Tietze, L. F., Hübsch, T.; Ott, Ch.; Kuchta, G.; Buback, M.
Intermolecular Hetero Diels-Alder-Reactions of Enamino Ketones - Effect of High Pressure on the Kinetics and Diastereoselectivity
Liebigs Ann. 1995, 1-7.
 

The hetero Diels-Alder reaction of the enamino ketones 1b and 1c 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, ΔV_o^≠, are determined to be -(43.3 ± 2.1) and -(43.9 ± 2.9) cm³ mol^-1 for the reactions of 1b and 1c at 100°C in dichloromethane solution. The activation enthalpies, ΔH^≠ for the two cycloadditions at 1500 bar are (64.4 ± 0.4) kJ mol^-1 and (64.0 ± 0.6) kJ mol^-1. The ΔΔV_o^≠ values are (3.8 ± 0.3) and (4.6 ± 0.3) cm³ mol^-1 and the ΔΔH^≠ values (1.5 ± 0.2) and (2.1 ± 0.3) kJ mol^-1, respectively.