Phenylcyclopropane energetics and characterization of its conjugate base: Phenyl substituent effects and the C-H bond dissociation energy of cyclopropane

Fattahi, A ; Sharif University of Technology

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  1. Type of Document: Article
  2. DOI: 10.1021/acs.joc.6b01718
  3. Publisher: American Chemical Society
  4. Abstract:
  5. The α-C-H bond dissociation energy (BDE) of phenylcyclopropane (1) was experimentally determined using Hess' law. An equilibrium acidity determination of 1 afforded ΔH°acid = 389.1 ± 0.8 kcal mol-1, and isotopic labeling established that the α-position of the three-membered ring is the favored deprotonation site. Interestingly, the structure of the base proved to be a key factor in correctly determining the proper ionization site (i.e., secondary amide ions are needed, and primary ones and OH- lead to incorrect conclusions since they scramble the deuterium label). An experimental measurement of the electron affinity of 1-phenylcyclopropyl radical (EA = 17.5 ± 2.8 kcal mol-1) was combined with the ionization energy of hydrogen (313.6 kcal mol-1) to afford BDE = 93.0 ± 2.9 kcal mol-1. This enabled the effect of the phenyl substituent to be evaluated and compared to other situations where it is attached to an sp3- or sp2-hybridized carbon center. M06-2X, CCSD(T), G4, and W1BD computations were also carried out, and a revised C-H BDE for cyclopropane of 108.9 ± 1.0 kcal mol-1 is recommended
  6. Keywords:
  7. Carbon ; Dissociation ; Electron affinity ; Ionization ; Propane ; Acidity determination ; C-H bond dissociation energy ; Isotopic labeling ; Key factors ; Phenyl substituents ; Secondary amides ; Three-membered rings ; Bond strength (chemical)
  8. Source: Journal of Organic Chemistry ; Volume 81, Issue 19 , 2016 , Pages 9175-9179 ; 00223263 (ISSN)
  9. URL: http://pubs.acs.org/doi/abs/10.1021/acs.joc.6b01718