- Transition-State Structure Variation in the Diels-Alder Reaction from Secondary Deuterium Kinetic Isotope Effects: The Reaction of Nearly Symmetrical Dienes and Dienophiles Is Nearly Synchronous
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Secondary deuterium kinetic isotope effects (KIEs) in the reaction of acrylonitrile, fumaronitrile, vinylidene cyanide, dimethyl fumarate and maleate, and methyl trans-β-cyanoacrylate with isoprene were determined by competition with isoprene-d0 (d0), 4,4-dideuterioisoprene (4,4-d2), and 1,1,4,4-tetradeuterioisoprene (d4).The d4 experiment gives the KIE for 1,1-dideuterioisoprene from the KIEs for 4,4-d2.The KIEs for bond making to the β site of acrylonitrile and vinylidene cyanide are much more inverse than those at the α bond making site but are less than the maximum value expected, which indicates an early, unsymmetrical, concerted transition state.The effects wit fumaronitrile are the same for both 1,1- and 4,4-dideuterioisoprene, indicating equivalent effects at both sites and not bonding to the preferred C-1 site of isoprene most of the time.The KIEs with methyl trans-β-cyanoacrylate are inverse at both bond-making sites to the dienophile and both nearly one-third of the maximum value expected.The results with nearly symmetrical addends are not consistent with a two-step reaction, particularly one with the second step being rate determining, or with a concerted reaction with a highly unsymmetrical transition state, but rather with a concerted pathway that is nearly synchronous.The KIEs with methyl fumarate and maleate suggest an unsymmetrical transition state, but they are within experimental error of a symmetrical transition state.
- Gajewski, Joseph J.,Peterson, Karen B.,Kagel, John R.,Huang, Y. C. Jason
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p. 9078 - 9081
(2007/10/02)
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- Structural and vibrational studies of 1,1,3,3-tetracyanopropane and 2,2,4,4,6-pentacyanocyclohexenamine
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1,1,3,3-Tetracyanopropane, 1, was prepared in low yields by a literature method with 2,2,4,4,6-pentacyanocyclohexenamine, 2, as a major by-product.The products were examined by X-ray crystallography. 1 has an orthorombic space group,Pbcn (No. 60) with cel
- Bell, R. A.,Brown, B. E.,Duarte, M.,Howard-Lock, H. E.,Lock, C. J. L.
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p. 261 - 270
(2007/10/02)
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