Ruthenium-Catalyzed Ring Expansion Reaction of 1-Acetylenylcyclobutanols with Methyl Vinyl Ketone

Abstract

A novel type of ruthenium-catalyzed cascade ring expansion reaction is reported. A 1,2-rearrangement of acetylenylcyclo­butanol followed by carbon-carbon bond formation with methyl vinyl ketone proceeds in one-pot process. This reaction enables to synthesize 2-alkylidenecyclopentanones in a stereoselective manner using appropriate ruthenium catalysts.

Key words

alkynes - ruthenium - ring expansion - domino reaction - stereoselective synthesis

References and Notes

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• Having isopropenyl group:
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• Having propargyl group:
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• Having acetylenyl group:
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• 7 Having 1,3-butadienyl group: Yoshida M. Sugimoto K. Ihara M. Org. Lett.  2004,  6:  1979 
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• 9 Yoshida M. Sugimoto K. Ihara M. Tetrahedron Lett.  2001,  42:  3877 
• 10 Ruthenium-catalyzed dimerization-ring expansion reaction has been previously reported by us. See: Yoshida M. Sugimoto K. Ihara M. ARKIVOC  2003,  (viii):  35 
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• 19 The cause of E-selectivity in this reaction is not clear, but isomerization of the Z-vinylruthenium 4 to the E-isomer could occur via ruthenium alkylidene complex because of the presence of coordinative methoxy group at the 2-position. An example of the reaction via ruthenium alkylidene complex, see: Trost BM. Müller TJJ. Martinez J. J. Am. Chem. Soc.  1995,  117:  1888 
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11

It is assumed that Lewis acid plays the role as an activating reagent of MVK. The addition of Brønsted acid (CSA), Brønsted base (K₂CO₃, pyridine) or Lewis base (PPh₃) caused a decrease in the product yield and/or an elongation of the reaction time.

12

General Procedure for the Ruthenium-Catalyzed Ring Expansion Reaction (entry 12 in Table 1). To a stirred solution of ethynylcyclobutanol (1a, 47.7 mg, 202 µmol) in toluene (1.0 mL) were added MVK (70.0 mg, 1.01 mmol), anhyd CeCl₃ (10.0 mg, 40.4 µmol) and CpRu(PPh₃)₂Cl (14.7 mg, 20.2 µmol) at r.t., and stirring was continued for 2 h at 80 °C. After filtration of the reaction mixture using EtOAc with small amount mixture of silica gel and a Celite^® pad followed by evaporation of the eluate, the residue was chromatographed on silica gel with hexane-EtOAc (92:8 v/v) as eluent to give the (Z)-cyclopentanone (Z)-2a (33.4 mg, 54%) and (E)-cyclopentanone (E)-2a (8.8 mg, 14%) as colorless oil, respectively.
Compound (Z)-2a: IR (neat): 1713, 1639 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 0.87 (6 H, t, J = 7.2 Hz), 1.13-1.46 (16 H, m), 1.75 (2 H, t, J = 7.6 Hz), 2.14 (3 H, s), 2.49 (2 H, td, J = 7.6, 2.0 Hz), 2.54 (2 H, t, J = 7.2 Hz), 2.90 (2 H, dt, J = 7.6, 7.2 Hz), 5.97 (1 H, tt, J = 7.6, 2.0 Hz). ¹³C NMR (150 MHz, CDCl₃): δ = 14.0, 22.4, 22.5, 23.7, 27.3, 29.6, 30.7, 32.5, 35.2, 43.1, 53.6, 136.7, 138.7, 208.2, 212.1. MS: m/z = 306 [M⁺]. HRMS: m/z calcd for C₂₀H₃₄O₂ [M⁺]: 306.2559; found: 306.2554.
Compound (E)-2a: IR (neat): 1715, 1651 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 0.86 (6 H, t, J = 6.8 Hz), 1.14-1.47 (16 H, m), 1.79 (2 H, t, J = 7.2 Hz), 2.16 (3 H, s), 2.40 (2 H, dt, J = 7.5, 2.0 Hz), 2.61 (2 H, t, J = 7.0 Hz), 6.45 (1 H, tt, J = 7.5, 2.0 Hz). ¹³C NMR (125 MHz, CDCl₃): δ = 14.0, 22.5, 23.2, 23.6, 23.7, 29.9, 30.7, 32.5, 35.2, 41.9, 52.5, 134.3, 138.7, 207.1, 210.4. MS: m/z 306 [M⁺]. HRMS:
m/z calcd for C₂₀H₃₄O₂ [M⁺]: 306.2559; found: 306.2572.

16

MOPAC 2000/AM1 calculation indicated that (E)-2a is thermodynamically more stable than (Z)-2a by 0.79 kcal/mol.