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Synlett 2008(13): 2056-2060  
DOI: 10.1055/s-2008-1077950
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Cobalt-Mediated [2+2+2] Cycloaddition of Alkynyl Boronates to Indole and Pyrrole Double Bonds

Sabine Amslingera, Corinne Aubertb, Vincent Gandonb, Max Malacriab, Elisa Paredesa, K. Peter C. Vollhardt*a
a Department of Chemistry, University of California at Berkeley, Berkeley, CA 94720-1460, USA
Fax: +1(510)6435208; e-Mail: kpcv@berkeley.edu;
b Laboratoire de Chimie Organique (UMR CNRS 7611), Institut de Chimie Moléculaire (FR 2769), Université Pierre et Marie Curie-Paris 6, case 229, 4 place Jussieu, 75252 Paris cedex 05, France
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Publication History

Received 10 May 2008
Publication Date:
15 July 2008 (online)

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Abstract

The complex [CpCo(C2H4)2] facilitates the [2+2+2] cycloaddition of borylalkynes to N-(4-pentynoyl)indole and -pyrrole to furnish heterocyclofused borylcyclohexadienes regioselectively.

Keywords

alkynes - cycloadditions - heterocycles - polycycles - ­transition metals

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12

Representative Cyclization Conditions; Synthesis of 5: To a degassed solution of N-(4-pentynoyl) indole (1;6g,¹4 197 mg, 1 mmol) and 4,4,5,5-tetramethyl-2-(3-methylbut-1-ynyl)[1,3,2]dioxaborolane (2; 242 mg, 1.25 mmol) in anhyd THF (15 mL) at 0 ˚C under an atmosphere of argon was added η5-cyclopentadienylbis(ethene)cobalt (150 mg, 0.833 mmol) in anhyd, degassed THF (6 mL) over a period of 3 h via syringe pump. The ensuing dark-red solution was treated with bright yellow Fe(NO3)3˙9H2O (337 mg, 0.834 mmol) in THF-H2O (5 mL, 3:1). After stirring for 5 min at 0 ˚C, the brown mixture was poured into iced water (50 mL) and the solution was extracted with CH2Cl2 (3 × 100 mL), followed by sat. aq NaHCO3 (3 × 150 mL). The extracts were dried (MgSO4), filtered through a short silica gel column to give a red-orange foam, and then subjected to flash chromatography on silica gel (hexanes-acetone, 9:1) to yield 5 (189 mg, 58%) as a white solid; mp 91-92 ˚C.
All new compounds gave satisfactory elemental analytical and/or HRMS data. For selected spectra, see: 3: ¹H NMR (500 MHz, CDCl3): δ = 1.21 (d, J = 7.0 Hz, 3 H, (CH 3)2CH], 1.25 (s, 6 H, Me), 1.32 (s, 6 H, Me), 1.48 [d, J = 6.5 Hz, 3 H, (CH 3)2CH], 1.80-1.85 (m, 1 H, CH2), 1.98-2.03 (m, 1 H, CH2), 2.52-2.66 [m, 3 H, CH2, (CH3)2CH], 2.55 (d, J = 8.0 Hz, 1 H, CH), 3.24 (d, J = 7.5 Hz, 1 H, CH), 4.52 (s, 5 H, Cp), 5.01 (s, 1 H, CH), 6.98 (t, J = 7.5 Hz, 1 H, CHarom), 7.23 (t, J = 7.5 Hz, 1 H, CHarom), 7.77 (d, J = 7.5 Hz, 1 H, CHarom), 8.20 (d, J = 7.5 Hz, 1 H, CHarom). ¹³C NMR (126 MHz, CDCl3): δ = 20.5, 24.5, 26.0, 26.1, 30.4, 31.5, 32.5, 45.2, 63.4, 64.9, 72.1, 81.1, 83.4, 106.4, 116.1, 122.8, 126.4, 127.6, 135.2, 142.7, 169.8; C-B not detected. MS (FAB, NBA): m/z (%) = 515 (100) [M+].
4: ¹H NMR (500 MHz, CDCl3): δ = 0.62 [d, J = 6.5 Hz, 3 H, (CH 3)2CH], 1.29 [d, J = 7.0 Hz, 3 H, (CH 3)2CH], 1.33 (s, 6 H, Me), 1.35 (s, 6 H, Me), 1.75-1.81 (m, 1 H, CH2), 2.28 (m, 1 H, CH2), 2.43-2.48 (m, 1 H, CH2), 2.69-2.76 (m, 1 H, CH2), 3.08 [sep, J = 6.5 Hz, 1 H, (CH3)2CH], 3.83 (d, J = 11.0 Hz, 1 H, CH), 4.41 (d, J = 11.5 Hz, 1 H, CH), 4.67 (s, 1 H, CH), 4.68 (s, 5 H, Cp), 6.83 (t, J = 7.0 Hz, 1 H, CHarom), 6.99-7.04 (m, 2 H, CHarom), 7.68 (d, J = 7.5 Hz, 1 H, CHarom). ¹³C NMR (126 MHz, CDCl3): δ = 19.5, 24.8, 25.5, 25.8, 29.5, 31.3, 33.3, 44.4, 62.1, 67.1, 76.3, 81.1, 82.8, 109.7, 115.5, 123.2, 124.4, 127.0, 133.6, 140.8, 169.8; C-B not detected. MS (FAB, NBA): m/z (%) = 515 (100) [M+].
5: ¹H NMR (400 MHz, CDCl3): δ = 0.93 [d, J = 6.8 Hz, 3 H, (CH 3)2CH], 0.96 [d, J = 6.8 Hz, 3 H, (CH 3)2CH], 1.32 (s, 6 H, Me), 1.35 (s, 6 H, Me), 2.47-2.57 (m, 2 H, CH2), 2.65-2.81 (m, 2 H, CH2), 3.46 [sep, J = 6.9 Hz, 1 H, (CH3)2CH], 4.57 (d, J = 12.8 Hz, 1 H, CH), 5.01 (dd, J = 1.2, 13.6 Hz, 1 H, CH), 5.88 (br s, 1 H, CH), 6.98 (t, J = 7.2 Hz, 1 H, CHarom), 7.16 (t, J = 7.8 Hz, 1 H, CHarom), 7.31 (d, J = 7.6 Hz, 1 H, CHarom), 8.12 (d, J = 7.6 Hz, 1 H, CHarom). ¹³C NMR (101 MHz, CDCl3): δ = 21.0, 21.5, 24.8, 25.1, 26.0, 30.9, 32.8, 41.2, 58.6, 83.3, 115.9, 118.4, 123.6, 124.4, 127.5, 133.8, 136.0, 140.1, 152.1, 170.2; C-B not detected. MS (70 eV, EI): m/z (%) = 391 (100) [M+], 286 (35), 260 (30). IR (neat): 2977, 1673, 1594, 1480, 1461, 1390, 1317, 1289, 1269, 1143, 1017, 857, 752, 732, 672 cm-¹.
7: ¹H NMR (500 MHz, CDCl3): δ = 0.85 (t, J = 6.8 Hz, 3 H, Me), 1.25 (m, 8 H, CH2), 1.32 (s, 6 H, Me), 1.35 (s, 6 H, Me), 2.19-2.23 (m, 1 H, CH2), 2.47-2.58 (m, 3 H, CH2), 2.66-2.79 (m, 2 H, CH2), 4.57 (d, J = 14.0 Hz, 1 H, CH), 5.01 (d, J = 13.5 Hz, 1 H, CH), 5.70 (s, 1 H, CH), 6.97 (t, J = 7.0 Hz, 1 H, CHarom), 7.15 (t, J = 7.8 Hz, 1 H, CHarom), 7.33 (d, J = 7.5 Hz, 1 H, CHarom), 8.12 (d, J = 8.0 Hz, 1 H, CHarom). ¹³C NMR (126 MHz, CDCl3): δ = 14.1, 22.6, 25.0, 25.7 (2 × C), 29.1, 29.8, 31.7, 32.7, 35.4, 41.2, 58.6, 83.3, 115.9, 123.6, 123.7, 124.5, 127.5, 133.5, 136.0, 140.1, 148.2, 170.3; C-B not detected. MS (70 eV, EI): m/z (%) = 433 (20) [M+], 432 (40), 431 (100), 361 (45), 285 (95). IR (neat): 2976, 2926, 2855, 1673, 1594, 1479, 1459, 1396, 1379, 1328, 1317, 1268, 1143, 1020, 857, 752 cm-¹.
9: Diastereomer 1: ¹H NMR (500 MHz, CDCl3): δ = 1.32 (s, 6 H, Me), 1.36 (s, 6 H, Me), 1.50 (m, 2 H, CH2), 1.57 (m, 2 H, CH2), 1.68 (m, 1 H, CH2), 1.80 (m, 1 H, CH2), 2.52-2.58 (m, 2 H, CH2), 2.67-2.80 (m, 2 H, CH2), 3.44-3.50 (m, 1 H, CH2), 3.83-3.89 (m, 1 H, CH2), 4.20 (d, J = 11.5 Hz, 1 H, CH2), 4.58-4.67 (m, 3 H, CH, CHO, CH2), 5.04 (br d, J = 13.5 Hz, 1 H, CH), 6.00 (s, 1 H, CH), 6.97 (t, J = 7.5 Hz, 1 H, CHarom), 7.17 (t, J = 7.8 Hz, 1 H, CHarom), 7.34 (d, J = 7.5 Hz, 1 H, CHarom), 8.14 (d, J = 8.0 Hz, 1 H, CHarom). ¹³C NMR (126 MHz, CDCl3): δ = 19.5, 25.0 (2 × C), 25.4, 25.9, 30.7, 32.7, 41.3, 58.5, 62.2, 67.5, 83.8, 97.7, 110.1 (br, C-B), 116.1, 120.9, 123.6, 24.4, 127.7, 133.7, 135.1, 140.1, 143.0, 170.1. MS (70 eV, EI): m/z (%) = 463 (16) [M+], 462 (6), 461 (10), 363 (32), 361 (52). IR (neat): 3053, 2977, 2941, 2870, 1671, 1595, 1480, 1460, 1391, 1381, 1321, 1269, 1202, 1142, 1076, 1022, 869, 754, 736 cm-¹. Diastereomer 2: ¹H NMR (500 MHz, CDCl3): δ = 1.32 (s, 6 H, Me), 1.36 (s, 6 H, Me), 1.48-1.59 (m, 4 H, CH2), 1.66-1.83 (m, 2 H, CH2), 2.52-2.58 (m, 2 H, CH2), 2.67-2.80 (m, 2 H, CH2), 3.44-3.50 (m, 1 H, CH2), 3.83-3.89 (m, 1 H, CH2), 4.17 (d, J = 12.0 Hz, 1 H, CH2), 4.58-4.67 (m, 3 H, CH, CHO, CH2), 5.03 (d, J = 13.5 Hz, 1 H, CH), 6.00 (s, 1 H, CH), 6.97 (t, J = 7.5 Hz, 1 H, CHarom), 7.17 (t, J = 7.8 Hz, 1 H, CHarom), 7.36 (d, J = 8.0 Hz, 1 H, CHarom), 8.14 (d, J = 8.0 Hz, 1 H, CHarom). ¹³C NMR (126 MHz, CDCl3): δ = 19.4, 24.9, 25.0, 25.4, 25.9, 30.5, 32.7, 41.3, 58.5, 62.1, 68.0, 83.8, 98.6, 110.2 (br, C-B), 116.1, 120.9, 123.6, 124.5, 127.7, 133.7, 135.2, 140.1, 143.3, 170.1.
11/12 mixture, major regioisomer 11: ¹H NMR (400 MHz, CDCl3): δ = 0.18 (s, 9 H, SiMe3), 1.33 (s, 6 H, Me), 1.37 (s, 6 H, Me), 2.45-3.03 (m, 4 H, CH2), 4.60 (d, J = 14.0 Hz, 1 H, CH), 5.01 (br d, J = 14.0 Hz, 1 H, CH), 6.01 (br s, 1 H, CH), 6.98 (t, J = 7.4 Hz, 1 H, CHarom), 7.17 (t, J = 7.8 Hz, 1 H, CHarom), 7.38 (d, J = 7.6 Hz, 1 H, CHarom), 8.14 (d, J = 7.6 Hz, 1 H, CHarom). ¹³C NMR (101 MHz, CDCl3): δ = 0.75, 24.6, 25.6, 26.1, 32.8, 42.1, 58.4, 83.9, 116.0, 122.7, 123.6, 124.6, 127.5, 130.8, 135.7, 140.2, 147.6, 170.5; C-B not detected. Minor regioisomer 12: ¹H NMR (400 MHz, CDCl3): δ = 0.36 (s, 9 H, SiMe3), 1.25 (s, 6 H, Me), 1.26 (s, 6 H, Me), 2.45-3.03 (m, 4 H, CH2), 4.52 (d, J = 12.8 Hz, 1 H, CH), 5.01 (dd, J = 1.6, 14.0 Hz, 1 H, CH), 6.11 (s, 1 H, CH), 6.98 (dt, J = 1.2, 7.4 Hz, 1 H, CHarom), 7.17 (t, J = 7.8 Hz, 1 H, CHarom), 7.38 (d, J = 7.6 Hz, 1 H, CHarom), 8.14 (d, J = 7.6 Hz, 1 H, CHarom). ¹³C NMR (101 MHz, CDCl3): δ = 1.38, 24.8, 25.0, 25.1, 32.7, 43.3, 59.5, 83.8, 115.7, 122.5, 123.5, 124.3, 127.8, 129.1, 134.9, 140.2, 147.6, 170.5; C-B not detected. MS (70 eV, EI): m/z (%) = 421 (52) [M+], 420 (66), 404 (28), 348 (22), 347 (20), 322 (30), 278 (65), 83 (100). IR (neat): 2979, 1667, 1594, 1537, 1481, 1460, 1398, 1317, 1247, 1216, 1142, 1091, 1058, 1020, 983, 960, 844, 755, 701, 667 cm-¹.
14/15 mixture, major regioisomer 14: ¹H NMR (500 MHz, CDCl3): δ = 1.07 (s, 6 H, Me), 1.13 (s, 6 H, Me), 2.60 (m, 2 H, CH2), 2.79 (m, 2 H, CH2), 4.70 (d, J = 13.5 Hz, 1 H, CH), 5.11 (d, J = 13.5 Hz, 1 H, CH), 6.07 (br s, 1 H, CH), 7.05 (t, J = 7.5 Hz, 1 H, CHarom), 7.20-7.40 (m, 6 H, CHarom), 7.52 (d, J = 7.5 Hz, 1 H, CHarom), 8.21 (d, J = 8.0 Hz, 1 H, CHarom). Minor regioisomer 15: ¹H NMR (500 MHz, CDCl3): δ = 1.16 (s, 6 H, Me), 1.20 (s, 6 H, Me), 2.60 (m, 2 H, CH2), 2.79 (m, 2 H, CH2), 4.81 (d, J = 14.0 Hz, 1 H, CH), 5.17 (d, J = 14.0 Hz, 1 H, CH), 5.95 (br s, 1 H, CH), 6.67 (t, J = 7.5 Hz, 1 H, CHarom), 7.12 (t, J = 7.5 Hz, 1 H, CHarom), 7.20-7.40 (m, 6 H, CHarom), 8.17 (d, J = 8.0 Hz, 1 H, CHarom). ¹³C NMR (126 MHz, CDCl3; 14/15 mixture): δ = 24.2, 24.6 (2 × C), 24.7, 26.0, 26.1, 30.9, 32.8, 33.1, 36.6, 41.9, 44.0, 58.1, 58.9, 83.5, 83.6, 116.0, 116.2, 120.8, 123.0, 123.1, 123.8, 124.7, 125.0, 127.1, 127.2, 127.7, 127.8, 127.9 (2 × C), 129.0, 132.2, 132.4, 134.9, 140.2, 140.3, 141.4, 142.8, 144.7, 144.9, 169.4, 169.9; C-B (2 × C) not detected. MS (70 eV, EI):
m/z (%) = 425 (18) [M+], 424 (33), 423 (86), 307 (52), 297 (60), 169 (41), 84 (100). IR (neat): 3015, 2979, 2930, 1705, 1655, 1594, 1482, 1408, 1367, 1317, 1141, 1066, 1023, 853, 755, 702, 667 cm-¹.
17: ¹H NMR (500 MHz, CDCl3): δ = 0.88 [d, J = 7.0 Hz, 3 H, (CH 3)2CH], 0.91 [d, J = 7.0 Hz, 3 H, (CH 3)2CH], 1.15 (s, 6 H, Me), 1.20 (s, 6 H, Me), 2.47 (m, 2 H, CH2), 2.57 (dd,
J = 6.4, 14.3 Hz, 1 H, CH2), 2.66 (dd, J = 6.4, 13.0 Hz, 1 H, CH2), 3.34 [sep, J = 7.0 Hz, 1 H, (CH3)2CH], 4.18 (br d, J = 14.6 Hz, 1 H, CH), 4.70 (d, J = 14.4 Hz, 1 H, CH), 5.36 (m, 1 H, CH), 5.83 (br s, 1 H, CH), 6.80 (m, 1 H, CH). ¹³C NMR (126 MHz, CDCl3): δ = 21.2, 24.4, 24.7, 24.9, 26.6, 30.9, 31.6, 45.4, 55.5, 82.9, 117.4, 117.9, 125.0, 132.5, 150.8, 167.1; C-B not detected. MS (70 eV, EI): m/z (%) = 341 (12) [M+], 340 (11), 339 (30), 326 (11), 298 (10), 224 (6), 198 (8), 185 (9), 129 (16), 83 (100). IR (neat): 2976, 2872, 1716, 1643, 1518, 1468, 1412, 1371, 1220, 1144, 1110, 1078, 1016, 983, 951, 924, 885, 858, 831, 787, 744, 674 cm-¹.
19: ¹H NMR (500 MHz, CDCl3): δ = 1.04 [d, J = 6.9 Hz, 6 H, (CH 3)2CH], 1.21 (s, 12 H, Me), 1.95 (quint, J = 6.0 Hz, 2 H, CH2), 2.50 (sep, J = 7.0 Hz, 1 H, CH), 2.54 (td, J = 2.0, 6.0 Hz, 2 H, CH2), 3.95 (t, J = 6.0 Hz, 2 H, CH2), 5.38 (s, 1 H, CH), 6.16 (m, 1 H, CH), 6.42 (m, 1 H, CH), 6.59 (m, 1 H, H-2), 6.61 (br s, 1 H, CH). ¹³C NMR (126 MHz, CDCl3): δ = 21.7, 23.9, 24.9, 25.6, 37.2, 45.5, 82.7, 103.1, 108.3, 119.9, 120.7, 129.6, 131.8, 165.0; C-B not detected. MS (70 eV, EI): m/z (%) = 327 (10) [M+], 312 (10), 284 (26), 184 (40), 59 (100). IR (neat): 3100, 2972, 2871, 1695, 1620, 1458, 1380, 1331, 1259, 1107, 1077, 970, 851, 756, 722, 667 cm-¹.