- A convenient pinacol coupling of diaryl ketones with B2pin2viapyridine catalysis
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A convenient, pyridine-boryl radical-mediated pinacol coupling of diaryl ketones is developed. In contrast to the conventional pinacol coupling that requires sensitive reducing metal, the current method employs a stable diboron reagent and pyridine Lewis base catalyst for the generation of a ketyl radical. The newly developed process is operationally simple, and the desired diols are produced with excellent efficiency in up to 99% yield within 1 hour. The superior reactivity of diaryl ketone was observed over monoaryl carbonyl compounds and analyzed by DFT calculations, which suggests the necessity of both aromatic rings for the maximum stabilization of the transition states.
- Jo, Junhyuk,Kim, Seonyul,Choi, Jun-Ho,Chung, Won-Jin
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supporting information
p. 1360 - 1363
(2021/02/22)
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- Light-enabled metal-free pinacol coupling by hydrazine
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Efficient carbon-carbon bond formation is of great importance in modern organic synthetic chemistry. The pinacol coupling discovered over a century ago is still one of the most efficient coupling reactions to build the C-C bond in one step. However, traditional pinacol coupling often requires over-stoichiometric amounts of active metals as reductants, causing long-lasting metal waste issues and sustainability concerns. A great scientific challenge is to design a metal-free approach to the pinacol coupling reaction. Herein, we describe a light-driven pinacol coupling protocol without use of any metals, but with N2H4, used as a clean non-metallic hydrogen-atom-transfer (HAT) reductant. In this transformation, only traceless non-toxic N2 and H2 gases were produced as by-products with a relatively broad aromatic ketone scope and good functional group tolerance. A combined experimental and computational investigation of the mechanism suggests that this novel pinacol coupling reaction proceeds via a HAT process between photo-excited ketone and N2H4, instead of the common single-electron-transfer (SET) process for metal reductants.
- Qiu, Zihang,Pham, Hanh D. M.,Li, Jianbin,Li, Chen-Chen,Castillo-Pazos, Durbis J.,Khaliullin, Rustam Z.,Li, Chao-Jun
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p. 10937 - 10943
(2019/12/23)
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- MeOH or H2O as efficient additive to switch the reactivity of allylSmBr towards carbonyl compounds
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A variety of carbonyl compounds were treated by allylSmBr (allylSmBr) with MeOH as the cosolvent to have further insights on the previously reported reductive coupling of aryl ketones mediated by Sm/alkyl halide/MeOH. The results demonstrate that the real reducing species in Sm/alkyl halide/MeOH system should be allylSmBr, and MeOH has elegantly switched the reactivity of allylSmBr from being nucleophilic to being good reductive coupling reagent. Besides, H2O was also found to be a useful additive to realize the pinacol coupling of aliphatic aldehydes and ketones promoted by allylSmBr.
- Li, Jianyong,Niu, Qingsheng,Li, Shanchan,Sun, Yuehao,Zhou, Qian,Lv, Xin,Wang, Xiaoxia
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supporting information
p. 1250 - 1253
(2017/03/10)
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- Microphotochemistry: 4,4′-dimethoxybenzophenone mediated photodecarboxylation reactions involving phthalimides
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A series of 4,4'-dimethoxybenzophenone mediated intra- and intermolecular photodecarboxylation reactions involving phthalimides have been examined under microflow conditions. Conversion rates, isolated yields and chemoselectivities were compared to analogous reactions in a batch photoreactor. In all cases investigated, the microreactions gave superior results thus proving the superiority of microphotochemistry over conventional technologies.
- Shvydkiv, Oksana,Nolan, Kieran,Oelgemoeller, Michael
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supporting information; experimental part
p. 1055 - 1063
(2011/10/04)
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- Photoreduction of benzophenones by amines in room-temperature ionic liquids
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(equation presented) The amine-mediated photoreduction of benzophenones in room temperature ionic liquids was investigated. Unlike the analogous reaction in organic solvents, the photoreduction produces mainly the corresponding benzhydrol in most cases. Because the reaction consumes only 1 equiv of amine and the solvent can be easily recycled, the photoreduction allows a very clean method for the conversion of benzophenones to benzhydrols.
- Reynolds, John L.,Erdner, Kimberly R.,Jones, Paul B.
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p. 917 - 919
(2007/10/03)
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- Direct measurement of ultrafast carbon-carbon cleavage rates via the subpicosecond charge-transfer activation of pinacols
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Highly transient (benzpinacol) cation radicals (D2+.) and their ultrafast mesolytic fragmentations to the diarylhydroxymethyl radical (D.) and cation (D+) are directly observed on the early picosecond time scale upon the charge-transfer photoactivation of the intermolecular donor-acceptor complexes of pinacol donors with methyl viologen. Ultrashort lifetimes of the cation radicals with τ ~ 10 ps obtain (for the first time) from quantitative kinetics analysis of the time-resolved spectroscopic results. These rapid C-C bond scissions successfully compete with back-electron transfer, which normally predominates on this time scale, and lead to exceptionally high efficiencies for the oxidative fragmentation of pinacols.
- Bockman,Hubig,Kochi
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p. 6542 - 6547
(2007/10/03)
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- Intrinsic reactivities of amino acids towards photoalkylation with benzophenone - A study preliminary to photolabelling of the transmembrane protein glycophorin A
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A systematic study of the photoalkylation of amino acids by benzophenone, a standard photosensitive probe of biomolecules, was performed addressing for the first time chemo, regio-, and stereoselectivities. The high reactivity of the capto-dative substituted α-carbon, particularly in glycine, could be demonstrated as well as the chemical lability of the α-coupling products. Methionine was shown to be favoured both in the sense of reactivity as well as product stability. Preliminary to a project directed towards the elucidation of the topography of glycophorin A in membranes, the present model experiments focussed on the ten amino acids that constitute the transmembrane part of this protein.
- Deseke, Eckart,Nakatani, Yoichi,Ourisson, Guy
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p. 243 - 251
(2007/10/03)
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- Surface modification of CdS quantum dots with fluorinated thiophenol
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CdS nanocrystallites prepared by capping with pentafluorothiophenol, 2,3,5,6-tetrafluorothiophenol or 4-fluorothiophenol are characterized as quantum dots by TEM, FTIR, NMR, UV-VIS and fluorescence spectroscopy. The crystalline size tends to increase with an increase in the number of fluorine atoms in the capping molecules while maintaining high solubility in organic solvents, with solubility in alcohols depending on the number of fluorine atoms in the capping molecules. Pentafluorophenyl-capped CdS nanocrystallites have the highest solubilities in alcohols, and exhibit quantum dot photocatalysis in methanol, leading to the efficient two-electron transfer photoreduction under visible-light irradiation.
- Hosokawa, Hiroji,Ogata, Tomoyuki,Wada, Yuji,Murakoshi, Kei,Sakata, Takao,Mori, Hirotaro,Yanagida, Shozo
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p. 4575 - 4580
(2007/10/03)
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- Reactions of pinacols with one-electron oxidants
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Oxidation of the tetraarylpinacols (Ar2COH)2, 1a-e, in which Ar = C6H5 (1a), 4-ClC6H4 (1b), 4-MeC6H4 (1c), 4-MeOC6H4 (1d) and 4-Me2NC6H4 (1e), by thianthrene cation radical (Th?+) in CH3-CN and in CH2Cl2 led quantitatively to the corresponding diaryl ketones Ar2C=O (2a-e), provided a sufficient amount of base, 2,6-di-tert-butyl-4-methylpyridine (DTBMP), was present to prevent presumed acid-catalyzed rearrangement. In the case of 1e, continued oxidation of 2e was also observed. Oxidation of 1a by (4-BrC6H4)3N?+SbCl 6- and (4-BrC6H4)3N?+SbF 6- (Ar3N?+) occurred analogously. Evidence for the catalytic, cation-radical rearrangement of 1a by Ar3N?+ (reported in earlier literature) and by Th?+ could not be found. Quantitative oxidation of 1a to 2a and of 1d to 2d was obtained also with NOBF4, again provided that sufficient DTBMP was present to prevent acid-catalyzed rearrangement. Catalytic, oxidative rearrangement of 1d at room temperature and (as reported in earlier literature) at -5 °C was not observed. Oxidation was also observed of 2,3-diphenyl-2,3-butanediol (3) to acetophenone (9) and of 1,1-dimethyl-2,2-diphenylethanediol (4) to 2a and acetone by Th?+. Oxidation of 2,3-dimethyl-2,3-butanediol (5) by Th?+ was not observed. Instead, even in the presence of DTBMP, pinacolone (10) and tetramethyloxirane (11) were formed, through, it is proposed, a mechanism involving complexation with Th?+.
- Han, Dong Sul,Shine, Henry J.
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p. 3977 - 3982
(2007/10/03)
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- 2-benzoylbenzoic acid: A photolabile mask for alcohols and thiols
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Photolysis of 2-benzoylbenzoate esters of primary and secondary alcohols 1 in the presence of a hydrogen donor (2-isopropanol) or an electron donor (primary amines) produces the corresponding alcohol in high yield. The fate of the benzoate is dependent on the conditions used for the photoreaction. In 2-propanol, the ketyl radical that derives from photoreduction dimerizes, to afford the benzpinacol product 3,3"-diphenylbiphthalidyl, 5. In the presence of amines the product is 3-phenylphthalide, 6, a benzhydrol derivative which is the result of simple reduction of the ketone followed by lactonization. While the photoproduct of the benzoate - 2-propanol reaction results from anticipated free radical chemistry, the amine-promoted reaction appears to result from a second, "dark", electron transfer process. We conclude that 2-benzoylbenzoic acid is an effective photolabile protecting group for primary and secondary alcohols, and preliminary studies indicate that thiols can be protected in an analogous way. Studies on the effect of benzophenone substituents and reaction solvent on the benzhydrol:benzpinacol product ratio provide mechanistic insight into the process.
- Jones, Paul B.,Pollastri, Michael P.,Porter, Ned A.
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p. 9455 - 9461
(2007/10/03)
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- Photoinduced Electron Transfer in Pinacol Cleavage with Quinones via Highly Labile Cation Radicals. Direct Comparison of Charge-Transfer Excitation and Photosensitization
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Benzopinacol and related diphenylethane-like donors (D) form electron donor-acceptor (EDA) complexes with chloranil and similar benzoquinones (A), in which the deliberate irradiation of the charge-transfer absorption band (hνCT) leads to oxidative cleavage (retropinacol) via electron transfer.Photosensitization by excitation of the ?-?* band of chloranil and diffusive quenching also effects the C-C bond cleavage of the same donors.The photoefficiencies of both photochemical processes are quantitatively compared with respect to the lifetimes of the pinacol cation radicals (D+ radical), as determined by the competition from back electron transfer and diffusion.These photoinduced processes are considered in the context of electron-transfer for an equivalent thermal reaction which occurs in the dark with high-potential quinones and electron-rich pinacols.
- Perrier, Serge,Sankararaman, Seth,Kochi, Jay K.
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p. 825 - 837
(2007/10/02)
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- Novel reduction of carbonyl compounds with Al/NH3/halide under irradiation of ultrasonic wave
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Various carbonyl compounds, such as benzophenones and acetophenones, were reduced by Al/NH3/halide under ultrasonic wave irradiation to give the corresponding monohydric alcohols and/or pinacols in satisfactory yields. The addition of inorganic halides improved the selectivity in the formation of monohydric alcohols and pinacols.
- Sato,Nagaoka,Goto,Saito
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p. 290 - 292
(2007/10/02)
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- Visible-Light-Induced Two-Electron-Transfer Photoreductions on CdS: Effeects of Morphology
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Freshly prepared CdS suspensions (CdS-0) consisting of quantized particles and their loose aggregation catalyze photoreductions of aromatic ketones and olefins in methanol under visible light irradiation using triethylamine as sacrificial electron donor, yielding alcohols and dihydro compounds, respectively, which are more selective than photocatalysis of commercially available crystalline CdS (Aldrich) (CdS-Ald).Deuterium incorporation experiments in photolysis of dimethyl maleate in methanol-O-D revealed that CdS-0 catalyzes sequental two-electron-transfer photoreduction, affording dideuterated dimethyl succinate, while CdS-Ald induces both photoreduction and photoisomerization through disproportionation between one-electron-transfer-reduction intermediates, yielding much trideuterated dimethyl succinate and monodeuterated dimethyl fumarate and maleate.
- Shiragami, Tsutomu,Pac, Chyongjin,Yanagida, Shozo
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p. 504 - 506
(2007/10/02)
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- Stereochemistry and Side Products in Reductive Coupling of Alkyl Aryl Ketones to 1,2-Dialkyl-1,2-diarylethylenes
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The reductive coupling of alkyl aryl ketones 1 - 3 and 7 - 9 by low valent titanium salts yields predominantly the (Z)-isomers of 11 - 13 and 17 - 19.Evidence is given by 1H NMR spectroscopy.This behavior can be explained by ?-complex formation of phenyl rings with Ti0.Severe steric hindrance, however, favors the (E)-isomers ( -> 14 and 15).Donor groups in p-position, particularly, give increasing amounts of pinacols, 23 - 27, which undergo rearrangement to the ketones 28 and 29.
- Leimner, Juergen,Weyerstahl, Peter
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p. 3697 - 3705
(2007/10/02)
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