Microwave-Assisted Stille Reactions as a Powerful Tool for Building Polyheteroaryl Systems Bearing a (1H)-1,2,4-Triazole Moiety

Abstract

Stille couplings and the combination of Stille/Heck cross-coupling reactions provide useful access to tricyclic systems with valuable material properties from 3,5-dibromo-1,2,4-triazoles. The reactions can all be dramatically improved under microwave irradiation.

Stille reaction - microwave - (1H)-1,2,4-triazole - 1,2,3-triazole - electropolymerization

References and Notes

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<A NAME="RD27809ST-18">18</A>

General Procedure: A mixture of 3,5-dibromo-1-methyl-1H-1,2,4-triazole (1; 200 mg, 0.83 mmol), stannyl derivative (2; 1.08 mmol), PdCl₂(PPh₃)₂ (12 mg, 0.017 mmol) and LiCl (106 mg, 2.49 mmol) or CuI (15.8 mg, 0.083 mmol) was irradiated under argon in a Discover™ (CEM) focused microwave reactor at 110 ˚C for 15 min. The crude products 3a and 4a-g were purified by flash chromatography on silica gel (hexane-EtOAc).
5-Bromo-1-methyl-3-(thiophen-2-yl)-1 H -1,2,4-triazole (3a): Mp 54-55 ˚C; ^¹H NMR (CDCl₃): δ = 7.58 (dd, J = 3.7, 1.2 Hz, 1 H, H-3′), 7.28 (dd, J = 5.1, 1.2 Hz, 1 H, H-5′), 7.02 (dd, J = 5.1, 3.7 Hz, 1 H, H-4′), 3.84 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 159.0, 132.8, 129.7, 127.7, 126.8, 126.5, 36.5 (CH₃).
3-Bromo-1-methyl-5-(thiophen-2-yl)-1 H -1,2,4-triazole (4a): Mp 67-68 ˚C; ^¹H NMR (CDCl₃): δ = 7.54-7.62 (m, 2 H, H-3′′, H-5′′), 7.19 (dd, J = 5.1, 3.7 Hz, 1 H, H-4′′), 4.06 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 151.1, 138.7, 129.4, 129.0, 128.0, 127.9, 37.4 (CH₃).
3-Bromo-1-methyl-5-(1-methyl-1 H -pyrrol-2-yl)-1 H -1,2,4-triazole (4b): Mp 41-42 ˚C; ^¹H NMR (CDCl₃): δ = 6.74 (t, J = 1.8 Hz, 1 H, H-5′′), 6.41 (dd, J = 4.0, 1.5 Hz, 1 H, H-3′′), 6.14 (dd, J = 3.7, 2.6 Hz, 1 H, H-4′′), 4.06 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 149.9, 138.3, 126.8, 118.3, 113.1, 108.2, 37.1 (CH₃), 36.0 (CH₃).
5-(2,2′-Bithiophen-5-yl)-3-bromo-1-methyl-1 H -1,2,4-triazole (4c): Mp 152-154 ˚C; ^¹H NMR (CDCl₃): δ = 7.61 (d, J = 3.9 Hz, 1 H, H-3), 7.31 (dd, J = 5.1, 1.1 Hz, 1 H, H-3′), 7.27 (dd, J = 3.8, 1.1 Hz, 1 H, H-5′), 7.20 (d, J = 3.9 Hz, 1 H, H-4), 7.05 (dd, J = 5.1, 3.8 Hz, 1 H, H-4′); ^¹³C NMR (CDCl₃): δ = 150.7, 141.7, 138.7, 135.7, 129.5, 128.1, 125.9, 125.9, 125.0, 124.1, 35.5 (CH₃).
3-Bromo-1-methyl-5-phenyl-1 H -1,2,4-triazole (4d): Mp 120-121 ˚C; ^¹H NMR (CDCl₃): δ = 7.59-7.61 (m, 2 H, o-PhH), 7.44-7.46 (m, 3 H, m-PhH, p-PhH), 3.91 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 156.6, 139.1, 130.7, 129.0, 128.7, 126.7, 37.3 (CH₃).
3-Bromo-1-methyl-5-(phenylethynyl)-1 H -1,2,4-triazole (4e): Mp 119-120 ˚C; ^¹H NMR (CDCl₃): δ = 7.52-7.55 (m, 2 H, o-PhH), 7.28-7.94 (m, 3 H, m-PhH, p-PhH), 3.97 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 147.7, 140.3, 132.1, 132.0, 129.2, 128.7, 128.4, 121.7, 97.4, 89.5, 36.5 (CH₃). 2-(3-Bromo-1-methyl-1 H -1,2,4-triazol-5-yl)pyridine (4f): Mp 78-80 ˚C; ^¹H NMR (CDCl₃): δ = 8.68 (dd, J = 4.8, 1.8 Hz, 1 H, H-6), 8.20 (dd, J = 7.7, 1.1 Hz, 1 H, H-3), 7.85 (td, J = 7.7, 1.8 Hz, 1 H, H-4), 7.37 (ddd, J = 7.7, 4.8, 1.1 Hz, 1 H, H-5), 4.36 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 153.4, 148.9, 146.8, 138.3, 137.1, 124.5, 124.0, 39.3 (CH₃).
2-(3-Bromo-1-methyl-1 H -1,2,4-triazol-5-yl)pyrazine (4g): Mp 131-133 ˚C; ^¹H NMR (CDCl₃): δ = 9.45 (s, 1 H, H-3), 8.67, 8.64 (2 × s, 2 H, H-5 and H-6), 4.34 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 151.1, 145.2, 154.1, 143.1, 142.5, 138.8, 39.3 (CH₃).

<A NAME="RD27809ST-19">19</A>

General Procedure: A mixture of 3,5-dibromo-1-methyl-1H-1,2,4-triazole (1; 200 mg, 0.83 mmol), stannyl derivative (2; 2.16 mmol, 2.6 equiv), PdCl_{2 (}PPh₃)₂ (24 mg, 0.033 mmol) and LiCl (106 mg, 2.49 mmol) was irradiated under argon in a Discover™ (CEM) focused microwave reactor at 130 ˚C for the appropriate time. The crude products 5a-c were purified by flash chromatography on silica gel (hexane-EtOAc).
1-Methyl-3,5-di(thiophen-2-yl)-1 H -1,2,4-triazole (5a): Mp 107-108 ˚C; ^¹H NMR (CDCl₃): δ = 7.71 (dd, J = 3.7, 1.1 Hz, 1 H, H-3′), 7,54 (dd, J = 3.7, 1.1 Hz, 1 H, H-3′′), 7.53 (dd, J = 5.1, 1.1 Hz, 1 H, H-5′′), 7.34 (dd, J = 4.8, 1.1 Hz, 1 H, H-5′), 7.18 (dd, J = 5.1, 3.7 Hz, 1 H, H-4′′), 7.10 (dd, J = 5.1, 3.7 Hz, 1 H, H-4′), 4.07 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 157.3, 150.0, 133.5, 128.9, 128.7, 128.6, 127.9, 127.6, 126.3, 126.2, 37.1 (CH₃).
1-Methyl-3,5-bis(1-methyl-1 H -pyrrol-2-yl)-1 H -1,2,4-triazole (5b): Mp 78-79 ˚C; ^¹H NMR (CDCl₃): δ = 6.74 (t, J = 2.0 Hz, 1 H, H-5′′), 6.73 (dd, J = 2.0, 1.0 Hz, 1 H, H-5′), 6.62 (t, J = 2.0 Hz, 1 H, H-3′), 6.43 (dd, J = 4.4, 2.4 Hz, 1 H, H-3′′), 6.17 (dd, J = 4.0, 2.4 Hz, 1 H, H-4′′), 6.10 (dd, J = 3.4, 2.9 Hz, 1 H, H-4′), 3.94 (s, 3 H, CH₃), 3.91 (s, 3 H, CH₃), 3.85 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 155.9, 148.0, 126.1, 125.0, 124.8, 119.8, 112.3, 110.3, 108.0, 107.7, 36.7 (CH₃), 36.6 (CH₃), 36.1 (CH₃).
3,5-Di(2,2′-bithiophen-5-yl)-1-methyl-1 H -1,2,4-triazole (5c): Mp 146-148 ˚C; ^¹H NMR (CDCl₃): δ = 7.61 (d, J = 3.8 Hz, 1 H, H-3′′), 7.45 (d, J = 3.8 Hz, 1 H, H-3), 7.22-7.31 (m, 5 H, H-4, H-3′, H-5′, H-3′′′, H-5′′′), 7.17 (d, J = 3.8 Hz, 1 H, H-4′′), 7.07 (dd, J = 4.9, 3.8 Hz, 1 H), 7.03 (t, J = 4.3 Hz, 1 H, H-4′ and H-4′′′); ^¹³C NMR (CDCl₃): δ = 156.9, 149.7, 140.8, 138.2, 137.2, 136.1, 131.9, 129.0, 128.1, 127.9, 127.2, 126.9, 125.6, 124.8, 124.7, 124.2, 124.1, 124.0, 37.3 (CH₃).

<A NAME="RD27809ST-20">20</A>

General Procedure: A mixture of 5-monosubstituted triazole 4 (1 equiv), stannyl derivative (2; 1.3 equiv),
PdCl₂ (PPh₃)₂ (0.02 equiv) and LiCl (3 equiv) or CuI (0.1 equiv) was irradiated at the appropriate temperature and for the appropriate time under argon in a Discover™ (CEM) focused microwave reactor. The crude products 6a-c were purified by flash chromatography on silica gel (hexane-EtOAc).
1-Methyl-3-(1-methyl-1 H -pyrrol-2-yl)-5-(thiophen-2-yl)-1 H -1,2,4-triazole (6a): Mp 88-89 ˚C; ^¹H NMR (CDCl₃): δ = 7.53 (dd, J = 3.7, 1.5 Hz, 1 H, H-3′′), 7.52 (dd, J = 5.1, 1.1 Hz, 1 H, H-5′′), 7.18 (dd, J = 5.1, 3.7 Hz, 1 H, H-4′′), 6.81 (dd, J = 3.7, 1.8 Hz, 1 H, H-5′), 6.70 (t, J = 2.2 Hz, 1 H, H-3′), 6.17 (dd, J = 3.7, 2.6 Hz, 1 H, H-4′), 4.08 (s, 3 H, CH₃), 4.00 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 156.3, 149.1, 129.5, 128.5, 128.2, 127.8, 125.2, 124.4, 110.8, 107.8, 37.0 (CH₃), 36.5 (CH₃).
3-(2,2′-Bithiophen-5-yl)-1-methyl-5-(thiophen-2-yl)-1 H -1,2,4-triazole (6b): Mp 124-126 ˚C; ^¹H NMR (CDCl₃): δ = 7.62 (d, J = 3.8 Hz, 1 H, H-4′′), 7.57 (dd, J = 3.7, 1.1 Hz, 1 H, H-3′′′), 7.55 (dd, J = 5.1, 1.1 Hz, 1 H, H-5′′′), 7.24-7.23 (m, 2 H, H-3′, H-5′), 7.20 (dd, J = 5.1, 3.7 Hz, 1 H, H-4′′′), 7.07 (dd, J = 3.8 Hz, 1 H, H-3′′), 7.03 (dd, J = 3.8, 4.9 Hz, 1 H, H-4′), 4.09 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 156.9, 150.1, 138.2, 137.2, 131.9, 128.9, 128.8, 128.7, 127.9, 126.9, 124.7, 124.4, 124.0, 37.2 (CH₃).
2-[1-Methyl-3-(thiophen-2-yl)-1 H -1,2,4-triazol-5-yl]pyrazine (6c): Mp 139-140 ˚C; ^¹H NMR (CDCl₃): δ = 9.55 (d, J = 1.7 Hz, 1 H, H-3 Py), 8.64-8.67 (m, 2 H, H-5, H-6 Py), 7.75 (dd, J = 3.7, 1.1 Hz, H-3 thioph), 7.37 (dd, J = 5.1, 1.1 Hz, 1 H, H-5 thioph), 7.13 (dd, J = 5.1, 3.7 Hz, 1 H, H-4 thioph), 4.37 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 157.5, 150.1, 145.4, 144.7, 143.5, 143.0, 133.5, 127.7, 126.5, 126.3.

<A NAME="RD27809ST-21">21</A>

General Procedure: A mixture of 1-phenyl-4-(tributyl-stannyl)-1H-1,2,3-triazole (11; 100 mg, 0.23 mmol), halogenated derivative 12 (0.25 mmol), PdCl₂(PPh₃)₂ (3 mg, 0.004 mmol) and LiCl (29.3 mg, 0.69 mmol) was irradiated at the appropriate temperature and for the appropriate time under argon in a Discover™ (CEM) focused microwave reactor. The crude products 13a-g were purified by flash chromatography (hexane-EtOAc).
4-(3,5-Dimethylphenyl)-1-phenyl-1 H -1,2,3-triazole (13a): Mp 115-116 ˚C; ^¹H NMR (CDCl₃): δ = 8.1 (s, 1 H, H-5), 7.8 (d, J = 7.3 Hz, 2 H, o-PhH), 7.5 (m, 4 H, Ph′H), 7.4 (t, J = 7.3 Hz, 1 H, p-Ph′H), 7.0 (s, 1 H, p-PhH), 2.2 (s, 6 H, 2 × CH₃); ^¹³C NMR (CDCl₃): δ = 148.6, 138.5, 137.1, 130.1, 129.9, 128.7, 123.6, 120.5, 117.5, 21.3 (CH₃).
4-(2,6-Dimethylphenyl)-1-phenyl-1 H -1,2,3-triazole (13b): Colorless oil; ^¹H NMR (CDCl₃): δ = 7.9 (s, 1 H, H-5), 7.8 (d, J = 7.1 Hz, 2 H, o-PhH), 7.5 (t, J = 6.9 Hz, 2 H,
m-PhH), 7.4 (t, J = 7.3 Hz, 1 H, p-PhH), 7.2-7.1 (m, 3 H, PhH′), 2.3 (s, 6 H, 2 × CH₃); ^¹³C NMR (CDCl₃): δ = 146.2, 137.9, 137.1, 129.8, 129.6, 123.6, 120.3, 117.5, 20.8 (CH₃).
4-(3-Bromophenyl)-1-phenyl-1 H -1,2,3-triazole (13c): Yellow oil; ^¹H NMR (CDCl₃): δ = 8.2 (s, 1 H), 8.0 (m, 1 H), 7.8-7.7 (m, 3 H), 7.6-7.5 (m, 4 H), 7.3 (t, J = 7.3 Hz, 1 H, p-PhH); ^¹³C NMR (CDCl₃): δ = 147.0, 136.9, 132.3, 131.3, 130.5, 129.8, 128.9, 128.8, 124.4, 123.0, 120.6, 117.9.
4-(Naphthalen-1-yl)-1-phenyl-1 H -1,2,3-triazole (13d): Mp 106-107 ˚C; ^¹H NMR (CDCl₃): δ = 8.5-8.4 (m, 1 H, H-Naph), 8.2 (s, 1 H, H-5), 7.9-7.8 (m, 5 H), 7.6-7.4 (m, 6 H); ^¹³C NMR (CDCl₃): δ = 147.6, 137.1, 133.9, 131.2, 129.9, 129.8, 129.2, 128.8, 128.5, 127.7, 127.4, 126.7, 126.1, 125.4, 120.6.
2-(1-Phenyl-1 H -1,2,3-triazol-4-yl)pyrazine (13e): Colorless oil; ^¹H NMR (CDCl₃): δ = 9.5 (s, 1 H, H-Py), 8.6 (s, 1 H, H-5), 8.6-8.5 (m, 2 H, H-Py), 7.8 (d, J = 7.3 Hz, 2 H, o-PhH), 7.5 (t, J = 7.3 Hz, 2 H, m-PhH), 7.4 (t, J = 7.3 Hz, 1 H, p-PhH); ^¹³C NMR (CDCl₃): δ = 146.6, 145.7, 144.1, 143.9, 142.3, 136.7, 129.9, 129.2, 120.9, 120.6.
1,1′-Diphenyl-1 H ,1′ H -4,4′-bi(1,2,3-triazole) (13f): Mp 118-119 ˚C; ^¹H NMR (CDCl₃): δ = 8.6 (s, 1 H), 8.0 (s, 1 H), 7.7 (d, J = 7.3 Hz, 4 H, o-PhH), 7.5 (t, J = 7.3 Hz, 4 H,
m-PhH), 7.4 (d, J = 7.3 Hz, 2 H, p-PhH). ^¹³C NMR (CDCl₃): δ = 137.0, 134.4, 129.7, 128.7, 121.6, 120.6.
4-(3-Bromo-1-methyl-1 H -1,2,4-triazol-5-yl)-1-phenyl-1 H -1,2,3-triazole (13g): Mp 104-105 ˚C; ^¹H NMR (CDCl₃): δ = 8.5 (s, 1 H, H-5), 7.7 (d, J = 7.3 Hz, 2 H, o-PhH), 7.5 (t, J = 8 Hz, 2 H, m-PhH), 7.4 (d, J = 7.1 Hz, 1 H, p-PhH), 4.3 (s, 3 H, CH₃); ^¹³C NMR (CDCl₃): δ = 147.5, 138.9, 137.7, 136.3, 130.1, 129.6, 122.6, 120.6, 38.3 (CH₃).