Kinetics of Strain-Activated Intramolecular Diels-Alder Reactions Between Furan and Bicyclopropylidene as well as Methylenecyclopropane Moieties
✍ Scribed by Michael Buback; Thomas Heiner; Bernard Hermans; Christopher Kowollik; Sergei I. Kozhushkov; Armin de Meijere
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 430 KB
- Volume
- 1998
- Category
- Article
- ISSN
- 1434-193X
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✦ Synopsis
The intramolecular Diels-Alder reactions of the furans 1a, 1b, diene or dienophile moieties. Replacement of the H atom on the furan ring by the methoxy group turns ∆V ϶ 0 more nega-3a and 3b in dichloromethane solution at pressures up to 3 kbar have been studied by quantitative on-line FT-IR spec-tive by 7.4 ± 2.9 cm 3 mol Ϫ1 for 1a and 1b and by 8.1 ± 2.0 cm 3 mol Ϫ1 for 3a and 3b. Varying the dienophile moiety from troscopy. At identical pressure and temperature cyclization rates are significantly faster, by a factor of 8.1 and 6.8, for methylenecyclopropane to bicyclopropylidene lowers ∆V ϶ 0 by about 3.5 cm 3 mol Ϫ1 between 1a and 3a and by about 5.0 the b-type species of 1 and 3, respectively. The activation enthalpies of the four cyclizations are very similar, between cm 3 mol Ϫ1 between 1b and 3b, which demonstrates the larger steric hindrance due to the tetrasubstitution of the double 78.5 ± 0.6 and 82.5 ± 2.7 kJ mol Ϫ1 . The activation volumes, ∆V ϶ 0 , are in the range Ϫ28.4 ± 1.3 (1a) to Ϫ40.8 ± 2.0 cm 3 bond. Comparing the effects on cyclization rate associated with having either one or two cyclopropane units at the die-mol Ϫ1 (3b). These data show that intramolecular Diels-Alder reactions are not necessarily associated with less negative nophile double bond suggests that the rate enhancing influence of strain energy slightly overcompensates a reduction activation volumes than intermolecular Diels-Alder reactions. The size of ∆V ϶ 0 , however, appears to be clearly corre-in rate that is expected to go with the increased steric hindrance due to the second cyclopropane unit. lated with the type and the position of substituents at the