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(4 + 4) dimerization of 2,3-dimethylene-2,3-dihydrofuran: secondary deuterium kinetic isotope effect evidence of a two-step mechanism

Journal Article · · J. Am. Chem. Soc.; (United States)
DOI:https://doi.org/10.1021/ja00274a048· OSTI ID:5585181
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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), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 108:14; ISSN JACSA
Country of Publication:
United States
Language:
English

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Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400302* -- Organic Chemistry-- Isotope Effects-- (-1987)
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
DATA
DEUTERIUM
DIMERIZATION
ENTHALPY
ENTROPY
EXPERIMENTAL DATA
FORMATION HEAT
FURANS
HETEROCYCLIC COMPOUNDS
HYDROGEN ISOTOPES
INFORMATION
ISOTOPE EFFECTS
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