- Decarboxylative cross-nucleophile coupling via ligand-to-metal charge transfer photoexcitation of Cu(ii) carboxylates
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Reactions that enable carbon–nitrogen, carbon–oxygen and carbon–carbon bond formation lie at the heart of synthetic chemistry. However, substrate prefunctionalization is often needed to effect such transformations without forcing reaction conditions. The development of direct coupling methods for abundant feedstock chemicals is therefore highly desirable for the rapid construction of complex molecular scaffolds. Here we report a copper-mediated, net-oxidative decarboxylative coupling of carboxylic acids with diverse nucleophiles under visible-light irradiation. Preliminary mechanistic studies suggest that the relevant chromophore in this reaction is a Cu(ii) carboxylate species assembled in situ. We propose that visible-light excitation to a ligand-to-metal charge transfer (LMCT) state results in a radical decarboxylation process that initiates the oxidative cross-coupling. The reaction is applicable to a wide variety of coupling partners, including complex drug molecules, suggesting that this strategy for cross-nucleophile coupling would facilitate rapid compound library synthesis for the discovery of new pharmaceutical agents. [Figure not available: see fulltext.].
- Li, Qi Yukki,Gockel, Samuel N.,Lutovsky, Grace A.,DeGlopper, Kimberly S.,Baldwin, Neil J.,Bundesmann, Mark W.,Tucker, Joseph W.,Bagley, Scott W.,Yoon, Tehshik P.
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- Visible Light Promoted, Catalyst-Free Radical Carbohydroxylation and Carboetherification under Mild Biomimetic Conditions
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Metal and catalyst-free carbohydroxylations and carboetherifications at room temperature have been achieved by a combination of beneficial factors including high aryl diazonium concentration and visible light irradiation. The acceleration of the reaction by visible light irradiation is particularly remarkable against the background that neither the aryldiazonium salt nor the alkene show absorptions in the respective range of wavelength. These observations point to weak charge transfer interactions between diazonium salt and alkene, which are nevertheless able to considerably influence the reaction course. As highly promising perspective, many more aryldiazonium-based radical arylations might benefit from simple light irradiation without requiring a photocatalyst or particular additive.
- Altmann, Lisa-Marie,Zantop, Viviane,Wenisch, Pia,Diesendorf, Nina,Heinrich, Markus R.
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p. 2452 - 2462
(2020/12/30)
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- N -Arylbenzo[ b ]phenothiazines as Reducing Photoredox Catalysts for Nucleophilic Additions of Alcohols to Styrenes: Shift towards Visible Light
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N -Phenylphenothiazines are an important class of photoredox catalysts because they are synthetically well accessible, they allow the tuning of the optoelectronic properties by different substituents, and they have strong reduction properties for activation of alkenes. One of the major disadvantages of N -phenylphenothiazines, however, is the excitation at 365 nm in the UV-A light range. We synthesized three differently dialkylamino-substituted N -phenylbenzo[ b ]phenothiazines as alternative photoredox catalysts and applied them for the nucleo philic addition of alkohols to α-methyl styrene. The additional benzene ring shift the absorbance bathochromically and allows performing the photocatalyses by excitation at 385 nm and 405 nm. This type of photoredox catalysis tolerates other functional groups, as representatively shown for alcohols as substrates with C-C and C-N triple bonds.
- Seyfert, Fabienne,Wagenknecht, Hans-Achim
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supporting information
p. 582 - 586
(2021/01/25)
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- Reactivity and Product Analysis of a Pair of Cumyloxyl and tert-Butoxyl Radicals Generated in Photolysis of tert-Butyl Cumyl Peroxide
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Alkoxyl radicals play important roles in various fields of chemistry. Understanding their reactivity is essential to applying their chemistry for industrial and biological purposes. Hydrogen-atom transfer and C-C β-scission reactions have been reported from alkoxyl radicals. The ratios of these two processes were investigated using cumyloxyl (CumO?) and tert-butoxyl radicals (t-BuO?), respectively. However, the products generated from the pair of radicals have not been investigated in detail. In this study, CumO? and t-BuO? were simultaneously generated from the photolysis of tert-butyl cumyl peroxide to understand the chemical behavior of the pair of radicals by analyzing the products and their distribution. Electron paramagnetic resonance and/or transient absorption spectroscopy analyses of radicals, including CumO? and t-BuO?, provide more information about the radicals generated during the photolysis of tert-butyl cumyl peroxide. Furthermore, the photoproducts of (3-(tert-butylperoxy)pentane-3-yl)benzene demonstrated that the ether products were formed in in-cage reactions. The triplet-sensitized reaction induced by acetophenone, which is produced from CumO?, clarified that the spin state did not affect the product distribution.
- Oyama, Ryoko,Abe, Manabu
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p. 8627 - 8638
(2020/07/21)
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- N-Arylphenothiazines as strong donors for photoredox catalysis – pushing the frontiers of nucleophilic addition of alcohols to alkenes
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A new range of N-phenylphenothiazine derivatives was synthesized as potential photoredox catalysts to broaden the substrate scope for the nucleophilic addition of methanol to styrenes through photoredox catalysis. These N-phenylphenothiazines differ by their electron-donating and electron-withdrawing substituents at the phenyl group, covering both, σ and π-type groups, in order to modulate their absorbance and electrochemical characteristics. Among the synthesized compounds, alkylaminylated N-phenylpheno-thiazines were identified to be highly suitable for photoredox catalysis. The dialkylamino substituents of these N-phenylpheno-thiazines shift the estimated excited state reduction potential up to ?3.0 V (vs SCE). These highly reducing properties allow the addition of methanol to α-methylstyrene as less-activated substrate for this type of reaction. Without the help of an additive, the reaction conditions were optimized to achieve a quantitative yield for the Markovnivkov-type addition product after 20 h of irradiation.
- Speck, Fabienne,Rombach, David,Wagenknecht, Hans-Achim
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- Selective reaction of benzyl alcohols with HI gas: Iodination, reduction, and indane ring formations
-
Reactions of benzyl alcohols with HI in solvent-free conditions were examined. Three types of reactions (iodination, reduction, and ring formation) occurred depending on the degree of crowding around the benzyl position and the benzylic stabilization of substrates. Results also showed that the ring formation to give indanes proceeded efficiently when HI was used, and that compounds with electron-rich aromatic rings gave indane derivatives in good yields.
- Matsumoto, Shoji,Naito, Masafumi,Oseki, Takehisa,Akazome, Motohiro,Otani, Yasuhiko
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p. 7254 - 7259
(2017/11/24)
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- Hydrochloric acid as an efficient catalyst for intermolecular condensation of alcohols. A simple and highly efficient synthesis of unsymmetrical ethers from benzylic alcohols and alkanols
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Benzylic alcohols and diarylmethanols with electron-donating substituents in the aromatic ring reacted with aliphatic alcohols in the presence of a catalytic amount of HCl to give the corresponding alkyl arylmethyl ethers. The reactivity of diarylmethanols in the intermolecular dehydration depended on the nature of substituents in the aromatic rings and structure of aliphatic alcohol.
- Mochalov,Fedotov,Trofimova,Zefirov
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p. 1217 - 1231
(2015/11/09)
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- Carbocationic polymerization of isoprene using cumyl initiators: Progress in understanding side reactions
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The cationic polymerization of isoprene using cumyl chloride/B(C6F5)3 and cumyl ether/TiCl4 systems was investigated in dichloromethane or in dichloromethane/methylcyclohexane mixtures varying the polymerization conditions. Polymerizations were performed in all cases in the presence of a large excess of a proton trap (2,6-di-tert-butyl pyridine, dtBP) compared to initiator in order to suppress any protic side reactions. As a consequence, no polymerization went to completion. Independently of the reaction conditions, trans-1,4-oligomers were exclusively obtained with mainly an olefinic terminal group. It was highlighted that an important loss of double bonds yielding saturated parts was observed, even in the absence of protons, assuming that a great amount of double bond loss generally observed in isoprene cationic polymerization could be due to intramolecular cyclization reactions. Nevertheless, under particular conditions (low temperature and/or low polarity medium), branching and cross-linking reactions were also found responsible for double bond loss.
- Ouardad, Samira,Wirotius, Anne-Laure,Kostjuk, Sergei,Ganachaud, Fran?ois,Peruch, Frédéric
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p. 59218 - 59225
(2016/01/09)
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- Metal-free activation of dioxygen by graphene/g-C3N4 nanocomposites: Functional dyads for selective oxidation of saturated hydrocarbons
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Graphene sheet/polymeric carbon nitride nanocomposite (GSCN) functions as a metal-free catalyst to activate O2 for the selective oxidation of secondary C-H bonds of cyclohexane. By fine-tuning the weight ratio of graphene and carbon nitride components, GSCN offers good conversion and high selectivity to corresponding ketones. Besides its high stability, this catalyst also exhibits high chemoselectivity for secondary C-H bonds of various saturated alkanes and, therefore, should be useful in overcoming challenges confronted by metal-mediated catalysis.
- Li, Xin-Hao,Chen, Jie-Sheng,Wang, Xinchen,Sun, Jianhua,Antonietti, Markus
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supporting information; scheme or table
p. 8074 - 8077
(2011/07/08)
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- Indium(i)-catalyzed alkyl-allyl coupling between ethers and an allylborane
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An efficient method for alkyl-allyl cross-coupling between ethers and a 9-BBN-derived allylborane catalyzed by indium(i) triflate has been developed. The allylborane proved to be essential to obtain the desired products in high yields. The reaction displayed good substrate scope including high functional group tolerance. The Royal Society of Chemistry 2011.
- Dao, Hai Thanh,Schneider, Uwe,Kobayashi, Shu
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supporting information; experimental part
p. 692 - 694
(2011/03/22)
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- Silica-supported KHSO4: An efficient system for activation of aromatic terminal olefins
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Potassium hydrogen sulfate adsorbed on chromatography-grade silica gel activates electron-rich aromatic terminal olefins towards nucleophilic attack at the benzylic position by alcohols. Temperature plays a crucial role and facilitates suppressing nucleophilic reaction in favor of dimerization of the terminal olefin. Georg Thieme Verlag Stuttgart - New York.
- Das, Ram Nath,Sarma, Kuladip,Pathak, Madan Gopal,Goswami, Amrit
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supporting information; experimental part
p. 2908 - 2912
(2011/02/25)
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- Cationic platinum-catalyzed etherification by intra- and intermolecular dehydration of alcohols
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Catalytic etherification of diols proceeds to give various cyclic ethers by use of cationic platinum salt, which is in situ prepared from PtCl2 and AgSbF6. Etherification of benzylic alcohols is also possible by intermolecular dehydration. Both of intra- and intermolecular etherifications smoothly proceed even under an atmosphere of air.
- Shibata, Takanori,Fujiwara, Ryo,Ueno, Yasunori
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p. 152 - 154
(2007/10/03)
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- Highly efficient reduction of unactivated aryl and alkyl iodides by a ground-state neutral organic electron donor
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Electron-transfer reductions of unactivated aryl and alkyl iodides with a neutral ground-state organic molecule are reported. The reducing agent 1 is formed in two steps from N-methylbenzimidazole using very simple chemistry, and subsequent treatment of the iodoalkane or-arene with 1 affords cyclized products (see scheme).
- Murphy, John A.,Khan, Tanweer A.,Zhou, Sheng-Ze,Thomson, Douglas W.,Mahesh, Mohan
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p. 1356 - 1360
(2007/10/03)
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- BiBr3, an efficient catalyst for the benzylation of alcohols: 2-Phenyl- 2-propyl, a new benzyl-type protecting group
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The benzylation of aliphatic alcohols with various benzylic alcohols has been achieved in the presence of BiBr3 under mild conditions. 2- Phenylpropan-2-ol proved to be the most efficient and can be considered as a novel protecting group. (C) 2000 Elsevier Science Ltd.
- Boyer, Bernard,Keramane, El-Mehdi,Roque, Jean-Pierre,Pavia, André A.
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p. 2891 - 2894
(2007/10/03)
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- A liquid-liquid process for production of 2,4-diphenyl-4-methyl-1-pentene by dimerization of α-methylstyrene
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A liquid-liquid process for the dimerization of α-methylstyrene (AMS) in order to produce selectively the commercially useful 2,4-diphenyl-4-methyl-l-pentene has been developed. The aqueous phase contained sulfuric acid as a catalyst and methanol as an additive, and the organic phase was 97% α-methylstyrene (AMS). An AMS conversion of 55% was achieved in 7 h when the dimerization was conducted at 80 °C. The product dimer contained 93% 2,4-diphenyl-4-methyl-1-pentene and only 0.06% 1,1,3-triniethyl-3-phenylindan, the saturated dimer. The liquid-liquid process can be used commercially to produce selectively 2,4-diphenyl-4-methyl-1-pentene at locations where cation-exchange resins are not readily available.
- Chaudhuri, Basab
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p. 220 - 223
(2013/09/08)
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- Fluorescence Quenching Kinetics of Anthracene Covalently Bound to Poly(Methacrylic Acid): Midchain Labeling
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Poly(methacrylic acid) labeled by anthracene in the middle of the chain (A-m-PMA) has been synthesized by anionic polymerization of tert-butyl methacrylate followed by introduction of the difunctional terminating agent 9,10-bis(bromomethyl)anthracene. Deprotection to yield the polyacid using trifluoroacetic acid in the presence of thioanisole results in complete removal of the tert-butyl moieties without destruction of the anthracene chromophore. Steady-state and time-resolved fluorescence quenching by Tl+ in aqueous solution at pH 11 at different ionic strengths has been measured. The data have been analyzed and compared to data previously obtained for 9-ethanolanthracene pendant-labeled poly(methacrylic acid) (9EA-PMA) using the following kinetic models: Stern - Volmer; hindered-access; combined Stern - Volmer - Perrin model developed by Morishima et al. (Macromolecules 1993, 26, 4293). These two polymers exhibit very similar quenching behavior, with the most significant difference being that static quenching is more important for A-m-PMA than 9EA-PMA. The Morishima model has been modified to include a preferential binding constant (Kb) for Tl+ in the vicinity of the chromophore. The estimated values of Kb are significant, on the order of 12-21, depending on the polymer and ionic strength.
- Clements, John H.,Webber
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p. 2513 - 2523
(2007/10/03)
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- Evidence for divalent iodine (9-I-2) radical intermediates in the thermolysis of (tert-butylperoxy)iodanes. An unusually efficient deiodination of o-iodocumyl alcohols by cyclohexyl radicals
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1-(tert-Butylperoxy)-3,3-dimethyl-1H-1,2-benziodoxoles (2a and 2b) and 1-(tert-butylperoxy)-3,3-bis(trifluoromethyl)-5-methyl-1H-1,2-benziodo xole (2c) were prepared from chloroiodanes 1a-c and tert-butylhydroperoxide in the presence of potassium tert-butoxide in tetrahydrofuran. Products, kinetic data for the decomposition of 2 in cyclohexane, benzene, toluene, and acetonitrile (E(a) = 31.0 ± 1.0 kcal/mol, log A = 17.0 ± 0.5; 35-70 °C), and the increased rate of decomposition of 2c in benzene-d6 in the presence of a magnetic field (7 T) indicate that homolytic cleavage of the I-O bond in 2 with the formation of iodanyl (9-I-2) and tert-butylperoxyl radicals is the primary decomposition step. The nearly quantitative formation of iodocyclohexane during the decomposition of 2c in cyclohexane is due to the unexpected reaction of cyclohexyl radicals with 2-(2-iodo-5-methylphenyl)-1,1,1,3,3,3-hexafluoro-2-propanol, a primary decomposition product of 2c. The results of a separate study of the deiodination of o-iodocumyl alcohols (3) by cyclohexyl radicals are consistent with an S(H)2 type mechanism.
- Dolenc, Darko,Plesni?ar, Bo?o
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p. 2628 - 2632
(2007/10/03)
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- The mechanism of the double bond cleavage in the titanium zeolite-catalvzed oxidation of α-methylstyrene by hydrogen peroxide: The β-hydroperoxy alcohol as intermediate
-
It is unequivocally shown that acetophenone, an oxidation product in the titanium zeolite-catalyzed oxidation of α-methylstyrene, derives from 2-hydroperoxy-2-phenyl-l-propanol as intermediate, which was detected and isolated in this reaction for the first time. VCH Verlagsgesellschaft mbH, 1996.
- Adam, Waldemar,Corma, Avelino,Martinez, Agustin,Renz, Michael
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p. 1453 - 1455
(2007/10/03)
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- Mesolytic scission of C-C bonds as a probe for photoinduced electron transfer reactions of Quinones
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Photoinduced electron transfer reactions of chlorinated benzoquinones are investigated using bibenzylic donors that undergo rapid fragmentation upon oxidation. The fragmentation rates and the quantum yields are used to probe the dynamics of back-electron transfer (BET) in two types of radical ion pairs. The triplet ion pairs formed by interception of excited state quinones give products with high quantum yields. The singlet ion pairs formed by irradiation of the charge-transfer (CT) complexes between the quinones and the donors undergo reactions with significantly lower efficiency. The advantage of the first method (triplet quenching) over the CT-irradiation depends on the energetics of BET. It is large for reactions with relatively small ΔGet for BET and it decreases for reactions with more negative ΔGbet. The indirectly obtained rates of BET are in excellent agreement with literature data for similar, but unreactive systems, and the rates of C-C bond scission in radical cations generated in these systems are consistent with the thermodynamics of these processes.
- Maslak, Przemyslaw,Chapman Jr., William H.
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p. 2647 - 2656
(2007/10/03)
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- NUCLEOPHILIC SUBSTITUTION REACTIONS OF CUMYL CHLORIDES IN METHANOL
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Nucleophilic substitution reactions of cumyl chlorides with anilines in methanol have been investigated.Large negative ρY+ values in methanol and weak chloride common ion rate depression support a mechanism in which a preformed benzylic-type carbocation intermediate reacts with the solvent and the added nucleophile, aniline.All positive ρX (ρnuc) values observed and the estimated non-interactive point, ?Y+ = 0.72, strongly suggest that deprotonation of a mobile proton from the nucleophile occurs during the reaction.Fair linear correlation is exhibited for the plot of stability of carbocation, as expressed by the enthalpy of cation formation (AM1), and ?Y+, indicating that the thermodynamic driving force is nearly compensated for by the intrinsic barrier.
- Lee, Ikchoon,Koh, Han Joong,Hong, Sung Nam,Lee, Bon-Su
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p. 347 - 352
(2007/10/02)
-
- Photochemistry of 3H-indazoles in protic media. Benzyl cations via protonation of 2-methylene-3,5-cyclohexadienylidenes
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Photolysis of 3,3-disubstituted 3H-indazoles in protic media (ROH) gives rise to benzyl ethers, in addition to hydrocarbons (derivatives of benzocyclopropene, styrene, and fluorene) which are also found in aprotic solvents. In the presence of ROD, the benzyl ethers are formed with incorporation of deuterium into the ortho position, pointing to protonation of 2-methylene-3,5-cyclohexadienylidenes. Laser flash photolysis of 3H-indazoles generates diazo compounds and benzyl cations as transient intermediates.
- Fehr,Fehr, Olaf C.,Grapenthin,Grapenthin, Olaf,Kilian,Kilian, Joerg,Kirmse,Kirmse, Wolfgang,Steenken,Steenken, Steen
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p. 5887 - 5890
(2007/10/02)
-
- Photoinduced Single Electron Transfer Reactions of Bobenzyl and some of its Derivatives
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The photochemical reaction of bibenzyl 1, its 4,4'-dimethoxy derivative 2 and bicumyl 3 with some aromatic nitriles (leading to alkylation of the nitrile) and tetranitromethane (TNM) (leading to ring trinitromethylation or nitration) involve SET fragmentation of the radical cation (either C-H or C-C bond) and, in the case of TNM, also of the radical anion.
- Bardi, Luca,Fasani, Elisa,Albini, Angelo
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p. 545 - 550
(2007/10/02)
-
- A MILD PROCEDURE FOR ETHERIFICATION OF ALCOHOLS WITH PRIMARY ALKYL HALIDES IN THE PRESENCE OF SILVER TRIFLATE
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Alcohols were alkylated in good to excellent yield with primary alkyl halides by a method employing silver triflate and a non-nucleophilic amine base.
- Burk, Robert M.,Gac, Todd S.,Roof, Michael B.
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p. 8111 - 8112
(2007/10/02)
-
- Experiments and calculations for determination of the stabilities of benzyl, benzhydryl, and fluorenyl carbocations: Antiaromaticity revisited
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The following pKR values for the formation of benzyl, benzhydryl, and fluorenyl carbocations in 50:50 (v:v) trifluoroethanol/water at I = 0.50 (NaClO4) were determined as pKR = -log (kHOH[H2O]/kH), where kH is the second-order rate constant for acid-catalyzed reaction of the alcohol to form the carbocation and kHOH is the second-order rate constant for capture of the carbocation by water: (R+, pKR); PhCH2+, ≤-20; PhCH(Me)+, -15.4; PhC(Me)2+, -12.3; Ph2CH+, -11.7; Ph2C(Me)+, -9.3; 9-fluorenyl carbocation (9-Fl+), -15.9; 9-methyl-9-fluorenyl carbocation (9-Me-9-Fl+), -11.1. The pKR for Ph2CH+ is in fair agreement with the value estimated using acidity functions,1a but the pKR for 9-Me-9-Fl+ is ca. 4 units more positive than that from the acidity function method,1a so that the difference in the acidity of benzhydryl and fluorenyl carbocations is smaller than estimated in earlier work. The 12 π-electron cyclic fluorenyl system in 9-Fl+ and 9-Me-9-Fl+ causes only 5.7 kcal/mol and 2.4 kcal/mol, respectively, destabilization of the corresponding acyclic carbocations Ph2CH+ and Ph2C(Me)+. The pKR values show that "antiaromatic" destabilization of the 9-fluorenyl carbocations must be small. Ab initio calculations of the structures and energies of 9-Fl+ and Ph2CH+ and of the corresponding alcohols at the 3-21G//3-21G and 6-31G*//3-21G levels indicate that Ph2CH+ is ca. 8-10 kcal/mol more stable than 9-F1+, which is in good agreement with the stability difference calculated from the pKR data. This indicates that electronic factors play the major role in determining the relative energies of these carbocations. Force field calculations were performed to estimate the contribution of van der Waals and ring strains to the difference in the pKR values for Ph2CH+ and 9-Fl+. Assuming hypothetical structures for Ph2CH+ and 9-Fl+ which are free of van der Waals and ring strains, it is then estimated that there is an 8-11 kcal/mol decrease in π-electron stabilization on moving from Ph2CH+ (C2v) and 9-F1OH to 9-Fl+ and Ph2CHOH. It is concluded that 9-fluorenyl carbocations are not antiaromatic. The difference in the energy of the 9-fluorenyl and benzhydryl carbanions relative to the alcohols was calculated to be -13.2 kcal/mol at the 6-31G*//3-21G level. This difference is attributed to the difference in the energies of the HOMOs for the two carbanions.
- Amyes, Tina L.,Richard, John P.,Novak, Michael
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p. 8032 - 8041
(2007/10/02)
-
- ANODIC FUNCTIONALIZATION OF OLEFINS IN ALCOHOLS IN THE PRESENCE OF HALIDE SALTS
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Electrolysis of conjugated, unbranched arylolefins in the presence of alkali metal halides in alcohols affords 1-aryl-2-bromoketals in 60-90percent yields.Under these conditions, 2-methyl-1-phenylprop-1-ene is converted into 1-bromo-2-methyl-1-phenylprop-1-ene in 80percent yield, and arylolefins with no benzylidene hydrogens give 1-aryl-1-alkoxy-2-bromoalkanes.
- Elinson, M. N.,Makhova, I. V.,Nikishin, G. I.
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p. 112 - 118
(2007/10/02)
-
- Facile Carbon-Carbon Bond Heterolysis. SN1-E1 Reactions of t-Cumyl and t-Butyl Substituted Tricyanomethanes
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The tricyanomethanide ion C(CN)3- was found to serve as an efficient leaving group anion to generate unstable carbocations such as t-cumyl and t-butyl cations, the ability of carbon-terminated leaving groups so far examined being in the order C(CN)2N=NC6H4NO2-p C(CN)3 C(CN)2NO2.
- Hirota, Hiroshi,Mitsuhashi, Tsutomu
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p. 803 - 806
(2007/10/02)
-
- Photochemically Generated Ion and Radical Pairs. Self-Destructive Charge-Transfer Complexes
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Irradiation of CT complexes between 4-methoxy-4'-X-bicumenes, 1a-e (X = H, a; OMe, b; Me, c; CF3, d; CN,) and tetranitromethane (2) results in efficient generation of radical and ion pairs (Φ ca. 0.20).The identity of the final products depends on the dynamics of these pairs, which in turn is dramatically influenced by substitution and solvent.In all cases, 2*- dissociates rapidly ( - (4-) and NO2*.In CH2Cl2, 1a*- reacts with 4- (aromatic trinitromethylation) and undergoes C-C bond fragmentation before diffusional separation of fragments can occur.Thus, a tetrad of reactive intermediates (cumyl radical, NO2*, p-methoxycumyl cation and 4-) are produced in a single solvent cage.The cumyl radical is oxidized by 2, and both cumyl cations undergo α-trinitromethylation.The cumyl cations undergo trinitromethylation in the original solvent cage which is kinetically equivalent to a contact ion pair (CIP).In CH3CN the fragmentation is accompanied by aromatic nitration (radical collapse).In this solvent, the p-methoxycumyl cations produced by cleavage reaction undergo trinitromethylation at the CIP stage, but cumyl cations produced by thermal oxidation of the cumyl radicals are trapped by 4- at the solvent-separated ion pair stage.In CH2Cl2, 1b-c*+ undergo exclusively fragmentation, completely within the solvent cage.The cleavage of 1d-e*+ is much slower, and the radical cations undergo instead aromatic trinitromethylation.The observed substituent effect on the rate of cleavage ( ρ+ = -2.2) indicates significant charge transfer across the scissile bond in the transition state for this process.The products of these reactions are predominantly derived from ion annihilation.The radical coupling processes are limited to radical cation/radical collapse that lead to nitrated products.
- Maslak, Przemyslaw,Chapman, William H.
-
p. 6334 - 6347
(2007/10/02)
-
- Radical ions in photochemistry. Carbon-carbon bond cleavage of radical cations in solution: Theory and application
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The cleavage of radical cations of two series of alkanes, 1,1,2-triaryl- and 1,1,2,2-tetraarylalkanes, generated by photoinduced single electron transfer in acetonitrile-methanol, occurs with formation of radical and carbocation fragments. The radical cations of some unsymmetrically substituted alkanes cleave to give all four of the possible products, two hydrocarbons emanating from the radicals and two methyl ethers from the carbocations, in proportion to the oxidation potentials of the two possible radical fragments. There is an excellent linear correlation between the logarithm of the observed ratio of products and that calculated from the reported electrochemically determined oxidation potentials (r = 0.998, 5 points). The proportionality constant (1.27) for this relationship is close to unity which indicates that the product ratio is determined by the relative rates of cleavage in the two possible modes or by equilibration of the radicals and carbocations before separation of the geminate radical carbocation pair and not by equilibration upon reencounter of freely solvated radical and carbocation fragments. The effect of temperature on the relative oxidation potentials of the radicals studied is small and can be neglected when radicals of the same order (i.e., both secondary or both tertiary) and of similar size are compared. The ratio of products obtained upon cleavage of the radical cation at 25 °C can be used to determined standard oxidation potentials of radicals. The oxidation potential of the diphenylmethyl radical (0.350 V vs SCE) has been accepted as the primary standard and the (4-methyl-phenyl)phenylmethyl (0.265 V) and bis(4-methylphenyl)methyl (0.188 V) radicals are established as secondary standards against which the oxidation potentials of other radicals can be measured. Oxidation potentials of several 4-substituted cumyl radicals have been determined by this photochemical method. There is a good (r = 0.987, 5 points) linear correlation between the measured oxidation potentials and the σ+ substituent constants. The reaction constant is appropriately negative and large (p = -6).
- Popielarz,Arnold
-
p. 3068 - 3082
(2007/10/02)
-
- Gas-Phase Acid-Induced Nucleophilic Displacement Reactions. 7. Structural and Stereochemical Evidence for the Existence and the Relative Stability of Alkylenebenzenium Ions in the Gas Phase
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A comprehensive investigation on the existence and relative stability of gaseous 2,3-butylene- and 1,2-propylenebenzenium ions was carried out by establishing the structural features and the stereochemistry of acid-induced displacement by CH3OH on isomeric 3-phenylbutyl-2 onium and β-phenylpropyl onium intermediates.The latter were obtained in the gas phase from the reaction of radiolytically formed GA+ (GA+ = D3+, CnH5+ (n=1,2), i-C3H7+, and CH3FCH3+) acids with isomeric 3-phenyl-2-chlorobutanes and β-phenyl-Y-propanes (Y = Cl, OH).The analysis of the isomeric distribution of the neutral substitution products allows the establishment of extensive phenyl-group participation in the displacement process, occuring in competition with methyl and hydrogen 1,2-transfers.The participating ability of a phenyl moiety adjacent to the substitution center is found to depend essentially upon the configuration of the precursor and to be related to its gas-phase nucleophilicity.The occurence of relatively stable cyclic alkylenebenzenium ions as static intermediates in these displacement reactions is suggested by the particular isomeric and stereoisomeric distribution of the products and by its comparison with that obtained from open-chain isomeric ions.The results obtained from the present gas-phase experiments are discussed in the light of those from related gas-phase and solution studies.
- Fornarini, Simonetta,Sparapani, Cinzia,Speranza, Maurizio
-
-
- Allowing for the Non-ideality of the Liquid Phase in the Calculation of the Thermodynamic Parameters of a Reaction on the Basis of Chemical Equilibrium
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The equilibrium of the reaction of methanol with α-methylstyrene has been studied at 323-383 K on the KU-23 cation exchanger for various molar ratios of the reagents.The activity coefficients needed for the non-ideality corrections in this system were calculated by the UNIFAC method.It is shown that at molar ratios of the initial reagents greater than 2.0 the concentration constants can be used to calculate ΔrHoT and ΔrSoT for the synthesis of methyl cumyl ether.
- Rozhnov, A. M.,Safronov, V. V.,Alenin, V. I.,Sharonov, K. G.
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p. 1511 - 1513
(2007/10/02)
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- PHOTOSENSITIZED (ELECTRON TRANSFER) CARBON-CARBON BOND CLEAVAGE OF RADICAL CATIONS; THE DIPHENYLMETHYL SYSTEM
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The photosensitized (electron transfer) reaction of methyl 2,2-diphenylethyl ether (1), 1,1,2,2-tetraphenylethane (5), 2-methyl-1,1,2-triphenylpropane (6), and 2-methoxy-2-diphenylmethylnorbornane (11 endo and exo) with 1,4-dicyanobenzene (4) in acetonitrile-methanol leads to products indicating cleavage of an intermediate radical cation to give the diphenylmethyl radical and a carbocation.The diphenylmethyl radical is then reduced by the radical anion of the photosensitizer and protonated to yield diphenylmethane.The carbocation fragment reacts with methanol to yield ether and/or acetals.The effect of temperature on the efficiency of cleavage of 5 and 6 has been analyzed.The increase in efficiency observed at higher temperatures reflects an activation energy for the cleavage of the radical cations.In cases where no cleavage is observed, the activation energy for cleavage may be so high that back electron transfer from the radical anion of the photosensitizer is the dominant reaction.The C-C bond dissociation energies of the radical cations of 5 and 6 were estimated by analysis of the thermochemical cycle using the bond dissociation energies and the oxidation potentials of the neutral molecules and the oxidation potential of the diphenylmethyl and cumyl radicals.The direction of cleavage of the radical cation is explained in terms of the relative oxidation potentials of the two possible radicals.
- Okamoto, Akio,Snow, Miles S.,Arnold, Donald R.
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p. 6175 - 6187
(2007/10/02)
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- Facile Heterolysis of a Carbon-Carbon Bond. Arylazodicyanomethanides as the Leaving Group Capable of Generating tert-Cumyl Cation and the Hydrogen-Bond-Insusceptible Behavior of the Leaving Group Anions
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Decompositions of (p-nitrophenyl)azo-tert-cumylmalonitrile in polar solvents were found to proceed via the heterolysis of a carbon-carbon bond to generate tert-cumyl cation and the conjugate base of (p-nitrophenyl)hydrazonomalononitrile.The major products arising from tert-cumyl cation are as follows: tert-cumyl methyl ether in methanol, the N-cumylpyridinium hydrazonide in pyridine, and α-methylstyrene in Me2SO and in DMF.The reactions in MeCN and in acetone afford a rearranged product, N-(tert-cumyl)(p-nitrophenyl)hydrazonomalononitrile, which gradually undergoes heterolysis as well.Decompositions of the azo compound and the rearragend product are faster in Me2SO than in methanol, being in conflict with the usual trend of solvent-ionizing power.This phenomenon is explained in terms of extensive charge dispersal of the leaving group anion wich prevents hydrogen bonding with a protic solvent molecule.It is pointed out that the behavior of the conjugate base of (p-nitrophenyl)hydrazonomalononitrile as the leaving group anion is closely connected with the function of FCCP (the p-CF3O derivative of the hydrazone) as one of the best uncouplers of phosphorylation in mitochondrial systems.
- Mitsuhashi Tsutomu
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p. 2394 - 2400
(2007/10/02)
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- Photochemical Electron-transfer Reaction between Aromatic Olefins and Metal Ions. Dependence of the Reaction Course on the Structure of the Olefins
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To investigate the photochemical electron transfer from aromatic olefins to copper(II) or iron(III) ions, a series of substituted styrenes (1) was irradiated in the presence of copper(II) or iron(II) salts in methanol, which gave dimethoxylated monomers (2) and one or more of three types of dimethoxylated dimers .The formation of these products are reasonably attributed to the participation of hte cation radicals of the olefins generated by electron transfer from excited olefins to the methal ions.Substituents on the substrates are found to govern the reaction products.
- Kojima, Masanobu,Sakuragi, Hirochika,Tokumaru, Katsumi
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p. 521 - 524
(2007/10/02)
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- Nucleofugality of the Sulfinate Group in Carbocation-Forming Processes
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The solvolytic reactivity of a variety of sulfones and sulfinate esters has been determined which allows one to place the sulfinate leaving group in a relative nucleofugality scale.Cumyl trifluoromethyl sulfone (1) reacts in a variety of solvents to give substitution products.The mOTs value of 0.82 is indicative of the involvement of the cumyl cation (9) formed in kc process.In terms of rate, 1 is 170 times less reactive than cumyl chloride but 286 times more reactive than cumyl p-nitrobenzoate.The analogous cumyl methyl sulfone (2) and cumyl phenyl sulfone (3) solvolyze approximately 107 times more slowly than 1.The sulfinate esters cumyl methanesulfinate (6) and cumyl p-toluenesulfinate (7) are considerably more reactive than the analogous sulfones.Methanolysis of 6 was also subject to acid catalysis, where a mechanism analogous to the AAL1 mechanism of hydrolysis of esters of carboxylic acids was suggested.The less hindered α-phenethyl trifluoromethyl sulfone (4) and p-methoxybenzyl trifluoromethyl sulfone (5) solvolyzed at rates that approached those of the analogous p-nitrobenzoates.This was indicative of the importance of relief of steric congestion in solvolyses of the more hindered tertiary sulfone 1.
- Creary, Xavier
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p. 5080 - 5084
(2007/10/02)
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- Radical ions in photochemistry. 16. The photosensitized (electron transfer) carbon-carbon bond-cleavage reaction of radical cations
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The photosensitized (electron transfer) carbon-carbon bond cleavage of 1,1,2,2-tetraphenylethane (1b), 2-(4-methoxyphenyl)-1,1-diphenyl-2-methylpropane (1c), 1,1,2-triphenyl-2-methylpropane (1d), and 2-(4-trifluoromethylphenyl)-1,1-diphenyl-2-methylpropane (1e) has been studied with 1,4-dicyanobenzene (2) serving as the electron-accepting sensitizer.The oxidation potential of 1b-e have been measured by cyclic voltammetry.Estimation of the free-energy associated with the electron transfer between these donors (1b-e) and the first excited singlet state of 2, using the Weller equation, indicates the process is favorable in every case.There is, in every case, a one-to-one correspondence of products derived from reaction of the carbocation fragment and from the carbanion derived from the initially formed radical fragment.The efficiency of the reaction of 1b, which gives a good yield of diphenylmethane (3) and methyl diphenylmethyl ether, has been studied as a function of temperature.The observed activation energy (7.2 kcal mol-1) is equated to the bond dissociation energy of the central bond of the radical cation (1b).The ratio of products from the unsymmetric compounds is dependent upon the redox potentials of the fragment radicals and carbocations.
- Okamoto, Akio,Arnold, Donald R.
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p. 2340 - 2342
(2007/10/02)
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- THE METHOXYMERCURATION OF SOME STYRENE DERIVATIVES
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Product analyses have been carried out and reaction rates and activation parameters determined for the methoxymercuration of some styrene derivatives and related compounds in methanol.The behaviour of α- and β-alkyl substituted styrenes generally parallels that of the similarly substituted vinylferrocenes.The anomalous reactivity of β,β-dimethylstyrene is suggested to arise from the structural features of this compound.The reaction of the stilbenes was found to be stereospecific.The results are consistent with an unsymmetrically bridged structure for the cationic intermediate possibly involving varying degrees of bridging depending on substrate.
- Bassetti, Mauro,Floris, Barbara,Illuminati, Gabriello
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p. 351 - 362
(2007/10/02)
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- Sodium borohydride reduction of alkoxythallated compounds of olefins
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Alkaline NaBH4 reduction of alkoxythallated compounds of olefins gives a mixture of the parent olefins, alkyl ethers, and dialkylthallium compounds; hydrogen for replacement of thallium arises from the solvent and not from NaBH4.
- Uemura, Sakae,Tabata, Akira,Okano, Masaya
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p. 517 - 518
(2007/10/12)
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