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Modulating the radical reactivity of phenyl radicals with the help of distonic charges: it is all about electrostatic catalysis.

ABSTRACT: This manuscript reports the modulation of H-abstraction reactivity of phenyl radicals by (positive and negative) distonic ions. Specifically, we focus on the origins of this modulating effect: can the charged functional groups truly be described as "extreme forms of electron-withdrawing/donating substituents" - implying a through-bond mechanism - as argued in the literature, or is the modulation mainly caused by through-space effects? Our analysis indicates that the effect of the remote charges can be mimicked almost perfectly with the help of a purely electrostatic treatment, i.e. replacing the charged functional groups by a simple uniform electric field is sufficient to recover the quantitative reactivity trends. Hence, through-space effects dominate, whereas through-bond effects play a minor role at best. We elucidate our results through a careful Valence Bond (VB) analysis and demonstrate that such a qualitative analysis not only reveals through-space dominance, but also demonstrates a remarkable reversal in the reactivity trends of a given polarity upon a rational modification of the reaction partner. As such, our findings demonstrate that VB theory can lead to productive predictions about the behaviour of distonic radical ions.

SUBMITTER: Mondal T

PROVIDER: S-EPMC8179573 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Two new monomeric Cu(II) alkoxide complexes were prepared and fully characterized as models for intermediates in copper/radical mediated alcohol oxidation catalysis: Tp(tBuR)Cu(II)OCH2CF3 with Tp(tBu) = hydro-tris(3-tert-butyl-pyrazol-1-yl)borate 1 or Tp(tBuMe) = hydro-tris(3-tert-butyl-5-methyl-pyrazol-1-yl)borate 2. These complexes were made as models for potential intermediates in enzymatic and synthetic catalytic cycles for alcohol oxidation. However, the alkoxide ligands are not readily oxidized by loss of H; instead, these complexes were found to be hydrogen atom acceptors. They oxidize the hydroxylamine TEMPOH, 2,4,6-tri-t-butylphenol, and 1,4-cyclohexadiene to the nitroxyl radical, phenoxyl radical, and benzene, with formation of HOCH2CF3 (TFE) and the Cu(I) complexes Tp(tBuR)Cu(I)-MeCN in dichloromethane/1% MeCN or 1/2 [Tp(tBuR)Cu(I)]2 in toluene. On the basis of thermodynamics and kinetics arguments, these reactions likely proceed through concerted proton-electron transfer mechanisms. Thermochemical analyses give lower limits for the "effective bond dissociation free energies (BDFE)" of the O-H bonds in 1/2[Tp(tBuR)Cu(I)]2 + TFE and upper limits for the free energies associated with alkoxide oxidations via hydrogen atom transfer (effective alkoxide ?-C-H BDFEs). These values are summations of the free energies of multiple chemical steps, which include the energetically favorable formation of 1/2[Tp(tBuR)Cu(I)]2. The effective alkoxide ?-C-H bonds are very weak, BDFE ? 38 ± 4 kcal mol(-1) for 1 and ?44 ± 5 kcal mol(-1) for 2 (gas-phase estimates), because C-H homolysis is thermodynamically coupled to one electron transfer to Cu(II) as well as the favorable formation of the 1/2[Tp(tBuR)Cu(I)]2 dimer. Treating 1 with the H atom acceptor (t)Bu3ArO(•) did not result in the expected alkoxide oxidation to an aldehyde, but rather net 2,2,2-trifluoroethoxyl radical transfer occurred to generate an unusual 2-substituted dienone-ether product. Treating 2 with (t)Bu3ArO(•) gives no reaction, despite evidence that overall ligand oxidation and formation of 1/2[Tp(tBuMe)Cu(I)]2 is significantly exoergic. The origin of this lack of reactivity may be due to insufficient weakening of the alcohol ?-C-H bond upon complexation to copper.

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Modulating the radical reactivity of phenyl radicals with the help of distonic charges: it is all about electrostatic catalysis.

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