- Container Chemistry: Manipulating excited state behavior of organic guests within cavitands that form capsules in water
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Two new cavitands substituted with acid and alcohol groups (tetra-acid tetra-alcohol [TATA] and inverted TATA [iTATA]) bearing the same molecular skeleton as octa acid (OA) have been synthesized and their use as photochemical reaction containers explored. Isothermal calorimetric titration experiments suggest that the inclusion of organic molecules within these cavitands is driven both by favorable ΔH and ΔS and the substituents at the portals have little role to play. Comparison of the 2 new cavitands with the previous results on OA reveals that the presence of benzoate anion at the top periphery is essential for the cavitand to be a triplet sensitizer. Polarity within the water-soluble capsules, resulting from TATA and iTATA, was found to be close to that of ethylacetate and hydrocarbons, similar to that of OA. Photophysical studies with anthracene and camphorthione as guests disclose that the capsules made of 2 molecules of cavitands do not disassemble in the time scale of the excited states of the above guests (S1 in the case of anthracene and T1 in the case of camphorthione). Capsules ability to confine guests and the resulting photochemical intermediates has been tested by examining the photochemistry of 1-phenyl-3-para-tolyl-2-propanone. The radicals resulting from the Norrish type 1 cleavage of 1-phenyl-3-para-tolyl-2-propanone did not escape the cage and gave products, resulting from 100% cage effect. Availability of TATA and iTATA along with already reported similar cavitands expands the list of water-soluble capsule forming cavitands that could be used as molecular containers.
- Jagadesan, Pradeepkumar,Samanta, Shampa R.,Choudhury, Rajib,Ramamurthy, Vaidhyanathan
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Read Online
- Cation-π interactions in the gas phase methylation of α,ωdiphenylalkanes
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The methylation of α,ω-diphenylalkanes (C6H5(CH2)nC6H5 , n = 1-6) has been performed in the gas phase using Me2Cl+ ions as alkylating species and toluene as reference substrate. Both in radiolytic experiments at atmospheric pressure and in FT-ICR measurements at 10-8 Torr, the selected diphenylalkanes reacted faster than toluene, the highest reactivity displayed by 1,3-diphenylpropane. The kinetic pattern of the reaction, conforming to the established scheme of an electrophilic alkylation reaction, is consistent with a rate-determining formation of the σ-complex intermediate, at variance with the tert-butylation of the same series of compounds by Me3C+ ions, occurring at the collisional encounter rate. The kinetic features are explained by a marked effect due to the presence of the second aryl ring, providing additional stabilization of both the ion-neutral collision complex and the σ complex with respect to toluene. Both factors contribute to the δEa of ca. 8 kcal mol-1 for the competition of 1,3-diphenylpropane and toluene found in the temperature dependence study of the Me2Cl+ reaction.
- Chiavarino, Barbara,Crestoni, Maria E.,Fornarini, Simonetta,Kuck, Dietmar
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Read Online
- Borane evolution and its application to organic synthesis using the phase-vanishing method
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Although borane is a useful reagent, it is difficult to handle. In this study, borane was generated in situ from NaBH4 or nBu4NBH4 with several oxidants using a phase-vanishing (PV) method. The borane generated was directly reacted with alkenes, affording the desired alcohols in good yields after oxidation with H2O2 under basic conditions. The selective reduction of carboxylic acids with the evolved borane was examined. The organoboranes generated by the PV method successfully underwent Suzuki–Miyaura coupling. Using this PV system, reactions with borane can be carried out easily and safely in a common test tube.
- Soga, Nene,Yoshiki, Tomo,Sato, Aoi,Kawamoto, Takuji,Ryu, Ilhyong,Matsubara, Hiroshi
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- HIGHLY SELECTIVE ELECTROCHEMICAL HYDROGENATION OF ALKYNES
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Disclosed are electrochemical methods to prepare an alkane or an alkene, such as a cis- alkene, from an alkyne, or an alkane from an alkene. The method utilizes an electrochemical cell having a cathode and an anode and a reactor.
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Page/Page column 7
(2020/10/20)
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- "bulky-Yet-Flexible" α-Diimine Palladium-Catalyzed Reductive Heck Cross-Coupling: Highly Anti-Markovnikov-Selective Hydroarylation of Alkene in Air
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To pursue a highly regioselective and efficient reductive Heck reaction, a series of moisture-and air-stable α-diimine palladium precatalysts were rationally designed, readily synthesized, and fully characterized. The relationship between the structures of the palladium complexes and the catalytic properties was investigated. It was revealed that the"bulky-yet-flexible"palladium complexes allowed highly anti-Markovnikov-selective hydroarylation of alkenes with (hetero)aryl bromides under aerobic conditions. Further synthetic application of the present protocol could provide rapid and straightforward access to functional and biologically active molecules.
- Yang, Xu-Wen,Li, Dong-Hui,Song, A-Xiang,Liu, Feng-Shou
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p. 11750 - 11765
(2020/10/23)
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- Exhaustive Reduction of Esters Enabled by Nickel Catalysis
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We report a one-step procedure to directly reduce unactivated aryl esters into their corresponding tolyl derivatives. This is achieved by an organosilane-mediated ester hydrosilylation reaction and subsequent Ni/NHC-catalyzed hydrogenolysis. The resulting conditions provide a direct and efficient alternative to multi-step procedures for this transformation that often require the use of hazardous metal hydrides. Applications in the synthesis of -CD3-containing products, derivatization of bioactive molecules, and chemoselective reduction in the presence of other C-O bonds are demonstrated.
- Cook, Adam,Prakash, Sekar,Zheng, Yan-Long,Newman, Stephen G.
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supporting information
p. 8109 - 8115
(2020/05/20)
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- Aqueous phase semihydrogenation of alkynes over Ni-Fe bimetallic catalysts
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Bimetallic Ni-Fe catalysts (Ni/Fe, 1?:?1, 1?:?3, and 3?:?1) are synthesized and explored for their catalytic activity in semihydrogenation of internal alkynes using H2 gas in water-ethanol solution. Our findings revealed that over the Ni1Fe3 catalyst a high diastereoselectivity for Z-alkenes with a high conversion for a wide range of internal alkynes can be achieved at moderate reaction temperature (40 °C). Notably, the selectivity for the Z-alkenes is enhanced in the presence of n-butyl amine as an additive. Deuterium labeling experiments evidenced that H2 gas becomes dissociated homolytically over the catalyst surface to hydrogenate alkynes to alkenes. Synthesized catalysts were successfully characterized by HR-TEM, SEM, XPS, EDS, P-XRD and H2-TPD.
- Awasthi, Mahendra K.,Barman, Sudipta Roy,Behrens, Silke,Rai, Rohit K.,Singh, Sanjay K.,Singh, Vipin K.
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p. 4968 - 4980
(2020/08/19)
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- Nickel-catalyzed anti-Markovnikov hydroarylation of alkenes
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We have developed a nickel-catalyzed hydroarylation of alkenes using aryl halides as coupling partners. Excellent anti-Markovnikov selectivity is achieved with aryl-substituted alkenes and enol ethers. We also show that hydroarylation occurs with alkyl substituted alkenes to yield linear products. Preliminary examination of the reaction mechanism suggests irreversible hydrometallation as the selectivity determining step of the hydroarylation.
- Nguyen, Julia,Chong, Andrea,Lalic, Gojko
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p. 3231 - 3236
(2019/03/21)
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- Pd-Catalyzed reductive heck reaction of olefins with aryl bromides for Csp2-Csp3 bond formation
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We developed a Pd-catalyzed intermolecular reductive Heck reaction to construct Csp2-Csp3 bonds between aryl bromides and olefins. Various styrene derivatives, acyclic and cyclic alkenes, were well tolerated to couple with varied aryl bromides in linear selectivity. Kinetic and deuterium labeling experiments suggested that i-PrOH provides a hydride through β-H elimination.
- Jin, Liqun,Qian, Jiaxia,Sun, Nan,Hu, Baoxiang,Shen, Zhenlu,Hu, Xinquan
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supporting information
p. 5752 - 5755
(2018/06/07)
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- Development of a Palladium-Catalyzed Process for the Synthesis of Z-Alkenes by Sequential Sonogashira–Hydrogenation Reaction
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A novel and selective sequential one-pot protocol for the synthesis of Z-alkenes via Sonogashira–semihydrogenation is reported. The efficiency of the methodology is increased by utilizing PdCl2/BuPAd2 as homogeneous catalyst for the Sonogashira coupling and subsequently transforming the transition metal complex into a heterogeneous Pd hydrogenation catalyst. This methodology represents one of the rare examples directly combining homogeneous and heterogeneous catalysis.
- Hancker, S?ren,Neumann, Helfried,Beller, Matthias
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p. 5253 - 5259
(2018/09/14)
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- Reactivity of the diphosphinodithio ligated nickel(0) complex toward alkyl halides and resultant nickel(i) and nickel(ii)-alkyl complexes
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Diphosphinodithio ligated complexes of nickel(0), nickel(i) and nickel(ii)-alkyl with a reactivity relevant to the C-C bond formation were described. Stoichiometric reactions of the nickel(0) complex, [(P2S2)Ni] ([1]0, P2S2 = (Ph2PC6H4CH2S)2(C2H4)), with alkyl halides (RX) such as C6H5CH2Br, C2H3CH2Br, C2H5I and (CH3)2CHI were investigated, from which the products were found to be highly dependent on the nature of RX used. Oxidative addition of C2H3CH2Br to [1]0 provides the stable Ni(ii)-alkyl complexes [1-allyl]+. The reaction of [1]0 with C6H5CH2Br proceeds through a radical pathway resulting in the formation of the nickel(i) complex [1]+ and an organic homo-coupled product 1,2-diphenylethane. Oxidative addition of C2H5I or (CH3)2CHI to [1]0 can be achieved but it competes with the halogen atom abstraction reaction as found for C6H5CH2Br. [1]0 was shown to be an active catalyst for the coupling reactions of primary halides and alkyl Grignard reagents.
- Zhang, Ailing,Wang, Congxiao,Lai, Xiaoyu,Zhai, Xiaofang,Pang, Maofu,Tung, Chen-Ho,Wang, Wenguang
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supporting information
p. 15757 - 15764
(2018/11/23)
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- Harnessing Alkyl Amines as Electrophiles for Nickel-Catalyzed Cross Couplings via C-N Bond Activation
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We developed a strategy to harness alkyl amines as alkylating agents via C-N bond activation. This Suzuki-Miyaura cross coupling of alkylpyridinium salts, readily formed from primary amines, is the first example of a metal-catalyzed cross coupling via C-N bond activation of an amine with an unactivated alkyl group. This reaction enjoys broad scope and functional group tolerance. Primary and secondary alkyl groups can be installed. Preliminary studies suggest a NiI/NiIII catalytic cycle.
- Basch, Corey H.,Liao, Jennie,Xu, Jianyu,Piane, Jacob J.,Watson, Mary P.
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p. 5313 - 5316
(2017/04/27)
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- High Loading of Pd Nanoparticles by Interior Functionalization of MOFs for Heterogeneous Catalysis
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In this report, the issue related to nanoparticle (NP) agglomeration upon increasing their loading amount into metal-organic frameworks (MOFs) has been addressed by functionalization of MOFs with alkyne groups. The alkynophilicity of the Pd2+ (or other noble metals) ions has been utilized successfully for significant loading of Pd NPs into alkyne functionalized MOFs. It has been shown here that the size and loading amount of Pd NPs are highly dependent on the surface area and pore width of the MOFs. The loading amount of Pd NPs was increased monotonically without altering their size distribution on a particular MOF. Importantly, the distinct role of alkyne groups for Pd2+ stabilization has also been demonstrated by performing a control experiment considering a MOF without an alkyne moiety. The preparation of NPs involved two distinct steps viz. adsorption of metal ions inside MOFs and reduction of metal ions. Both of these steps were monitored by microscopic techniques. This report also demonstrates the applicability of Pd@MOF NPs as extremely efficient heterogeneous catalysts for Heck-coupling and hydrogenation reactions of aryl bromides or iodides and alkenes, respectively.
- Gole, Bappaditya,Sanyal, Udishnu,Banerjee, Rahul,Mukherjee, Partha Sarathi
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p. 2345 - 2354
(2016/03/19)
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- Reduction of diphenylacetylene using Al powder in the presence of noble metal catalysts in water
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Diphenylacetylenes can be reduced to the corresponding diphenylethanes (2) in water in excellent yield using Al powder and Pd/C at 60?°C for 3?h in a sealed tube. In addition, the complete reduction of both aromatic rings required 80?°C for 15?h with Al powder in the presence of Pt/C. However, the nature of hydrogenated product formed was found to be strongly influenced by the reaction temperature, time, volume of water and the amount of catalyst being employed.
- Rayhan, Ummey,Kowser, Zannatul,Redshaw, Carl,Yamato, Takehiko
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supporting information
p. 6943 - 6947
(2016/10/14)
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- The copper-catalysed Suzuki-Miyaura coupling of alkylboron reagents: disproportionation of anionic (alkyl)(alkoxy)borates to anionic dialkylborates prior to transmetalation
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We report the first example of CuI-catalysed coupling of alkylboron reagents with aryl and heteroaryl iodides that affords products in good to excellent yields. Preliminary mechanistic studies with alkylborates indicate that the anionic (alkoxy)(alkyl)borates, generated from alkyllithium and alkoxyboron reagents, undergo disproportionation to anionic dialkylborates and that both anionic alkylborates are active for transmetalation to a CuI-catalyst. Results from a radical clock experiment and the Hammett plot imply that the reaction likely proceeds via a non-radical pathway.
- Basnet, Prakash,Thapa, Surendra,Dickie, Diane A.,Giri, Ramesh
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supporting information
p. 11072 - 11075
(2016/09/19)
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- Plasma synthesis of oxidized graphene foam supporting Pd nanoparticles as a new catalyst for one-pot synthesis of dibenzyls
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We have developed an environmentally-friendly method for the synthesis of Pd nanoparticle (Pd NPs) decorated different graphene supports, and the morphology and structure of the hybrids are characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and elemental mappings. Four hybrid materials based on graphene foam (GF), oxidized graphene foam (OGF), graphene oxide (GO) and reduced graphene oxide (RGO) have been used to catalyze Heck coupling reactions, and the effect of support on the activity of the hybrid material has been studied. Our results have revealed that Pd NP decorated OGF (Pd/OGF) is the most active catalyst, showing better performance than the commercial Pd/C catalyst. More importantly, the Pd/OGF catalyst has been successfully used for one-pot synthesis of dibenzyls with different aryl bromides and olefins, which has simplified the separation and purification process and realized a green organic synthesis process. This journal is
- Ren, Liang,Yang, Fan,Wang, Chunxia,Li, Yongfeng,Liu, Hailing,Tu, Zhiqiang,Zhang, Liqiang,Liu, Zhichang,Gao, Jinsen,Xu, Chunming
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p. 63048 - 63054
(2015/02/19)
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- Cross-coupling of alkyl halides with aryl or alkyl Grignards catalyzed by dinuclear Ni(ii) complexes containing functionalized tripodal amine-pyrazolyl ligands
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Structurally distinctive dinuclear Ni(ii) complexes with furan or thiophene tethered amine-pyrazolyl tripodal hybrid ligands have been synthesized and crystallographically characterized. All complexes are catalytically active towards cross-coupling of aryl/alkyl Grignard reagents with β-H containing alkyl halides at room temperature in the presence of N,N,N',N'- tetramethylethylenediamine (TMEDA). The catalytic efficacy of the complexes is dependent on the tether substituent at the central amine. Two species, Ni(ii) TMEDA and Mg(ii) TMEDA complexes, have been isolated from the catalytic reaction mixtures under different conditions. Some ligand-stabilized Ni(ii) and Mg(ii) bimetallic species have also been identified in the ESI-MS spectra.
- Xue, Fei,Zhao, Jin,Hor, T. S. Andy
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p. 5150 - 5158
(2013/04/10)
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- Iron-catalysed alkene hydrogenation and reductive cross-coupling using a bench-stable iron(II) pre-catalyst
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Operationally simple, iron-catalysed hydrogenation and reductive cross-coupling protocols have been developed using a bench-stable iron(ii) pre-catalyst. The hydrogenation of 18 alkenes (50-99%) and reductive cross-coupling of vinyl halides with aryl- and alkyl Grignard reagents (8 examples, 18-99%) is reported using 3 mol% pre-catalyst and hydrogen as stoichiometric reductant (1-50 bar).
- Frank, Dominik J.,Guiet, Lea,Kaeslin, Alexander,Murphy, Elliot,Thomas, Stephen P.
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p. 25698 - 25701
(2013/12/04)
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- In-cage and out-of-cage combinations of benzylic radical pairs in the glassy and melted states of poly(alkyl methacrylate)s
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Norrish type 1 reactions of 1-(4-methylphenyl)-3-phenyl-2-propanone (ACOB1) have been used to probe structural and morphological properties of a series of poly(alkyl methacrylate)s (PAMAs, where the alkyl is ethyl, butyl, isobutyl, cyclohexyl, and hexadecyl) below and above their glass transition (or melting) temperatures. The PAMAs investigated cover a wide range of glass transition temperatures and structure types. The ratio of in-cage to the sum of in-cage and out-of-cage recombinations of the triplet benzylic radical-pairs generated upon irradiation of ACOB1 (Fc) have been calculated from relative photoproduct yields at different temperatures and are compared with the free hole volumes within the polymers as calculated from positronium annihilation lifetime spectroscopy. Laser flash photolysis experiments to follow the growth and decay of the radicals have also been conducted in order to correlate the steady-state irradiation results with the radical pair recombination processes (i.e., in-cage and out-of-cage). The changes in Fc as a function of PAMA type and phase (temperature) can be correlated with chain relaxation rates and the nature of the polymer side chains, but not hole free volumes. These results are compared with those from our previous work, conducted in polyethylenes with differing degrees of crystallinity, where hole free volume was the primary factor in controlling Fc.
- Abraham, Shibu,Ghosh, Indrajit,Nau, Werner M.,Chesta, Carlos,Pas, Steven J.,Hill, Anita J.,Weiss, Richard G.
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p. 914 - 924
(2015/03/03)
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- Interfacial regions governing internal cavities of dendrimers. Studies of poly(alkyl aryl ether) dendrimers constituted with linkers of varying alkyl chain length
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This report deals with a study of the properties of internal cavities of dendritic macromolecules that are capable of encapsulating and mediating photoreactions of guest molecules. The internal cavity structures of dendrimers are determined by the interfacial regions between the aqueous exterior and hydrocarbon like interior constituted by the linkers that connect symmetrically sited branch points constituting the dendrimer and head groups that cap the dendrimers. Phloroglucinol-based poly(alkyl aryl ether) dendrimers constituted with a homologous series of alkyl linkers were undertaken for the current study. Twelve dendrimers within first, second, and third generations, having ethyl, n-propyl, n-butyl, and n-pentyl groups as the linkers and hydroxyl groups at peripheries in each generation, were synthesized. Encapsulation of pyrene and coumarins by aqueous basic solutions of dendrimers were monitored by UV-vis and fluorescence spectroscopies, which showed that a lower generation dendrimer with an optimal alkyl linker presented better encapsulation abilities than a higher generation dendrimer. Norrish type I photoreaction of dibenzyl ketone was carried out within the above series of dendrimers to probe their abilities to hold guests and reactive intermediate radical pairs within themselves. The extent of cage effect from the series of third generation dendrimers was observed to be higher with dendrimers having an n-pentyl group as the linker.
- Natarajan, Baskar,Gupta, Shipra,Ramamurthy, Vaidhyanathan,Jayaraman, Narayanaswamy
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experimental part
p. 4018 - 4026
(2011/06/27)
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- Unsurpassed cage effect for the photolysis of dibenzyl ketones in water-soluble dendrimers
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Amphiphilic water-soluble poly(alkyl aryl ether) dendrimers Gn (n = 1-3) with charge-neutral tetraethylene glycol monomethyl ethers at their periphery were synthesized as microreactors to control the photochemical reactions of dibenzyl ketone derivatives in aqueous solutions. Photophysical studies demonstrated that Gn can encapsulate organic molecules and provide a hydrophobic microenvironment. The product distribution of photolysis of dibenzyl ketone derivatives can be successfully controlled by encapsulating the substrates within dendrimers, and an unsurpassed cage effect of 1.00 is reached in high generation dendrimers, revealing that a thick and compact "shell" was formed at the periphery of the dendrimers. The cage effect is also significantly influenced by the substituent at the para-position of the guest molecules. The higher generation dendrimers exhibit a better confined microenvironment and the aggregates possess more compact cavities to "lock" the guests than the corresponding unimolecular dendrimers. After photolysis, the separation of products can be easily achieved by extracting from the dendrimer solutions and the dendrimers are simply recovered and reused.
- Yuan, Zhao,Chen, Jinping,Zeng, Yi,Li, Ying-Ying,Han, Yongbin,Li, Yi
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experimental part
p. 6256 - 6264
(2011/10/09)
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- Functional group tolerant Kumada-Corriu-Tamao coupling of nonactivated alkyl halides with aryl and heteroaryl nucleophiles: Catalysis by a nickel pincer complex permits the coupling of functionalized Grignard reagents
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A nickel(II) pincer complex [(MeNN2)NiCl] (1) catalyzes Kumada-Corriu-Tamao cross coupling of nonactivated alkyl halides with aryl and heteroaryl Grignard reagents. The coupling of octyl bromide with phenylmagnesium chloride was used as a test reaction. Using 3 mol % of 1 as the precatalyst and THF as the solvent, and in the presence of a catalytic amount of TMEDA, the coupling product was obtained in a high yield. The reaction conditions could be applied to cross coupling of other primary and secondary alkyl bromides and iodides. The coupling is tolerant to a wide range of functional groups. Therefore, alkyl halides containing ester, amide, ether, thioether, alcohol, pyrrole, indole, furan, nitrile, conjugated enone, and aryl halide moieties were coupled to give high isolated yields of products in which these units stay intact. For the coupling of ester-containing substrates, O-TMEDA is a better additive than TMEDA. The reaction protocol proves to be efficient for the coupling of Knochel-type functionalized Grignard reagents. Thus aryl Grignard reagents containing electron-deficient and/or sensitive ester, nitrile, amide, and CF3 substituents could be successfully coupled to nonactivated and functionalized alkyl iodides. The catalysis is also efficient for the coupling of alkyl iodides with functionalized heteroaryl Grignard reagents, giving rise to pyridine-, thiophene-, pyrazole-, furan-containing molecules with additional functionalities. Concerning the mechanism of the catalysis, [(MeNN2)Ni-(hetero)Ar] was identified as an intermediate, and the activation of alkyl halides was found to take place through a radical-rebound process.
- Vechorkin, Oleg,Proust, Valerie,Hu, Xile
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supporting information; experimental part
p. 9756 - 9766
(2011/03/19)
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- Palladium-catalyzed heck coupling-hydrogenation: Highly efficient one-pot synthesis of dibenzyls and alkyl phenyl esters
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An efficient method for the synthesis of industrially important dibenzyls and alkyl phenyl esters via sequential Heck coupling and hydrogenation of the alkenyl double bond in one pot with a single recyclable catalyst under mild conditions has been realised. The catalyst was recovered by simple filteration and reused for several cycles with consistent activity.
- Kantam, M. Lakshmi,Chakravarti, Rajashree,Chintareddy, Venkat Reddy,Sreedhar,Bhargava, Suresh
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supporting information; experimental part
p. 2544 - 2550
(2009/08/14)
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- New insights into the mechanism of triplet radical-pair combinations. The persistent radical effect masks the distinction between in-cage and out-of-cage processes
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Steady-state and laser-pulsed irradiations of dibenzyl ketone (ACOB 0) and derivatives with a p-methyl or a p-hexadecyl chain (ACOB 1 and ACOB16, respectively) have been conducted in polyethylene films with 0, 46, and 68% crystallinities. Calculation of the fractions of in-cage combinations of the triplet benzylic radical-pair intermediates based on photoproduct yields, Fc, from ACOB 16 are shown to be incorrect as a result of the kinetic consequences of drastically different diffusion coefficients for the benzyl and p-hexadecylbenzyl radicals. Careful analyses of the transient absorption traces, based upon a new model developed here, allow the correct cage effects to be determined even from ACOB0. The model also permits the rate constants for radical-pair combinations and escape from their cage of origin to be calculated using either an iterative fitting procedure or a very simple one which requires only k-CO and the intensities of the transient absorption immediately after the flash and after the in-cage portion of reaction by the benzylic radicals is completed. Values of the rate constant for decarbonylation of the initially formed arylacetyl radicals, k-CO, have been measured from the rise portions of the laser-flash transient absorption traces. They confirm the assertion from results in liquid alkane media that decarbonylation rates are independent of microviscosity. The data separate components of a reaction from an (in-cage) "cage effect" and an (out-of-cage) "persistent radical effect" that are responsible for formation of AB-type (i.e., decarbonylated) products. The effects here are a consequence of vastly different rates of diffusion for coreacting A· and B· benzylic radicals rather than segregation of the radicals in different parts of a hetereogeneous environment (which leads to an excess of AA and BB products). Heretofore, observation of exclusive formation of AB products has been attributed to in-cage combinations of geminate radical pairs. We show that not to be the case here and provide methodologies which may be used for testing the importance of the "persistent radical effect" component of reaction.
- Chesta, Carlos A.,Mohanty, Jyotirmayee,Nau, Werner M.,Bhattacharjee, Urbashi,Weiss, Richard G.
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p. 5012 - 5022
(2008/02/07)
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- Stereoselective debromination and selective reduction of vic-dibromides with nickel boride
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A simple procedure is reported for the stereoselective debromination of vic-dibromides with nickel boride at ambient temperature. Debromination with concomitant reduction of vic-dibromides to give dihydro products in a one-pot reaction is also reported. α,β-Dibromoketones can also be converted to their corresponding alcohols.
- Khurana, Jitender M.,Kandpal, Bhaskar M.,Kukreja, Gagan,Sharma, Purnima
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p. 1019 - 1023
(2007/10/03)
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- Alkali ion exchanged nafion as a confining medium for photochemical reactions
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Methanol-swollen Nafion beads were used as microreactors to control the photochemical reaction pathways. Product selectivity in three unimolecular reactions, namely, the photo-Fries rearrangement of naphthyl esters, Norrish Type I reaction of 1-phenyl-3-p-tolyl-propan-2-one and Norrish Type I and Type II reactions of benzoin alkyl ethers were examined. The influence of cations over the photodimerization of acenaphthylene and cross-photodimerization between acenaphthylene and N-benzyl maleimide included within Nafion were also examined. The photochemical behaviors of the above substrates were significantly altered within Nafion compared with their solution photochemistry. Of particular interest, the product distributions were found to depend on the counter cations of Nafion.
- Arumugam, Selvanathan,Kaanumalle, Lakshmi S.,Ramamurthy
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p. 139 - 145
(2008/02/04)
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- Dendrimers as photochemical reaction media. Photochemical behavior of unimolecular and bimolecular reactions in water-soluble dendrimers
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Synthesis and studies of poly(alkyl aryl ether) dendrimers, possessing carboxylic acid functionalities at their peripheries, are reported. 5-Bromopentyloxy methylisophthalate was utilized as the monomer to 0-alkylate the phenolic hydroxyl groups of poly(alkyl aryl ether) dendrimers. Dendrimers of first, second, and third generations, possessing 6, 12, and 24 carboxylic acids, respectively, were thus prepared. These dendrimers were soluble in alkaline aqueous solutions, and the ensuing microenvironmental properties of the aqueous solutions were assessed by pyrene solubilization studies. Upon establishing the presence of nonpolar microenvironments within the dendritic structures, solubilizations of few organic substrates were conducted and their photochemical behaviors were assessed. Specifically, the photolysis of 1-phenyl-3-p-tolyl-propan-2-one and benzoin ethyl ether and photodimerization of acenaphthylene were conducted. These studies revealed that the product distribution and the "cage effect" were more distinct and efficient for the third generation dendrimer, than for the first and second generation dendrimers. The photochemical studies of carboxylic acid functionalized dendrimers were compared to that of hydroxyl group terminated poly(alkyl aryl ether) dendrimers.
- Kaanumalle, Lakshmi S.,Ramesh,Maddipatla, V. S. N. Murthy,Nithyanandhan, Jayaraj,Jayaraman, Narayanaswamy,Ramamurthy
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p. 5062 - 5069
(2007/10/03)
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- Amphiphilic homopolymer as a reaction medium in water: Product selectivity within polymeric nanopockets
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A styrene-based water-soluble polymer has been explored for its use as a host for lipophilic substrates in aqueous medium. Unimolecular reactions, namely, photo-Fries rearrangement of naphthyl esters, α-cleavage reaction of 1-phenyl-3-p-tolyl-propan-2-one, and Norrish type I and type II reactions of benzoin alkyl ethers were examined. We find that the hydrophobic domains generated by the polymer not only restrict the mobility of the radicals but also modestly incarcerate the substrate, intermediates, and products during the time scale of the reactions. Comparative studies of the same photoreactions in micelles formed from small molecule surfactants and an amphiphilic diblock copolymer demonstrate that the styrene-based water-soluble polymer aggregates in aqueous medium offer better selectivity.
- Arumugam, Selvanathan,Vutukuri, Dharma Rao,Thayumanavan,Ramamurthy
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p. 13200 - 13206
(2007/10/03)
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- An empirical study of the effect of the variables in a flash vacuum pyrolysis (FVP) experiment
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The effect of the variation of the experimental parameters on the conversion of precursor to products in a typical flash vacuum pyrolysis (FVP) experiment was investigated empirically. Temperature-conversion plots can be used to optimise FVP conditions and their mechanistic significance is exemplified. At a given temperature, the conversion can be increased by an increase in the background pressure, or by packing a section of the furnace tube with inert material (particularly when placed at the trap end of the furnace tube) or by employing a catalyst. Despite the prevailing view that only intramolecular reactions take place by FVP, it has been shown by a 'dual-FVP' cross-over experiment that the dimerisation of benzyl radicals occurs in the gas-phase, before the cold trap, under standard conditions. However, reduction in through-put rate, increase in furnace temperature and reduction in background pressure all reduce the amount of gas-phase coupling.
- Duffy, Emma F.,Foot, Jonathan S.,McNab, Hamish,Milligan, Andrew A.
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p. 2677 - 2683
(2007/10/03)
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- Heterogeneous Palladium-Catalysed Heck Reaction of Aryl Chlorides and Styrene in Water under Mild Conditions
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The Heck reaction of chlorobenzene and substituted chlorobenzenes with styrene is performed in water using catalytic palladium on carbon. The presence of a reducing agent, a mild base, and a phase-transfer catalyst enhances the reactivity of chloroarenes. Transfer hydrogenation of the stilbene double bond to yield 1,2-diphenylethane is also observed in situ. The competing reactions in this system are haloaryl homocoupling and hydrodehalogenation. The effects of various process parameters on substrate conversion and product selectivity are examined for PhCl and PhBr and the unique role of the reducing agent is discussed. A series of filtration experiments indicate that, in this system, the catalysis is heterogeneous and not due to palladium leaching.
- Mukhopadhyay, Sudip,Rothenberg, Gadi,Joshi, Ashutosh,Baidossi, Mubeen,Sasson, Yoel
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p. 348 - 354
(2007/10/03)
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- Ansa macrolides as molecular workbenches for stereoselective additions to achiral (E) olefins
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Highly stereocontrolled additions to achiral acyclic (E) olefins are achieved via incorporation into an ansa macrolide with a nonracemic stilbene diol (molecular workbench approach).
- Mulzer, Johann,Boehm, Ingo,Bats, Jan-W
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p. 9643 - 9646
(2007/10/03)
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- Flash vacuum pyrolysis of stabilised phosphorus ylides. Part 12. Extrusion of Ph3P from sulfonyl ylides and reactivity of the resulting sulfonyl carbenes
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Twelve sulfonyl stabilised phosphorus ylides have been prepared and their behaviour upon flash vacuum pyrolysis at 600°C has been examined. Examples with an arylsulfonyl substituent undergo loss of Ph3PO to give intractable products but those with an arylmethylsulfonyl substituent separately lose Ph3P and SO2 to give products consistent with the intermediacy of sulfonyl carbenes. X-Ray structure determinations of one ylide from each series show a more significant P-O non-bonding interaction in the first case, providing some explanation for the different thermal reactivity.
- Aitken, R. Alan,Drysdale, Martin J.,Ferguson, George,Lough, Alan J.
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p. 875 - 880
(2007/10/03)
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- Zeolite as a reagent and as a catalyst: Reduction and isomerization of alkenes by Ca Y
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Depending on the number of Bronsted acid sites present, Ca Y can act either as a reagent and reduce stilbenes to 1,2-diarylethanes or act as a catalyst and isomerize cis-stilbenes to the more stable trans form; Lewis acid sites generated by the activation process yield radical cations from stilbenes but these do not yield any stable products.
- Pitchumani,Joy, Abraham,Prevost, Nicolette,Ramamurthy
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p. 127 - 128
(2007/10/03)
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- Role of water in intrazeolite photochemistry
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The photochemical and photophysical behavior of several ketones included within Na X zeolites is dependent upon the adsorbed water within the zeolite. Results suggest that when the zeolite is dry (water free), the guest ketones reside within supercages and when the internal structure is filled with water the guest molecules are displaced to the external surface.
- Zhang,Turro,Johnston,Ramamurthy
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p. 4861 - 4864
(2007/10/03)
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- Photochemistry of ketones adsorbed on porous silica
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The Phetochemical reactions of several families of ketones adsorbed on various forms of silica have been investigated. The influence of surface coverage, silica average pore diameter, temperature, applied magnetic fields, and additives on the product distributions have been determined. The results are consistent with a model in which the silica surface consists of two major regions, one consisting of stronger binding sites (active silanol groups) and the other consisting of weaker binding sites (free silanol groups). The relative proportions of these two types of sites are related to the pore size of the silica, smaller pores correlating with a larger fraction of the stronger binding sites and larger pores correlating with a smaller fraction of the stronger binding sites. At low coverage, the initial location of a ketone in one of the two binding sites is an important factor in determining the product distributions. Some preliminary results are reported on the time resolved spectroscopy of ketones adsorbed on silica investigated by diffuse reflectance absorption spectrescopy.
- Turro, Nicholas J.
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p. 1589 - 1616
(2007/10/02)
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- SIZE, SHAPE, AND SITE SELECTIVITIES IN THE PHOTOCHEMICAL REACTIONS OF MOLECULES ADSORBED ON PENTASIL ZEOLITES. EFFECTS OF COADSORBED WATER
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The photochemistry of methylbenzyl benzyl ketones (ACOB) in the presence of pentasil zeolites follows strikingly different pathways due to the location of the adsorbed ketone.The product distribution, in terms of the cage effect (efficiency of geminate radical combination), demonstrates the effects of sorption and diffusion on the radical species produced by photolysis. p-ACOB is readily adsorbed within the pentasil framework and produces p-AB as the primary product.In contrast, the photolysis product distributions of o-ACOB can be dramatically varied depending upon the extent of its adsorption into the framework.By addition of a nonreactive titrant, such as water, after the ketone adsorption, the photolysis product distributions can be systematically varied depending upon the aluminum content of the framework.The observed results are completely described by considerations of (a) the size and shape sorption of the pentasil zeolites, (b) the sorption of water by the hydrophilic sites of the pentasil zeolites (which depend upon the framework aluminum content), and (c) the hydrophobic characteristics of the pentasil channels which do not contain framework aluminum.
- Turro, Nicholas J.,Cheng, Chen-Chih,Abrams, Lloyd,Corbin, David R.
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p. 2449 - 2456
(2007/10/02)
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- Additive Effects on the CIDNP, Cage Effect, and Exit Rate of Micellized Radical Pairs
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The effects of additives on the recombination efficiency and exit rates of micellized benzyl radicals have been studied by steady-state CIDNP, steady-state product analysis, and time-resolved optical absorption techniques.The results indicate that the efficiency of triplet geminate pair recombination increases monotonically as the micellar volume increases.Aggregation numbers of SDS micelles were deduced as a function of additive concentration from CIDNP measurements and were found to be consistent with literature values.
- Turro, N. J.,Zimmt, M. B.,Lei, X. G.,Gould, I. R.,Nitsche, K. S.,Cha, Y.
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p. 4544 - 4548
(2007/10/02)
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- Modification of Chemical Reactivity via Inclusion Complex Formation: Photochemistry of Dibenzyl Ketones and Benzyl Phenylacetates
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In the context of employing "inclusion complexes" as a medium for organic photochemical reactions, we have investigated the photochemical behavior of dibenzyl ketones and benzyl phenylacetates using deoxycholic acid (DCA), Dianin's compound, and cyclodextrin (CD) as hosts in the solid state.Results on cage effect suggest that the translational motion of the benzyl radical pairs is restricted in all three media and totally in Dianin's compound.Products resulting from rearrangement of dibenzyl ketones were formed upon photolysis in Dianin's compound and cyclodextrin and were absent in deoxycholic acid.The absence of rearrangement in DCA and its presence to varied extents in Dianin's compound and cyclodextrin is suggested to be an indication of the restriction imposed by the host on the reorientational process of geminate radical pairs.
- Rao, Bantu Nageswara,Turro, Nicholas J.,Ramamurthy, Vaidhyanathan
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p. 460 - 464
(2007/10/02)
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- Photolysis of Dibenzyl Ketones Adsorbed on Zeolite Molecular Sieves. Correlation of Observed Cage Effects with Carbonyl 13C Enrichment Efficiencies
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The photolysis of 3-(4-methylphenyl)-1-phenyl-2-propanone (1) (4-MeDBK) and 1,3-diphenyl-2-propanone-13C2 (2') (DBK-13C) has been carried out on several commonly available zeolites.Dramatic changes in product distributions for 4-MeDBK (1) photolysis were observed on the several zeolites.Percent cage effects were calculated and rationalized in terms of molecular mobility of photogenerated benzyl radicals in the void volumes of zeolites.Percent cage effects correlate well with 13C-enrichment efficiencies for DBK-13C photolysis on these zeolites and, with the yield of a DBK isomer (PMAP); the para-coupling product of the primary geminate radical pair (Figure 3) in the photolysis of DBK-13C.
- Turro, Nicholas J.,Wan, Peter
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p. 678 - 682
(2007/10/02)
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- MECHANISTIC STUDIES OF THE PHOTODECOMPOSITION OF ARYLMETHYL SULFONES IN HOMOGENEOUS AND MICELLAR SOLUTIONS.
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The mechanism of photodecomposition of aryl sulfones has been investigated by using both steady-state and time-resolved techniques. Direct evidence for radical and triplet-state intermediates is provided. A correlation is found between the dynamics of reaction of the intermediate triplet states with the stabilities of the intermediate radicals.
- Gould,Tung,Turro,Givens,Matuszewski
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p. 1789 - 1793
(2007/10/02)
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- Photolyses of Dibenzyl Ketones in Liquid-Crystalline Media. The Fate of Benzyl Radical Pairs in Various Anisotropic Environments
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Photolyses of 1-(4-methylphenyl)-3-phenylpropan-2-one (1a) and 1,3-diphenylpropan-2-one (1b) have been conducted in solid, liquid-crystalline, and isotropic phases of a 35/65 (w/w) mixture of cholesteryl chloride/cholesteryl nonanoate (CCl/CN), trans,trans-4'-n-butylbicyclohexane-4-carbonitrile (BCCN), or n-butyl stearate (BS).The fraction of in-cage recombination of benzyl radicals (Fc) produced upon decarbonylation of 1a depends strongly upon solvent phase.The effects of magnetic field upon Fc for 1a and the degree of isotopic enrichment of photolyzed 1b in mesophases are described.The results in BS, especially, indicate that solvent-containment barriers, whose origin does not lie in phase discontinuities or large polarity differences, can influence strongly Fc.
- Hrovat, David A.,Liu, Jerry H.,Turro, Nicholas J.,Weiss, Richard G.
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p. 5291 - 5295
(2007/10/02)
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- Carbon-Hydrogen vs. Carbon-Carbon Bond Cleavage of 1,2-Diarylethane Radical Cations in Acetonitrile-Water
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Radical cations of 1,2-diarylethanes and 1-phenyl-2-arylethanes (Ar = phenyl, p-tolyl, p-anisyl) were generated in acidic 70percent acetonitrile-water by Cu2+-catalyzed peroxydisulfate oxidation.The radical cations fragment mainly by loss of benzylic protons (C-H cleavage) rather than by alkyl C-C bond cleavage.The radical cation of 2,3-dimethyl-2,3-diphenylbutane fragments efficiently by C-C cleavage, forming cumyl radical and cumyl cation.Oxidations of bibenzyl-bicumyl mixtures show selective oxidation of bicumyl dependent on total substrate concentration, providing evidence of equilibrating radical cations and showing that bicumyl fragments faster than bibenzyl loses protons.The effects of reaction conditions and substrate structure on reactivity are discussed.
- Camaioni, Donald M.,Franz, James A.
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p. 1607 - 1613
(2007/10/02)
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- The Reactivity of Organophosphorus Compounds. Part 35. Pyrolysis of 3-Aryl- and 3-Alkyl-2,3-dihydro-1,3,2-benzoxazaphosph(V)oles
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Pyrolysis of 2,2-diphenyl-2-methoxy derivatives of the title compounds in the gas phase gives phosphinates or phosphinamidates derived from the substrate by formal loss of methylene.Control experiments show that the reaction involves radical cleavage initiated by rupture of the oxazaphosph(V)ole ring.Pyrolysis of 3-aryl-2,2-dimethoxy-2-phenyl derivatives of the title compounds under similar conditions results in the formation of 3-aryl-2,3-dihydro-2-oxo-2-phenyl-1,3,2-benzoxazaphospholes.No significant products could be isolated from 2,2,2-trimethoxy derivatives, though vigorous conditions were required for their complete breakdown.The pyrolysis of the corresponding 2,2-diphenyl-2-ethoxy, 2,2-diethoxy-2-phenyl, and 2,2,2-triethoxy compounds gave higher yields of similar products, possibly via a six-centre concerted elimination.
- Cadogan, J. I. G.,Husband, James B.,McNab, Hamish
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p. 1489 - 1495
(2007/10/02)
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- Micellar Systems as ''Supercages'' for Reactions of Geminate Radical Pairs. Magnetic Effects
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The photochemistry of dibenzyl ketone (DBK) and other molecules capable of producing benzyl radicals and substituted benzyl radicals has been investigated in micellar systems.The cage effect (percent of unscavengeable radical pairs produced by photolysis) was measured under a variety of conditions, and the results are compared with those obtained in homogeneous organic solvents.For example, parameters such as mean occupancy of ketone, detergent type and concentration, O2 concentration, additives, temperature, applied magnetic field, and pressure have been varied and investigated as to their influence on the magnitude of cage effect.In addition to modifying its environment, structural modification of the DBK by incorporation of 2H and 13C isotopes, hydrophobic groups, and heavy atoms was performed to investigate the impact of these variations on the cage effect in micellar systems.Isotopic substitution of 2H or 13C leads to results on both the quantum yields for reactions and on the percent cage that were consistent with expectations of magnetic isotope effects.Hydrophobic groups substituted in the 4-position of DBK were found to cause a substantial increase in the cage effect and yet retain the magnetic-field-dependent character found in the parent DBK.Incorporation of Br in the 4-position of DBK was found to enhance the cage effect but at the same time cause the cage effect to become magnetic field independent.Substitution of α-naphthyl for phenyl in DBK also produced magnetic-field-independent behavior, in addition to a dramatic decrease in the efficiency of photolysis.
- Turro, Nicholas J.,Weed, Gregory C.
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p. 1861 - 1868
(2007/10/02)
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- BOND DISSOCIATION ENERGIES IN 2-METHOXY-3-PHENYLPROPIONITRILE AND IN BIBENZYL; CAPTO-DATIVE STABILIZATION OF THE CYANO METHOXY METHYL RADICAL
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In the temperature region of 800-900 K the rate of C-C bond homolysis in 2-methoxy-3-phenylpropionitrile (1a) is 2-3 times faster than in bibenzyl (2a), from which it is inferred that the "capto-dative" stabilization of the .CH(OMe)CN radical is ca. 4 kcal/mol.
- Louw, Robert,Bunk, Jaap Joosting
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p. 119 - 120
(2007/10/02)
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- Chemistry of the tert-Butyl Radical: Polar Character, ρ Value for Reaction with Toluenes, and the Effect of Radical Polarity on the Ration of Benzylic Hydrogen Abstraction to Addition to Aromatic Rings
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We have reexamined the reactions of tert-butyl radicals with toluenes and have obtained a ρ value of 0.49 +/- 0.04 at 80 deg C.The new, independent system involves the quantification of all of the principal products from the reaction of tert-butyl radicals with mixtures of toluene and substituted toluene.Five major products contain benzyl fragments and are formed in significant yields: two symmetrical bibenzyls, the cross bibenzyl, and the two neopentylbenzenes that result from combination of tert-butyl and the two benzyl radicals.Attack on the side chain is a major reaction of free tert-butyl radicals and is the only significant reaction that they undergo other than cage and termination reactions. tert-Butyl radicals do not add to the ring of toluenes.Data on the relative rates of addition to benzenes and hydrogen abstraction from toluenes are collected for a series of radicals including hydrogen atoms; methyl, isopropyl, and tert-butyl radicals; and p-nitrophenyl, p-bromophenyl, and phenyl radicals.The data demonstrate that more electrophilic radicals have a larger tendency to add to rings whereas more nucleophilic radicals have a larger tendency to abstract benzylic hydrogens.
- Pryor, William A.,Tang, Felicia Y.,Tang, Robert H.,Church, Daniel F.
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p. 2885 - 2891
(2007/10/02)
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