Energetic Differences between the Five- and Six-Membered Ring Hydrocarbons: Strain Energies in the Parent and Radical Molecules

Abstract

The C-H bond dissociation enthalpies (BDEs) for the five- and six-membered ring alkanes, alkenes, and dienes were investigated and discussed in terms of conventional strain energies (SEs). New determinations are reported for cyclopentane and cyclohexane by time-resolved photoacoustic calorimetry and quantum chemistry methods. The C-H BDEs for the alkenes yielding the alkyl radicals cyclopenten-4-yl and cyclohexen-4-yl and the alpha-C-H BDE in cyclopentene were also calculated. The s-homodesmotic model was used to determine SEs for both the parent molecules and the radicals. When the appropriate s-homodesmotic model is chosen, the obtained SEs are in good agreement with the ones derived from group additivity schemes. The different BDEs in the title molecules are explained by the calculated SEs in the parent molecules and their radicals: (1) BDEs leading to alkyl radicals are ca. 10 kJ mol (-1) lower in cyclopentane and cyclopentene than in cyclohexane and cyclohexene, due to a smaller eclipsing strain in the five-membered radicals relative to the parent molecules (six-membered hydrocarbons and their radicals are essentially strain free). (2) C-H BDEs in cyclopentene and cyclohexene leading to the allyl radicals are similar because cyclopenten-3-yl has almost as much strain as its parent molecule, due to a synperiplanar configuration. (3) The C-H BDE in 1,3-cyclopentadiene is 27 kJ mol (-1) higher than in 1,4-cyclohexadiene due to the stabilizing effect of the conjugated double bond in 1,3-cyclopentadiene and not to a destabilization of the cyclopentadienyl radical. The chemical insight afforded by group additivity methods in choosing the correct model for SE estimation is highlighted.

Function-Oriented Biosynthesis of β-Lactone Proteasome Inhibitors in Salinispora tropica

“…Among the saturated alicycles, the cyclopentyl ring exhibited the highest affinity to the β5 subunit of the proteasome and the corresponding derivative 5 was about three times more active than the cyclohexyl analogue 4 25. The difference in bioactivity vanished after insertion of a double bond into both ring systems adjacent to C-6, introducing a new stereocenter and altering bond lengths as well as bond angles as compared to the saturated 5- and 6-membered rings 26. By confining the ring flexibility, both 1 and 10 are oriented in a position, which perfectly meets the structural requirements for binding to the CT-L site.…”

Cyclopentane combustion chemistry. Part I: Mechanism development and computational kinetics

“…In our detailed mechanism, the unimolecular decomposition pathways of cyclopentane include fission reactions to form 1-pentene, ethene + propene, and cyclopentyl radical + H. Similar to the high-temperature cyclopentane mechanisms of Tian et al [16] and Sirjean et al [15], the rate coefficients of these reactions are taken from the shocktube experiments of Tsang [17]. The same rate coefficients were used for the unimolecular decomposition reaction of cyclopentene to 1,4-pentadiene as for cyclopentane, except, the activation energies were corrected to account for the difference in bond dissociation energies (BDE) between vinylic-secondary (cyclopentene) and secondary-secondary (cyclopentane) bonds, as well as for the difference between cyclopentene and cyclopentane ring strain: 0.8 kcal mol -1 , calculated at the CBS-QB3 level of theory [29]. These ring strain values were extracted by Ritter [30] from thermodynamic evaluations of cyclic species by Dorofeeva et al [31].…”

Force Distribution Analysis of Mechanochemically Reactive Dimethylcyclobutene

“…Similarly, we expect FM-FDA to be helpful in the identification of the coordinate(s) along which mechanical force could regulate or direct the reaction to a desired product. Although the notion of molecular stress, such as ring stress in cyclic compounds [16,19,[49][50][51][52] or steric stress in inorganic complexes or proteins, [53][54][55] is commonly employed to explain reactivity, FM-FDA allows, to the best of our knowledge for the first time, the quantification of such molecular stress and reveals its sources.…”