Department of ChemistryUniversity of York, York, YO10 5DD,

UK

Epoxidation of Cyclopentene, Cyclohexene and Cycloheptene by Acetylperoxyl Radicals

References Acknowledgements(1) Stark, M. S. J. Phys. Chem. 1997, 101, 8296; Stark, M. S., J. Am. Chem. Soc. 2000, 122, 4162. DMSS would like to thank the EPSRC for financial support.(2) Ruiz Diaz, R.; Selby, K.; Waddington, D. J. J. Chem. Soc. Perkin Trans. 2 1977, 360.(3) Baldwin, R. R.; Stout, D. R.; Walker, R. W. J. Chem. Soc. Faraday Trans. 1 1984, 80, 3481.(4) Ruiz Diaz, R.; Selby, K.; Waddington, D. J. J. Chem. Soc. Perkin Trans. 2 1975, 758.(5) Parr, R. G.; Pearson, R. G. J. Am. Chem. Soc. 1983, 105, 7512.(6) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902.

J. R. Lindsay Smith, D. M. S. Smith, M. S. Stark and D. J. Waddington

IntroductionTo 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.

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

on formation of the transition state (EC).2-5

Effect of Ring Strain on Activation EnergyThe 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 148 kJ mol-1.

AM1 Geometries for the Addition of Acetylperoxyl Radicals to Cyclopentene

Experimental and ResultsThese reactions were studied via the co-oxidation of

acetaldehyde (to produce acetylperoxyl radicals), the

cyclic alkene and a previously studied reference alkene,

cis-2-butene.

Arrhenius parameters determined over the temperature

range 373 - 433 K are given in Table 1.

Parameters for cis-2-butene are also given

for comparison,4 as it would be expected

that

a relatively unstrained cycloalkene would

behave in a similar manner to this non-

cyclic alkene.

This is indeed found to be so, with no

statistically significant difference between

cyclohexene and cis-2-butene.

AdductTransition State Cyclopentene Oxide

Alkene log10(A / dm3 mol-1 s-1) Eact / kJ mol-1

cyclopentene 9.7±0.6 31.2±5.0

cyclohexene 7.7±0.7 17.4±5.3

cycloheptene 8.8±0.8 25.5±6.5

cis-2-butene4 8.1±0.5 22.9±3.8

Table 1: Arrhenius parameters for the addition of acetylperoxylradicals to cyclic alkenes and cis-2-butene.

Effect of Ring Strain on A-factorsCyclopentene, which is planar and consequently highly strained, has a significantly larger pre-exponential factor and activation energy 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.

Alkene C–O Transition State ( ) ΔN Transition State log10(A/Acyclohexene )Exp 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 relativeentropies of reaction for the addition of acetylperoxyl radicals to cyclic alkenes.

Alkene C–O Transition State ( ) ΔN Transition State log10(A/Acyclohexene )Exp 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

The measured activation energy for

the relatively unstrained cyclohexene

fits in well with this correlation

between EC and activation energy.1

Only cyclohexene could be included

in this correlation, as electron

affinities for cyclopentene and

cycloheptene have not yet been

determined.

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