- Photoredox catalysis on unactivated substrates with strongly reducing iridium photosensitizers
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Photoredox catalysis has emerged as a powerful strategy in synthetic organic chemistry, but substrates that are difficult to reduce either require complex reaction conditions or are not amenable at all to photoredox transformations. In this work, we show that strong bis-cyclometalated iridium photoreductants with electron-rich β-diketiminate (NacNac) ancillary ligands enable high-yielding photoredox transformations of challenging substrates with very simple reaction conditions that require only a single sacrificial reagent. Using blue or green visible-light activation we demonstrate a variety of reactions, which include hydrodehalogenation, cyclization, intramolecular radical addition, and prenylationviaradical-mediated pathways, with optimized conditions that only require the photocatalyst and a sacrificial reductant/hydrogen atom donor. Many of these reactions involve organobromide and organochloride substrates which in the past have had limited utility in photoredox catalysis. This work paves the way for the continued expansion of the substrate scope in photoredox catalysis.
- Shon, Jong-Hwa,Kim, Dooyoung,Rathnayake, Manjula D.,Sittel, Steven,Weaver, Jimmie,Teets, Thomas S.
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p. 4069 - 4078
(2021/04/06)
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- An efficient preparation of chroman derivatives from 3-aryl-1-propanols and related compounds with 1,3-diiodo-5,5-dimethylhydantoin under irradiation conditions
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Treatment of various 3-aryl-1-propanols with 1,3-diiodo-5,5- dimethylhydantoin (DIH) in ethyl acetate or 1,2-dichloroethane under irradiation with a tungsten lamp gave the corresponding chroman derivatives in good to moderate yields. The present reaction proceeds via the initial formation of an alkoxyl radical and the radical cyclization onto the aromatic ring, followed by the oxidation of the formed radical intermediate with DIH to provide the chroman derivative. The same treatment of o-biphenyldimethylcarbinol, o-phenylbenzoic acid, and o-alkylbenzoic acids with DIH provided the corresponding chroman derivatives and lactone derivatives in good yields, respectively. Georg Thieme Verlag Stuttgart - New York.
- Furuyama, Shusuke,Togo, Hideo
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scheme or table
p. 2325 - 2329
(2010/11/16)
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- Intermediates for use in the preparation of vitamin e
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Novel intermediate compounds which can be used in the preparation of phytone and Vitamin E and a process for the preparation thereof. A process for the preparation of phytone and Vitamin E from these compounds is also claimed.
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- Photochemistry of o-Allylphenol. Identification of the Minor Products and New Mechanistic Proposals
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The photochemistry of o-allylphenol (1) in cyclohexane has been reinvetigated.Besides the previously reported cyclic ethers 2 and 3, seven additional minor photoproducts have been detected.Spectroscopic methods, coupled with independent synthesis, have allowed their identification as 2-methylbenzofuran (5), o-propylphenol (8), the epoxide 4, the dihydroxy compound 9, the cyclohexyl ether 6, o-(cyclohexylmethyl)phenol (10), and the dimer 7.Their formation is rationalized through new mechanistic pathways, which involve initial intermolecular electron and/or proton transfer between two molecules of o-allylphenol, as well as di-?-methane rearrangement.Key intermediates appear to be radical V, carbenium ion IX, and carbene XI.This is supported by photolysis of o-allylphenyl acetate (11), which leads to the formation of a radical pair, followed by in cage recombination to the photo-Fries products 12 and 13 or, alternatively, diffusion of the radicals out of the solvent cage to afford the minor products 2, 5, and 6, identical to those obtained by photolysis of 1.
- Miranda, Miguel A.,Tormos, Rosa
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p. 3304 - 3307
(2007/10/02)
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- o-Benzoquinone Methide: An Intermediate in the Gas-Phase Pyrolysis of Chroman
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Gas-phase pyrolysis of chroman gives o-cresol, benzofuran, and styrene in a 4:2:1 ratio at 413 deg C.In the presence of 2-butene, a moderately stereospecific 4 + 2 reaction occurs with the likely intermediate, o-benzoquinone methide, 1.The addition of 2-butene increased the amount of reaction and decreased the extent of formation of benzofuran and styrene.Addition of hydrogen gas increased the amount of hydrodealkoxylation product, styrene, and gave more of its reduction product, ethylbenzene, at the expense of benzofuran.Only a slight increase in the amount of o-cresol was observed upon addition of hydrogen gas.No dehydrochroman was observed in the reactions, and pyrolysis of 4H- and 2H-benzopyran gave no o-cresol, but instead gave styrene, benzofuran, 1-indanone, and some chroman.
- Paul, Gitendra C.,Gajewski, Joseph J.
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p. 5060 - 5062
(2007/10/02)
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- Cyclisation of 3-Phenylpropan-1-ol via Alkoxy Radical and Aryl Radical-cation Intermediates. A Product and E.S.R. Study
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A comparison is made of the cyclisation reaction of the 3-phenyl-1-propoxyl radical (2) generated by metal-catalysed reduction of 3-phenylpropyl hydroperoxide, with that of the aryl radical cation +. (3) obtained by oxidising 3-phenylpropan-1-ol (1) with SO4-. II-S2O82- and thermolysis of S2O82->.Significant yields of the cyclised product 3,4-dihydro-2H-1-benzopyran (7) were obtained from the radical (2) and radical cation (3) only in the presence of CuII ions which are proposed to oxidise the intermediate cyclised radicals.Reaction of the alcohol (1) with AgI-S2O82- and HO. III-H2O2> was also investigated, as were e.s.r. spectra of intermediates which formed from its reaction with SO4-. and HO..The results are interpreted in terms of the reaction of the 3-phenyl-1-propoxyl radical (2) via competing 1,2-hydrogen shift and cyclisation ; generation of the radical cation +. (3) (from either SO4-. or HO./H+) is followed by cyclisation via nucleophilic attack to give a cyclic radical intermediate which can undergo one-electron oxidation to give (7), ring-open to give the alkoxyl radical (2), or at low pH revert to the radical cation (3) to give the benzylic radical (15).
- Gilbert, Bruce C.,McCleland, Cedric W.
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p. 1545 - 1552
(2007/10/02)
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- Rate Constants for the Cyclisation of Some Aryl Radicals bearing Unsaturated ortho-Substituents
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Rate constants and Arrhenius parameters have been determined for ring-closure of o-alkenyl- and o-alkenyloxy-aryl radicals, and for deuterium atom transfer from tributyltin deuteride.
- Abeywickrema, Anil N.,Beckwith, Athelstan L. J.
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p. 464 - 465
(2007/10/02)
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- Hydroxyalkylation with α-Hydroperoxydiazenes. Alcohols from Olefins and Carbonyl Compounds from Enol Ethers
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Alkyl(1-hydroxy-1-methylethyl)diazenes 2a-f were prepared in solution by autoxidation of the corresponding hydrazones of acetone.Thermolysis of the diazenes at 50-80 deg C in alkenes leads to alcohols.For example, 2b decomposes in 1,1-diphenylethene to afford 5-hydroxy-5,5-diphenylpentanenitrile.Alkenes with abstractable allylic hydrogens gave analogous products, but in very low yield.Thermolysis of a diazene 2 in an enol ether solvent leads to an aldehyde or a ketone.Thus, 2a decomposes in 1-ethoxyethene and in 2-methoxypropene to afford, respectively, 4,4,4-trifluorobutanal and 5,5,5-trifluoro-2-pentanone.Yields lie in range from 50percent to 70percent.The thermolysis of 2 in alkenes involves radical intermediates and radical chain hydroxyalkylation of alkene double bonds.In one chain-propagating step, R*, generated from 2, adds to the alkene.The adduct radical so formed propagates by inducing decomposition of 2 by attack at hydroxyl oxygen.According to this mechanism, initial products from enol ethers are hemiacetals or hemiketals which do not survive the reaction conditions but decompose to the corresponding carbonyl compounds.Preliminary evidence for this mechanism is presented.
- Osei-Twum, Emmanuel Y.,McCallion, Doug,Nazran, Avtar S.,Panicucci, Rick,Risbood, Prabhakar A.,Warkentin, John
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p. 336 - 342
(2007/10/02)
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- Oxidation of Arylalkanols by S2O82--CuII
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Products of oxidation of a series of arylalkanols by S2O82--CuII in acetic acid and acetonitrile are consistent with initial oxidation to aryl radical cations followed by either loss of a benzylic proton or C-C bond scission and subsequent oxidation of intermediate benzylic radicals by CuII to final products. 1-Arylalkanols (ArCHOHR)react by both paths, C-C bond scission increasing with stability of the radical R.. 2-Arylalkanols (ArCH2CHOHR) give chiefly C-C bond scission to benzyl radicals and RCHO.The 3- and 4-arylalkanols undergo chiefly proton loss, and the resulting radicals are oxidized with cyclization to chromans and 2-phenylfurans, respectively, both of which may by oxidized in turn to further products.
- Walling, Cheves,El-Taliawi, Gamil M.,Zhao, Chengxue
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p. 4914 - 4917
(2007/10/02)
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- Methylase Models: Studies on General-Base vs. Nucleophilic Catalysis in the Intramolecular Alkylation of Phenols
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The ortho substituted phenols, 1 and 3, have been synthesized as models for the O-methylation of catecholamines, as catalyzed by catechol O-methyltransferase.The decomposition of 1 and 3 was studied at 40 deg C over a wide range of pH in both oxyanion and
- Knipe, Jay O.,Vasquez, Peter J.,Coward, James K.
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p. 3202 - 3209
(2007/10/02)
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