Use of scandium tris(trifluoromethanesulfonate) as a Lewis acid catalyst in supercritical carbon dioxide: efficient Diels–Alder reactions and pressure dependent enhancement of endo:exo stereoselectivity

Abstract

The Diels–Alder reaction between various acrylates and cyclopentadiene in supercritical CO₂ are catalysed by scandium tris(trifluoromethanesulfonate); optimisation of CO₂ density leads to increased endo∶exo selectivities compared to those obtained in conventional solvents.

References

1. A. A. Clifford, Fundamentals of Supercritical Fluids, Oxford University Press, 1998; A. A. Clifford, in Supercritical Fluids, ed. E. Kiran and J. M. H. Levelt Sengers, Kluwer, 1994, 449; see also ref. 3 and 4 and references cited therein.
2. See for example: L. E. McMahon, P. Timmins, A. C. Williams and P. York, J. Pharm. Sci., 1996, 85, 1064; J. W. Tom, G. B. Lim, P. G. Debendetti and R. K. Prud'homme, in Supercritical Fluid Engineering Science - Fundamentals and Applications, ed. E. Kiran and J. F. Brennecke ACS Symposium Series 514, 1993, ch. 11; P. G. Debendetti, Supercritical Fluids, ed. E. Kiran and J. M. H. Levelt Sengers, Kluwer, 1994, 719.
3. R. S. Oakes, A. A. Clifford, K. D. Bartle, M. Thornton-Pett and C. M. Rayner, Chem. Commun., 1999, 247.
4. A. A. Clifford, K. Pople, W. J. Gaskill, K. D. Bartle and C. M. Rayner, J. Chem. Soc., Faraday Trans., 1998, 94, 1451.
5. For a recent review see: C. P. Dell, J. Chem. Soc., Perkin Trans. 1, 1998, 3873.
6. ref. 1 and references cited thereln. Protic acid catalysed process have been reported including our own work on sulfur oxidation (ref. 3) see also: M. G. Hitzler, F. R. Smail, S. K. Ross and M. Poliakoff, Chem. Commun., 1998, 359.
7. N. Shezad, R. S. Oakes, A. A. Clifford and C. M. Rayner, Tetrahedron Lett., 1999, 40, 2221.
8. For a recent review see: S. Kobayashi, Eur. J. Chem., 1999, 15.
9. Apparatus used is as previously described in ref. 3. Typical procedure: scandium tris(trifluoromethanesulfonate)(95 mg, 0.18 mmol, 6.5 mol%) was placed in a pressure vessel which was then sealed and charged to 50 bar pressure with CO₂. Agitation and heating was commenced, and a period of 20 min allowed for equilibration of conditions. n-Butyl acrylate (270 µl, 3.0 mmol) was injected into the system and the pressure increased to 80 bar. Cyclopentadiene (500 µl, 6.0 mmol, freshly cracked monomer) was injected and the pressure increased to the desired level. After 15 h, agitation and heating was stopped and the pressure released through a trap of diethyl ether (25 ml). The system was cleaned by further diethyl ether washes (2 × 1 ml). Filtration through Celite followed by solvent removal yielded the crude product mixture from which diastereomeric ratios were measured by ¹H NMR. Conversions were typically > 90%, and isolated yields > 80%. Density calibration was achieved by metering known amounts of carbon dioxide into the reaction vessel under standard reaction conditions (including reagents) using an ISCO 100D programmable syringe pump. Knowing the amount of carbon dioxide, the reactor volume and by observing the pressure, a calibration curve can be plotted for density versus pressure at constant temperature. .
10. See for example D. Andrew, B. T. Des Islet, A. Margaritis and A. C. Weedon, J. Am. Chem. Soc., 1995, 117, 6132; Y. Ikushima, N. Saito and M. Arai, J. Phys. Chem., 1992, 96, 2293.