TY - JOUR
T1 - Phenylcyclopropane Energetics and Characterization of Its Conjugate Base
T2 - Phenyl Substituent Effects and the C-H Bond Dissociation Energy of Cyclopropane
AU - Fattahi, Alireza
AU - Lis, Lev
AU - Kass, Steven R.
N1 - Funding Information:
Generous support from the National Science Foundation (CHE-1361766) and the Minnesota Supercomputer Institute for Advanced Computational Research are gratefully acknowledged.
PY - 2016/10/7
Y1 - 2016/10/7
N2 - 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.
AB - 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.
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U2 - 10.1021/acs.joc.6b01718
DO - 10.1021/acs.joc.6b01718
M3 - Article
C2 - 27598540
AN - SCOPUS:84990948103
SN - 0022-3263
VL - 81
SP - 9175
EP - 9179
JO - Journal of Organic Chemistry
JF - Journal of Organic Chemistry
IS - 19
ER -