ChemInform Abstract: ELECTRON SPIN RESONANCE STUDIES IN AQUEOUS SOLUTION. FRAGMENTATION OF RADICAL INTERMEDIATES DERIVED FROM BETA‐AMINO ALCOHOLS

Radiation Stability of Cations in Ionic Liquids. 5. Task-Specific Ionic Liquids Consisting of Biocompatible Cations and the Puzzle of Radiation Hypersensitivity

“…We suggest that the main peculiarity of this system is the interplay of radical chemistry with the crystal structure. As has been noticed by Foster and co-workers, in α-Chol Cl, the cations form a chain along crystallographic axis c , as shown in SI Scheme 2S. The cations in this chain are aligned along the crystallographic axis, but the direction of the alignment alternates from one chain to another.…”

“…The preferential formation of a H-loss radical at the carbon-2 atom follows the general pattern for aminated compounds; , it is attributed to the kinetic control of H atom abstraction. In the gas phase (SI Table 3S), we estimate that the X–H bonds have energies of 4.19, 4.73, 4.92, and 4.93 eV for C 2 –H, C 1 –H, C N –H, and O–H bonds, respectively, so H–C 2 abstraction is greatly preferred energetically.…”

“…Relatively high decomposition yields were also observed for Me 3 N + CH 2 CO 2 H Cl and Me 3 N + (CH 2 ) 3 OH Cl crystals (SI Table 1S), whereas the radicals derived from the corresponding ions do not appear to have fragmentation reactions analogous to reaction . Furthermore, shortly after Symons suggested his chain reaction mechanism, it was found that the presumably unstable Me 3 N + CH 2 C • HOH radical cation ,, (like many other similar species) , can be prepared and observed in aqueous solutions through the reaction of hydroxyl radicals with corresponding aminoethanols; there was no indication that these radicals decayed via very rapid reaction except perhaps in strongly acidic solutions (pH < 1); see section for further discussion.…”

“…For choline, the one-electron oxidation can potentially involve all of the C–H bonds in this cation. In aqueous solution, the HO • radical selectively oxidizes (alkyl- or H- substituted) R 1 R 2 R 3 N + CH 2 CH 2 OH cations to R 1 R 2 R 3 N + CH 2 • CHOH radical cation. ,− , This carbon-2 centered radical species (Scheme ) is stable in solution; only for Et 3 N + CH 2 CH 2 OH were some secondary Et 3 N +• radicals observed in the reaction mixture. Foster and West suggested that reaction in choline containing crystals proceeds via the formation of Me 3 N and a CH 2 CHOH +• intermediate that forms only in such crystals, but this seems very unlikely energetically.…”

“…Calvin challenged his colleagues to explain these surprising observations. − The task proved to be unexpectedly difficult. Twenty years later, using his new evidence from electron paramagnetic resonance (EPR) spectroscopy, Symons suggested the currently accepted answer to Calvin’s challenge. , The crucial insight is that radiolytic oxidation of choline can yield an unstable Me 3 N + CH 2 C • HOH radical that fragments along the C–N bond and releases vinyl alcohol, the enol form of acetaldehyde (see reaction Scheme 1S in the Supporting Information). Me 3 normalN + CH 2 normalC • HOH → Me 3 normalN + • + CH 2  CHOH …”