Enhancement of the Oxidizing Power of an Oxoammonium Salt by Electronic Modification of a Distal Group

Abstract

The multigram preparation and characterization of a novel TEMPO-based oxoammonium salt, 2,2,6,6-tetramethyl-4-(2,2,2-trifluoroacetamido)-1-oxopiperidinium tetrafluoroborate (5), and its corresponding nitroxide (4) are reported. The solubility profile of 5 in solvents commonly used for alcohol oxidations differs substantially from that of Bobbitt's salt, 4-acetamido-2,2,6,6-tetramethyl-1-oxopiperidinium tetrafluoroborate (1). The rates of oxidation of a representative series of primary, secondary, and benzylic alcohols by 1 and 5 in acetonitrile solvent at room temperature have been determined, and oxoammonium salt 5 has been found to oxidize alcohols more rapidly than does 1. The rate of oxidation of meta- and para-substituted benzylic alcohols by either 1 or 5 displays a strong linear correlation to Hammett parameters (r > 0.99) with slopes (ρ) of -2.7 and -2.8, respectively, indicating that the rate-limiting step in the oxidations involves hydride abstraction from the carbinol carbon of the alcohol substrate.

Structural Effects on the pH-Dependent Redox Properties of Organic Nitroxyls: Pourbaix Diagrams for TEMPO, ABNO, and Three TEMPO Analogs

“…In light of these observations, design or selection of an optimal nitroxyl catalyst will depend on whether the catalyst cycles between oxoammonium/nitroxyl or nitroxyl/hydroxylamine manifolds. In the former case, substituent inductive effects can be expected to tune the redox potential, as has been widely documented in the literature. ,,, In the latter case, little variation is seen in the O–H bond strength of the various TEMPOH analogs. The ring strain present in ABNOH, however, contributes to a stronger O–H bond strength and offers an opportunity to tune the PCET reactivity of nitroxyl/hydroxylamine catalysts.…”

“…One electron oxidation of the nitroxyls affords oxoammonium ions, which are key intermediates in catalytic alcohol oxidation reactions (Scheme ). Such reactions represent some of the most widely used applications of organic nitroxyls. − The reaction of an alcohol with an oxoammonium species results in its two-electron reduction to a hydroxylamine species, and many different secondary oxidants have been used to reoxidize the hydroxylamine to the oxoammonium and enable catalytic turnover.…”

A Practical Oxidation Experiment for Undergraduate Students: Bobbitt’s Salt as a “Green” Alternative

“…It is posited that the reaction proceeds via a hydride-transfer mechanism from the α-C–H bond of alcohols to the electrophilic oxygen of the oxoammonium salt under nonbase-assisted conditions (Figure ). The transition state of this process is similar to the putative oxocarbenium ion formed, and thus, the rates of oxidation reflect the typical cation stability taught in introductory organic chemistry . In reality, the mechanism is likely a concerted asynchronous process wherein hydride transfer and deprotonation happen in the same step, but for ease of student understanding, it is often presented as two separate steps (Figure ).…”

Ionic Liquid Stabilized 2,2,6,6-Tetramethylpiperidine 1-Oxyl Catalysis for Alcohol Oxidation

“…Another critical feature observed is the variances among the anodic current of different alcohols. Specifically, the anodic current of primary alcohols is considerably higher than that of secondary alcohols, likely the result of the lower p K a of primary alcohols and small steric hindrance. , In addition, electron-rich substrates (e.g., benzylic alcohols) exhibit larger anodic currents than aliphatic alcohols. , …”

“…37,38 In addition, electronrich substrates (e.g., benzylic alcohols) exhibit larger anodic currents than aliphatic alcohols. 39,40 The differences in the oxidation reactivities of alcohols highlight the need for maintaining the stability of TEMPObased composites, particularly for substrates with low oxidation rates. For example, the pyrene-TEMPO used in this work has a current density of 8.3 mA/cm 2 (at 0.8 V vs SCE) for benzyl alcohol.…”