A Catalytic Asymmetric Fujiwara-Moritani Cyclization

Abstract

An enantioselective intramolecular Fujiwara-Moritani reaction involving direct C-H functionalization using modified PyOX ligands is reported for the first time. Screening of several ligand-oxidant combinations eventually produced a stereogenic quaternary carbon with promising enantiocontrol (54% ee) and in good chemical yield (71%).

Key words

asymmetric catalysis - cyclizations - Fujiwara-Moritani reaction - oxidative Heck reaction - palladium

References and Notes

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• Detailed procedures and characterization data will be reported elsewhere. See:
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• 23a

  Analytical Data for ( Z )- or ( E )-1-Methyl-3-(3-methylpent-3-enyl) indole [( Z )-1 or ( E )-1] ^¹4a
  (Z)-1 (Z/E > 99:1): ^¹H NMR (300 MHz, CDCl₃): δ = 1.62 (d, J = 6.7 Hz, 3 H), 1.82-1.86 (m, 3 H), 2.48 (br t, J = 8.4 Hz, 2 H), 2.83-2.91 (m, 2 H), 3.78 (s, 3 H), 5.32 (q, J = 6.7 Hz, 1 H), 6.90 (s, 1 H), 7.13-7.20 (m, 1 H), 7.24-7.31 (m, 1 H), 7.31-7.36 (m, 1 H), 7.68 (ddd, J = 7.8, 1.7, 0.9 Hz, 1 H) ppm. ^¹³C NMR (75 MHz, CDCl₃): δ = 13.3, 23.4, 23.5, 32.5, 32.5, 109.1, 115.3, 118.5, 118.9, 119.2, 121.4, 125.8, 127.9, 136.1, 137.0 ppm.
  (E)-1 (E:Z > 99:1): ^¹H NMR (300 MHz, CDCl₃): δ = 1.67 (dq, J = 6.7, 1.0 Hz, 3 H), 1.77 (m, 3 H), 2.39-2.49 (m, 2 H), 2.87-2.94 (m, 2 H), 3.78 (s, 3 H), 5.37 (qq, J = 6.8, 1.3 Hz, 1 H), 6.88 (s, 1 H), 7.16 (ddd, J = 8.0, 6.8, 1.3 Hz, 1 H), 7.27 (ddd, J = 8.2, 6.8, 1.2 Hz, 1 H), 7.33 (ddd, J = 8.2, 1.1, 0.9 Hz, 1 H), 7.66 (ddd, J = 7.8, 0.9 Hz, 1 H) ppm. ^¹³C NMR (75 MHz, CDCl₃): δ = 13.4, 15.8, 24.0, 32.5, 40.5, 109.0, 115.3, 118.4, 118.4, 119.0, 121.3, 125.8, 127.9, 135.9, 136.9 ppm.

• 23b

  Analytical Data for (-)-3,4-Dimethyl-3-vinyl-1,2,3,4-tetrahydrocyclopenta[ b ]indole [(-)-2]
  [α]_D ^²0 -17.7 (c 0.565, CHCl₃) for 44% ee (Table  [³] , entry 2). HPLC (Daicel Chiralcel IB column using n-heptane at 15 ˚C): t _R = 13.9 min (minor enantiomer) and 16.1 min (major enantiomer). IR (film): 3052 (m), 2926 (s), 2854 (s), 1465 (s) cm^-¹. ^¹H NMR (300 MHz, CDCl₃): δ = 1.50 (s, 3 H), 2.36 (ddd, J = 13.0, 7.1, 6.1 Hz, 1 H), 2.47-2.58 (m, 1 H), 2.83 (m, 2 H), 3.64 (s, 3 H), 4.98 (dd, J = 17.4, 1.3 Hz, 1 H), 5.06 (dd, J = 10.5, 1.2 Hz, 1 H), 6.09 (dd, J = 17.4, 10.5 Hz, 1 H), 7.09 (ddd, J = 7.5, 7.1 Hz, 1.3 Hz, 1 H), 7.16 (ddd, J = 8.2, 7.0, 1.4 Hz, 1 H), 7.26 (ddd, J = 8.1, 0.9, 0.8 Hz, 1 H), 7.47 (ddd, J = 7.6, 1.3, 0.7 Hz, 1 H) ppm. ^¹³C NMR (75 MHz, CDCl₃): δ = 22.5, 23.7, 30.0, 46.0, 46.3, 109.3, 111.8, 117.3, 118.8, 119.0, 120.2, 123.9, 141.6, 145.1, 148.7 ppm. ESI-HRMS: m/z calcd for C₁₅H₁₇NNa: 234.1259; found: 234.1253. Anal. Calcd for C₁₅H₁₇N: C, 85.26; H, 8.11; N, 6.63. Found: C, 84.97; H, 8.20; N, 6.54.

21

General Procedure for Fujiwara-Moritani Cyclization A flame-dried reaction vessel equipped with a magnetic stir bar is charged with Pd(OAc)₂ (10 mol%) as well as the indicated ligand L2-8 (30 mol%) and subsequently evacuated and then backfilled with O₂ if used as oxidant (three cycles). Otherwise, a solution of the indole and tert-amyl alcohol (0.125 M) and AcOH (0.25 mL per mL of tert-amyl alcohol) are consecutively added. The reaction mixture is maintained at r.t. until a homogeneous tawny solution forms. Then, the oxidant (1.0 equiv), PhCO₃ t-Bu or 1,4-benzoquinone, is added, and the resulting reaction mixture is heated at 80 ˚C for 15 h (under O₂ atmosphere with O₂ as the oxidant). After cooling to r.t., the reaction mixture is diluted with MTBE and poured into H₂O. The organic layer is separated and washed with sat. aq NaHCO₃ and brine. The aqueous phase is extracted with MTBE and the combined organic extracts are dried over Na₂SO₄. The solvents are evaporated under reduced pressure, and the residue is purified by flash column chromatography on SiO₂ using cyclohexane-MTBE solvent mixtures.