| Abstract
| - The title ion reacts in 1:1 DF−SbF5 and exchanges upto five protium atoms with deuterium. Theincorporation of label was measured by GC−MS analysis of the methylcrotonate formed by methanolquenching. The isotopomer distribution at about 60% conversion,which shows a minimum for thed1 and a maximum for thed4 species, indicates that the intermediatedication with the secondcharge at C(3) loses a proton faster from C(4) than from C(2).Formation of the pentadeuteriocrotonyl cation indicates that the 1,4-dication (acyl primary alkyl) orthe 1,2-dication must intervenein the process. Computer modeling of the kinetics for the multipleexchange process to fit theexperimental deuterium distribution allowed determination of therelative rate constants and isotopeeffects (KIEs) for the formation of the carbocations from alkenoylcations (β-secondary KIE) andelimination from carbodications to alkenoyl cations (primary KIE).An exceptionally largeβ-secondary KIE of ca. 2.0/hydrogen was found for the formation ofthe dication. A small primaryisotope effect of ca. 1.5 was found for elimination from the dicationsto the alkenoyl cations.Elimination from the 1,3-acylalkyl dication to form thenonconjugated 3-butenoyl cation is 6−7times faster than elimination to the conjugated 2-butenoyl cation.The rate ratio for the conversionof 3-butenoyl cation to the 1,4-dication (primary alkyl cation) and1,3-dication (secondary alkylcation) is (0.025−0.030):1, whereas the relative rate of theformation of the 1,2-acylalkyl dication(the alternative route of achieving pentadeuteration) iszero.
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