- Photoreduction of Pt(IV) chloro complexes: Substrate chlorination by a triplet excited state
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The Pt(IV) complexes trans-Pt(PEt3)2(Cl) 3(R) 2 (R = Cl, Ph, 9-phenanthryl, 2-trifluoromethylphenyl, 4-trifluoromethylphenyl, 3-perylenyl) were prepared by chlorination of the Pt(II) complexes trans-Pt(PEt3)2(R)(Cl) 1 with Cl 2(g) or PhICl2. Mixed bromo-chloro complexes trans,trans-Pt(PEt3)2(Cl)2(Br)(R) (R = 9-phenanthryl, 4-trifluoromethylphenyl), trans,cis-Pt(PEt3) 2(Cl)2(Br)(4-trifluoromethylphenyl), trans,trans- Pt(PEt3)2(Br)2(Cl)(R) (R = 9-phenanthryl), and trans,cis-Pt(PEt3)2(Br)2(Cl)(4- trifluoromethylphenyl) were obtained by halide exchange or by oxidative addition of Br2 to 1 or Cl2 to trans-Pt(PEt3) 2(R)(Br). Except for 2 (R = Ph, 4-trifluoromethylphenyl), all of the Pt(IV) complexes are photosensitive to UV light and undergo net halogen reductive elimination to give Pt(II) products, trans-Pt(PEt3) 2(R)(X) (X = Cl, Br). Chlorine trapping experiments with alkenes indicate a reductive-elimination mechanism that does not involve molecular chlorine and is sensitive to steric effects at the Pt center. DFT calculations suggest a radical pathway involving 3LMCT excited states. Emission from a triplet is observed in glassy 2-methyltetrahydrofuran at 77 K where photoreductive elimination is markedly slowed.
- Perera, Tharushi A.,Masjedi, Mehdi,Sharp, Paul R.
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supporting information
p. 7608 - 7621
(2014/08/05)
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- Pyridine-assisted chlorinations and oxidations by palladium(IV)
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The reactivity of the bis-NHC complex LPdIVCl4 (L = κ2-[R-NHCCH2NHC-R] with R = C14H 29) in chlorinations and oxidations of organic substrates was considerably increased in the presence of pyridine. For alkene chlorinations, this effect was due to the in situ formation of highly reactive LPd IVCl3(py)+, which was able to transfer Cl + to the C=C bond in a ligand-mediated process (devoid of π complexation), which did not require py dissociation. The enhanced reactivity in the presence of pyridine also extended to the oxidation of secondary and benzylic alcohols under mild conditions in a reaction where py served as a base, broadening the known scope of reactivity for PdIV complexes. LPdIVCl3(py)+ could be formed from Cl -/py exchange or from the oxidation of LPdIICl(py) + by Cl2. Taking advantage of the enhanced reactivities that pyridine coordination imparted on both PdII and PdIV complexes allowed for the catalytic chlorination of styrene with LPd IVCl4 as a sacrificial oxidant, thereby establishing the principal feasibility of PdII/PdIV catalyses that obviates PdII activations of the substrate.
- McCall, A. Scott,Kraft, Stefan
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scheme or table
p. 3527 - 3538
(2012/06/16)
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- Bis- N -heterocyclic carbene palladium(IV) tetrachloride complexes: Synthesis, reactivity, and mechanisms of direct chlorinations and oxidations of organic substrates
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This Article describes the preparation and isolation of novel octahedral CH2-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula LPdIVCl4 [L = (NHC)CH 2(NHC)] from LPdIICl2 and Cl2. In intermolecular, nonchelation-controlled transformations LPdIVCl 4 reacted with alkenes and alkynes to 1,2-dichlorination adducts. Aromatic, benzylic, and aliphatic C-H bonds were converted into C-Cl bonds. Detailed mechanistic investigations in the dichlorinations of alkenes were conducted on the 18VE PdIV complex. Positive solvent effects as well as kinetic measurements probing the impact of cyclohexene and chloride concentrations on the rate of alkene chlorination support a PdIV-Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl+-transfer from the pentacoordinated PdIV-intermediate LPdIVCl 3+ to olefins. 1-Hexene/3-hexene competition experiments rule out both the formation of π-complexes along the reaction coordinate as well as in situ generated Cl2 from a reductive elimination process. Instead, a ligand-mediated direct Cl+-transfer from LPd IVCl3+ to the π-system is likely to occur. Similarly, C-H bond chlorinations proceed via an electrophilic process with in situ formed LPdIVCl3+. The presence of a large excess of added Cl- slows cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both PdIV-Cl ionization and Cl+-transfer from LPdIVCl3 +. 1H NMR titrations, T1 relaxation time measurements, binding isotherms, and Job plot analysis point to the formation of a trifurcated Cl-...H-C bond in the NHC-ligand periphery as a supramolecular cause for the accelerated chemical events involving the metal center.
- McCall, A. Scott,Wang, Hongwang,Desper, John M.,Kraft, Stefan
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supporting information; experimental part
p. 1832 - 1848
(2011/04/15)
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- MECHANISMS OF FREE-RADICAL REACTIONS. XXIV. QUANTITATIVE DESCRIPTION OF THE POLAR EFFECTS OF SUBSTITUENTS ON THE KINETICS OF THE FREE-RADICAL CHLORINATION OF ALIPHATIC COMPOUNDS BY N-CHLOROPIPERIDINE
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The free-radical chlorination of 1-substituted alkanes with electron-withdrawing substituents by N-chloropiperidine in trifluoroacetic acid was studied by the method of competing reactions, and the relative rate constants were obtained for all positions of the substrates.The data on the position selectivity can be described satisfactorily by means of an electrostatic model of the polar effect of the substituent, calculated according to the Kirkwood-Westheimer equation.The obtained characteristics of the electrostatic effect can be successfully applied to calculation of the substrate selectivity and the intermolecular relative rate constants for all the positions, beginning with the third.The Taft equation is unsuitable for description of the effect of substituents on the reaction rate.
- Dneprovskii, A. S.,Mil'tsov, S. A.,Arbuzov, P. V.
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p. 1826 - 1835
(2007/10/02)
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- THE LONG-RANGE ACTION OF THE POLAR EFFECT OF SUBSTITUENTS ON THE ABSTRACTION OF HYDROGEN IN FREE-RADICAL CHLORINATION PROCESSES
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The free-radical chlorination of 1-chloroalkanes between C3 and C6 at 263 deg K was studied under conditions with wide variation in the concentrations of the substrates in benzene.By analysis of the products from chlorination of the pure substrates it was shown that the deactivating effect of the polar substituent does not extend beyond the third carbon atom and is mainly determined by the inductive effect.In the transition to an aromatic solvent the deactivating effect on the substituent extends to the fourth carbon atom.The results are substantiated in terms of a contribution from dipole-dipole interaction between the substituent and the polar form of the transition state to the polar effect of the substituents.
- Aver'yanov, V. A.,Ruban, S. G.,Klykova, M. B.,Golubev, V. E.
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p. 435 - 439
(2007/10/02)
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- MECHANISMS OF FREE-RADICAL REACTIONS. XIX. SELECTIVITY OF THE FREE-RADICAL CHLORINATION OF 1-CHLOROALKANES BY N-CHLOROPIPERIDINE
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The free-radical chlorination of 1-chloroalkanes ClH2l+1Cl (l = 4-7) by N-chloropiperidine was studied by the method of competing reactions, and the relative constants for all the positions of the investigated substrates were determined.The chlorination is a highly electrophilic process, and the effect of the substituents is transmitted through at least six C-C bonds.The results can be described satisfactorily by means of an electrostatic model of the polar effect of the substituents according to the Kirkwood-Westheimer equation.At the same time an attempt to describe the obtained data by means of the Taft equation led to unsatisfactory results.
- Dneprovskii, A. S.,Mil'tsov, S. A.
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p. 265 - 269
(2007/10/02)
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- CHLORINATING CHARACTERISTICS OF THE COMPLEX OF N,N-DIMETHYLDICHLOROMETHYLENEIMINIUM CHLORIDE WITH CHLORINE
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The chlorinating characteristics of the complex (I) of N,N-dimethyldichloromethyleneiminium chloride with chlorine were studied for the case of its reactions with olefins (1-hexene, cyclohexene, tetrachloroethylene), unsaturated alcohols (allyl alcohol), and phenols (phenol, p-cresol).It was found that the ionic chlorination of the olefins by the complex takes place at high rates and with a high degree of selectivity.With the hydroxy compounds there are parallel reactions, in which the chlorinating characteristics of the complex (I) and the reactivity of the N,N-dimethyldichloromethyleneiminium chloride appear.Chlorination of the phenols by the complex (I) takes place at the ortho position, and this leads to the formation of di(o-chloroacyl) carbonates (with the complex and the phenol in a ratio of 1:2) or o-chloroaryl dimethylcarbamates (with the reagents in a ratio of 1:1).
- Mironova, D. F.,Stetsyuk, G. A.,Kukhar', V. P.
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p. 2319 - 2323
(2007/10/02)
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- MECHANISM OF RADICAL TELOMERIZATION OF ETHYLENE WITH 1,1-DICHLOROETHANE
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The mechanism of the radical telomerization of ethylene with 1,2-dichloroethane was investigated. 1,2-Dichloroethane reacts with telomeric alkyl radicals with cleavage both of the C - H bond and of the C - Cl bond, and the ratio of the corresponding rate constants is 2.8 (100 deg C).It was shown that, in addition to the chlorine-containing telomers, the mixture of products from the reaction initiated by benzoyl peroxide contains alkyl- and dialkylbenzenes.
- Dzheiranishvili, M. S.,Levinskii, M. B.,Chkhubianishvili, N. G.,Afanas'ev, I. B.
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p. 445 - 447
(2007/10/02)
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