Synlett 2013; 24(16): 2095-2101
DOI: 10.1055/s-0033-1339657
letter
© Georg Thieme Verlag Stuttgart · New York

Efficient C-7 or C-3/C-7 Direct Arylation of Tri- or Disubstituted Imidazo[1,2-b]pyrazoles

Sandrine Grosse
a  Institut de Chimie Organique et Analytique (ICOA), Université d’Orléans, UMR-CNRS 7311, BP 6759, rue de Chartres, 45067 Orléans cedex 2, France   Fax: +33(38)417281   Email: [email protected]
,
Christelle Pillard
b  Greenpharma S.A.S, 3, allée du Titane, 45100 Orléans, France
,
Philippe Bernard
b  Greenpharma S.A.S, 3, allée du Titane, 45100 Orléans, France
,
Gérald Guillaumet*
a  Institut de Chimie Organique et Analytique (ICOA), Université d’Orléans, UMR-CNRS 7311, BP 6759, rue de Chartres, 45067 Orléans cedex 2, France   Fax: +33(38)417281   Email: [email protected]
› Author Affiliations
Further Information

Publication History

Received: 24 June 2013

Accepted after revision: 24 July 2013

Publication Date:
28 August 2013 (online)


Abstract

A novel and efficient method of C-7 direct arylation of the imidazo[1,2-b]pyrazole core, never described to date, is presented in this paper. Series of electron-rich or electron-poor aryl and heteroaryl groups were easily introduced. The corresponding products were obtained in moderate to excellent yields thanks to this ­pallado-catalyzed and microwave-assisted process. A one-pot ­double C-3 and C-7 direct coupling is also reported.

Supporting Information

 
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  • 15 General Procedure A; C-7 Direct Arylation of the Imidazo[1,2-b]pyrazoles 1–5: A microwave vial containing a stirring bar was loaded with imidazo[1,2-b]pyrazole 15 in 1,4-dioxane, (hetero)aryl bromide or chloride (2.0 equiv), tricyclohexylphosphine tetrafluoroborate (0.20 equiv) and cesium carbonate (2.0 equiv). The tube was evacuated and backfilled with dry argon twice. Palladium acetate (0.10 equiv) was added and the mixture was submitted to microwave irradiation with stirring at 160 °C for 4 h. It was then cooled to r.t., and 1,4-dioxane was removed under reduced pressure. The residue was purified by flash chromatography to provide the desired products 1a5b. 2-(4-Methoxyphenyl)-1-methyl-6-phenyl-3-(4-tolyl)-7-[4-(trifluoromethyl)phenyl]-1H-imidazo[1,2-b]pyrazole (1b): The reaction was carried out as described in general procedure A using imidazo[1,2-b]pyrazole 1 (100 mg, 0.254 mmol), palladium acetate (5.7 mg, 0.0254 mmol), tricyclohexylphosphine tetrafluoroborate (18.7 mg, 0.0508 mmol), cesium carbonate (142 mg, 0.508 mmol) and 4-bromobenzotrifluoride (114 mg, 72 μL, 0.508 mmol) in 1,4-dioxane (2 mL). Standard workup followed by flash chromatography (CH2Cl2–petroleum ether, 1:1) yielded 1b as a pale yellow solid (123 mg, 90%); mp 226–228 °C. 1H NMR (400 MHz, CDCl3): δ = 7.71 (d, J = 8.1 Hz, 2 H, HAr), 7.58 (d, J = 8.4 Hz, 2 H, HAr), 7.53 (d, J = 7.9 Hz, 2 H, HAr), 7.47 (d, J = 8.1 Hz, 2 H, HAr), 7.33 (d, J = 8.5 Hz, 2 H, HAr), 7.25–7.29 (m, 3 H, HAr), 7.12 (d, J = 8.1 Hz, 2 H, HAr), 7.00 (d, J = 8.5 Hz, 2 H, HAr), 3.88 (s, 3 H, OMe), 3.30 (s, 3 H, NMe), 2.32 (s, 3 H, Me). 13C NMR (101 MHz, CDCl3): δ = 160.31 (Cq), 152.26 (Cq), 140.07 (Cq), 137.17 (Cq), 136.82 (Cq), 134.27 (Cq), 132.46 (CHAr), 130.97 (CHAr), 129.19 (Cq), 129.03 (CHAr), 128.87 (CHAr), 128.18 (CHAr), 127.87 (2 J C–F = 33.0 Hz, Cq), 127.42 (CHAr), 127.14 (CHAr), 125.63 (Cq), 125.14 (3 J C–F = 3.72 Hz, CHAr), 124.41 (1 J C–F = 273 Hz, Cq), 121.13 (Cq), 118.42 (Cq), 114.64 (CHAr), 94.33 (Cq), 55.34 (OMe), 31.73 (NMe), 21.29 (Me). IR (neat): 1603, 1322, 1118, 1066, 838, 697 cm–1. HRMS (ESI): m/z [M + H]+ calcd for C33H27F3N3O: 538.21007; found: 538.21007. HRMS (ESI): m/z [M + Na]+ calcd for C33H27F3N3O: 560.19202; found: 560.19113. General Procedure B; One-Pot C-3 and C-7 Direct Arylation of Imidazo[1,2-b]pyrazole 6: A microwave vial containing a stirring bar was loaded with imidazo[1,2-b]pyrazole 6 in 1,4-dioxane, (hetero)aryl bromide (3.0 equiv), tricyclohexylphosphine tetrafluoroborate (0.20 equiv) and cesium carbonate (4.0 equiv). The tube was evacuated and back-filled with dry argon twice. Palladium acetate (0.10 equiv) was added and the mixture was submitted to microwave irradiation with stirring at 160 °C for 4 h. It was then cooled to r.t., and 1,4-dioxane was removed under reduced pressure. The residue was purified by flash chromatography to provide the desired products 1a, 6a and 6b. 2-(4-Methoxyphenyl)-1-methyl-6-phenyl-3,7-bis[4-(trifluoromethyl)phenyl]-1H-imidazo[1,2-b]pyrazole (6a): The reaction was carried out as described in general procedure B using imidazo[1,2-b]pyrazole 6 (100 mg, 0.329 mmol), palladium acetate (8.04 mg, 0.0329 mmol), tricyclohexylphosphine tetrafluoroborate (24.0 mg, 0.0658 mmol), caesium carbonate (369 mg, 1.32 mmol) and 4-bromobenzotrifluoride (222 mg, 138 μL, 0.987 mmol) in 1,4-dioxane (2 mL). Standard workup followed by flash chromatography (CH2Cl2–petroleum ether, 3:7) yielded 6a as a white solid (151 mg, 78%); mp 220–222 °C. 1H NMR (400 MHz, CDCl3): δ = 8.01 (d, J = 8.4 Hz, 2 H, HAr), 7.47–7.64 (m, 8 H, HAr), 7.29–7.41 (m, 5 H, HAr), 7.07 (d, J = 8.4 Hz, 2 H, HAr), 3.92 (s, 3 H, OMe), 3.33 (s, 3 H, NCH3). 13C NMR (101 MHz, CDCl3): δ = 160.76 (Cq), 152.49 (Cq), 140.08 (Cq), 136.81 (Cq), 133.96 (Cq), 132.34 (CHAr), 132.28 (Cq), 131.09 (Cq), 131.05 (CHAr), 128.78 (CHAr), 128.37 (2 J C–F = 32.6 Hz, Cq), 128.27 (CHAr), 128.20 (2 J C–F = 32.8 Hz, Cq), 127.64 (CHAr), 126.73 (CHAr), 125.25 (3 J C–F = 3.70 Hz, 2 × CHAr), 124.36 (1 J C–F = 273 Hz, Cq), 124.18 (1 J C–F = 273 Hz, Cq), 120.39 (Cq), 117.03 (Cq), 114.99 (CHAr), 94.65 (Cq), 55.39 (OMe), 31.74 (NMe). IR (neat): 1613, 1320, 1249, 1162, 1103, 1076, 1064, 1016, 834 cm–1. HRMS (ESI): m/z [M + H] calcd for C33H24F6N3O: 592.18181; found: 592.18163.