Abstract
The direct transesterification of carboxylic esters is efficiently catalyzed with Sc(OTf)3 (10 mol%) in boiling alcoholic solvent. Methyl, ethyl, isopropyl, and allyl esters were prepared from a broad range of different substrates in high yields. The application of microwave irradiation led to significantly reduced reaction times.
Key words
carboxylic esters - catalysis - scandium - transesterification
References and Notes
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Typical Experimental Procedure.
A solution of the ester (1.00 mmol) in the respective alcohol (10 mL) was treated with 49.0 mg (0.10 mmol, 10 mol%) Sc(OTf)3 and refluxed for the indicated reaction time. The reaction mixture was cooled to r.t. and filtered over a short plug of silica gel. All volatiles were evaporated in vacuo and the residue was purified by chromatography with mixtures of Et2O and pentane. All esters prepared were characterized by NMR, IR, and MS and gave spectroscopic data matching the reported data. The ee values of the (S)-mandelate esters were determined by HPLC on a chiral OD-phase with hexane-i-PrOH = 90:10, flow rate 1.0 mL/min. The ee of (R)-methyl β-hydroxy butyrate was determined by 19F NMR analysis of the corresponding Mosher ester. For the micro-wave experiments a microwave reactor ‘microPREP A’ (MLS GmbH, Germany) with a single magnetron (max. 1200 W, pulsed irradiation, 2.45 GHz), terminal 320 controller, and easy CONTROL 06 software was used. An internal ATC-FO 300 fiberoptic sensor inserted into the reaction vessel was employed to measure the temperature inside the reaction mixture accurately and adjust the temperature to the set temperature automatically.
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