Microwave-mediated and customery synthesis of N-benzoyl- or N-substituted benzoyl-N,N'-dialkylureas from arylcarboxylic acids and N,N'-disubstituted carbodiimides under solvent-free conditions

ARTICLE

Microwave-mediated and customery synthesis of N-benzoyl- or N-substituted benzoyl-N,N'-dialkylureas from arylcarboxylic acids and N,N'-disubstituted carbodiimides under solvent-free conditions

Ricardo A. W. Neves Filho; Ronaldo N. de Oliveira; Rajendra M. Srivastava^*

Departamento de Química Fundamental, Universidade Federal de Pernambuco, Cidade Universitária, 50740-540 Recife-PE, Brazil

ABSTRACT

An easy, efficient and quick synthesis of N-benzoyl-(and N-substituted benzoyl)-N,N'dialkylureas, in good yields, under solvent-free conditions is described. First, the reaction between a carboxylic acid and a disubstituted carbodiimide was carried out employing microwave radiation, and later on the same reaction was performed separately under dry heating without any radiation. The yields are comparable in both cases.

Keywords: microwave irradiation, N,N'-dialkylureas, carbodiimides, solvent-free conditions

RESUMO

Uma síntese fácil, eficiente e rápida de N-benzoil-(e benzoil N-substituída)-N,N'-dialquiluréias foi realizada com bons rendimentos em condições livre de solvente. Primeiramente, a reação entre um ácido carboxílico e uma carbodiimida dissubstituída foi feita empregando radiação de microondas, e depois a mesma reação foi realizada sob aquecimento convencional. Os rendimentos são comparáveis em ambos os casos.

Introduction

N-Acylureas exhibit a wide range of biological activities, and a number of them possesses analgesic, antiinflammatory, anthelmintic, antifungal, and larvicidal properties.¹ Another unusual N-acylurea, cabergoline, acts as an agonist mainly at dopamine (D1)-D2 receptors, and is used as monotherapy in early untreated Parkinson's disease (PD) and as adjunctive therapy to L-dopa in PD.² The efficacy of cabergoline has been evaluated in many clinical trials.^3-5 The synthesis of cabergoline, an N-acylurea and a potent prolactin inhibitor, has been reported by Ashford et al. in 2002,⁶ and in the same year, its effectiveness in treating acromegaly has also been announced.⁷ Furthermore, some acylureas have found application in agriculture because they hamper the growth and reproduction of the fall armyworm and house fly.⁸ In fact, these ureas also serve as intermediates for their transformation into amides and esters.⁹ Because of these properties, N-acylureas are of interest as potential drugs, and also as starting intermediates for the preparation of amides and esters, hence our interest in synthesizing such compounds.

Discovering new high-yielding, selective reactions is vital for the advancement of synthetic organic chemistry. Reactions that generate products with a minimum of operations are not only noteworthy but are fundamental for synthesizing new compounds in a shorter time. For achieving this goal, microwave-mediated reaction has great advantage, and during the last decade this technique experienced tremendous growth and development.¹⁰ Because of its cleanliness and reduced work-up, we decided to examine the reaction of benzoic acid and substituted benzoic acids with N,N'-dicyclohexyl- and N,N'diisopropylcarbodiimides employing microwave radiation.

A literature pursuit revealed the existence of a sole example involving the preparation of 1-benzoyl-1,3diisopropylurea employing benzoic acid and N,N'diisopropylcarbodiimide in a mixture of CH₂Cl₂ / DMF (9:1), containg DMAP as a catalyst, under microwave radiation for 60 min.¹¹ This reaction was performed in a sealed tube having a pressure of 3.0 bar and at 90 ºC. The usual method for synthesizing N-acylureas involves: i) reactions between N-substituted amides and isocyanates in refluxing toluene for 24h;¹² ii) benzoyl peroxide and dicyclohexylcarbodiimide in methanol again at reflux temperature for 12h,¹³ and iii) carboxylic acids and N,N'disubstituted carbodiimides in solution.¹⁴ In 1991, Corriu et al.¹⁵ reported that N-acylureas can also be obtained from pentacoordinated silicon hydride and N, N'-dialkylcarbodiimides in two steps. In this article, we wish to describe two methods, one by microwave irradiation and the other by dry heating, both without the use of any solvent. None of these procedures have been found in the literature. These protocols are speedy, eco-friendly and involve simple purification of the products. Therefore, this type of reaction can be considered as a part of green chemistry.

Results and Discussion

In the present work, N,N'-dicyclohexyl- and N,N'diisopropylcarbodiimides have been allowed to react with aromatic carboxylic acids under the influence of microwave irradiation using a domestic oven (Scheme 1).¹⁶ In general, it took from 5-12 min. for completion of the reaction and the temperature varied between 100-114 ºC. In each case the reaction between the reagents cited above furnished mainly the desired N-aroylureas, acid anhydrides and N,N'-disubstituted ureas.

The formation of such products by the reaction of an acid and a carbodiimide employing usual methods has been known since 1962.¹⁷ We observed the reaction rate enhancement when the phenyl ring of the acid carried an electron-withdrawing group. Increased reaction rate has also been reported earlier by Šlebioda.¹⁸ He studied the substituent effect in the reaction of dicyclohexylcarbodiimide with substituted benzoic acids in buffered solution and found the velocity increment of the reaction containing electron-withdrawing substituent in the phenyl ring.

In order to compare the results described above, the reactants were mixed and heated in a glass tube in solventless condition without microwaves. The reaction took place at 110 ºC, but required a little longer time for its completion (see Table 1). With this, it is concluded that N-benzoyl- or substituted aroylureas can easily be obtained either from benzoic acid and substituted benzoic acids and a diimide. There is no report in the literature regarding the dry conventional heating for obtaining N-acylureas. Table 1 contains, besides other details, the yields of our two methods and their comparison with the literature values.

Separation of compounds 3a-i and 4a-g was achieved by liquid chromatography over silica gel. The yields of the chromatographically pure products are provided in the experimental section. The infrared and NMR spectra of the isolated substances agreed with the structure.

Conclusions

In summary, this study has highlighted the ready generation of N-aroyl-N,N'-disubstituted ureas from benzoic and substituted benzoic acids and N,N'disubstituted carbodiimides under solvent-free conditions in a domestic microwave oven. This opens up the scope for synthesizing a variety of acylureas using such radiation technique. Also, the classical method for the above-cited preparation by dry heating is an achievement.

Experimental

General experimental procedures

Melting points were determined on an Electrothermal (Mel-Temp) apparatus (Model No. 1002D) and are uncorrected. All reactions were monitored by TLC analysis (TLC plates contained GF₂₅₄, Merck). IR spectra were measured with a IFS66 Bruker spectrometer employing either a KBr disc or a nujol mull. ¹H NMR spectra were recorded with a Varian unity plus 300 MHz spectrometer using CDCl₃ as solvent and SiMe₄ as an internal standard. All reactions were conducted in a domestic microwave oven Model Sanyo EM-300B, 220/650W/2450 MHz.

Synthesis of N-benzoyl- or N-substituded benzoyl-N,N' dialkylureas

A suitable aromatic carboxylic acid 1a-g (1.0 mmol) and an appropriate carbodiimide (1.1 mmol of DCC and 2.0 mmol of DIC. ²³ were well triturated and placed in a small glass test tube. Two separate experiments were carried out. (i) microwave irradiation: the mixture was irradiated between 5-12 min. in an unmodified domestic microwave oven (100% potency, 650W), cooled and the components were purified by chromatography. (ii) Conventional heating experiments: the mixture was heated in a preheated oil bath maintained at 110 ºC for a certain length of time and cooled. In both cases the crude product was treated with chloroform and filtered to remove the insoluble urea. Chloroform: ethyl acetate (8.0:2.0) for entries 1-7 and chloroform: ethyl acetate (9.0:1.0) for entries 8 and 9 were used to develope the tlc plates followed by their revelation under ultraviolet light. Two spots were observed – one due symmetrical anhydrides with R_f values » 0.60 and N-acyl ureas having R_f values » 0.43. The chloroform solution of the products was applied to a thick-layer chromatographic plate and developed using the above-mentioned solvent system. Work-up furnished chromatographically pure N-acyl ureas 3a-i and symmetrical anhydrides 4a-g.

1-Benzoyl-1,3-dicyclohexylurea (3a)

Colorless crystals from chloroform, m.p. 165-166 ºC (51% from MW method and 56% from conventional method); Lit.¹³ m.p. 160-161 ºC, crystals from methanol-water (79%); Lit.¹³ m.p. 164-165.5 ºC, crystals from ethanol (92%); Lit.⁹ m.p. 175-175.5 ºC (76%). IR and ¹H NMR spectra gave the same absorptions as reported earlier.⁹

1,3-Dicyclohexyl-1-(3-nitrobenzoyl)-urea (3b)

Yellow crystals from chloroform, m.p. 175-176 ºC (66% from MW method and 65% from conventional method); Lit.²⁴ m.p. 175 ºC (56%). IR: 3315, 2922, 2853, 1700, 1646, 1532, 1346, 1235 cm^-1. ¹H NMR: d 0.81-1.96 (m, 20H, 10 CH₂), 3.42-3.51 (m, 1H, CH), 4.07-4.19 (m, 1H, CH'), 6.09 (bd, 1H, NH, J = 7.5 Hz), 7.59 (t, 1H, J 8.1 Hz, arom.), 7.89 (dd, 1H, J 8.1 and J 0.9 Hz, arom.), 8.30 (dd, 1H, J 8.1 and 2.1 Hz, arom.), 8.40 (t, 1H, J 2.1 Hz, arom.).

1,3-Dicyclohexyl-1-(4-nitrobenzoyl)-urea (3c)

Yellow crystals from ethyl acetate, m.p. 203-204 ºC (70% from MW method and 72% from conventional method); Lit.²⁴ m.p. 204 ºC (86%). IR: 3295, 2928, 2855, 1699, 1646, 1519, 1343, 1237 cm^-1. ¹H NMR: d 0.83-2.04 (m, 20H, 10CH₂), 3.42-3.56 (m, 1H, CH), 4.03 (dddd, 1H, CH', J_{1ax,2ax (1ax,6ax)} 12.0 Hz and J_{1ax,2eq (1ax,6eq)} 3.6 Hz), 6.11 (bd, 1H, NH, J 6.6 Hz), 7.07 (d, 2H, J 8.9 Hz, arom.), 8.27 (d, 2H, J 8.9 Hz, arom.).

1,3-Dicyclohexyl-1-(3,5-dinitrobenzoyl)-urea (3d)

Yellow crystals from chloroform, m.p. 177-179 ºC (73% from MW method and 76% from conventional method); Lit.²⁵ m.p. 177-179 ºC. IR: 3290, 2935, 2857, 1675, 1543, 1343 cm^-1. ¹H NMR: d 0.97-1.97 (m, 20H, 10 CH₂), 3.39-3.57 (m, 1H, CH), 4.24 (dddd, 1H, CH', J_1ax,2ax 12.3 Hz and J 3.6 Hz), 5.88 (bd, 1H, NH,(1ax,6ax)1ax,2eq (1ax,6eq) J 8.1 Hz), 7.26 (t, 1H, J 1.0 Hz, arom.), 8.73 (t, 1H, J 1.0 Hz, arom.), 9.10 (t, 1H, J 1.8 Hz, arom.).

1,3-Dicyclohexyl-1-(3-methyl,4-nitrobenzoyl)-urea (3e)

Colorless crystals from chloroform, m.p. 133-134 ºC (63% from MW method and 60% from conventional method); Lit.²⁴ m.p. 133 ºC (41%). IR: 3311, 2923, 2850, 1701, 1651, 1593 cm^-1. ¹H NMR: d 0.84-2.11 (m, 20H, 10 CH₂), 2.61 (s, 3H, CH₃), 3.45-3.51 (m, 1H, CH), 4.06 (dddd, 1H, CH', J_1ax,2ax 11.7 Hz and J 3.6 Hz), 6.25 (bs, 1H, NH), (1ax,6ax)1ax,2eq (1ax,6eq)7.50 (m, 2H, arom.), 7.69 (d, 1H, J 8.4 Hz, arom.).

1,3-Dicyclohexyl-1-(4-fluorobenzoyl)-urea (3f)

Colorless crystals from chloroform, m.p. 177-178 ºC (67% from MW method and 64% from conventional method); Lit.²⁴ m.p. 178 ºC (48%). IR: 3277, 2935, 2857, 1709, 1620, 1541 cm^-1. ¹H NMR: d 0.83-2.05 (m, 20H, 10 CH₂), 3.42-3.58 (m, 1H, CH), 4.10 (dddd, 1H, CH', J_1ax,2ax 12.0 Hz and J 3.6 Hz), 5.92 (bd, 1H, NH, (1ax,6ax)1ax,2eq (1ax,6eq) J 6.3 Hz), 7.08 (dd, 2H, ³J_H,F 8.6 Hz and ³J_H,H 8.6 Hz, arom.), 7.57 (dd, 2H, ³J_H,H 8.7 Hz and ⁴J_H,F 5.1 Hz, arom.).

1,3-Dicyclohexyl-1-(4-methoxybenzoyl)-urea (3g)

Colorless crystals from chloroform, m.p. 149-150 ºC (55% from MW method and 53% from conventional method); Lit.¹⁵ m.p. 151 ºC (71%). IR and ¹H NMR spectra gave the same absorptions as reported earlier.

1-Benzoyl-1,3-diisopropylurea (3h)

Colorless crystals from chloroform, m.p. 111-112 ºC (61% from MW method and 54% from conventional method); Lit.¹¹ m.p. 114 ºC (58%). IR and ¹H NMR spectra gave the same absorptions as reported earlier.

1,3-Diisopropyl-1-(4-nitrobenzoyl)-urea (3i)

Yellow crystals from chloroform, m.p. 129-130 ºC (72% from MW method and 76% from conventional method); Lit.¹⁵ m.p. 130 ºC (71%). IR and ¹H NMR spectra gave the same absorptions as reported earlier.

Benzoic anhydride (4a)

Colorless crystals from chloroform, m.p. 42-43 ºC (49% from MW method and 44% from conventional method); Lit.²⁶ m.p. 42-44 ºC. IR and ¹H NMR spectra gave the same absorptions as reported earlier.

3-Nitrobenzoic anhydride (4b)

Yellow crystals from chloroform, m.p. 160-162 ºC (34% from MW method and 35% from conventional method); Lit.²⁷ m.p. 162-163 ºC. IR and ¹H NMR spectra gave the same absorptions as reported earlier.

4-Nitrobenzoic Anhydride (4c)

Yellow crystals from chloroform, m.p. 172-173 ºC (30 from MW method and 28% from conventional method); Lit.²⁶ m.p. 172-176 ºC. IR and ¹H NMR spectra gave the same absorptions as reported earlier.

3,5-Dinitrobenzoic anhydride (4d)

Yellow crystals from chloroform, m.p. 107-108 ºC (27% from MW method and 24% from conventional method); Lit.²⁸ m.p. 108-109 ºC. IR and ¹H NMR spectra gave the same absorptions as reported earlier.

4-Methylbenzoic anhydride (4e)

Colorless crystals from chloroform, m.p. 77-78 ºC (37% from MW method and 40% from conventional method); Lit.²⁶ m.p. 78-80 ºC. IR and ¹H NMR spectra gave the same absorptions as reported earlier.

4-Fluorobenzoic anhydride (4f)

Colorless crystals from chloroform, m.p. 107-108 ºC (33% from MW method and 36% from conventional method); Lit.²⁹ m.p. 108-110 ºC (21%). ¹H NMR: d 7.15 (ddd, 2H, ³J 8.7 Hz and ³J 8.7, and ⁴J 2.1 Hz), 8.13 (dd, 2H, ³J 8.7 Hz and ⁴J 2.1 Hz).

4-Methoxybenzoic anhydride (4g)

Colorless crystals from chloroform, m.p. 84-88 ºC (45% from MW method and 47% from conventional method); Lit.²⁴ m.p. 88-93 ºC. IR and ¹H NMR spectra gave the same absorptions as reported earlier.

Acknowledgments

The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for providing financial help. One of us (R.A.W.N.F.) is thankful to Programa Institucional de Bolsas de Iniciação Científica (PIBIC)/CNPq for a research fellowship since August, 2004 until May, 2006.

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Received: February 27, 2007

Web Release Date: November 15, 2007

Publication Dates

• Publication in this collection
  08 Jan 2008
• Date of issue
  2007

History

• Accepted
  15 Nov 2007
• Received
  27 Feb 2007