Microwave-Assisted Organic Synthesis in Near-Critical Water at 300 °C. A Proof-of-Concept Study.

Jennifer M. Kremsner and C. Oliver Kappe, Karl-Franzens-University Graz

Date Posted: Monday, September 18, 2006

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Chemical processes often employ large amounts of hazardous and toxic solvents. Therefore, organic chemists are requested to investigate clean, economical an environmentally safer methodologies.

One of the most promising approaches uses water as an alternative reaction medium. Besides reactions in water under ambient or slightly elevated temperature (<100 °C) the so-called near-critical (also termed subcritical) region of water at temperatures between 200–300 °C is of great importance to organic synthesis.

At 250 °C, water exhibits a density and polarity similar to those of acetonitrile at room temperature. The dielectric constant of water (e’) drops rapidly with temperature, and at 250 °C has fallen from 78.5 (at 25 °C) to 27.5.

Most importantly, the ionic product of water is increased by three orders of magnitude on going from room temperature to 250 °C. Near critical water (NCW) can therefore act as an acid, base, or acid-base bicatalyst without the need for costly and cumbersome neutralization and catalyst regeneration.

Herein we demonstrate the feasibility of performing microwave-assisted organic transformations like the hydrolysis of esters or amides, the hydration of alkynes, Diels Alder cycloadditions, pinacol rearrangements and the Fischer indole synthesis without the addition of an acid or basic catalyst in the 270–300 °C temperature range.[1]

For this purpose, a dedicated multimode microwave reactor (2.45 GHz, 1400 W) was utilized allowing to perform reactions in water as solvent in heavy-walled quartz reaction vessels on a 15–400 mL scale with operation limits of 80 bars of pressure and close to 300 °C temperature.[2] 

[1]  Kremsner, J. M.; Kappe, C. O. Eur. J. Org. Chem. 2005, in press.
[2] Stadler, A.;  Yousefi, B. H.; Dallinger, D.; Walla, P.; Van der Eycken, E.; Kaval, N.; Kappe, C. O. Org. Process Res. Dev., 2003, 7, 707.

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