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Inter‐ and Intramolecular Hetero Diels‐Alder Reactions, Part XXVI. Diastereoselectivity and Kinetics of Intermolecular Hetero Diels‐Alder Reactions under High Pressure. A Significant Pressure‐Induced Increase in Stereoselectivity
Author(s) -
Buback Michael,
Tost Winfried,
Hübsch Thomas,
Voß Edgar,
Tietze Lutz F.
Publication year - 1989
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19891220625
Subject(s) - chemistry , dichloromethane , intramolecular force , cycloaddition , stereoselectivity , diels–alder reaction , adduct , intermolecular force , selectivity , medicinal chemistry , ether , kinetics , stereochemistry , organic chemistry , molecule , catalysis , solvent , physics , quantum mechanics
Abstract The Hetero Diels‐Alder reaction of the enamino ketones 5a – d and ethyl vinyl ether ( 2 ) to give the dihydropyrans 6a – d and 7a – d is studied in dichloromethane and in heptane/isodurene under high pressure up to 7 kbar at temperatures between 0.5 and 130°C. The kinetics is measured by on‐line FT‐IR spectroscopy. The cycloaddition shows a remarkable pressure‐dependent increase of diastereoselectivity in favour of the cis adducts 6a – c with the largest effect found for the reaction of 5a and the smallest for 5c . The pressure‐averaged overall activation volumes Δ V ≠ in dichloromethane are determined to be between –(23.4± 1.0) and –(24.2±1.0) cm 3 /mol. The ΔΔ V ≠ values for the cycloaddition of 5a , 5b , and 5c are –(5.9±0.5), –(3.9±0.1), and –(2.4±0.2) cm 3 /mol, respectively, and the ΔΔ H ≠ values are –(8.1±1.7), –(8.7±2.7), and –(10.0±0.9 kJ/mol, respectively. Because of the favourable ΔΔ V ≠ and ΔΔ H ≠ , the selectivity of the reaction of 5a to give 6a / 7a can be increased from 1.67: 1.00 at 90°C and 1 bar to 13.6:1.0 at 0.5°C and 6 kbar. This example shows that a significant and synthetic useful increase of diastereoselectivity in chemical reactions is possible by applying high pressure.