Transcript Epoxidation of Cyclopentene, Cyclohexene and Cycloheptene by Acetylperoxyl Radicals

Epoxidation of Cyclopentene, Cyclohexene and Cycloheptene by Acetylperoxyl Radicals
J. R. Lindsay Smith, D. M. S. Smith, M. S. Stark and D. J. Waddington
Department of Chemistry
University of York, York, YO10 5DD, UK
Introduction
Transition State
Adduct
Cyclopentene Oxide
To further the current understanding of how radicals add to C=C double bonds,eg. 1-3
a series of cyclic alkenes (cyclopentene, cyclohexene and cycloheptene) were reacted
with acetylperoxyl radicals.
The example given here is for CH3C(O)O2 addition to cyclopentene, which ultimately
forms the product cyclopentene oxide.
AM1 Geometries for the Addition of Acetylperoxyl Radicals to Cyclopentene
Experimental and Results
Alkene
log10(A / dm3 mol-1 s-1)
Eact / kJ mol-1
cyclopentene
9.7±0.6
31.2±5.0
cyclic alkene and a previously studied reference alkene,
cyclohexene
7.7±0.7
17.4±5.3
a relatively unstrained cycloalkene would behave
cis-2-butene.
cycloheptene
8.8±0.8
25.5±6.5
in a similar manner to this non-cyclic alkene.
Arrhenius parameters determined over the temperature
cis-2-butene4
8.1±0.5
22.9±3.8
This is indeed found to be so, with no
These reactions were studied via the co-oxidation of
acetaldehyde (to produce acetylperoxyl radicals), the
Rate parameters for cis-2-butene are also given
for comparison,4 as it would be expected that
range 373 - 433 K are given in Table 1.
Table 1:
statistically significant difference between the
Arrhenius parameters for the addition of acetylperoxyl
radicals to cyclic alkenes and cis-2-butene.
parameters for cyclohexene and cis-2-butene.
Correlation of Activation Energy with Charge Transfer
Activation energies for the addition of peroxyl radicals to
non-cyclic, unsubstituted mono-alkenes all correlate well
with estimates of the energy released by charge transfer
The measured activation energy for the
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relatively unstrained cyclohexene fits in
well with this correlation between EC and
on formation of the transition state (EC).2-5
activation energy.1
Only cyclohexene could be included in this
correlation, as electron affinities for
cyclopentene and cycloheptene have not
yet been determined.
Effect of Ring Strain on A-factors
Cyclopentene, which is planar and consequently highly strained, has a significantly larger pre-exponential factor and activation energy for its reaction with acetylperoxyl than either
cyclohexene or cis-2-butene.
Geometries for the transition states and isolated reactants calculated at the AM1 level6 indicate that radical addition to cyclopentene can release a significant amount of entropy and
consequently increase the pre-exponential factor for the reaction. The calculated ratio of the A-factors for cyclopentene with respect to cyclohexene is comparable with the experimental
determination.
An alternative interpretation is that the transition state for addition to cyclopentene is comparatively early with the C-O bond length being slightly longer than for cyclohexene.
Consequently, the transition state is looser, consistent with a relatively large pre-exponential factor in comparison with addition to cyclohexene.
C–O Transition State () ΔN Transition State log10(A/Acyclohexene )Exp
Alkene
log10(A/Acyclohexene )AM1
cyclopentene
1.951
0.170
2.0±0.9
1.8
cycloheptene
1.936
0.180
1.1±1.1
1.0
cyclohexene
1.931
0.185
(1)
(1)
Table 2:
C–O bond lengths and Charge Transfer (ΔN) at the transition state (AM1 level)6 and relative
entropies of reaction for the addition of acetylperoxyl radicals to cyclic alkenes.
Effect of Ring Strain on Activation Energy
The degree of charge transfer at the transition state is determined by the degree of overlap between the free electron orbital of the radical, and the  orbital of the C=C double bond.
Everything else being equal, the longer the C-O bond at the transition state, the less the overlap, hence there is less charge transfer and less energy released by this charge transfer.
Consequently the barrier for radical addition to cyclopentene is higher than for addition to cyclohexene by 148 kJ mol-1.
References
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Stark, M. S. J. Phys. Chem. 1997, 101, 8296; Stark, M. S., J. Am. Chem. Soc. 2000, 122, 4162.
Ruiz Diaz, R.; Selby, K.; Waddington, D. J. J. Chem. Soc. Perkin Trans. 2 1977, 360.
Baldwin, R. R.; Stout, D. R.; Walker, R. W. J. Chem. Soc. Faraday Trans. 1 1984, 80, 3481.
Ruiz Diaz, R.; Selby, K.; Waddington, D. J. J. Chem. Soc. Perkin Trans. 2 1975, 758.
Parr, R. G.; Pearson, R. G. J. Am. Chem. Soc. 1983, 105, 7512.
Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902.
Acknowledgements
DMSS would like to thank the EPSRC for financial support.