EFFECT OF HIGH-PRESSURE ON THE STEREOSELECTIVITY OF INTERMOLECULAR HETERO-DIELS-ALDER REACTIONS

The hetero Diels-Alder reaction of enamino ketone 1 with the vinyl ethers 7 - 11 leading to the dihydropyrans 12a - e and 13a - e is studied in dichloromethane under high pressures up to 7 kbar. The kinetics is measured by on-line FT-IR spectroscopy up to 3 kbar. For the first time individual rate coefficients k(cis) and k(trans) have been determined. Pressure-averaged activation volumes, DELTAV(not-equal)BAR, for the overall reaction leading to the formation of the two diastereomers in dichloromethane solution between -(19.1 +/- 1.7) and -(25.4 +/- 1.6) cm3 Mol-1 are measured. The corresponding activation enthalpies, AH(not-equal), are between (62.2 +/- 1.6) and (76.3 +/- 3.6) kJ mol-1. The cycloaddition shows a remarkable increase in diastereoselectivity in favour of the cis adducts 12 a - e toward higher pressure. The pressure-averaged differences in activation volume, DELTADELTAV(not-equal)BAR, for the two reaction pathways leading to cis and trans diastereomers are found to be between -(4.5 +/- 0.3) and -(6.9 +/- 0.7) cm3 mol-1. The corresponding differences in activation enthalpy, DELTADELTAH(not-equal), are between -(4.7 +/- 1.4) and -(9.9 +/- 1.6) kJ mol-1. Combination of the overall rate coefficient k k(cis) + k(tran) for the reaction leading to the two diastereomers with the measured product ratio, c(cis)/C(trans), which is identified with the ratio of the rate coefficients k(cis/k(trans) yields the individual coefficients k(cis) and k(trans). The rate coefficient k(cis) is strongly influenced by the steric demand of the substituents on both diene and dienophile whereas k(trans) varies to a smaller extent, and the observed changes appear to be mainly due to the electronic properties of the substituents.