1,2-H Atom Rearrangements in Benzyloxyl Radicals

Abstract

The rate constants for solvent-assisted 1,2-H atom rearrangements in para-substituted benzyloxyl radicals were studied with density functional theory. The rate of the radical rearrangement, calculated through transition state theory with Eckhart tunneling corrections, was shown to be drastically impacted by the presence of both implicit and explicit solvent molecules, with a quantitative agreement with laser flash photolysis studies for a variety of electron-donating and -withdrawing substituents. The rate of rearrangement was found to be correlated to the distance between the rearranging hydrogen atom and the α-carbon in the transition state, which could be modified through the para substituent and the type of assisting solvent molecule (e.g., water, ethanol, methanol, acetic acid, or a mixture of the latter). Natural bond orbital analysis showed that the rearrangement does not proceed through a hydrogen radical but through a quasi-proton exchange and charge transfer between the benzyl carbon and the adjacent oxygen atom. Energetic and spin population results indicated that electron-withdrawing groups induce faster rearrangement kinetics. Understanding 1,2-H atom shifts in benzyloxyl radicals are essential for tuning the rate of superoxide production in aqueous systems, as the resonance-stabilized carbon radical produced from the rearrangement can bind oxygen and decompose to produce superoxide radical anion, an important reactive intermediate in environmental and biological systems.

Indium-promoted butenolide synthesis through consecutive C–C and C–O bond formations in aqueous tetrahydrofuran enabled by radicals

“…Isomerization of the oxygen-centred radical to the carbon-centred radical through a formal 1,2-H shift is well documented and a recent theoretical study has revealed that water accelerates this process through a cyclic transition state involving two water molecules. 16 As shown in Fig. 1, in our case from radical INT-1 with two water molecules ( INT-1 ·2H 2 O), the cyclic transition state TS-1 ·2H 2 O with an activation barrier ΔΔ G of only +7.3 kcal mol −1 was found.…”

“…Isomerization of the oxygen-centred radical to the carboncentred radical through a formal 1,2-H shift is well documented and a recent theoretical study has revealed that water accelerates this process through a cyclic transition state involving two water molecules. 16 As shown in ΔΔG of only +7.3 kcal mol −1 was found. In contrast, a much higher activation energy (+23.9 kcal mol −1 ) is required for the 1,2-H shift of the isolated radical species (TS-1).…”

Structure and Reactivity of Alloxan Monohydrate in the Liquid Phase

“…The stabilization of the activation energy barrier for hydrogen transfer due to the solvent has been seen elsewhere for protic solvents. Additionally, the incorporation of additional solvent molecules, specifically for water, to assist the hydrogen transfer mechanism has been shown previously to further stabilize the transition state. − Because DMSO is not a protic solvent and the hydrogens are nonacidic, this type of proton-assisted transfer is not favorable and the energy barrier for hydration increases to 62.4 kcal/mol (Figure f). The trend for the general lowering of TS energies for the protic solvent was also looked at with an implicit solvation of water and acetic acid, and the results are similar to that seen in the gas phase (Table S4).…”

Radical‐Pair Formation in Hydrocarbon (Aut)Oxidation

“…Aside from the two pathways shown in Figure (radical recombination to yield PC4 vs. separation of the complex PC3 to form radicals 7 and 4 and water), additional reaction pathways are conceivable. This may, for example, include the water‐mediated isomerization of the benzyloxyl radical ( 4 ) to the hydroxybenzyl radical through a formal 1,2‐hydrogen shift (not shown) . The relevance of this pathway will, among other things, depend on the polarity of the surrounding solvent and its ability to engage in hydrogen‐bonding interactions with the radical 4 .…”

“…This may,f or example, include the water-mediated isomerization of the benzyloxyl radical (4)t ot he hydroxybenzyl radical through af ormal 1,2-hydrogen shift (not shown). [43][44][45] The relevance of this pathway will, among other things,d epend on the polarity of the surrounding solventa nd its ability to engage in hydrogen-bonding interactions with the radical 4. [46][47][48][49][50] For the sake of completeness, it should also be mentioned that the toluenes hown in Figure 3a sa na ctive reaction partner can potentially also "solvate" the hydroperoxide 3 while it undergoes OÀOb ond homolysis and then react with the generated HOC radicali nas econd step along the lines shown in Figure 2.…”