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

Abstract

The electrophilic addition of Br2 to specifically deuterated cyclohexenes (1 − 5) 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.

Semiconductor Surface-Induced 1,3-Hydrogen Shift: The Role of Covalent vs Zwitterionic Character

“…In particular, the cyclohexene FTIR spectrum has alkane C-H peaks at 2834, 2857, and 2925 cm -1 and an alkene peak at 3020 cm -1 . 39 It is notable that the CdC stretching peak at ∼1654 cm -1 is very weak (it is only visible when the spectrum is enlarged, see Figure 5b), which is consistent with previously reported cyclohexene spectra. 39 Table 1 summarizes the relevant infrared peaks observed for this work.…”

“…FTIR peaks at 2834, 2857, and 2925 cm -1 for cyclohexene are similar to peaks that have been attributed to C-H stretching of the four sp 3 -hybridized C atoms. 39 The intact cyclohexene FTIR spectrum (Figure 8a) almost exactly overlaps the spectrum for 1,2-CHD/Si(001) after a 10 cm -1 shift (Figure 8b), within the noise of our data. Since small changes in ring structure usually lead to significant differences in spacing and peak intensity, the similarity in relative position and intensity of FTIR peaks for 1,2-CHD/Si(001) compared to those for cyclohexene suggests that the structure of the ring is the same for the two species.…”

Electrophilic Bromination of Ethylene and Ethylene-d4. A Combined Experimental and Theoretical Study

“…Secondary dkie's have been determined for a variety of electrophilic processes including brominations of styrenes, stilbenes, 7,7‘-norbornylidenenorbornane, and cyclohexene , Strausz and co-workers have offered another explanation for the inverse dkie observed for thiirane formation from addition of S atoms to ethylene, namely the creation of new isotopically sensitive vibrations in the transition state that were not present in the ground state .…”

“…4 Nevertheless, because of its simplicity, high degree of symmetry, and the fact that one can substitute all four positions with deuterium, ethylene is an ideal candidate for experimental investigation of the secondary deuterium kinetic isotope effects (dkie) for various reactions which are also amenable to detailed theoretical analysis. Secondary dkie's have been determined for a variety of electrophilic processes 5 including brominations of styrenes, 6 stilbenes, 7 7,7′-norbornylidenenorbornane, 8 and cyclohexene. 9 In general, the observed effect for R-substitution is inverse which is commonly explained by invoking isotope-sensitive changes in the out of plane bending frequencies of the R-C-L bonds of the alkene in passing from an sp 2 ground state to an sp 3 -like transition state.…”

Functionalization of Diamond(100) by Diels−Alder Chemistry

“…While the spectral similarities provide further evidence that a [4 + 2] surface adduct is formed on C(100), they also introduce provoking questions. For example, although the Diels−Alder product lacks the four C4 and C5 hydrogens of cyclohexene, it exhibits modes at 2860 and 2930 cm -1 which have been previously assigned to the C4 and C5 CH 2 stretching modes of cyclohexene . These additional peaks in the adsorbate spectrum may arise from overtones or combination bands, or alternatively they may imply either that multiple conformations of the Diels−Alder product are formed at the surface or that a side product is also present.…”