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Chemicals >> Oxidizing Agents > Peroxides

Hydrogen peroxide

See also: Hydrogenperoxide urea adduct, Sodium perborate, Sodium percarbonate


Name Reactions


Baeyer-Villiger Oxidation


Follow-up reaction of Brown Hydroboration


Ozonolysis


Recent Literature


An efficient bismuth tribromide catalyzed oxidation of various alcohols with aqueous hydrogen peroxide provides carbonyl compounds in good yields.
M.-k. Han, S. Kim, S. T. Kim, J. C. Lee, Synlett, 2015, 26, 2434-2436.


A clean and safe method for the dihydroxylation of alkenes under organic-solvent- and metal-free conditions was developed. The resin-supported sulfonic acid is easily recycled.
Y. Usui, K. Sato, M. Tanaka, Angew. Chem. Int. Ed., 2003, 42, 5623-5625.


As the highly volatile OsO4 is quite toxic, the development of alternative catalysts for cis-1,2-dihydroxylation of alkenes is needed. A complex of a nitrogen-based tetradentate ligand, tris(2-pyridylmethyl)amine (tpa), and osmium acts as a very efficient turnover catalyst for syn-selective dihydroxylation of various alkenes in aqueous media using hydrogen peroxide as oxidant.
H. Sugimoto, K. Kitayama, S. Mori, S. Itho, J. Am. Chem. Soc., 2012, 134, 19270-19280.


Convenient methods for the preparation of stable and non-volatile mono- and dichloroborane adducts of dioxane from dioxane-BCl3 and NaBH4 in the presence of catalytic amounts of tri- or tetraglyme were developed. The dioxane-monochloroborane adduct hydroborates representative olefins cleanly and rapidly and lead to the corresponding alcohols in quantitative yields after oxidation.
J. V. B. Kanth, H. C. Brown, J. Org. Chem, 2001, 66, 5359-5365.


A rhodium-catalyzed enantioselective syn addition of bis(catecholato)diboron to simple alkenes in the presence of (S)-Quinap provides enantioenriched 1,2-diols after subsequent oxidation. The substrate scope, the reaction mechanism, and competing pathways are discussed.
S. Trudeau, J. M. Morgan, M. Shrestha, J. P. Morken, J. Org. Chem., 2005, 70, 9538-9544.


Pt-catalyzed enantioselective addition of bis(pinacolato)diboron (B2(pin)2) to conjugated dienes enables an asymmetric 1,4-dihydroxylation of 1,3-dienes. Dienes which are unable to adopt the S-cis conformation are unreactive. For most substrates, 1,4-addition is the predominant pathway.
H. E. Burks, L. T. Kliman, J. P. Morken, J. Am. Chem. Soc., 2009, 131, 9134-9135.


A catalytic stereoselective 1,4-diboration of conjugated dienes with B2(pin)2 and the presence of Ni(cod)2 and PCy3 as the catalyst roceeds efficiently at low catalyst loadings and broadens the substrate scope of current methods for catalytic diene diboration by including internal and sterically hindered. The intermediate allylboronate was oxidized to the stereodefined allylic 1,4-diol.
R. J. Ely, J. P. Morken, Org. Lett., 2010, 12, 4348-4351.


The use of aqueous hydrogen peroxide as oxidizing agent and molecular iodine as catalyst enables a mild and efficient methodology for the ipso-hydroxylation of arylboronic acids to phenols. The reactions were performed at room temperature in short reaction time under metal-, ligand- and base-free conditions.
A. Gogoi, U. Bora, Synlett, 2012, 23, 1079-1081.


Rapid, efficient methods enable the preparation of phenols from the oxidation of arylhydrosilanes. Electron-rich aromatics benefit from silane activation via oxidation to the methoxysilane using homogeneous or heterogeneous transition metal catalysis. A combination of these two oxidations into a streamlined flow procedure involves minimal processing of reaction intermediates.
E. J. Rayment, N. Summerhill, E. A. Anderson, J. Org. Chem., 2012, 77, 7052-7060.


The combination of sBuLi and TMEDA in CPME at -60 °C enables deprotonation of unactivated, chiral secondary dialkyl TIB esters. These carbanions were reacted with a range of neopentyl boronic esters which, after 1,2-metalate rearrangement and oxidation, gave a range of tertiary alcohols in high yield and high ee. Further functional group transformations of the tertiary boronic esters were demonstrated.
A. P. Pulis, D. J. Blair, E. Torres, V. K. Aggarwal, J. Am. Chem. Soc., 2013, 135, 16054-16057.


Lithiated epoxides react stereospecifically with boronates to give syn-1,2-diols, a process that can be used iteratively to create triols containing four stereogenic centers.
E. Vedrenne, O. A. Wallner, M. Vitale, F. Schmidt, V. K. Aggarwal, Org. Lett., 2009, 11, 165-168.


A direct, mild ketohydroxylation of various 1-aryl-1-alkenes with H2O2, catalyzed by the inexpensive 12-tungstophosphoric acid/cetylpyridinium chloride system, gave acyloins in good yields and high regioselectivies.
Y. Zhang, Z. Shen, J. Tang, Y. Zhang, L. Kong, Y. Zhang, Org. Biomol. Chem., 2006, 4, 1478-1482.


A non-heme iron complex catalyzes highly enantioselective epoxidation of olefins with H2O2 in the presence of catalytic amounts of carboxylic acid additives. Ligand and carboxylic acid synergistically cooperate in promoting efficient O-O cleavage and creating highly chemo- and enantioselective epoxidizing species which provide a broad range of epoxides in synthetically valuable yields and short reaction times.
O. Cussó, I. Garcia-Bosch, X. Ribas, J. Lloret-Fillol, M. Costas, J. Am. Chem. Soc., 2013, 135, 14871-14878.


An efficient epoxidation of a broad range of olefins using hydrogen peroxide as the oxidant has been accomplished in the presence of acetic acid and a manganese catalyst that exhibits an uncommon chemoselectivity.
I. Garcia-Bosch, X. Ribas, M. Costas, Adv. Synth. Catal., 2008, 351, 348-352.


A tungsten-bishydroxamic acid complex promotes a simple, efficient, and environmentally friendly asymmetric epoxidation of allylic, and homoallylic alcohols at room temperature using aqueous hydrogen peroxide as oxidant.
C. Wang, H. Yamamoto, J. Am. Chem. Soc., 2014, 136, 1222-1225.


Homoallylic alcohols were efficiently epoxidized to the corresponding 3,4-epoxy alcohols in excellent yields in the presence of methyltrioxorhenium (MTO) as catalyst, aqueous hydrogen peroxide as the terminal oxidant, and 3-methylpyrazole as an additive. Organic solvent-free conditions accelerate the reaction.
S. Yamazaki, J. Org. Chem., 2012, 77, 9884-9888.


A chiral bisaryl-silyl-protected pyrrolidine acts as a very selective epoxidation organocatalyst using simple oxidation agents. The scope of the reaction is demonstrated by the formation of optically active α,β-epoxy aldehydes in high yields and enantioselectivities. The asymmetric epoxidation reactions proceed also under environmental friendly reaction conditions in, for example, water mixtures of alcohols.
M. Marigo, J. Franzen, T. B. Poulsen, W. Zhuang, K. A. Jorgensen, J. Am. Chem. Soc., 2005, 127, 6284-6289.


The asymmetric Payne oxidation of N-sulfonyl aldimines catalyzed by a P-spiro chiral triaminoiminophosphorane enables a practical synthesis of optically active N-sulfonyl oxaziridines with high efficiency and an excellent level of enantioselectivity. The versatility of this method was demonstrated by the diastereoselective kinetic oxidation of racemic α-chiral N-sulfonyl imines.
R. Tsutsumi, S. Kim, D. Uraguchi, T. Ooi, Synthesis, 2014, 46, 871-878.


VO(acac)2 catalyzes the oxidation of aromatic and aliphatic aldehydes to the corresponding acids efficiently and selectively in the presence of hydrogen peroxide as an oxidant. This method offers functional-group compatibility, easy workup procedure, and a short reaction time. The performance of titania-supported VO(acac)2 in the oxidation of aldehydes was also investigated.
D. Talukdar, K. Sharma, S. K. Bharadwaj, A. J. Thakur, Synlett, 2013, 24, 963-966.


A copper-catalyzed O-methylation of carboxylic acids using dimethyl sulfoxide (DMSO) as the methyl source exhibits a broad substrate scope and excellent functional group tolerance. Mechanistic studies indicate that a methyl radical is generated from dimethyl sulfoxide.
J. Jia, Q. Jiang, A. Zhao, B. Xu, Q. Liu, W.-P. Luo, C.-C. Guo, Synthesis, 2016, 48, 421-428.


A combined amino- and N-heterocyclic carbene (NHC)-catalyzed one-pot reaction sequence using commercially available catalysts at low catalyst loadings gives β-hydroxy and β-amino esters in high yield and excellent enantiopurity. The generation of quaternary stereocenters and application in gram-scale synthesis were also realized, with no requirements of inert or anhydrous reaction conditions.
H. Jiang, B. Gschwend, Ł. Albrecht, K. A. Jørgensen, Org. Lett., 2010, 12, 5052-5055.


An effective epoxidation of lipophilic alkenes using hydrogen peroxide was accomplished with a manganese sulfate/bicarbonate catalytic system in an ionic liquid at room temperature.
K.-H. Tong, K.-Y. Wong, T. H. Chan, Org. Lett., 2003, 5, 3423-3425.


An epoxidation of alkenes using hydrogen peroxide as the terminal oxidant is promoted by catalytic amounts (1.0-0.1 mol %) of manganese(2+) salts, and must be performed using at least catalytic amounts of bicarbonate buffer. Various aryl-substituted, cyclic, and trialkyl-substituted alkenes were epoxidized under these conditions using 10 equiv of hydrogen peroxide, but monoalkyl-alkenes were not. Additives such as sodium acetate and salicylic acid enhanced the rate of the desired epoxidation reaction by 2-3 times. Possible mechanisms for the reaction are discussed.
B. S. Lane, M. Vogt, V. J. DeRosa, K. Burgess, J. Am. Chem. Soc., 2002, 124, 11946-11954.


Aryl benzyl selenoxides are efficient catalysts for the epoxidation of various olefinic substrates and the Baeyer-Villiger oxidation of aldehydes and ketones with hydrogen peroxide.
M. A. Goodman, M. R. Detty, Synlett, 2006, 1100-1104.


A highly enantioselective catalytic epoxidation of α,β-unsaturated diaryl enones was achieved with high chemical yield by using aqueous hydrogen peroxide in the presence of a guanidine-urea bifunctional organocatalyst. The catalyst performs cooperatively by interaction of the guanidine group with hydrogen peroxide and the urea group with the enone or vice versa.
S. Tanaka, K. Nagasawa Synlett, 2009, 667-670.


Using cinchona alkaloid-derived primary amines as catalysts and aqueous hydrogen peroxide as the oxidant, highly enantioselective Weitz-Scheffer-type epoxidation and hydroperoxidation reactions of α,β-unsaturated carbonyl compounds take place. Acyclic enones, cyclic enones, and α-branched enals can be converted. Intermediates have been characterized by MS and NMR. DFT calculations explain the activation of H2O2.
O. Lifchits, M. Mahlau, C. M. Reisinger, A. Lee, C. Farès, I. Polyak, G. Gopakumar, W. Thiel, B. List, J. Am. Chem. Soc., 2013, 135, 6677-6693.


Chiral primary amine salts catalyze highly enantioselective epoxidations of cyclic enones with hydrogen peroxide.
X. Wang, C. M. Reisinger, B. List, J. Am. Chem. Soc., 2008, 130, 6070-6071.


M. A. Goodman, M. R. Detty, Synlett, 2006, 1100-1104.


Aldehydes undergo oxidative transformation to the methyl esters in methanol as solvent upon treatment with catalytic amounts of vanadium pentoxide in combination with hydrogen peroxide. This method features mild reaction conditions, short reaction times, high efficiencies, cost-effectiveness, and facile isolation of the desired products.
R. Gopinath, B. Patel, Org. Lett., 2000, 2, 577-579.


A convenient and efficient oxidation of hydroxylated and methoxylated benzaldehydes and acetophenones to the corresponding phenols uses hydrogen peroxide and methyltrioxorhenium as catalyst in ionic liquids [bmim]BF4 and [bmim]PF6.
R. Bernini, A. Coratti, G. Provenzano, G. Fabrizi, D. Tofani, Tetrahedron, 2005, 61, 1821-1825.


In the presence of catalytic amounts of phosphomolybdic acid (PMA), ethereal hydrogen peroxide reacted readily with a range of epoxides at ambient temperature, giving corresponding β-hydroxyhydroperoxides in good yields.
Y. Li, H.-D. Hao, Y. Wu, Org. Lett., 2009, 11, 2691-2694.


The acid-catalyzed reaction of β,δ-triketones with hydrogen peroxide produces tricyclic peroxides selectively in good yields via the monoperoxidation of the carbonyl groups in β-position and the transformation of the δ-carbonyl group into an acetal. The resulting peroxides can be easily isolated from the reaction mixture.
A. O. Terent'ev, I. A. Yaremenko, V. V. Chernyshev, V. M. Dembitsky, G. I. Nikishin, J. Org. Chem., 2012, 77, 1833-1842.


Nitroso arenes are prepared in good yield from anilines by oxidation with H2O2 catalysed with MoO3/KOH, ammonium molybdate or other molybdenum salts. Further oxidation to nitro arenes is also described.
A. Defoin, Synthesis, 2004, 706-710.


Oxidation of sulfides with 30% hydrogen peroxide catalyzed by tantalum carbide provides the corresponding sulfoxides in high yields, whereas niobium carbide as catalyst efficiently affords the corresponding sulfones. Both catalysts can easily be recovered and reused without losing their activity.
M. Kirihara, A. Itou, T. Noguchi, J. Yamamoto, Synlett, 2010, 1557-1561.


M. Kirihara, A. Itou, T. Noguchi, J. Yamamoto, Synlett, 2010, 1557-1561.


A versatile procedure oxidizes sulfanes to sulfoxides without any overoxidation to sulfones using a combination of hydrogen peroxide and triflic acid. This method tolerates oxidatively sensitive functional groups.
M. M. Khodaei, K. Bahrami, A. Karimi, Synthesis, 2008, 1682-1684.


Confined chiral Brønsted acids catalyze asymmetric oxidations of a broad range of sulfides to sulfoxides with hydrogen peroxide. The wide generality and high enantioselectivity of the developed method is comparable even to the best metal-based systems.
S. Liao, I. Čorić, Q. Wang, B. List, J. Am. Chem. Soc., 2012, 134, 10765-10768.


A porphyrin-inspired manganese-catalyzed asymmetric sulfoxidation method  enables a rapide oxidation of a broad range of sulfides in high yields with excellent enantioselectivities in the presence of hydrogen peroxide.
W. Dai, J. Li, B. Chen, G. Li, Y. Lv, L. Wang, S. Gao, Org. Lett., 2013, 15, 5658-5661.


Various aromatic and aliphatic sulfides are selectively oxidized to sulfoxides and sulfones in good to excellent yields using 30% H2O2 in the presence of a recyclable silica-based tungstate interphase catalyst at room temperature.
B. Karimi, M. Ghoreishi-Nezhad, J. H. Clark, Org. Lett., 2005, 7, 625-628.


Sc(OTf)3 is an efficient catalyst for the hydrogen peroxide mediated monooxidation of alkyl-aryl sulfides and methyl cysteine containing peptides. The method is high yielding, compatible with many widely used protecting groups, suitable for solid-phase applications and proceeds with minimum over-oxidation.
M. Matteucci, G. Bhalay, M. Bradley, Org. Lett., 2003, 5, 235-237.


An air and moisture tolerant complex of Ti(IV) with a C3-symmetric triphenolate amine ligand efficiently catalyzes sulfoxidation reactions at room temperature without previous activation using aqueous hydrogen peroxide as oxidant.
M. Mba, L. J. Prins, G. Licini, Org. Lett., 2007, 9, 21-24.


The combination of very high ee values with high yield, the consequence of an efficient initial asymmetric oxidation followed by an efficient kinetic resolution, makes the reported system very practical for the asymmetric oxidation of simple akyl aryl sulfides.
C. Drago, L. Caggiano, R. F. W. Jackson, Angew. Chem. Int. Ed., 2005, 44, 7221-7223.


A chiral Fe(salan) complex serves as an efficient catalyst for asymmetric oxidation of sulfides using hydrogen peroxide in water without surfactant. Not only alkyl aryl sulfides but also various methyl alkyl sulfides were oxidized to the corresponding sulfoxides with high enantioselectivities.
H. Egami, T. Katsuki, J. Am. Chem. Soc., 2007, 129, 8940-8941.


The asymmetric oxidation of sulfides to chiral sulfoxides with hydrogen peroxide in good yield and high enantioselectivity has been catalyzed very effectively by a chiral vanadium-salan complex. The efficient kinetic resolution of racemic sulfoxides catalyzed by the vanadium-salan system is also described.
J. Sun, C. Zhu, Z. Dai, M. Xang, Y. Pan, H. Hu, J. Org. Chem., 2004, 69, 8500-8503.


The combination of H2O2 and SOCl2 is a highly reactive reagent for the direct oxidative conversion of thiol derivatives to the corresponding sulfonyl chlorides through oxidative chlorination. Upon reaction with amines, the corresponding sulfonamides were obtained in excellent yields in very short reaction times.
K. Bahrami, M. M. Khodaei, M. Soheilizad, J. Org. Chem., 2009, 74, 9287-9291.


Hydrogen peroxide, in the presence of zirconium tetrachloride, is a very efficient reagent for the direct oxidative conversion of thiols and disulfides into the corresponding sulfonyl chlorides with high purity through oxidative chlorination. Excellent yields, very short reaction times, mild reaction conditions, and the avoidance of harsh reagents are the main advantages of this method.
K. Bahrami, M. M. Khodaei, M. Soheilizad, Synlett, 2009, 2773-2776.


Thiols were effectively oxidized into disulfides by reacting with hydrogen peroxide in the presence of a catalytic amount of iodide ion or iodine.
M. Kirihara, Y. Asai, S. Ogawa, T. Noguchi, A. Hatano, Y. Hirai, Synthesis, 2007, 3286-3289.


Simple, mild, and environmentally friendly procedures for the direct conversion of dithioesters into either carboxylic acids or esters using hydrogen peroxide under alkaline conditions are described.
F. Grellepois, C. Portella, Synthesis, 2008, 3443-3446.


The hydrogen peroxide-zirconium(IV) chloride reagent system is efficient and general for the conversion of thioamides to amides in short reaction times and good chemoselectivity, and allows a simple workup that precludes the use of toxic solvents.
K. Bahrami, M. M. Khodaei, Y. Tirandaz, Synthesis, 2009, 369-371.


The synthesis of a planar-chiral bisflavin catalyst (1) and its use in asymmetric Bayer-Villiger-Oxidations is described.
S. Murahashi, S. Ono, Y. Imada, Angew. Chem. Int. Ed., 2002, 41, 2366-2368.


α-Acetoxylation of ketones catalyzed by iodobenzene using^acetic anhydride and 30% aqueous hydrogen peroxide as the oxidant is an effective and economical method for the preparation of α-acetoxy ketones in good yields.
J. Sheng, Y. Li, M. Tang, B. Gao, G. Huang, Synthesis, 2007, 1165-1168.


Ketones or ketals were readily converted into the corresponding gem-dihydroperoxides in high yields by treatment with ethereal H2O2 at ambient temperature in the presence of phosphomolybdic acid (PMA) as catalyst.
Y. Li, H.-D. Hao, Q. Zhang, Y. Wu, Org. Lett., 2009, 11, 1615-1618.


1-Arylethanones and related compounds are rapidly brominated in dioxane with the H2O2-HBr aq system, resulting in the replacement of two hydrogen atoms in the methyl group with bromine. The reaction is also accompanied by bromination of the aromatic ring provided that the latter contains electron-donating substituents.
A. O. Terent'ev, S. V. Khodykin, I. B. Krylov, Y. N. Ogibin, G. I. Nikishin, Synthesis, 2006, 1087-1092.


Active methylene compounds can be chemoselectively brominated in high yields using potassium bromide, hydrochloric acid, and hydrogen peroxide at room temperature.
M. Kirihara, S. Ogawa, T. Noguchi, K. Okubo, Y. Monma, I. Shimizu, R. Shimosaki, A. Hatano, Y. Hirai, Synlett, 2006, 2287-2289.


Pd-catalyzed enantioselective diborations of prochiral allenes followed by allylation reactions with primary imines provide vinyl boronates which may be oxidized to give nonracemic Mannich products. Alternatively, enantiomerically enriched homoallylic amine derivatives may be obtained by protonation and Suzuki cross-coupling of the vinyl boronate.
J. D. Sieber, J. P. Morken, J. Am. Chem. Soc., 2006, 128, 74-75.


A simple, mild and efficient procedure cleaves a wide range of ketoximes and aldoximes to the corresponding carbonyl compounds in an aqueous medium using catalytic amounts of potassium bromide and ammonium heptamolybdate tetrahydrate in combination with hydrogen peroxide.
N. C. Ganguly, S. K. Barik, Synthesis, 2008, 425-428.


A selective and efficient oxidative iodination of electron rich arenes was carried out with one equivalent of KI and two equivalents of 30% hydrogen peroxide in MeOH in the presence of strong acid.
J. Iskra, S. Stavber, M. Zupan, Synthesis, 2004, 1869-1873.


An efficient and highly enantioselective Payne-type oxidation of N-sulfonyl imines exhibits broad substrate generality and unique chemoselectivity based on the combined use of hydrogen peroxide and trichloroacetonitrile under the catalysis of P-spiro chiral triaminoiminophosphorane.
D. Uraguchi, R. Tsutsumi, T. Ooi, J. Am. Chem. Soc., 2013, 135, 8161-8164.


A mild, environmentally friendly, and efficient process enables the synthesis of 2-imidazolines in high yield by reaction of aldehydes with ethylenediamine using hydrogen peroxide as an oxidant in the presence of sodium iodide and anhydrous magnesium sulfate.
G.-y. Bai, K. Xu, G.-f. Chen, Y.-h. Yang, T.-y. Li, Synthesis, 2011, 1599-1603.


A 2,2,2-trifluoroacetophenone-catalyzed oxidation of allyloximes enables a green and efficient synthesis of isoxazolines utilizing H2O2 as the oxidant. A variety of substitution patterns, both aromatic and aliphatic moieties, are well tolerated, leading to isoxazolines in good yields.
I. Triandafillidi, C. G. Kokotos, Org. Lett., 2017, 19, 106-109.


A convenient method for the synthesis of 2-substituted benzimidazoles and benzothizoles offers short reaction times, large-scale synthesis, easy and quick isolation of the products, excellent chemoselectivity, and excellent yields as main advantages.
K. Bahrami, M. M. Khodaei, F. Naali, J. Org. Chem., 2008, 73, 6835-6837.


A simple and efficient procedure for the synthesis of substituted benzimidazoles through a one-pot condensation of o-phenylenediamines with aryl aldehydes in the presence of H2O2 and HCl in acetonitrile at room temperature features short reaction time, easy and quick isolation of the products, and excellent yields.
K. Bahrami, M. M. Khodaei, I. Kavianinia, Synthesis, 2007, 417-427.


An oxidative cleavage of alkyl C-Pd bond by H2O2 enables a palladium-catalyzed intramolecular aminohydroxylation to give various heterocycles with good yields and excellent diastereoselectivities. Facile transformation of these products provided a powerful tool toward the synthesis of 2-amino-1,3-diols and 3-ol amino acids. Preliminary mechanistic studies revealed that a SN2 type attack of water at a high-valent Pd center is involved.
H. Zhu, P. Chen, G. Liu, J. Am. Chem. Soc., 2014, 136, 1766-1769.


An efficient and environmentally friendly conversion of α-hydroxy N-arylamides into isatins in very good yields proceeded smoothly under metal-free conditions in the presence of hydrogen peroxide as oxidant. This convenient method offers broad substrate scope.
J. Li, X. Cheng, X. Ma, G. Lv, Z. Zhan, M. Guan, Y. Wu, Synlett, 2016, 27, 2485-2488.


A simple protocol for the deprotection of 1,3-dithianes and 1,3-dithiolanes showed tolerance for a number of phenol and amino protecting groups using 30% aqueous hydrogen peroxide activated by iodine catalyst (5 mol%) in water in the presence of sodium dodecyl sulfate (SDS) under essentially neutral conditions without any detectable overoxidation.
N. G. Ganguly, S. K. Barik, Synthesis, 2009, 1393-1399.


Phosphorylation of amines, alcohols, and sulfoximines provides various phosphoramidates, phosphorus triesters and sulfoximine-derived phosphoramidates using molecular iodine as a catalyst and H2O2 as the sole oxidant under mild reaction conditions.
J. Dhineshkumar, K. R. Prabhu, Org. Lett., 2013, 15, 6062-6065.