- Iron-catalyzed pinacol coupling of aryl ketones with a phenyltitanium reagent: A new type of catalytic reaction
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A reaction of aryl ketones with phenyltitanium triisopropoxide ([PhTi(Oi-Pr)3]) in the presence of [Fe(acac)3] as a catalyst (1 mol%) gave the corresponding pinacols in high yields. The catalytic cycle of this process involves an iron-catalyzed disproportionation of [PhTi(Oi-Pr) 3] into biphenyl and a lowvalent titanium species.
- Hayashi, Tamio,Sasaki, Keigo
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supporting information; experimental part
p. 492 - 494
(2011/06/25)
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- Enantioselective cathodic reduction of some prochiral ketones in the presence of (1R,2S)-(-)-N,N-dimethylephedrinium tetrafluoroborate at a mercury pool cathode
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The enantioselective cathodic reduction of some prochiral ketones viz. acetophenone (I), propiophenone (II), butyrophenone (III), valerophenone (IV), isobutyrophenone (V), and pivalophenone (VI) has been accomplished at a mercury pool cathode in N,N-dimethylformamide (DMF)-water (90:10) using tetrabutylammonium fluoroborate (TBA·BF4) as a supporting electrolyte in the presence of (1R,2S)-(-)-N,N-dimethylephedrinium tetrafluoroborate (DET). Cyclic voltammetric investigations have been carried out and a probable mechanism of the process has been presented.
- Yadav, Ashok K.,Singh, Arpita
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p. 587 - 588
(2007/10/03)
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- Efficient method for the preparation of pinacols derived from aromatic and aliphatic ketones by using low-valent titanium reagents in dichloromethane-pivalonitrile
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The reductive coupling reaction of aldehydes and ketones, including unsymmetrical aliphatic ketones, proceeded smoothly to give the corresponding pinacols in good to high yields under mild conditions by using combination of titanium(II) chloride and zinc or titanium(IV) chloride and zinc in dichloromethane-pivalonitrile. Meso-selective formation of the coupling products was observed in the cases of some aliphatic ketones. The diastereoselectivities of coupling products depend on both difference of bulkiness of 2-, and 2'-substituents of carbonyl group of the reactant, and overall steric effect around the carbonyl groups.
- Kagayama,Igarashi,Mukaiyama
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p. 657 - 665
(2007/10/03)
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- Diastereoselective pinacol coupling of alkyl aryl ketones with rare earth metals in the presence of chlorosilanes
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Rare earth metals (Ln) are found to act as useful reducing agents for the pinacol coupling reaction of alkyl aryl ketones in the presence of chlorosilanes. Although the hitherto known pinacol coupling reaction using rare earth reducing agents generally exhibits very low diastereoselectivities, the present pinacol coupling by use of a Ln/R3SiCl system usually indicates higher diastereoselectivities. In particular, a Yb/Me3SiCl system attains the diastereoselective pinacol coupling of primary alkyl aryl ketones with the dl/meso ratio of 8/2~9/1.
- Ogawa, Akiya,Takeuchi, Hiroki,Hirao, Toshikazu
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p. 7113 - 7114
(2007/10/03)
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- Rate Constants for Degenerate Hydrogen Atom Exchange between α-Hydroxy Radicals and Ketones
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When a phenyl ketone is irradiated in the presence of 1-phenylethanol, the photoproducts include acetophenone pinacol, the pinacol from the other phenyl ketone, a mixed pinacol, and acetophenone, which is formed by irreversible hydrogen atom exchange between the hemipinacol radical of acetophenone and the other ketone.Rate constants for this hydrogen atom exchange between the hemipinacol radical of acetophenone and three other ketones were determined by measuring how acetophenone yields depend on the concentration of the other ketone.Comparable measurements were made for p-chloroacetophenone formation by irradiating acetophenone in the presence of 1-(p-chlorophenyl)ethanol.As the starting ketone concentration increases, so does the amount of exchange relative to pinacol, while the pinacol content reflects decreasing amounts of the original alcohol.Exchange is measurable at ketone concentrations below 0.01 M and is complete by 0.1 M.Rate constants kex of 3.7, 6.3, 4.2, and 8.6 * 103 M-1 s-1 were deduced for hydrogen transfer to propiophenone, isobutyrophenone, p-methylacetophenone, and p-chloroacetophenone, respectively, based on a competing rate constant for radical coupling of 2 * 109 M-1 s-1.Equilibrium constants for the hydrogen transfer were determined from the product ratios obtained by irradiating a mixture of two ketones with 2-propanol; from these k-ex values of 11.5, 57, 13, and 1.8 * 103 M-1 s-1, respectively, were deduced for hydrogen transfer to acetophenone from the four other ketone hemipinacol radicals.These exchange rate constants depend more on the structure of the radical than on that of the ketone.Actual quantum yields for pinacol formation do not exceed 50 percent ; this maximum quantum efficiency rises to 71 percent for 1-phenylethanol-O-d.From this inverse isotope effect it is concluded that half the reaction of triplet acetophenone with 1-phenylethanol involves abstraction of an OH hydrogen followed by disproportionation of the initial radical pair back to reactants.
- Wagner, Peter J.,Zhang, Yuanda,Puchalski, Allen E.
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p. 13368 - 13374
(2007/10/02)
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- Studies on the Occurrence of Hydrogen Transfer, 73. - The Reduction of Prochiral Alkyl (Aryl) Ketones with Alkali Metal Amalgams Forming Carbinols and Pinakols
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The factors are determined which influence the stereochemistry and the ratio (C/P) in the reduction of acetophenone with lithium, sodium, and potassium amalgam (Li/Hg, Na/Hg, K/Hg) leading to carbinol (C) and pinacol(P).The C/P ratio is influenced a) by proton donators, b) by quaternaryammonium salts, c) by bis-tertiary amines.In the presence of optically active quaternary ammonium salts and optically active bis-tertiary amines a low optical induction is observed both in pinacol and carbinol.An anomal dimerization product 20 is formed in the reduction of pivalophenone with Li/Hg.
- Horner, Leopold,Dickerhof, Karlheinz
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p. 1240 - 1258
(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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