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Electrophilic Bromination of Specifically Deuterated Cyclohexenes:  A Combined Experimental and Theoretical Investigation

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journal contribution
posted on 07.03.1998, 00:00 by H. Slebocka-Tilk, A. Neverov, S. Motallebi, R. S. Brown, O. Donini, J. L. Gainsforth, M. Klobukowski
The electrophilic addition of Br2 to specifically deuterated cyclohexenes (15) was studied methanol (MeOH) by stopped-flow kinetics in order to determine a deuterium kinetic isotope effect (DKIE) for the various isotopomers. The DKIE for bromination of the isotopomers was also determined by a mass spectrometric method where exactly known quantities of two of the cyclohexenes were incompletely brominated in MeOH and where the ratio of the remaining isotopomers was determined. A computational study using density functional theory (DFT) was undertaken to examine the equilibrium isotope effect (EIE) for the equilibrium involving the formation of the cyclohexenyl bromonium ion from cyclohexene plus Br2. The agreement between experiment and theory is remarkably good and indicates that, for perdeuteriocyclohexene, the inverse DKIE and EIE of ∼1.5 can be partitioned two-thirds to the two vinyl CH's and one-third to the four homoallylic CH's, the four allylic CH's contributing negligibly to the overall effect. The computational study also indicates that there is extensive mixing of the CC and CH vibrational modes and that it is not possible to identify all the individual modes responsible for the large inverse EIE. Analysis of the computational data indicates that the isotopic effects may be divided into two groups; those associated with the deuteriums on the vinyl positions and those associated with the remaining allylic and homoallylic carbons. In the former, the inverse EIE is due to the changes in all bending modes. For the latter, the isotopically sensitive modes are those of all ten C−H stretches with changes in bending frequencies being unimportant. The bending vibrational modes were found to be strongly coupled.