2,2,6,6-Tetramethylpiperidinyloxy is a stable nitroxyl radical, which serves in oxidations as catalyst. By substitution in position 4 (4-hydroxy-TEMPO, 4-acetamido-TEMPO) its reactivity can be steered.
For example, processes with oxygen and 5 mol-% TEMPO permit environmentally benign reactions as alternatives to some oxidations, where so far e.g. chrome reagents were used.
2-azaadamantane N-Oxyl (AZADO), 9-Azabicyclo[3.3.1]nonane N-Oxyl (ABNO), 9-Azanoradamantane N-oxyl (Nor-AZADO) form a less hindered class of nitroxyl radicals and exhibit an enhanced reactivity compared with TEMPO.
A convenient method enables the preparation of a silica gel supported TEMPO catalyst. The catalyst prepared from [4-hydroxy-TEMPO + NaCl]/SiO2 was used for an aerobic oxidation of alcohols to carbonyls under mild reaction conditions in the presence of Fe(NO3)3 • 9 H2O. Alcohols were converted to the corresponding carbonyls in good to excellent yields. After a simple filtration, the catalyst can be reused at least six times.
N. Tamura, T. Aoyama, T. Takido, M. Kodomari, Synlett, 2012, 1397-1407.
A rapid oxidation of primary and secondary alcohols using catalytic amounts of TEMPO and Yb(OTf)3 in combination with a stoichiometric amount of iodosylbenzene afforded carbonyl compounds in excellent yields without over-oxidation. Oxidation of primary alcohols in the presence of secondary alcohols proceeded with good selectivity.
J.-M. Vatèle, Synlett, 2006, 2055-2058.
An aerobic oxidation of primary and secondary alcohols to aldehydes and ketones using TEMPO-CuCl as catalyst in the ionic liquid [bmin][PF6] has been developed. The system needs no bubbling of O2 due to its good solubility in the ionic liquid. The resulting aldehydes (with no traces of carboxylic acids) and ketones can be extracted with organic solvents. The ionic liquid can be reused after washing with water and drying under high vacuum (8 runs for the oxidation of benzyl alcohol: yields of 72%, 70, 68, 70, 65, 64, 62, and 60).
I. A. Ansari, R. Gree, Org. Lett., 2001, 1507-1509.
A highly efficient 2,2,6,6-tetramethylpiperidin-1-yloxy (TEMPO) catalyzed reaction using recyclable 1-chloro-1,2-benziodoxol-3(1H)-one as the terminal oxidant allows the conversion of various alcohols to their corresponding carbonyl compounds in high to excellent yields at room temperature in ethyl acetate, which is an environmentally friendly organic solvent.
X.-Q. Li, C. Zhang, Synthesis, 2009, 1163-1169.
The combination of TEMPO and CAN can be used for the aerobic oxidation of benzylic and allylic alcohols into their corresponding carbonyl compounds. However, steric hindrance has been observed to impede the reaction with some substituted allylic systems. The present method is superior to others currently available due to its relatively short reaction times and excellent yields.
S. S. Kim, H. C. Jung, Synthesis, 2003, 2135-2137.
The reaction of KBrO3 and NH2OH • HCl in situ generates NOx and Br anion, which allows in the presence of 2,2,6,6-tetramethylpiperidine-N-oxide (TEMPO) an activation of dioxygen to oxidize various benzylic alcohols quantitatively to their corresponding carbonyl compounds under mild conditions.
G. Yang, W. Wang, W. Zhu, C. An, X. Gao, M. Song, Synlett, 2010, 437-440.
TEMPO-derived reagents tagged with multiple perfluoroalkyl chains and triazole moieties promote the oxidation of alcohols to aldehydes in organic solvent/water mixtures with reaction rates comparable to homogeneous TEMPO reagents, but can be easily recovered by liquid/emulsion filtration.
A. Gheorghe, T. Chinnusamy, E. Cuevas-Yañez, P. Hilgers, O. Reiser, Org. Lett., 2008, 10, 4171-4174.
A four-component system consisting of acetamido-TEMPO/Cu(ClO4)2/TMDP/DABCO in DMSO allows an efficient room-temperature aerobic alcohol oxidation of various alcohols into their corresponding aldehydes or ketones in good to excellent yields. The catalytic system can be recycled.
N. Jiang, A. J. Ragauskas, J. Org. Chem., 2006, 71, 7087-7090.
The system Cu(ClO4)2/acetamido-TEMPO/DMAP catalyses the room-temperature aerobic oxidation of primary alcohols to aldehydes in the ionic liquid [bmpy]PF6. The catalysts can be recycled and reused.
N. Jiang, A. J. Ragauskas, Org. Lett., 2005, 7, 3689-3692.
A highly efficient and mild procedure for the oxidation of different types of alcohols uses TEMPO as catalyst, iodobenzene dichloride as stoichiometric oxidant, and pyridine as base. Oxidation of 1,2-diols gives α-hydroxy ketones or α-diketones depending on the amount of oxidant used. High yielding procedures for the preparation of iodoarene dichlorides have been developed.
X.-F. Zhao, C. Zhang, Synthesis, 2007, 551-557.
The use of NaClO/TEMPO/Co(OAc)2 enabled a benzylic oxidation of alkyl arenes to yield various aromatic aldehydes and ketones in very good yields. The reaction reactivity, selectivity, and scope of the reaction were investigated.
C. Jin, L. Zhang, W. Su, Synlett, 2011, 1435-1438.
A one-pot two-step sequence involving an oxidation/imine-iminium formation/reduction allowed the N-alkylation of amines by alcohols. Optically active alcohols and amines can be converted without any epimerization.
C. Guérin, V. Bellosta, G. Guillamot, J. Cossy, Org. Lett., 2011, 13, 3478-3481.
A sequential one-pot synthesis for the oxidation of primary and secondary tert-butyldimethylsilyl (TBDMS) ethers, using the presence of PhIO or PhI(OAc)2 and catalytic amounts of metal triflates and TEMPO in THF or acetonitrile tolerates acid-sensitive protecting groups and leaves tert-butyldiphenylsilyl ethers and phenolic TBDMS groups untouched.
B. Barnych, J.-M. Vatèle, Synlett, 2011, 2048-2052.
A Cu-catalyzed oxidative amidation-diketonization reaction of terminal alkynes leads to α-ketoamides. In this copper-catalyzed radical process, O2 not only participates as the ideal oxidant but also undergoes dioxygen activation under ambient conditions.
C. Zhang, N. Jiao, J. Am. Chem. Soc., 2010, 132, 28-29.
Passerini three-component reaction under catalytic aerobic conditions allows the conversion of alcohols instead of aldehydes. The reaction of alcohols, isocyanides, and carboxylic acids in toluene in the presence of a catalytic amount of cupric chloride, NaNO2, and TEMPO afforded, under an oxygen atmosphere, the P-3CR adducts in good yields.
J. Brioche, G. Masson, J. Zhu, Org. Lett., 2010, 12, 1432-1435.
A highly convenient organocatalytic method for the mono-oxidation of unprotected glycosides relies on the chemoselective properties of TEMPO in combination with trichloroisocyanuric acid under very mild, basic conditions. The resulting dialdo-glycosides are efficiently purified with the use of solid-phase imine capture.
M. Angelin, M. Hermansson, H. Dong, O. Ramström, Eur. J. Org. Chem., 2006, 4323-4326.
An efficient and highly selective method for the oxidative conversion of primary amines to the corresponding nitriles using trichloroisocyanuric acid in the presence of catalytic TEMPO provides a new entry to the synthesis of various aliphatic, aromatic and heterocyclic nitriles in excellent yield.
F.-E. Chen, Y.-Y. Kuang, H.-F. Dai, L. Lu, M. Huo, Synthesis, 2003, 2629-2631.
Iodine was compared to other positive halogens as terminal oxidant in chemoselective oxidations of alcohols using catalytic TEMPO and was shown to be superior in cases of electron-rich and heteroaromatic benzylic alcohols.
R. A. Miller, R. S. Hoerrner, Org. Lett., 2003, 5, 285-287.
A novel, metal-free oxidation system for the catalytic synthesis of aldehydes and ketones using TEMPO and a quarternary ammonium salt as catalysts and Oxone as oxidant proved especially successful for the synthesis of ketones. The mild conditions tolerate even sensitive silyl protective groups which can otherwise be cleaved in the presence of Oxone.
C. Bolm, A. S. Magnus, J. P. Hildebrand, Org. Lett., 2000, 2, 1173-1175.
1 mol-% TEMPO and a catalytic amount of 1,3-dibromo-5,5-dimethylhydantoin and NaNO2 is a highly efficient catalytic system for the aerobic oxidations of benzylic alcohols in water.
R. Liu, C. Dong, X. Liang, X. Wang, X. Hu, J. Org. Chem., 2005, 70, 239-244.
Benzylic ethers are oxidatively cleaved by 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate in wet MeCN at room temperature to give the corresponding aromatic aldehydes and alcohols in high yield. Primary and secondary alkyl alcohols are further oxidized to give carboxylic acids and ketones, respectively.
P. P. Pradhan, J. M. Bobbitt, W. F. Bailey, J. Org. Chem., 2009, 74, 9501-9504.
Oxoammonium salts enable a practical and highly efficient oxidative rearrangement of tertiary allylic alcohols to β-substituted α,β-unsaturated carbonyl compounds. Acyclic substrates as well as medium membered ring substrates and macrocyclic substrates can be oxidized.
M. Shibuya, M. Tomizawa, Y. Iwabuchi, J. Org. Chem., 2008, 73, 4750-4752.
CuCl/DABCO/4-HO-TEMPO as the catalysts and oxygen as the terminal oxidant enabled an efficient aerobic oxidative synthesis of 2-substituted quinazolines and 4H-3,1-benzoxazines from the one-pot reaction of aldehydes with 2-aminobenzylamines and 2-aminobenzyl alcohols, respectively.
B. Han, X.-L. Yang, C. Wang, Y.-W. Bai, T.-C. Pan, X. Chen, W. Yu, J. Org. Chem., 2012, 77, 1136-1142.
A one-pot dehydrogenative Povarov/oxidation tandem reaction of N-alkyl anilines with mono- and 1,2-disubstituted aryl and alkyl olefins enables the synthesis of a various substituted quinolines. The simple protocol uses cheap and benign iron(III)chloride as the Lewis acid catalyst and a TEMPO oxoammonium salt as a nontoxic, mild, efficient oxidant.
H. Richter, O. G. Mancheño, Org. Lett., 2011, 13, 6066-6069.
Cross coupling of ortho-substituted aryl Grignard reagents with alkynyl Grignard reagents can be performed without adding any transition metal in the presence of 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO) as an environmentally benign organic oxidant. Importantly, functional groups such as esters, amides, and cyanides are tolerated.
M. S. Maji, S. Murarka, A. Studer, Org. Lett., 2010, 12, 3878-3881.