ABNO vs Keto-ABNO: Performance Comparison & Selection Guide

In the field of nitroxide radical catalysis, the 9-azabicyclo[3.3.1]nonane (ABNO) family has attracted wide attention because of its unusual bicyclic scaffold. Two members are used most often: ABNO (9-azabicyclo[3.3.1]nonane N-oxyl, CAS No. 31785-68-9) and Keto-ABNO (9-azabicyclo[3.3.1]nonan-3-one N-oxyl, CAS No. 7123-92-4). They share the same molecular skeleton. The key difference is whether C-3 carries a ketone group. That single ketone group leads to clear differences in oxidation potential, catalytic activity, and application scope. This article compares the two from the perspectives of molecular structure, oxidation performance, catalytic activity, and typical applications, to help with selection.

1. Molecular Structure: One Ketone Group Makes the Difference

ABNO and Keto-ABNO share the same bicyclic[3.3.1]nonane skeleton. The key difference is at C-3: Keto-ABNO has a ketone group (C=O), while ABNO has a methylene group (CH2) at that position.

This structural difference has two effects.

First, it raises the oxidation potential. The ketone group is electron-withdrawing. Through an inductive effect, it lowers electron density at the nitroxide radical active center. This makes the oxoammonium form easier to generate and significantly increases the oxidation potential. Keto-ABNO has an oxidation potential of +521 mV (vs. Ag/Ag+), ABNO +419 mV, and TEMPO only +320 mV.

Second, it changes the reversible three-state interconversion. Keto-ABNO can reversibly switch among hydroxylamine, N-oxyl radical, and oxoammonium states. This makes its catalytic cycle flexible and efficient. ABNO also has three-state switching ability, but its lower oxidation potential limits the reactivity of its oxoammonium form. Some studies show that under electrochemical oxidation conditions, ABNO does not degrade because it lacks the ketone group, and its stability is good.

The molecular formulas are: ABNO, C8H14NO (molecular weight 140.20); Keto-ABNO, C8H12NO2 (molecular weight 154.19). In appearance, ABNO is a brown to reddish-brown solid, while Keto-ABNO is a yellow crystalline solid.

2. Oxidation Potential and Thermodynamic Driving Force

Oxidation potential is a core parameter for nitroxide radical catalytic activity. The difference between ABNO andKeto-ABNO on this metric directly determines their substrate scope.

Keto-ABNO's oxidation potential is about 102 mV higher than that of ABNO. This matters in catalysis: a higher oxidation potential means stronger thermodynamic driving force, so it can oxidize substrate types that ABNO  cannot handle efficiently.

Studies show that Keto-ABNO's higher reactivity relative to ABNO comes from its higher redox potential. In aerobic oxidation of aldehydes, Keto-ABNO reacts with substrates noticeably faster than ABNO. The slower oxidation rate with ABNO leads to significant formation of the by-product acetophenone. Likewise, in photoelectrocatalytic oxidation of lignin model compounds, Keto-ABNO reaches 100% conversion at room temperature in 24 hours, while TEMPO reaches only 86%.

3. Catalytic Activity and Substrate Scope

ABNO and Keto-ABNO are both unhindered stable nitroxide radicals. Compared with TEMPO, their bicyclic scaffolds fully expose the nitroxide radical active site, so they can efficiently approach various alcohol substrates. But their substrate scopes differ in a graded way.

ABNO substrate scope

ABNO efficiently catalyzes aerobic oxidation of primary alcohols, secondary alcohols, allylic alcohols, benzylic alcohols, and aliphatic alcohols. A catalytic system with Cu(I) salts can efficiently oxidize various primary and secondary alcohols at room temperature under air. With only 0.01 mol% catalyst loading, ABNO gives 81% product yield, while TEMPO gives only 23% under the same conditions. In addition, a catalytic system combining ABNO with Fe(NO3)3·9H2O can also efficiently catalyze alcohol oxidation at room temperature under ambient air.

Keto-ABNO substrate expansion

With its higher oxidation potential, Keto-ABNO further expands beyond ABNO's substrate scope. It efficiently oxidizes unactivated aliphatic secondary alcohols—a substrate class that TEMPO systems struggle with. Keto-ABNO can also be used for aerobic oxidation of aldehydes to carboxylic acids and chemoselective conversion of α-hydroxy acids to α-keto acids, scenarios that ABNO handles poorly. In a Bi(NO3)3/Keto-ABNO catalytic system, various primary and secondary alcohols are smoothly oxidized to the corresponding aldehydes and ketones under mild conditions. In a gram-scale experiment, the isolated yield for oxidation of 1-phenylethanol to acetophenone reaches 94%.

Catalyst accessibility

Notably, both ABNO and Keto-ABNO can be prepared in 3–4 synthetic steps on a multigram scale. Their synthetic accessibility is significantly better than that of AZADO derivatives. This is also an important reason why Stahl and other researchers chose ABNO and Keto-ABNO rather than AZADO when optimizing catalytic systems.

4. Comparison of Typical Applications

Typical applications of ABNO

ABNO is mainly used for aerobic oxidation of conventional alcohols. With air or oxygen as the terminal oxidant, ABNO efficiently converts primary and secondary alcohols to aldehydes or ketones under mild conditions. In an aqueous micellar system with NaNO2 as co-catalyst, ABNO converts secondary alcohols and some primary alcohols to the corresponding carbonyl compounds, with a calculated E-factor of only 1.47, far below the 63.4 of traditional methods. In the synthesis of sodium pyruvate, air is the oxidant, ABNO is the catalyst, and sodium nitrite is the co-catalyst. Ethyl pyruvate is first formed by catalytic oxidation, then hydrolyzed and neutralized to sodium pyruvate. The yield of ethyl pyruvate reaches 92%, and the yield of sodium pyruvate reaches 93%.

ABNO also shows good prospects in electrochemical oxidation. Under bleach-mediated conditions, ABNO's reoxidation rate is about 7 times faster than that of 4-acetamido-TEMPO, and it exists mainly in the oxoammonium form during turnover, ensuring sustained catalytic activity.

Typical applications of Keto-ABNO

Keto-ABNO has a broader application scope. Beyond conventional alcohol oxidation, it extends to the following areas:

Aerobic oxidation of aldehydes to carboxylic acids. Keto-ABNO shows a faster reaction rate than ABNO  in aerobic oxidation of aldehydes. In an asymmetric hydroformylation/oxidation tandem reaction, the Keto-ABNO-catalyzed oxidation step maintains high enantiomeric purity of the product. The total yield from styrene to (R)-2-phenylpropanoic acid reaches 89%, with an ee of 72–73%.

Aerobic oxidation of amines to imines. In 2012, the Kanai group at the University of Tokyo reported Keto-ABNO as a catalyst for aerobic oxidation of amines. With a CuBr–ligand complex as co-catalyst, it efficiently catalyzes conversion of amines to imines under mild conditions. Using a chiral copper–bisoxazoline complex further enables catalytic asymmetric amino acid synthesis from glycine derivatives and nitroalkanes.

Protein modification. Keto-ABNO's applications have expanded from small-molecule synthesis to chemical biology. Combined with water-soluble copper complex catalysts, Keto-ABNO enables serine-selective oxidative cleavage in proteins. Keto-ABNO can also selectively form adducts with tryptophan residues, enabling tryptophan-selective bioconjugation.

5. Selection Guidance

ABNO and Keto-ABNO are not simple substitutes for each other. They are differentiated choices for different application needs.

When to choose ABNO:

• Aerobic oxidation of conventional primary and secondary alcohols with relatively simple substrate structures

• Processes sensitive to catalyst cost

• Catalytic preparation of specific products such as sodium pyruvate

• Use as a mediator in electrochemical oxidation reactions

When to choose Keto-ABNO:

• Oxidation of unactivated aliphatic secondary alcohols

• Efficient conversion of aldehydes to carboxylic acids

• Aerobic oxidation of amines to imines and asymmetric amino acid synthesis

• Chemical biology applications such as protein modification

• Catalytic systems requiring higher oxidation potential

ABNO and Keto-ABNO are two nitroxide radicals built on the same skeleton, and they cover different levels of demand from routine oxidation to frontier applications. ABNO  meets routine alcohol oxidation processes with good catalytic activity and economy. Keto-ABNO, with its higher oxidation potential and broader substrate scope, shows unique advantages in unactivated secondary alcohol oxidation, aldehyde oxidation, amine oxidation, and protein modification. A clear understanding of their performance differences and application boundaries helps select the most suitable catalytic tool for a given process.