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

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 (S_NAr) and preferentially occurs in a meta-tele fashion.

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

References

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8

Only one enantiomer of racemic compounds is depicted.

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13

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.

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