Inter‐ and intramolecular hetero Diels‐Alder reactions, 51. Intermolecular hetero Diels‐Alder reactions of enamino ketones. Effect of high pressure on the kinetics and diastereoselectivity

Abstract

The hetero Diels-Alder reaction of the enamino ketones l b and l c 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, AVZ, are deter-mined to be -(43.3 f 2.1) and -(43.9 f 2.9) cm3 mol-' for the reactions of l b and l c at 100°C in dichloromethane solution. The activation enthalpies, AH+ for the two cycloadditions at 1500 bar are (64.4 f 0.4) kJ mol-I and (64.0 f 0.6) kJ mol-I. The AAV; values are (3.8 f 0.3) and (4.6 _+ 0.3) cm3 mol-' and the AAH* values (1.5 f 0.2) and (2.1 f 0.3) kJ mol-I, respectively.

Activation and Reaction Volumes in Solution. 3

“…The groups of Buback and Tietze have published their results with many Diels-Alder pairs. [512][513][514][515][516][517][518] The data are generally in the range now expected for the [4+2] cycloaddition of R,β-unsaturated ketones to vinyl ethers, but two points are worth noting. First, where both cis and trans stereoisomers are produced, the former uniformly have the more compact transition states.…”

Correlation between pressure and steric interactions in organic reactions

“…[38][39][40][41] Changes in diastereoselectivity due to steric hindrance may be expected with an increase in pressure. Large DDV* values (up to K11 cm 3 mol K1 ) were determined in these cycloadditions.…”