- Chemistry of gem-Dihalocyclopropanes. XXI. An Example of Competition between 1,3-Insertion and the Vinylcyclopropylidene/Cyclopentenylidene Rearrangement
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gem-Dibromotrimethylvinylcyclopropane was treated with ethereal methyllithium at four temperatures ranging from -78 deg C to +20 deg C.At -40 deg C or higher, 1,5,5-trimethylcyclopentadiene was formed as the main product along with three isomeric bicyclobutanes; the ratio was practically independent of temperature.The compounds are believed to originate from the same intermediate cyclopropylidene by a carbene-carbene rearrangement and 1,3-insertion reactions, respectively.At -78 deg C a substantial amount of dimers was formed as well, indicating an unusual stabilization of the intermediate α-bromo lithium derivative.No allene was detected in any of the reactions.Possible factors that govern the product distribution are discussed.
- Brun, Roger,Grace, David S. B.,Holm, Kjetil H.,Skatteboel, Lars
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- Electronic Effects of Polar Substituents in the Gas Phase Unimolecular Elimination of 4-Substituted Isobornyl Acetates
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The rates of gas-phase elimination of a series of 4-substituted isobornyl acetates have been measured in the presence of propene between 209-370 deg C and within a pressure range 37-153 Torr; a static system with seasoned vessels was used.The reaction are unimolecular, homogeneous, and obey a first-order rate law; electron-withdrawing substituents at C(4) cause a diminution in the elimination rate, such that ρ1 -0.69 in a Hammett treatment.The temperature dependence of the rate coefficients is illustrated for the Arrhenius equation for isobornyl acetate: logk/s-1=(12.82+/-0.13)-(189.2+/-1.5)kJmol-1/2.303RT.The results are consistent with a cyclic transition state in which C(2)-O cleavage is more advanced than that of C(3)-H.The major primary product of elimination of isobornyl acetate is bornylene, which partially isomerises to camphene and tricyclene under the reaction conditions.Bornylene also undergoes a retro-Diels-Alder reaction to yield the trimethylcyclopentadiene and ethylene.
- Chuchani, Gabriel,Hernandez A, Jose A.,Morris, David G.,Shepherd, Alistair G.
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p. 917 - 922
(2007/10/02)
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