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Organic & Biomolecular Chemistry Organic & Biomolecular Chemistry Accepted Manuscript This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/obc Page 1 of 12 Organic & Biomolecular Chemistry Journal Name RSCPublishing ARTICLE A Powerful Combination: Recent Achievements on Using TBAI and TBHP as Oxidation System Cite this: DOI: 10.1039/x0xx00000x Xiao-Feng Wu,a,b* Jin-Long Gong,a and Xinxin Qia Manuscript Received 00th January 2012, The recent achievements on using TBAI (tetrabutylammonium iodide) and TBHP (tert-butyl Accepted 00th January 2012 hydroperoxide) as oxidation system have been summarized and discussed. DOI: 10.1039/x0xx00000x www.rsc.org/ Introduction Accepted Oxidative transformation is one of the fundamental reactions in TBAI-catalyzed C-C bonds formation modern organic synthesis, which have experienced impressive [1] progress during the last decades. Among the numerous The group of Shia reported an intramolecular radical cascade of achievements, epoxidation reactions are representative example the α-cyano-TMS (TMS- = Me3Si-)/aryl-capped alkynyl aryl [2] from academic point of view while p-xylene oxidation is vital alkyl ketones in 2012.[7] The reaction using TBAI as their [3] from the point of industrial importance. Epoxidation have catalyst and TBHP as the oxidant, a variety of [6,6,5] tricyclic been verified by the Nobel Prize of Chemistry in 2001; while frameworks were constructed which containing a high level of terephthalic acid is an aromatic carboxylic acid core to functionalization efficiently (Scheme 1). One main issue is the polyester fibers production which mainly produced by p-xylene reaction should be performed in benzene. Regarding the oxidation. However, large percentage of the known procedures reaction mechanism, this cascade process was proposed to be demands the presence of transition metals as catalysts. initiated with abstracting H-atom, α to both cyano and carbonyl th Chemistry Since the end of 20 century, sustainable development has been groups, by the free radical species t-BuO• or t-BuOO• generated taken as one of the main targets of our society development. by a catalytic cycle of oxidants TBHP and I2. Under this background, ‘Green Chemistry’ started to be considered more and more by chemists in developing new Scheme 1. Intramolecular radical cyclization. synthetic methodologies.[4] In the respect of oxidation reactions, catalytic systems without the need of metal catalysts are more O O 1 CN R appealing than the traditional transition metal relied oxidative R2 CN TBAI (20 mol%), TBHP (1.1 equiv.) transformations. Recently, iodide or hypervalent iodine- o Benzene, 80 C, 0.5-1h promoted organic reactions have received considerable R1 R R2 R attentions which have already experienced impressive R3 R3 advancements during the past few years.[5] Although these 15 examples - . - 72-98% reactions can avoid the usage of metal salts, the stability and tBuOOH + I tBuO + I2 + OH - toxicity of these compounds lead those transformations have to + + . - tBuOOH I2 OH tBuOO + I + H2O be paid special attentions. More recently, the using of O O tetrabutylammonium iodide (TBAI) as catalyst with the Biomolecular CN CN combination of tert-butyl hydroperoxide (TBHP) as a powerful tBuO. or tBuOO. oxidation system has received unique attentions. In the light of R1 1 R & R2 R2 the advantages of TBAI, such as inexpensive, stable and etc., R3 R3 increasing efforts are being put on this topic. Even though the populating of TBAI catalyst in organic synthesis, a general [6] review on this topic is still absent. Taking all these points into O 1 1 CN R O R R2 CN 2 consideration and our own interests on TBAI-catalyzed . R oxidations, we started to prepare a review on this topic. The -H main achievements on this topic will be discussed and 3 catalogued by the bonds formed (C-C, C-N, C-O, C-S, N-N). R R3 The reaction mechanisms will be discussed and compared. Selected examples of substrates will be listed as well and a Nachtsheim and co-workers developed an iodocyclization Organic [8] personal outlook will be given at the end. reaction of o-alkynylphenyl carboxaldehydes. Highly This journal is © The Royal Society of Chemistry 2013 J. Name., 2013, 00, 1-3 | 1 Organic & Biomolecular Chemistry Page 2 of 12 ARTICLE Journal Name substituted 1-naphthalenones were prepared in high yields (up Wang and co-workers developed an interesting TBAI-catalyzed to 91%). In their catalyst system, tetrabutylammonium iodide C3-formylation of indoles with N-methylaniline as the (TBAI) was applied as the electrophilic iodine source, Oxone source.[11] This method can be applied to N-H and N-substituted was used as a non-nucleophilic co-oxidant, and HFIP indoles without using toxic phosphorus oxychloride and (1,1,1,3,3,3-hexafluoro-2-propanol) as a fluorinated protic transition metal catalyst. Good yields of formylated indoles solvent. TBHP, which was intensively used as co-oxidant in were produced (Scheme 3). tert-Butyl peroxybenzoate (TBPB) previous (hypo)iodite-catalyzed reactions, seemed to be a was used as the oxidant. Notably, under the optimized reaction nonsuitable co-oxidant due to the nucleophilic properties of the conditions, TBHP was completely ineffective at 80 oC, and - o emerging t-BuO anion which could undergo undesired only a trace product was obtained at 100 C. The use of H2O2 nucleophilic addition to the reaction intermediate. led to no product formation either at 80 oC or 100 oC. The yield At the beginning of 2013, Itoh and co-workers reported a dropped from 82% to 15% in the absence of nBu4NI, which molecular iodine-catalyzed cross-dehydrogenative coupling indicated that the use of nBu4NI was critical for the success of (CDC) reaction between tertiary amine and a carbon this reaction. When a radical inhibitor, TEMPO (2,2,6,6- nucleophile using hydrogen peroxide as the terminal oxidant.[9] tetramethylpiperidine-N-oxyl), was added into the reaction The corresponding aza-Henry products were formed in good system, the yield of the desired product decreased dramatically Manuscript yields using 0.1 equiv. of molecular iodine and 2 equiv. of to 30% under the optimized conditions. Taking all these into o H2O2 at 40 C (Scheme 2a). TBAI was tested as catalyst under consideration, the author proposed the reaction started with the the standard conditions as well, 24% of the desired product was reaction between an iodide (I-) ion and TBPB which provide a formed. However, when the authors tested their model system benzoyloxy radical (or tert-butyloxy radical) and iodine (I2). with 1 equiv. of molecular iodine in the absence of aq. Soon later, the same group found that replace N-methylaniline hydrogen peroxide, only 16% yield of the desired produced was with 4-substituted-N,N-dimethylanilines, a C3-formylation and formed and recovered 75% of their starting material. This result N-aminomethylation of indoles occurred using potassium suggested that the oxidation of amines requires both molecular iodide as the best catalyst (Scheme 3).[12] TBAI could gave iodine and hydrogen peroxide. NaI/H2O2/acid as a known moderate yield of the desired product in model study. In this system for generated HOI could gave the desired products in 65% two methodologies, pivalic acid was used as the additive as it yield as well. Additionally, BHT [2,6-bis(1,1-dimethylethyl)-4- has been shown to suppress decomposition of indoles under Accepted methylphenol] as a radical inhibitor was added in their reaction oxidative conditions.[13] system as well, the reaction was scarcely suppressed and gave 71% of the desired product. Based on these observations, they Scheme 3. TBAI-catalyzed formylation of indoles. proposed a HOI catalyzed reaction mechanism. In the same CHO year, the same group reported an improved reaction system NHMe [10] mo e u v. (Scheme 2b). They succeed to avoid the usage of H2O2 and TBAI (10 l%), TBPB (4 q i ) 17 examples Piv H 5 e uiv. DM o 7-82% further decreased the loading of iodine. Molecular oxygen was N O ( q ), SO, 80 C N R R applied as the terminal oxidant, and visible light from a R' R' general-purpose fluorescent lamp was necessary. In this new system, TBAI, CaI , KI, MgI and etc. can all be applied as the CHO 2 2 NMe2 catalyst and gave the corresponding product in good yield. In Chemistry KI 20 mol% , TBPB 6 equiv. ( ) ( ) 10 exam les the case when one equivalent of molecular iodine was used in o N p N PivOH (5 equiv.), DMSO, 60 C R 32-84% the absence of molecular oxygen and visible light irradiation, R H R' N only a trace amount of the corresponding product was obtained. R' The same as their precious system, BHT did not inhibit the tBuOOBz + I- u . + z . + reaction, which exclude the radical mechanism. tB O B O I2 Ar r Ar u .
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