TEMPO‐tert‐Butyl Nitrite: An Efficient Catalytic System for Aerobic Oxidation of Alcohols

Abstract

A metal-free catalytic system consisting of 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) and tert-butyl nitrite has been developed to activate molecular oxygen for the aerobic oxidation of alcohols. A variety of active and non-active alcohols were oxidized to their corresponding carbonyl compounds in high selectivity and yields.Keywords: alcohols; tert-butyl nitrite; molecular oxygen; oxidation; TEMPO; 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) Selective oxidation of alcohols is an important reaction in organic chemistry. [1,2] Among the stoichiometric oxidants, molecular oxygen is a preferred terminal oxidant from both economical and environmental perspectives. We have previously developed a three-component catalyst system, consisting of 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO), Br 2 and NaNO 2 , to activate molecular oxygen for the selective aerobic oxidation of alcohols to their corresponding carbonyl compounds.[3] The method hinges upon a key finding that the inorganic salt, NaNO 2 , could serve as an NO equivalent to activate molecular oxygen under acidic conditiona. This concept has been extended and applied for a broad array of substrates with good selectivities.[4] However, most of these methods require a Br source as the co-catalyst to bridge the two catalytic cycles between TEMPO cation/TEMPOH and NO/ NO 2 . As a result, those substrates bearing an olefin moiety are not favored due to side reactions of HBr or Br 2 with the carbon-carbon double bonds.[5] Moreover, the aerobic oxidation of aliphatic primary alcohols remains a challenge because their propensity toward being overoxidized into acids and/or ortho esters under these conditions. [6] Therefore, it is desirable to develop of an efficient and selective catalyst system for the aerobic oxidation of a broad range of alcohols. Although the catalytic system TEMPO/NaNO 2 /HCl was recently developed for selective aerobic oxidation of a variety of primary and secondary alcohols under mild conditions, [7] its application to substrates containing acid-sensitive functional groups might be limited because a high loading of HCl (10-16 mol%) was used. Very recently, we have reported an efficient aerobic oxidation of various active alcohols with high turnover number (TON) under almost neat condition with TEMPO/ tert-butyl nitrite (TBN)/aqueous HBr as the catalytic system. [8] In this system, the organic nitrite, TBN, served as an efficient NO equivalent to activate molecular oxygen and enabled the aerobic oxidation at very high-volume efficiency.Generally, the TEMPO cation, which was initially formed via oxidation of TEMPO, acts as the active oxidant in the TEMPO-catalyzed oxidation of alcohols. According to the literature, the electrode potential (E 0 ) of TEMPO cation/TEMPO is 0.75 V, [2h] while E 0 of N 2 O 4 /NO is 1.03 V. [9] Based on this information, we reckoned that NO 2 can oxidize TEMPO into the TEMPO cation to initialize the oxidation (Scheme 1). Under high temperatures TBN can release NO or NO 2 because of its thermal instability. NO...

Improved synthesis of 2-norbornanone

“…Semmelhack [25] and He [26] provided twocomponent catalyst systems (TEMPO/CuCl and TEMPO/TBN). Besides, Hoover [27] discussed the mechanism of copper(I)/TEMPO-catalyzed aerobic alcohol oxidation.…”

TEMPO and Carboxylic Acid Functionalized Imidazolium Salts/Sodium Nitrite: An Efficient, Reusable, Transition Metal‐Free Catalytic System for Aerobic Oxidation of Alcohols

“…Then, a TEMPO/TBN doublecomponent and sequential bicycle catalyst system was successfully exploited for alcohol oxidations, and even has been applicable to substrates containing acid-sen-sitive functional groups without the need of the toxic halogen source and acid. [10] The TEMPO used could not be recovered, however. On the other hand, Karimi et al developed a heterogeneous catalyst system, consisting of the SBA-15-supported TEMPO/ Bu 4 NBr/NaNO 2 under acidic conditions, allowing the supported TEMPO to be recoverable.…”

Symbiotic Catalysis Relay: Molecular Oxygen Activation Catalyzed by Multiple Small Molecules at Ambient Temperature and its Mechanism

“…[10] Further research showed that the thermal decomposition of tert-butyl nitrite (TBN) could also be suitable for the aerobic oxidation of active alcohols, although the efficiency was considerably lower compared to the reaction in the presence of H + . [11] However, a high temperature is necessary in these systems to compensate for the kinetic barrier between NO 2 and TEMPO + . Recently, the use of TEMPO as a catalyst to promote the aerobic oxidation of alcohol has been reported.…”