120417-46-1Relevant articles and documents
Diastereoselectivity and Kinetics of Intermolecular Hetero Diels-Alder Reactions under High Pressure. A Significant Pressure-Induced Increase in Stereoselectivity
Buback, Michael,Tost, Winfried,Huebsch, Thomas,Voss, Edgar,Tietze, Lutz F.
, p. 1179 - 1186 (2007/10/02)
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 deg 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 ΔV(excit.) in dichloromethane are determined to be between -(23.4+/-1.0) and -(24.2+/-1.0) cm3/mol.The ΔΔV(excit.) values for the cycloaddition of 5a, 5b, and 5c are -(5.9+/-0.5), -(3.9+/-0.1), and -(2.4+/-0.2) cm3/mol, respectively, and the ΔΔH(excit.) values are -(8.1+/-1.7), -(8.7+/-2.7), and -(10.0+/-0.9) kJ/mol, respectively.Because of the favourable ΔΔV(excit.) and ΔΔH(excit.), the selectivity of the reaction of 5a to give 6a/7a can be increased from 1.67:1.00 at 90 deg C and 1 bar to 13.6:1.0 at 0.5 deg 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. - Keywords: Diastereoselectivity; Hetero Diels-Alder reactions; Enamino ketones; High-pressure kinetics; Pyrans, dihydro-
