Premium
Kinetics of Strain‐Activated Intramolecular Diels‐Alder Reactions Between Furan and Bicyclopropylidene as well as Methylenecyclopropane Moieties
Author(s) -
Buback Michael,
Heiner Thomas,
Hermans Bernard,
Kowollik Christopher,
Kozhushkov Sergei I.,
de Meijere Armin
Publication year - 1998
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/(sici)1099-0690(199801)1998:1<107::aid-ejoc107>3.0.co;2-q
Subject(s) - chemistry , intramolecular force , methylenecyclopropane , furan , moiety , steric effects , intermolecular force , diels–alder reaction , cyclopropane , ring strain , medicinal chemistry , intramolecular reaction , diene , stereochemistry , ring (chemistry) , organic chemistry , catalysis , molecule , natural rubber
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, Δ V 0 ≠ , are in the range −28.4 ± 1.3 ( 1a ) to −40.8 ± 2.0 cm 3 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 Δ V 0 ≠ , 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 Δ V 0 ≠ more negative by 7.4 ± 2.9 cm 3 mol −1 for 1a and 1b and by 8.1 ± 2.0 cm 3 mol −1 for 3a and 3b . Varying the dienophile moiety from methylenecyclopropane to bicyclopropylidene lowers Δ V 0 ≠ by about 3.5 cm 3 mol −1 between 1a and 3a and by about 5.0 cm 3 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.