(4 + 4) dimerization of 2,3-dimethylene-2,3-dihydrofuran: secondary deuterium kinetic isotope effect evidence of a two-step mechanism
2,3-Dimethylene-2,3-dihydrofuran (1a), prepared by the flash vacuum pyrolysis of 2-methyl-3-furylmethyl benzoate (2a), rapidly dimerizes to form almost exclusively the (4 + 4) head-to-head dimer 3a. Rate constants k/sub a/, k/sub b/, k/sub c/, and k/sub d/ were measured at several temperatures for the dimerization, a (4 + 4) cycloaddition, of 1a, 2-methylene-3-dideuteriomethylene-2,3-dihydrofuran (1b), 2-dideuteriomethylene-3-methylene-2,3-dihydrofuran (1c), and 2-3-bis(dideuteriomethylene)-2,3-dihydrofuran (1d), respectively. At -30.0/sup 0/C these rate constants in L mol/sup -1/s/sup -1/ were found to be k/sub /a = 6.223 +/- 0.458 x 10/sup -4/, k/sub b/ = 1.147 +- 0.087 x 10/sup -3/, k/sub c/ = 6.750 +- 0.602 x 10/sup -4/, and k/sub d/ = 1.169 +- 0.086 x 10/sup -3/, which indicates that k/sub a/ approx. k/sub c/ and k/sub b//k/sub a/ approx. = k/sub d//k/sub a/ = 1.86. It was found that the (4 + 4) dimerization rate of 1a is virtually the same in solvents ranging from 5:1 to 1:5 CS/sub 2//CDCl/sub 3/, a result not consistent with the involvement of a zwitterionic intermediate. The secondary deuterium kinetic isotope effect results and the insignificant solvent effects lead to the conclusion that the (4 + 4) dimerization of 1a proceeds via a stepwise mechanism involving a diradical intermediate. The activation enthalpy, ..delta..H/sup double dagger/, for the 9 dimerization of 1a at 25/sup 0/C is 10.2 +/- 0.3 kcal mol/sup -1/ and the activation entropy, ..delta..S/sup double dagger/, is -30.9 +/- 1.2 entropy units.
- Research Organization:
- Iowa State Univ., Ames
- DOE Contract Number:
- W-7405-ENG-82
- OSTI ID:
- 5585181
- Journal Information:
- J. Am. Chem. Soc.; (United States), Vol. 108:14
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
DEUTERIUM
ISOTOPE EFFECTS
FURANS
DIMERIZATION
CHEMICAL REACTION KINETICS
ENTROPY
EXPERIMENTAL DATA
FORMATION HEAT
CHEMICAL REACTIONS
DATA
ENTHALPY
HETEROCYCLIC COMPOUNDS
HYDROGEN ISOTOPES
INFORMATION
ISOTOPES
KINETICS
LIGHT NUCLEI
NUCLEI
NUMERICAL DATA
ODD-ODD NUCLEI
ORGANIC COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
POLYMERIZATION
REACTION HEAT
REACTION KINETICS
STABLE ISOTOPES
THERMODYNAMIC PROPERTIES
400302* - Organic Chemistry- Isotope Effects- (-1987)