Synlett 2004(6): 991-994  
DOI: 10.1055/s-2004-822883
LETTER
© Georg Thieme Verlag Stuttgart · New York

New Persistent Heteroarylmethyl Radicals Resulting from the Intramolecular Addition of Aryloxymethyl Radicals onto Ketenimines. Synthesis of 2H-1,4-Benzoxazines

Mateo Alajarína, Angel Vidal*a, María-Mar Ortína, Delia Bautistab
a Departamento de Química Orgánica, Facultad de Química, Universidad de Murcia, Campus de Espinardo, 30100, Murcia, Spain
Fax: +34(968)364149; e-Mail: vidal@um.es;
b Servicio Universitario de Instrumentación Científica, Universidad de Murcia, Campus de Espinardo, 30100, Murcia, Spain
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Publikationsverlauf

Received 16 February 2004
Publikationsdatum:
25. März 2004 (online)

Abstract

A novel method for producing aryloxymethyl radicals, based on the treatment of (phenylseleno)methyl aryl ethers with tris(trimethylsilyl)silane and AIBN is disclosed, as well as the intramolecular addition of such radicals onto C,C-disubstituted ketenimines. The persistent α-(2H-1,4-benzoxazin-3-yl)benzyl radicals resulting from such cyclization processes undergo cross-coupling with the 1-cyano-1-methylethyl radical arising from the thermal ­decomposition of AIBN to give 2H-1,4-benzoxazines. These radical cyclizations are controlled by the persistent radical effect. The crystal and molecular structure of benzoxazine 10a has been solved by X-ray analysis.

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Typical Procedure: A solution of the corresponding ketenimine 8 (1.5 mmol) in anhyd benzene (100 mL) was heated under nitrogen at reflux temperature and tris(trimethylsilyl)silane (0.56 g, 2.25 mmol) and AIBN (0.098 g, 0.6 mmol) were added. Further additions of tris(trimethylsilyl)silane and AIBN were made as follows: 1) after 4 h since the first addition, tris(trimethylsilyl)silane (0.19 g, 0.75 mmol) and AIBN (0.098 g, 0.6 mmol) and 2) 4 h later, tris(trimethylsilyl)silane (0.37 g, 1.5 mmol) and AIBN (0.098 g, 0.6 mmol). After 16 h since the last addition the solvent was removed under reduced pressure and the crude material was chromatographed on a silica gel column, using hexanes/EtOAc (9:1) as eluent.
1,4-Benzoxazine 9f: Rf = 0.48; yield 27%; colorless prisms (Et2O); mp 177-178 °C. IR (nujol): 2234, 1625, 1258, 1220, 1164, 1117, 1074, 1044, 964, 889, 827, 741, 707 cm-1. 1H NMR (300 MHz, CDCl3): δ = 1.23 (s, 3 H), 1.50 (s, 3 H), 3.39 (d, 1 H, J = 13.5 Hz), 4.28 (d, 1 H, J = 13.5 Hz), 6.79 (d, 1 H, J = 8.7 Hz), 7.11 (dd, 1 H, J = 8.7, 2.7 Hz), 7.41 (very broad s, 8 H), 7.61 (d, 1 H, J = 2.7 Hz), 7.91 (broad s, 2 H). 13C NMR (75 MHz, CDCl3): δ = 25.3, 27.6, 37.3 (s), 63.6, 64.2 (s), 116.7, 127.2 (s), 127.4, 127.7 (s), 128.3, 128.5, 128.6, 128.9, 130.0, 131.1, 133.9 (s), 136.4 (s), 137.0 (s), 145.2 (s), 164.9 (s). MS: m/z (relative intensity) = 402 (3) [M+ + 2], 400 (8) [M+], 332 (100). Anal. Calcd for C25H21ClN2O: C, 74.90; H, 5.28; N, 6.99. Found: C, 74.77; H, 5.21; N, 7.11.
1,4-Benzoxazine 10f: Rf = 0.36; yield 55%; colorless prisms (Et2O); mp 143-144 °C. IR (nujol): 2233, 1624, 1600, 1579, 1494, 1299, 1261, 1236, 1198, 1128, 971, 870, 819, 763, 709, 668 cm-1. 1H NMR (300 MHz, CDCl3): δ = 1.47 (s, 3 H), 1.51 (s, 3 H), 4.77 (d, 1 H, J = 11.7 Hz), 4.92 (d, 1 H, J = 11.7 Hz), 6.82 (d, 1 H, J = 8.7 Hz), 6.99 (dd, 1 H, J = 8.7, 2.4 Hz), 7.08-7.11 (m, 2 H), 7.15 (d, 1 H, J = 2.4 Hz), 7.16-7.19 (m, 2 H), 7.28-7.35 (m, 6 H). 13C NMR (75 MHz, CDCl3): δ = 27.8, 28.9, 56.8 (s), 65.2, 118.3, 121.8 (s), 123.4, 124.7, 125.1 (s), 128.1, 128.2, 128.4, 128.5, 129.8, 130.6, 132.4 (s), 139.5 (s), 140.2 (s), 142.8 (s), 147.5 (s). MS: m/z (relative intensity) = 402 (2) [M+ + 2], 400 (5) [M+], 332 (100). Anal. Calcd for C25H21ClN2O: C, 74.90; H, 5.28; N, 6.99. Found: C, 74.76; H, 5.18; N, 7.08.

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Crystallographic data for the structure 10a have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number CCDC 229317. Copies of the data can be obtained on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (e-mail: deposit@ccdc.cam.ac.uk).