ABNO in Electrocatalytic Oxidation

Basic Information

Common Chinese name: 9-Azabicyclo[3.3.1]nonane N-oxyl

English name: 9-Azabicyclo[3.3.1]nonane N-oxyl

CAS No.: 31785-68-9

Molecular formula: C8H14NO*

Molecular weight: 140.20286

EINECS No.: 803-697-7

9-Azabicyclo[3.3.1]nonane N-oxyl (ABNO, CAS No. 31785-68-9) is a stable nitroxide radical with a bicyclic structure. Over the past few years, it has been drawing attention in electrocatalytic oxidation and is becoming a catalyst of interest for green organic synthesis. Compared with the classic TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl), ABNO’s compact bicyclic scaffold and unhindered molecular structure give it higher catalytic activity and a broader substrate scope in electrochemical oxidation.

1. Electrochemical Behavior of ABNO

In ABNO, the nitroxyl radical active site is fully exposed, and the molecule can switch quickly and reversibly between the nitroxyl and oxoammonium states. During electrochemical oxidation, ABNO undergoes single-electron oxidation at the anode to form ABNO⁺, a stronger oxidant that then oxidizes the substrate efficiently.

ABNO’s redox potential (E° = 0.80 V vs NHE) is noticeably higher than that of TEMPO (0.65 V vs NHE). That gives it more thermodynamic driving force in electrocatalytic oxidation. Cyclic voltammetry studies confirm that, under the same conditions, ABNO shows much higher electrochemical activity than TEMPO for the oxidation of 1-phenylethanol. In some systems the half-wave potentials of ABNO and TEMPO are close, but ABNO’s steady-state catalytic current is clearly higher—which points to better catalytic efficiency in actual electrolysis.

2. Electrocatalytic Oxidation of Secondary Alcohols

ABNO really stands out when it comes to electrocatalytic oxidation of secondary alcohols to ketones.

With ABNO as the catalyst and 2,6-lutidine (2,6-dimethylpyridine) as the base in acetonitrile, secondary alcohols can be converted efficiently to the corresponding ketones at room temperature. Under constant-current electrolysis, a range of secondary alcohols can be converted to ketones with isolated yields of 80–95%. Studies show that formation of ABNO⁺ is the key step, and 2,6-lutidine acts as a proton acceptor and plays a crucial role in ketone formation.

Compared with TEMPO, ABNO has a clear advantage in electrocatalytic oxidation of secondary alcohols. TEMPO-based systems often perform poorly with sterically hindered secondary alcohols and require tedious re-optimization for each substrate. ABNO, with its unhindered bicyclic scaffold, can efficiently oxidize a wide range of substrates, including aliphatic secondary alcohols.

3. Electrocatalytic Self-Oxidative Coupling of Primary Amines

ABNO also has distinct value in the electrocatalytic self-oxidative coupling of primary amines.

Using ABNO as the electrocatalytic mediator in a NaClO4–acetonitrile solution, primary amines can undergo self-oxidative coupling to the corresponding imines. Cyclic voltammetry studies of ABNO’s electrocatalytic performance for primary amines show that, under the same conditions, ABNO has better electrocatalytic activity than TEMPO in this self-oxidative coupling reaction. In situ electrochemical infrared spectroscopy confirms that the intermediate is Ph—CHNH. Under optimized conditions, a range of aromatic primary amines can undergo ABNO-electrocatalyzed self-oxidative coupling to the corresponding imines in good yields.

4. Synergy with Other Catalytic Modes

ABNO’s electrochemical applications can also work together with other catalytic modes. In the electrochemical-assisted synthesis of covalent organic frameworks (COFs), for example, bromide ions are oxidized to bromine radicals, which then undergo single-electron transfer with ABNO to form ABNO⁺ and regenerate bromide ions; ABNO⁺ is then reduced back to ABNO, closing the catalytic cycle. This electrochemical–organocatalysis synergy opens up possibilities for ABNO in a wider range of electrochemical synthesis scenarios.

Thanks to its unhindered bicyclic scaffold, relatively high redox potential, and excellent electrochemical activity, ABNO shows unique value in secondary-alcohol electrocatalytic oxidation, primary-amine electrocatalytic self-oxidative coupling, and even selective protein bioconjugation. As green electrochemical synthesis keeps moving forward, ABNO looks set to expand further as an efficient and environmentally friendly electrocatalytic mediator.