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Synlett 2007(13): 2081-2085  
DOI: 10.1055/s-2007-984905
LETTER
© Georg Thieme Verlag Stuttgart · New York

Preparation of Polyfunctional Aryl Triazenes via a Direct Insertion Reaction of Zn in the Presence of LiCl

Ching-Yuan Liu, Paul Knochel*
Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, Haus F, 81377 München, Germany
Fax: +49(89)218077680; e-Mail: Paul.Knochel@cup.uni-muenchen.de;
Further Information

Publication History

Received 16 May 2007
Publication Date:
17 July 2007 (online)

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Abstract

A direct zinc insertion of functionalized iodo- or bromo­phenyl triazenes with zinc dust in the presence of LiCl generates the corresponding arylzinc derivatives which react with a variety of electrophiles such as allylic bromides, acid chlorides, TsCN, and aryl halides providing polyfunctional aryl triazenes in good yields. Two successive zinc insertion reactions into diiodophenyl triazenes have also been achieved.

Key words

zinc insertion - lithium chloride - functionalized tri­azenes - copper catalysis - Negishi cross-coupling

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Typical Procedure: Preparation of the Aryl Triazene 3b
Anhydrous LiCl (8 mmol) was placed in an Ar-flushed flask and dried for 5 min at 130 °C under vacuum (1 mbar). Zinc dust (8 mmol, 2.0 equiv, <10 micron, 98+%, Aldrich) was added under Ar and the mixture was dried again for 10 min at 130 °C under vacuum (1 mbar). The reaction flask was evacuated and refilled with Ar twice. Then THF (4 mL) was added and the zinc dust was activated with BrCH2CH2Br (5 mol%) and Me3SiCl (2 mol%) by warming the reaction mixture with a heat gun for a few minutes. 1-(2-Carbethoxy-4-iodophenylazo)pyrrolidine (2a, 1.5 g, 4 mmol) was added neat at r.t. and the reaction mixture was gradually heated to 50 °C. The insertion reaction was complete after 7 h (checked by GC analysis of reaction aliquots, the conversion was ca. 92%). Titration of an aliquot (1 mL) of the organo-zinc reagent with iodine indicated that the concentration of 1a was ca. 0.88 M. The solution of 1a (3.4 mL, 3 mmol) was carefully separated from the remaining zinc dust by using a syringe and transferred to another Ar-flushed flask. Benzoyl chloride (635 mg, 4.5 mmol) was added at -20 °C, followed by CuCN·2LiCl (3 mL of 1.0 M solution in THF, 3 mmol). The reaction mixture was gradually warmed to r.t. and stirred for 2 h and then quenched with sat. aq NH4Cl solution (5 mL). The aqueous phase was extracted with Et2O (3 × 5 mL) and concentrated in vacuo. Purification by flash chromatography (n-pentane-Et2O = 1:1) furnished the aryl triazene 3b (853 mg, 81%) as a pale red liquid (entry 2 of Table [1] ). 1H NMR (600 MHz, CDCl3, 25 °C): δ = 8.06 (s, 1 H), 7.85 (d, J = 8.6 Hz, 1 H), 7.76 (d, J = 8.4 Hz, 2 H), 7.57 (t, J = 7.5 Hz, 1 H), 7.52 (d, J = 8.6 Hz, 1 H), 7.44-7.49 (m, 2 H), 4.34 (q, J = 7.1 Hz, 2 H), 3.95 (br s, 2 H), 3.71 (br s, 2 H), 1.98-2.12 (m, 4 H), 1.35 (t, J = 7.1 Hz, 3 H) ppm. 13C NMR (150 MHz, CDCl3, 25 °C): δ = 195.3, 168.0, 153.2, 137.8, 133.1, 132.2, 131.9, 129.8, 128.4, 128.3, 126.4, 118.8, 61.1, 51.3, 46.9, 23.9, 23.5, 14.3 ppm. MS (70 eV, EI): m/z (%) = 351 (5) [M+], 281 (28), 254 (9), 225 (100), 209 (9), 177 (8), 141 (14), 105 (75), 77 (30). IR (neat): ν = 3061 (w), 2977 (w), 2874 (w), 1719 (s), 1651 (vs), 1595 (vs), 1394 (vs), 1302 (s), 1239 (s), 1071 (m), 959 (m) cm-1. HRMS (EI): m/z calcd for C20H21N3O3: 351.1583; found: 352.1651 [M + H]+.

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Aryl Triazene 3d
Mp 73.8-76.7 °C. 1H NMR (600 MHz, CDCl3, 25 °C): δ = 9.97 (s, 1 H), 8.01 (d, J = 7.9 Hz, 1 H), 7.61-7.66 (m, 2 H), 7.60 (s, 1 H), 7.44 (d, J = 8.2 Hz, 1 H), 7.40 (d, J = 7.9 Hz, 1 H), 7.21 (t, J = 8.2 Hz, 1 H), 3.95-4.01 (m, 2 H), 3.75-3.82 (m, 2 H), 2.00-2.12 (m, 4 H) ppm. 13C NMR (150 MHz, CDCl3, 25 °C): δ = 191.6, 153.6, 143.6, 134.8, 134.1, 133.9, 133.8, 133.6, 130.6, 128.3, 128.1, 117.6, 117.3, 107.4, 51.5, 47.3, 23.9, 23.4 ppm. MS (70 eV, EI): m/z (%) = 304 (43) [M+], 234 (72), 206 (100), 177 (42), 151 (41). IR (KBr): ν = 3057 (w), 2973 (w), 2873 (w), 2748 (w), 2223 (m), 1689 (s), 1596 (m), 1470 (m), 1384 (s), 1307 (s), 1267 (s), 1098 (m), 903 (w) cm-1. HRMS (EI): m/z calcd for C18H16N4O: 304.1324; found: 304.1328.

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Aryl Triazene 3e
Mp 120.3-122.2 °C. 1H NMR (600 MHz, CDCl3, 25 °C): δ = 7.60 (s, 1 H), 7.51 (d, J = 8.4 Hz, 1 H), 7.48 (d, J = 8.4 Hz, 1 H), 7.43 (d, J = 8.6 Hz, 2 H), 6.90 (d, J = 8.6 Hz, 2 H), 3.92 (br s, 2 H), 3.83 (s, 3 H), 3.49 (br s, 2 H), 1.95-2.00 (m, 4 H) ppm. 13C NMR (150 MHz, CDCl3, 25 °C): δ = 158.9, 151.5, 136.3, 134.2, 131.3, 131.0, 130.5, 119.6, 117.9, 113.0, 107.7, 55.2, 51.1, 46.9, 23.9, 23.3 ppm. MS (70 eV, EI): m/z (%) = 306 (22) [M+], 236 (11), 208 (100), 193 (32), 177 (6), 165 (19). IR (KBr): ν = 2948 (w), 2879 (w), 2830 (w), 2215 (s), 1595 (m), 1513 (m), 1479 (m), 1383 (s), 1243 (s), 1173 (s), 1030 (m) cm-1. HRMS (EI): m/z calcd for C18H18N4O: 306.1481; found: 306.1483.

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Aryl Triazene 6
Mp 133.5-135.0 °C. 1H NMR (300 MHz, CDCl3, 25 °C): δ = 7.76 (d, J = 7.1 Hz, 2 H), 7.63 (t, J = 7.1 Hz, 1 H), 7.49-7.53 (m, 2 H), 7.46 (d, J = 7.1 Hz, 2 H), 2.38 (s, 3 H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 195.8, 146.8, 139.3, 135.6, 134.7, 134.3, 132.3, 130.4, 128.9, 122.3, 119.1, 92.3, 20.7 ppm. MS (70 eV, EI): m/z (%) = 347 (100) [M+], 270 (22), 220 (34), 190 (11), 165 (6), 115 (15), 105 (98), 77 (40). IR (KBr): n = 3041 (w), 2922 (w), 2852 (w), 2230 (m), 1667 (vs), 1594 (m), 1578 (m), 1448 (m), 1412 (m), 1318 (s), 1212 (m), 1020 (m), 878 (m) cm-1. HRMS (EI): m/z calcd for C15H10INO: 346.9807; found: 346.9789.

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Aryl Triazene 7 Mp 151.4-152.6 °C. 1H NMR (600 MHz, CDCl3, 25 °C): δ = 7.38 (d, J = 8.4 Hz, 2 H), 7.32-7.36 (m, 4 H), 3.88 (br s, 2 H), 3.78 (br s, 2 H), 3.48 (br s, 2 H), 2.34 (s, 3 H), 1.98-2.03 (m, 4 H), 1.92-1.97 (m, 4 H) ppm. 13C NMR (150 MHz, CDCl3, 25 °C): δ = 150.3, 149.3, 136.5, 135.5, 135.4, 134.4, 133.0, 130.6, 119.5, 119.3, 103.7, 50.8, 46.6, 23.9, 23.8, 23.4, 20.6 ppm. MS (70 eV, EI): m/z (%) = 387 (100) [M+], 348 (11), 289 (59), 277 (53), 262 (49), 219 (65), 205 (12), 191 (72), 165 (13), 152 (25), 116 (10), 70 (19). IR (KBr): ν = 3051 (w), 2974 (w), 2874 (w), 2210 (w)1596 (w), 1397 (s), 1340 (s), 1312 (m), 1218 (m), 1148 (m), 970 (w) cm-1. HRMS (EI): m/z calcd for C22H25N7: 387.2171; found: 387.2174.