Iron- or Cobalt-Catalyzed Nazarov Cyclization: Asymmetric Reaction and Tandem Cyclization-Fluorination Reaction

Abstract

We succeeded in demonstrating the iron- or cobalt-catalyzed asymmetric Nazarov cyclization of divinyl ketones using iron or cobalt catalysts, which were prepared from Co(ClO₄)₂˙6H₂O, Fe(ClO₄)₂˙6H₂O, or Fe(OTf)₂ with a chiral pybox-type ligand. Furthermore, we found that Fe(OTf)₂ effectively catalyzed the tandem Nazarov cyclization-fluorination reaction of divinyl ketones.

Key words

iron catalyst - cobalt catalyst - Nazarov cyclization - ­fluorination

References and Notes

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7

Typical Procedure for the Asymmetric Nazarov Cyclization of 1a A solution of Fe(ClO₄)˙6H₂O (18.1 mg, 0.050 mmol) and (S,S)-tb-pybox (16.5 mg, 0.050 mmol) in anhyd CH₂Cl₂ (0.50 mL) was stirred at r.t. for 3 h, then 2-benzyliden-3-oxo-4,5-diphenylpent-4-enoic acid ethyl ester (1a, 38.2 mg 0.10 mmol) in anhyd CH₂Cl₂ (0.50 mL) and hexane (0.50 mL) was added slowly, then stirred at 60 ˚C for 15 h. The reaction mixture was quenched with H₂O, and extracted with EtOAc. The combined organic layers was washed with brine, dried over MgSO₄, and evaporated. The residue was chromatographed on silica gel (hexane-Et₂O = 7:3) to give 26.4 mg (69%) of 2a.
Analytical Data
[α]_D ^²4 +289 (c 1.0, CHCl₃). The enantiomeric purity was determined to be 83% ee by chiral HPLC analysis with a Daicel CHIRALPAK AD-H [hexane-2-PrOH (95:5), flow: 0.5 mL/min, 35 ˚C, t _major = 23 min, t _minor = 33 min]. The absolute configuration of the major enantiomer is (1R,5S) by the comparison of the reported data. ^¹H NMR (400 MHz, CDCl₃): δ = 1.31 (t, J = 6.8 Hz, 3 H), 3.64 (m, 1 H), 4.26 (m, 2 H), 5.02 (d, J = 2.8 Hz, 1 H), 7.10-7.34 (m, 15 H). ^¹³C NMR (125 MHz, CDCl₃): δ = 14.2, 50.9, 61.9, 62.5, 127.2, 127.7, 128.1, 128.2, 128.3, 128.9, 128.9, 129.6, 129.7, 131.2, 134.0, 138.7, 140.5, 168.3, 170.0, 199.4.

8

We also examined the reaction using other chiral pybox-type ligands, such as (S,S)-phe-pybox and (S,S)-inda-pybox, but enantioselectivities were 2% ee (92% yield) and 33% ee (22% yield), respectively.

9

Surprisingly, the reaction of 1a with Fe(OTf)₂ in toluene did not proceed.

11

The relative stereochemistry of 3 was estimated by comparison with the product from the reaction of 1 by Cu(OTf)₂, see ref. 10a.