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Synlett 2002(8): 1253-1256
DOI: 10.1055/s-2002-32969
LETTER
© Georg Thieme Verlag Stuttgart · New York

Electron Transfer Driven Transformations of Transition Metal π-Complexes: Samarium(II) Iodide Mediated Coupling of Fluoroarene-Cr(CO)3 Complexes with Ketones

Hans-Günther Schmalz*, Oliver Kiehl, Battsengel Gotov
Institut für Organische Chemie, Universität zu Köln, Greinstrasse 4, 50939 Köln, Germany
Fax: +49(221)4703064; e-Mail: schmalz@uni-koeln.de;
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Publikationsverlauf

Received 21 May 2002
Publikationsdatum:
25. Juli 2002 (online)

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Abstract

Fluoro-substituted η6-arene tricarbonylchromium complexes react with aliphatic ketones in the presence of SmI2 and tBuOH under C-C bond formation to give the corresponding tertiary benzylic alcohols in up to 77% yield. Chlorobenzene-Cr(CO)3 exhibit a similar reactivity. The reactions proceed through attack of a ketyl radical to the complexed arene followed by single electron reduction of the 17 VE intermediate. The overall reaction parallels a nucleophilic aromatic substitution of fluoride (SNAr) and preferentially occurs in a meta-tele fashion.

Key words

arene-chromium complexes - nucleophilic aromatic substitution - radical reactions - electron transfer - samarium(II) iodide

8

Only one enantiomer of racemic compounds is depicted.

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Typical Experimental Procedure: A flame dried Schlenk flask was charged with 15 mL of a 0.1 M solution of SmI2 in THF and HMPA (1.5 mL) under argon. After cooling to 0 °C, a solution of the fluorobenzene-Cr(CO)3 complex(11) (116 mg, 0.5 mmol), tBuOH (145 µL, 1.5 mmol) and acetone (45 µL, 0.6 mmol) in THF (3 mL) was added dropwise via cannula over a period of 5 min. The resulting mixture was stirred at 0 °C for 90 min and for 60 min at r.t. before it was quenched with saturated aqueous NaHCO3 (5 mL). After stirring for another 5 min, the reaction mixture was filtered through a short pad of Celite® and washed with MTBE. The organic layer was washed with saturated NaHCO3 solution, brine, dried over Na2SO4 and evaporated. The residue was purified by chromatography (20g of SiO2, CyH-EtOAc = 3:1) to give 105 mg (77%) of complex 12 as yellow crystalline compound. Mp 73 °C. 1H NMR (270 MHz, CDCl3): 1.56 (s, 6 H); 5.31 (t, 2 H, J = 6.5 Hz); 5.42 (t, 1 H, J = 6.5 Hz); 5.64 (d, 2 H, J = 6.5 Hz). 13C NMR (67.7 MHz, CDCl3): 31.3; 70.8; 91.3; 91.5; 93.4; 121.3; 232.53. FT-IR (ATR): 3576; 3483; 3092; 2985; 2932; 1948; 1892; 1854; 1457; 1415; 1366; 1168; 1154; 1096; 1054; 966; 861; 821; 662 cm-1. HRMS: for C12H12CrO4 calcd.: 272.0141; found: 272.0144.
Compound 13: Mp 98 °C. 1H NMR (400 MHz, CDCl3): 1.58-1.70 (m, 5 H); 1.70-1.86 (m, 5 H); 5.26 (t, 2 H, J = 6.5 Hz); 5.44 (t, 1 H, J = 6.5 Hz); 5.68 (d, 2 H, J = 6.5 Hz). 13C NMR (100 MHz, CDCl3): 21.8; 25.2; 38.9; 71.4; 90.8; 92.1; 94.0; 121.8; 233.4. FT-IR (ATR): 3582; 3093; 2936; 2859; 1959; 1869; 1449; 1411; 1350; 1309; 1261; 1203; 1153; 1123; 1034; 1022; 978; 849; 814; 688 cm-1. HRMS: for C15H16CrO4 calcd.: 312.0454; found: 312.0454.
Compound 14: Mp 71 °C. 1H NMR (270 MHz, CDCl3): 1.77-2.09 (m, 8 H); 5.31-5.43 (m, 3 H); 5.64 (dd, 2 H, J 1 = 6.5 Hz, J 2 = 2 Hz). 13C NMR (67.7 MHz, CDCl3): 24.3; 42.4; 81.6; 91.4; 92.1; 92.8; 118.7; 233.4. FT-IR (ATR): 3582; 3443; 3088; 2958; 2875; 1957; 1865; 1458; 1412; 1327; 1295; 1233; 1189; 1152; 1009; 949; 903; 882; 817; 661
cm-1. HRMS: for C14H14CrO4 calcd.: 298.0297; found: 298.0293.
Compound rac-15: Mp 119 °C. 1H NMR (400 MHz, CDCl3): 1.64 (s, 3 H); 1.97 (t, 2 H, J = 5.5 Hz); 3.79 (dtt, 1 H, J 1 = 10 Hz, J 2 = 5.5 Hz, J 3 = 5 Hz); 3.90 (dtt, 1 H, J 1 = 10 Hz, J 2 = 5.5 Hz, J 3 = 5 Hz); 5.25 (t, 1 H, J = 6 Hz); 5.29 (t, 1 H, J = 6.5 Hz); 5.43 (t, 1 H, J = 6 Hz); 5.45 (d, 1 H, J = 6 Hz); 5.80 (d, 1 H, J = 6.5 Hz). 13C NMR (100 MHz, CDCl3): 29.1; 44.9; 59.7; 70.6; 90.5; 90.6; 91.7; 92.0; 93.7; 120.0; 233.1. FT-IR (ATR): 3363; 2978; 2893; 1958; 1870; 1457; 1413; 1378; 1298; 1153; 1083; 1054; 1014; 965; 881; 816; 662
cm-1. HRMS: for C13H14CrO5 calcd.: 302.0246; found: 302.0247.
Compound rac-16: Mp 114 °C. 1H NMR (400 MHz, CDCl3): 1.60 (s, 3 H); 2.72 (d, 1 H, J = 16 Hz); 2.79 (d, 1 H, J = 16 Hz); 3.74 (s, 3 H); 5.22 (t, 2 H, J = 6.5 Hz); 5.46 (t, 1 H, J = 6.5 Hz); 5.62 (d, 1 H, J = 6.5 Hz); 5.68 (d, 1 H, J = 6.5 Hz). 13C NMR (100 MHz, CDCl3): 30.0; 46.6; 52.1; 70.9; 89.6; 89.7; 91.8; 92.1; 94.0; 117.9; 172.0; 232.7. FT-IR (ATR): 3482; 3092; 2954; 2927; 1958; 1866; 1854; 1713; 1457; 1438; 1412; 1374; 1342; 1207; 1180; 1095; 1069; 1049; 1009; 958; 857; 816; 660 cm-1. HRMS: for C14H14CrO6 calcd.: 330.0195; found: 330.0195.

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Preliminary attempts to employ aldehydes instead of ketones under similar conditions were not successful. In these cases, only the reduced aldehydes (alcohols) and some pinacol-type coupling products were formed, and the unchanged fluorobenzene-Cr(CO)3 derivatives could be reisolated.