Thermochemical parameters for organic radicals and radical ions. Part 1. The estimation of the pK_a of radical cations based on thermochemical calculations

Abstract

A. MARTIN DE P. NICHOLAS and DONALD R. ARNOLD. Can. J. Chem. 60, 2165Chem. 60, (1982. Three methods for estimating the pKa of a radical cation using thermochemical cycles are discussed. These methods enable the calculations of the pK, of radical cations which have not been determined experimentally. Radical cation acids are classified according to the nature of their respective conjugate bases. The application of these methods is illustrated for a o-acid (the benzene radical cation) and n-acid (the toluene radical cation). The potential of these methods to estimate relative and absolute solvation energies is also discussed. The use of thermochemical cycles in estimating standard oxidation potentials of organic compounds is outlined.Calculations show that the toluene radical cation is an extremely strong acid and the benzene radical cation a moderately strong acid in acetonitrile solution. Difficulties in directly determining the pK, value of aromatic hydrocarbon radical cations are also discussed.A. MARTIN DE P. NICHOLAS et DONALD R. ARNOLD. Can. J. Chem. 60, 2165Chem. 60, (1982. On discute de trois methodes d'evaluation du pKa d'un cation radicalaire en utilisant les cycles thermochimiques. Ces methodes permettent de calculer le pKa des cations radicalaires que l'on ne peut pas diterminer experimentalement. Les cations radicalaires acides sont classes selon la nature de leurs bases conjuguCes respectives. On illustre I'application de ces methodes dans le cas d'un acide o (le cation radicalaire du benzene) et dans le cas d'un acide n (le cation radicalaire du toluene). On discute egalement du potentiel de ces methodes en ce qui a trait B I'Cvaluation des Cnergies de solvatation relatives et absolues. On souligne I'utilisation des cycles thermochimiques dans I'Cvaluation des potentiels standard d'oxydation des composCs organiques.Les calculs indiquent que le cation radicalaire du toluene est un acide extrkmement fort et que le cation radicalaire du benzene est un acide moyennement fort en solution dans I'acetonitrile. On discute egalement des difficultCs rencontrees lors de la determination directe de la valeur du pKa des cations radicalaires des hydrocarbures aromatiques.[Traduit par le journal]

1,n-Radical ions. Photosensitized (electron transfer) and electrochemical oxidation of 1,1,2,2-tetraphenylcyclopropane

“…This spectrum can be compared to the spectrum of 9-which exhibits visible absorption maxima at 564 nm and 463 nm ( 14). Similarity is expected; the energy difference between the HOMO and the NBMO (non-bonding molecular orbital) in the cation is the same as the difference between the NBMO and the antibonding MO in the anion (34).…”

“…The reduction potential of 9' is -0.94 V (14). The reduction potentials of all of the acceptors which lead to the formation of 2 are greater than, or within 0.05 eV of, this value.…”

“…The free-energy change associated with the isomerization from the cyclopropyl to the allylic structure must also be estimated. In the unsubstituted C3H5' radical this value is approximately -25 kcal mol-' (14). The extra stabilization associated with the four phenyl groups4 should not account for more than -1423 kcal mol-I, so AGisorn should then be approximately -39f 3 kcal mol-I.…”

“…To determine the pK, of 1 ", the value of E;, is estimated from the E,, (1.48 V vs. NHE); AG,,(H+lsol is 11 kcal mol-' for acetonitrile (13); AGf(H)g is 48.6 kcal mol-', and AGBDE(RH) is approximately 96+3 kcal mol-' (14). The free-energy change associated with the isomerization from the cyclopropyl to the allylic structure must also be estimated.…”

Photoredox Catalysis in Organic Chemistry

“…Following this step, the α-amino C–H bonds become relatively acidic and can be readily removed by a weak base to generate an α-amino radical. 20 Subsequently, this radical can be oxidized to an iminium ion ( 3 ), a step in which a wide variety of terminal oxidants have been shown to be competent. An alternate mechanistic pathway is presented in Scheme 3, wherein amine radical cation 2 undergoes α-amino C–H abstraction by a radical anion (arising via reduction of an oxidant by the photocatalyst) to generate the same iminium species ( 3 ).…”