Rare Earth Salt Catalyzed Asymmetric Diels-Alder Reaction with a Chiral Dienophile in Supercritical Carbon Dioxide: Enhancement Effect on Stereoselectivity

Abstract

The rare earth(III) salt catalyzed asymmetric Diels-Alder reaction of cyclopentadiene with a chiral dienophile in supercritical carbon dioxide (scCO₂) proceeded rapidly to give the adduct with a higher diastereoselectivity than that in dichloromethane; optimization of the CO₂ density increased the de value up to 77%.

Key words

rare earth(III) salt - Diels-Alder - cyclopentadiene

References

• For reviews:
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• For a review:
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• 2b Oakes RS. Clifford AA. Rayner CM. J. Chem. Soc., Perkin Trans. 1  2001,  917 ; Pressure-dependant increase in stereoselectivity in scCO₂ was observed in the asymmetric oxidation of a sulfoxide
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• Hydroformylation:
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• Carbonylation:
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• 14a

  Ionic radius (Å): La³⁺ = 1.172, Yb³⁺ = 1.065, Sc³⁺ = 0.885.

• 14b

  Shannon R. D.; Acta Crystallogr.; 1976, A32: 751

• 16 Uncatalyzed diastereoselective Diels-Alder reaction of cyclopentadiene with a chiral dienophle in scCO₂ has been reported: Chapuis C. Kucharska A. Rzepecki P. Jurczak J. Helv. Chim. Acta  1998,  81:  2314 
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13

(1S,2R)-N-methylephedrin, (R)-borneol and 1,1,(2R)-triphenylethandiol were examined as a chiral auxiliary of the dienophile, but they gave poor selectivity (< 10% de).

15

Typical Experimental Procedure: La (OTf)₃ (30 mg, 0.05 mmo1) and 1 (91.6 mg, 0.5 mmol) were placed in the pressure vessel (10 mL) along with a magnetic stirring bar which was then sealed and charged to 6 MPa pressure with cooled CO₂ at r.t. Stirring and heating to 40 °C was started and a period of 15-20 min was allowed for equilibration. Freshly distilled 2 (82 µL, 1.0 mmol) was injected into the system and the pressure increased to the desired level (10 MPa). After 0.5 h, stirring and heating was stopped and the pressure released through a trap of ethyl acetate (30 mL) at -78 °C. The vessel was washed with more ethyl acetate (50 mL) and water (50 mL). The combined organic layer was washed with water and dried over MgSO₄. GC analysis of the solution revealed the presence of the stereoisomeric mixture of adducts (conversion 80%). The de of the endo-adduct was determined by GC (J&W DB-5, 30m): temperature program; 180 °C, 5 min, 4 °C/min, 200 °C 20 min, 110 kPa, retention time: 9.64 min (endo minor), 9.80 min(exo), 10.15 min (endo major). Evaporation of the solvent left a pale orange residue that was subjected to preparative TLC (hexane/ethyl acetate = 4/1, R_f = 0.4-0.5). ¹H NMR (400 MHz) δ = 0.85 (d, 6 H, J = 7.30 Hz), 1.4-2.6 (m, 5 H), 2.89 (s, 1 H), 3.32 (s, 1 H), 3.8-4.4 (m, 4 H), 5.73 (dd, 1 H, J = 2.7, 5.6 Hz), 6.20 (dd, 1 H, J = 3.2, 5.6 Hz).

17

When the reaction was carried out using larger amount of starting compounds (2.5 mmol) in scCO₂ (10 mL), yield and de value of the adduct were almost the same as the case with usual amount of them. From this result, it is reasonable to consider that all organic compounds dissolve in scCO₂ and the reaction system is homogeneous within the range of these quantities.

18

Yields of the addduct were almost independent from the pressure, 80-90%.

20

The use 30 mol% of Et₂AlCl to a dienophile gives an endo-adduct in 30% conversion with only 14% de. (See ref. [12] ).