Kinetics of strain-activated intramolecular Diels-Alder reactions between furan and bicyclopropylidene as well as methylenecyclopropane moieties

Buback, M., Heiner, T., Hermans, B., Kowollik, C., Kozhushkov, S. I., & De Meijere, A. (1998) Kinetics of strain-activated intramolecular Diels-Alder reactions between furan and bicyclopropylidene as well as methylenecyclopropane moieties. European Journal of Organic Chemistry.

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Abstract

The intramolecular Diels-Alder reactions of the furans 1a, 1b, 3a and 3b in dichloromethane solution at pressures up to 3 kbar have been studied by quantitative on-line FT-IR spectroscopy. At identical pressure and temperature cyclization rates are significantly faster, by a factor of 8.1 and 6.8, for the b-type species of 1 and 3, respectively. The activation enthalpies of the four cyclizations are very similar, between 78.5 ± 0.6 and 82.5 ± 2.7 kJ mol-1. The activation volumes, ΔV0 ≠, are in the range -28.4 ± 1.3 (1a) to -40.8 ± 2.0 cm3 mol-1 (3b). These data show that intramolecular Diels-Alder reactions are not necessarily associated with less negative activation volumes than intermolecular Diels-Alder reactions. The size of ΔV0 ≠, however, appears to be clearly correlated with the type and the position of substituents at the diene or dienophile moieties. Replacement of the H atom on the furan ring by the methoxy group turns ΔV0 ≠ more negative by 7.4 ± 2.9 cm3 mol-1 for 1a and 1b and by 8.1 ± 2.0 cm3 mol-1 for 3a and 3b. Varying the dienophile moiety from methylenecyclopropane to bicyclopropylidene lowers ΔV0 ≠ by about 3.5 cm3 mol-1 between 1a and 3a and by about 5.0 cm3 mol-1 between 1b and 3b, which demonstrates the larger steric hindrance due to the tetrasubstitution of the double bond. Comparing the effects on cyclization rate associated with having either one or two cyclopropane units at the dienophile double bond suggests that the rate enhancing influence of strain energy slightly overcompensates a reduction in rate that is expected to go with the increased steric hindrance due to the second cyclopropane unit.

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ID Code: 99034
Item Type: Journal Article
Refereed: Yes
Additional Information: Cited By :7
Export Date: 20 September 2016
CODEN: EJOCF
Correspondence Address: Buback, M.; Inst. für Physikalische Chemie, Georg-August-Univ. Göttingen, Tammannstraße 6, D-37077 Göttingen, Germany
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Keywords: Activation parameters, Cycloadditions, intramolecular, [4 + 2], High-pressure chemistry, Polycycles, Small ring systems, Strain activation, Volume profile
ISSN: 1434193X
Divisions: Current > Institutes > Institute for Future Environments
Current > QUT Faculties and Divisions > Science & Engineering Faculty
Deposited On: 22 Sep 2016 04:50
Last Modified: 28 Jun 2017 13:01

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