Solvent-induced selectivity of Williamson etherification in the pursuit of amides resistant against oxidative degradation

Abstract

Selective etherification produces precursors for amides resistant to oxidative degradation, i.e., showing reversible oxidation at 1.5 to 1.7 V vs. SCE.

On the Search of a Silver Bullet for the Preparation of Bioinspired Molecular Electrets with Propensity to Transfer Holes at High Potentials

“…As the field took off, the focus was on dipolar biomolecules on one hand, and on small polar organic conjugates on the other, limiting the range of CT to 2 nm, and the mechanism predominantly to tunneling or superexchange [ 1 , 6 , 10 ]. To take the field out of this confinement, we developed the concept for CT bioinspired molecular electrets ( Figure 1 ) [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ].…”

“…Ethers as R 2 substituents are electron-withdrawing enough to ensure stability of the Aa radical cations, but not too polarizing to lower their reduction potential to ≤1 V vs. SCE, as we observe for amines [ 20 , 24 ]. As we show, aromatic ether substituents such as R 2 , sterically locked in orthogonality with the Aa ring, lead to reversible electrochemical oxidation at about 1.7 V vs. SCE at scan rates of 100 mV s −1 or less [ 27 ]. Upon increasing the scan rates beyond 0.5 V s −1 , aliphatic and other aromatic ethers of Aa residues commence to show reversibility at a potential between 1.5 and 1.9 V vs. SCE [ 27 ].…”

“…As we show, aromatic ether substituents such as R 2 , sterically locked in orthogonality with the Aa ring, lead to reversible electrochemical oxidation at about 1.7 V vs. SCE at scan rates of 100 mV s −1 or less [ 27 ]. Upon increasing the scan rates beyond 0.5 V s −1 , aliphatic and other aromatic ethers of Aa residues commence to show reversibility at a potential between 1.5 and 1.9 V vs. SCE [ 27 ]. Considering the timescales of electrochemical measurements ensures that the oxidative degradation of the ether Aa residues showing even partial reversibility is orders of magnitude slower than the residence time of holes on them during CT through Aa electrets.…”

“…Considering the timescales of electrochemical measurements ensures that the oxidative degradation of the ether Aa residues showing even partial reversibility is orders of magnitude slower than the residence time of holes on them during CT through Aa electrets. Furthermore, the potentials exceeding 1.5 V vs. SCE are the highest known under which amides can undergo reversible, or even partially reversible, oxidation [ 27 ].…”

“…Employing nucleophilic aromatic substitution for the synthesis of alkyloxy NBAs, however, has proven utterly unfeasible. Fluorinated NBA derivatives can react with phenoxy nucleophiles in the presence of a carbonate base in organic solvent to produce aromatic ethers of NBA [ 27 ]. Unfortunately, the same NBAs decompose in the presence of alkyloxy nucleophiles, requiring strong bases or alkaline metals to generate them.…”