INTERMOLECULAR AND INTRAMOLECULAR HETERO-DIELS-ALDER REACTIONS .51. INTERMOLECULAR HETERO-DIELS-ALDER REACTIONS OF ENAMINO KETONES - EFFECT OF HIGH-PRESSURE ON THE KINETICS AND DIASTEREOSELECTIVITY

The hetero Diels-Alder reaction of the enamino ketones 1b and Ic with ethyl vinyl ether (2) in dichloromethane to give the dihydropyrans 3b/4b and 3c/4c is studied at high pressure up to 5 kbar. The kinetics is measured by on-line FT-IR spectroscopy up to 3 kbar, The cycloadditions show a relatively high pressure-dependent increase in diastereoselectivity in favor of the trans adducts 4b and 4c, respectively. The activation volumes at atmospheric pressure, Delta V-0(not equal), are determined to be -(43.3 +/- 2.1) and -(43.9 +/- 2.9) cm(3) mol(-1) for the reactions of Ib and Ic at 100 degrees C in dichloromethane solution. The activation enthalpies, Delta H-not equal for the two cycloadditions at 1500 bar are (64.4 +/- 0.4) kJ mol(-1) and (64.0 +/- 0.6) kJ mol(-1). The Delta Delta V-0(not equal) values are (3.8 +/- 0.3) and (4.6 +/- 0.3) cm(3) mol(-1) and the Delta Delta H-not equal values (1.5 +/- 0.2) and (2.1 +/- 0.3) kJ mol(-1), respectively.