 26.8 (NCH3), 126.2 (CH), 127.3 (CH), 130.8 (CH), 131.4 (CH), 133.6 (Cquat), 136.8 (Cquat), 165.8 (Cquat, CO). 4-Nitrobenzoic acid N-methyl-N-phenyl hydrazide (13d).—To a suspension of triphenylphosphine (PPh3, 980 mg, 3.74 mol) in dry CH3CN (12 mL), BrCCl3 (760 mg, 3.83 mmol) is added dropwise. The solution which turns dark yellow-brown is stirred at rt for 20 min. Then, 4- nitrobenzoic acid (1g, 525 mg, 3.14 mmol) is added and the mixture is stirred at 53 °C for 25 min. Thereafter, N-methyl-N-phenylhydrazine (12, 732 mg, 6.0 mmol) is added and the solution is stirred at 53 °C for 12 h. The cooled mixture is concentrated to dryness in vacuo, and the residue is subjected to column chromatography on silica gel (CH2Cl2-ether 10:1) to give 13d (665 mg, 78%) as a light yellow solid; IR (KBr/cm−1) ν 3233, 1659, 1597, 1521, 1498, 1345, 1281, 850, 748, 691; 1H NMR (400 MHz, DMSO-d6) δ 3.17 (3H, s, CH3), 6.74 (1H, t, 3J = 8.0 Hz), 6.81 (2H, d, 3J = 8.0 Hz), 7.20 (2H, dd, 3J = 8.0 Hz, 3J = 8.0 Hz), 8.09 (2H, d, 3J = 8.8 Hz), 8.32 (2H, d, 3J = 8.8 Hz), 10.93 (1H, s, NHCO); 13C NMR (100.5 MHz, DMSO-d6) δ 40.5 (NCH3), 112.9 (2C, CH), 119.1 (CH), 124.2 (2C, CH), 129.3[5] (2C, CH), 129.4 (2C, CH), 139.0 (C ), 149.8 (Cquat), 150.0 (Cquat), 164.7 (Cquat, NCO). N-4-Ethoxyphenylmaleimide (21a).—A solution of triphenylphosphine (1.89 g, 7.21 mmol) and bromotrichloromethane (BrCCl3, 1.56 g. 7.86 mmol) in dry CH3CN (20 mL) is stirred at rt for 30 min,

Proceedings 2019, 41, 4 3 of 12 during which in turns dark yellow. Then, maleanilic acid (20a) (1.35 g, 5.76 mmol) is added, and the resulting mixture is stirred at 70 °C for 40 min. Thereafter, dry triethylamine (750 mg, 7.42 mmol) is added dropwise over 15 min, and the resulting mixture is stirred at 70 °C for 8 h. The cooled mixture is added to water (50 mL), and the ensuing mixture is extracted with CH2Cl2 (3 × 35 mL). The combined organic phase is dried over anhydrous MgSO4 and evaporated in vacuo. The residue is subjected to rapid column chromatography on silica gel (CH2Cl2) to give 21a (1.0 g, 80%) a yellow, crystalline solid, mp. 134 °C (Lit. 136–137 °C [8]) IR (KBr/cm−1) ν 3095, 2936, 2877, 1702, 1518, 1256, 720, 686; 1H NMR (DMSO-d6): δ 1.32 (3H, t, 3J = 6.7 Hz), 4.03 (2H, q, 3J = 6.7 Hz), 7.00 (2H, d, 3J = 8.6 Hz), 7.20 (2H, s), 7.40 (2H, d, 3J = 8.6 Hz). Cinnamamide (11a).—A solution of triphenylphosphine (1.89 g, 7.21 mmol) and bromotrichloromethane (BrCCl3, 1.56 g. 7.86 mmol) in dry CH3CN (15 mL) is stirred at rt for 30 min., during which in turns dark yellow. Then, cinnamic acid (4a, 965 mg, 6.51 mmol) is added, and the resulting mixture is stirred at 70 °C for 40 min. Thereafter, the solution is cooled to rt, and aq. ammonia (10 mL, ammonium hydroxide, 25 wt %) is added dropwise to the solution. Thereafter, the reaction mixture is heated at 70 °C for 10 h. The cooled solution is added to water (50 mL) and the ensuing mixture is extracted with CHCl3 (2 × 30 mL). The combined organic phase is dried over anhydrous MgSO4 and concentrated in vacuo. The residue is subjected to column chromatography over silica gel (CH2Cl2-ether 10:1) to give 11a as a colorless solid (718 mg, 75%); mp. 148 °C (Lit. 148– 151 °C [9]); IR (KBr/cm−1) ν 3371, 3165, 1660, 1607, 1492, 1449, 1397, 1245, 1114, 968, 862, 566, 526, 476; 1H NMR (400 MHz, DMSO-d6) δ 6.59 (1H, d, 3J = 15.6 Hz), 7.09 (1H, bs, NH), 7.36–7.41 (3H, m), 7.53 (2H, d, 3J = 6.8 Hz), 7.56 (1H, bs, NH); 13C NMR (100.5 MHz, DMSO-d6) δ 122.6 (CH), 128.0 (2C, CH), 129.4 (2C, CH), 129.9 (CH), 135.2 (Cquat), 139.8 (CH), 167.3 (Cquat, CO).

3. Results and Discussion

Previously, it had been established that the reactions of the modified Appel reagent BrCCl3-PPh3 are similar to those of CCl4-PPh3. This includes the dehydration of aldoximes and amides to nitriles (1) and the amidation of carboxylic acids (4). Only the actual Appel reaction itself, i.e., the transformation of alkanols to haloalkanes with BrCCl3-PPh3 (10) is more complex, where under certain conditions the transfer of the bromo-substituent vs. the chloro-substituent is not selective (2). In the following, the authors tried to examine whether amidation reactions of carboxylic acids with the less nucleophilic anilines would proceed equally well as with amines. Also, the authors investigated the reactions with hydrazines. Here, the use of ammonium salts such as methylammonium chloride and hydrazinium salts such as hydrazinium sulfate was probed as well as was the reaction of carboxylic acids with aq. ammonia in the presence of BrCCl3-PPh3. Finally, the scope of an intramolecular imidation was looked at to transform maleianilic acids to N- arylmaleimides (5). The reaction of benzoic acids 1, treated with BrCCl3-PPh3, with anilines 2 proceeded straightforward to give the anilides 3 (Figure 1). The anilines were added slowly while the reaction mixture was at reflux. It was noted that the reactions are exothermic. The products were isolated by column chromatographic separation on silica gel. The reactions were also carried out successfully with cinnamic acids 4 (Figure 2) and phenylacetic acids 6 (Figure 3) as substrates. The release of methylamine in situ, within a reaction mixture of initially carboxylic acid, triphenylphosphine and bromotrichloromethane by equilibration with added triethylamine was seen to work well, providing the corresponding N-methyl amides in acceptable yields (Figure 4). The reaction of aq. ammonia with the in situ prepared acyl halides proceeded equally well, showing that the amidation of the acyl halides proceeded faster than their hydrolysis (Figures 5 and 6). For the reaction of the in situ prepared acyl halides, three available hydrazines/hydrazine salts available to us were chosen: 1-methyl-1-phenylhydrazine (12), 2,4-dinitrophenylhydrazine (14), and hydrazinium sulfate. 1-Methyl-1-phenylhydrazine is liquid, so its addition to the in situ prepared acyl halides poses no problem. The 2,4-dinitrophenylhydrazine (14) commercially available to us was dampened with water, most likely to lessen the chance of a detonation of the material. Thus,

Proceedings 2019, 41, 4 4 of 12 hydrazine 14 was dried in an oven at 37 °C for 48 h before use. Hydrazine itself was released in situ by equilibration of hydrazinium sulfate and triethylamine just as in the case of the use of methylammonium chloride (see above). In all cases, the reaction proceeded reasonably well, delivering acyl hydrazides 13 and 15 (Figures 7 and 8), and in the case of hydrazine itself, 1,2- bis(aroyl)hydrazines 16 were obtained (Figure 9). Finally, our work (5) on using the modified Appel reagent PPh3-BrCCl3 for a ring closure of maleianilic acids to N-maleimides was expanded slightly (Figure 10). While previously we had utilized anilines available to us commercially in these reactions, here, we prepared the anilines through a hydrolysis of the corresponding acetamides 17. The general published procedure (7) works very well and allows for the construction of a substituted acetamide before releasing the substituted aniline by hydrolysis.

1.) PPh3-BrCCl3 H CH3CN or CH2Cl2 R R N 2.) R1 CO2H R1 O 1 NH2 3 2

Br H N CO2H NH2 1a 2a O 3a (85%) Br Br H N Cl CO2H Cl NH2 1b 2a O Br 3b (87%) Br Br Br H

CO2H N NH2 1c 2a O 3c (83%) Br H3C H N Cl CO2H Cl NH2 1b 2b O 3d (88%) CH3

H3CO H N Cl CO2H Cl NH2 1b 2c O 3e (90%) OCH3

Figure 1. Reaction of benzoic acids with anilines in the presence of PPh3-BrCCl3.

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1.) PPh3-BrCCl3 CH3CN or CH2Cl2 H R R R1 2.) N 1 CO2H R

4 NH2 5 2 O

CO2H NH2 N 4a 2a O H3CO 5a (75%) Br

H3CO H3CO

CO2H NH2 N 4b 2c O 5b (76%) OCH3 H3CO

H3CO

CO2H NH2 N 4b 2d O 5c (73%)

Figure 2. Reaction of cinnamic acids with anilines in the presence of PPh3-BrCCl3.

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R1 1.) PPh3-BrCCl3 CH3CN or CH2Cl2 O R R

CO2H 2.) 1 R N H 6 NH2 7 2

OCH3

H CO 3 H N

CO2H NH2 6a O 2c 7a (82%) OCH3

H CO 3 H N

CO2H NH2 6b O 2c 7b (85%)

OCH3

Cl Cl H CO 3 Cl ClH N

CO2H NH2 6c O 2c 7c (73%)

OCH3

Cl H CO 3 Cl H N

CO2H NH2 6d O 2c 7d (85%)

OCH3

H CO 3 H N

CO2H NH 2 O

6e 2c 7e (70%)

Figure 3. Reaction of phenylacetic acids with anilines in the presence of PPh3-BrCCl3.

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1.) PPh3-BrCCl3 H CH2Cl2 R R N . 2.) CH3NH2 HCl(8) CH3 CO2H Et3N O 1 9

H . CH3NH2 HCl N Cl Cl CO2H 8 1b O 9a (83%)

H . CH3NH2 HCl N Br CO H Br 2 8 1d O 9b (79%)

H CO H3CO 3 H CH NH .HCl 3 2 N CO2H 8 1e O 9c (70%)

Br Br H CH NH .HCl 3 2 N CO2H 8 1f O

9d (68%)

Figure 4. Reaction of substituted benzoic acids with methylammonium chloride in the presence of

PPh3-BrCCl3.

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1.) PPh3-BrCCl3 O CH2Cl2 R R 2.) aq. 25 w% (CH2)n NH2 NH OH (CH)nCO2H 4 1 (n=0), 6 (n=1) 10 (n=0, 1)

NH4OH NH2 CO2H 1a O 10a (67%)

O2N O2N

NH4OH NH2 CO2H 1g O 10b (62%)

NH4OH NH2 Br Br CO2H 1e O 10c (69%)

NH4OH

6e CO2H CONH2 10d (60%)

Figure 5. Reaction of carboxylic acids with aq. ammonia in the presence of PPh3-BrCCl3.

1.) PPh3-BrCCl3 CH2Cl2 R R 2.) aq. 25 w% NH OH CONH CO2H 4 2 1 11

NH4OH CONH CO2H 2 4a 11a (75%)

H3CO H3CO

NH4OH

CO2H CONH2 4b 11b (74%) OCH3 OCH3

NH4OH

CO2H CONH2

OCH3 4c OCH3 11c (69%)

Figure 6. Reaction of cinnamic acids with aq. ammonia in the presence of PPh3-BrCCl3.

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1.) PPh3-BrCCl3 CH3CN R R H 2.) N

N(CH3)Ph NH2 CO2H N 13 1 12 O

H NH2 N CO2H N N(CH3)Ph 1a 12 O 13a (75%)

H NH2 N Cl CO2H N Cl N(CH3)Ph 1b 12 O 13b (78%)

H NH2 N Br CO2H N Br N(CH3)Ph

1d 12 O 13c (80%) O2N O2N

H NH2 N CO2H N N(CH3)Ph

1g 12 O 13d (78%)

Br Br

H NH2 N CO2H N N(CH3)Ph

1f 12 O 13e (76%)

H3CO H3CO

H NH2 N CO2H N N(CH3)Ph

1e 12 O 13f (85%)

Figure 7. Reaction of benzoic acids with 1-methyl-1-phenylhydrazine in the presence of PPh3 – BrCCl3.

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1.) PPh -BrCCl 3 3 O2N NO2 CH3CN R R O N H 2.) 2 N N H CO2H NH2 N 15 1 H O 14 NO2

Br O2N Br O2N NO2 H NH2 N N N CO2H H H 1c NO2 14 O 15a (66%) O2N O2N NO2

NH2 H N Cl CO H N 2 H Cl N 1b NO2 14 H O 15b (71%) O2N O2N NO2

NH2 H N N Br CO2H H Br N NO2 H 1d 14 O 15c (74%) O N H3CO 2 H3CO O2N NO2 H NH2 N N CO H H N 2 H NO2 14 1e O 15d (78%) OCH3 H3CO O N OCH 2 3 O2N NO2 OCH3 H NH2 N N N CO H H 2 NO H 2 14 O 1h 15e (65%) O N NO Cl O2N 2 2 H NH2 N CO2H N N H H NO2 14 O 1e Cl 15f (71%)

Figure 8. Reaction of benzoic acids and 4-chlorophenylacetic acid with 2,4-dinitrophenylhydrazine in

the presence of PPh3 – BrCCl3.

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1.) PPh3-BrCCl3 CH3CN O R R H 2.) NH2 N H2N . H2SO4 N CO H H 2 R then Et3N 16 1 O

Br Br NH O Br 2 H H2N . H2SO4 N N CO2H H 1c O 16a (48%)

NH2 O H2N . H H2SO4 N Cl Cl CO H Cl N 2 H 1b O 16b (45%) O NH2 H H2N . H SO N 2 4 N CO2H H O 1d 16c (47%)

H3CO H3CO NH O 2 H H2N . H SO N 2 4 N H CO2H O OCH3 1e 16d (48%)

O N O2N 2 O NH2 H H2N . N H2SO4 N H CO2H O NO2 1g 16e (38%) Br Br O H NH2 N H2N . N H2SO4 H CO H O 2 Br

1f 16f (45%)

Figure 9. Reaction of benzoic acids with in situ released hydrazine in the presence of PPh3 – BrCCl3 to yield 1,2-diacylhydrazines 16.

HO2C O H O NH H N 2 O N NaOH 19 MeOH-dioxane O THF O RO RO RO

17: R = C2H5 (17a), C3H7 (17b) 18: R = C2H5 (18a), C3H7 (18b) 20: R = C2H5 (20a), C3H7 (20b) (62 [20a] and 60% [20b] over two steps)

HO C 2 O

H N PPh3-BrCCl3 N OR CH3CN, Et3N O RO O

20: R = C2H5, C3H7 21: R = C2H5 (21a, 80%), C3H7 (21b, 78%) Figure 10. Reaction of maleianilic acids 20 to N-arylmaleimides 21.

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4. Conclusions

In conclusion, it can be said that acyl halides, prepared in situ by the action of PPh3-BrCCl3 on carboxylic acids can be transformed further in one pot by the reaction with anilines and hydrazines to anilides and hydrazides, respectively. Also, ammonium and hydrazinium salts can be used when trimethylamine is added as base. Furthermore, the reaction of maleianilic acids with PPh3-BrCCl3 provides N-arylmaleimides.

Author Contributions: Conceptualization, supervision, investigation, writing—original draft preparation, T.T.; investigation, A.A.-H.; investigation, A.A.-H.; writing—review and partial supervision, M.B.

Conflicts of Interest: The authors declare no conflict of interest.

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