Buback, M.; Gerke, K.; Ott, Ch.; Tietze, L. F.
Kinetics and Selectivity of the Intramolecular Hetero Diels-Alder Reaction of 1-Oxa-1,3-butadienes using a Benzylidene Barbituric Acid Derivate Effect of Pressure, Temperature, and Solvent
Chem. Ber. 1994, 127, 2241-2248.
 

The intramolecular hetero Diels-Alder reaction of the benzy-lidenebarbituric acid derivative 1 to give the ortho and meta products 2 and 3 is studied under high pressure up to 6 kbar in various solvents. The kinetics is measured by on-line FT-IR spectroscopy up to 3 kbar. The cycloaddition shows a pressure-dependent increase in regioselectivity in favour of the ortho adduct 2. The activation volumes, ΔV, are determined to be -(33.1· 1.2) and -(34.2· 1.5) cm3· mol-1 for the reactions in dichloromethane and tetrahydrofuran at 100°C, respectively. For the cycloaddition in toluene and acetonitrile the activation volumes are found to be -(13.4· 1.5) and -(17.0· 4.1) cm3· mol-1, respectively. Contrary to the large solvent effect on the activation volume, only a minor effect on the activation volume differences, ΔΔV, is observed. Measurement of the molar volumes of 1 and the cycloadducts 2 and 3 show a strong solvent dependency.