- Structure-reactivity relationship for alcohol oxidations via hydride transfer to a carbocationic oxidizing agent
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Second-order rate constants were determined for the oxidation of 27 alcohols (R1R2CHOH) by a carbocationic oxidizing agent, 9-phenylxanthylium ion, in acetontrile at 60°C. Alcohols include open-chain alkyl, cycloalkyl, and unsaturated alcohols. Kinetic isotope effects for the reaction of 1-phenylethanol were determined at three H/D positions of the alcohol (KIEα-D=3.9, KIEβ-D3=1.03, KIE OD=1.10). These KIE results are consistent with those we previously reported for the 2-propanol reaction, suggesting that these reactions follow a hydride-proton sequential transfer mechanism that involves a rate-limiting formation of the α-hydroxy carbocation intermediate. Structure-reactivity relationship for alcohol oxidations was deeply discussed on the basis of the observed structural effects on the formation of the carbocationic transition state (Cδ+-OH). Efficiencies of alcohol oxidations are largely dependent upon the alcohol structures. Steric hindrance effect and ring strain relief effect win over the electronic effect in determining the rates of the oxidations of open-chain alkyl and cycloalkyl alcohols. Unhindered secondary alkyl alcohols would be selectively oxidized in the presence of primary and hindered secondary alkyl alcohols. Strained C7-C11 cycloalkyl alcohols react faster than cyclohexyl alcohol, whereas the strained C5 and C12 alcohols react slower. Aromatic alcohols would be efficiently and selectively oxidized in the presence of aliphatic alcohols of comparable steric requirements. This structure-reactivity relationship for alcohol oxidations via hydride-transfer mechanism is hoped to provide a useful guidance for the selective oxidation of certain alcohol functional groups in organic synthesis. Copyright
- Lu, Yun,Bradshaw, Joshua,Zhao, Yu,Kuester, William,Kabotso, Daniel
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p. 1172 - 1178
(2013/01/12)
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- Mechanism of Homogeneously and Heterogeneously Catalysed Meerwein-Ponndorf-Verley-Oppenauer Reactions for the Racemisation of Secondary Alcohols
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The mechanism of hydrogen transfer from alcohols to ketones, catalysed by lanthanide(III) isopropoxides or zeolite Beta has been studied. For the lanthanide catalysed reactions, (S)-1-phenyl-(1-2H 1)ethanol and acetophenone were used as case studies to determine the reaction pathway for the hydrogen transfer. Upon complete racemisation all deuterium was present at the 1-position, indicating that the reaction exclusively takes place via a carbon-to-carbon hydrogen transfer. Zeolite Beta with different Si/Al ratios was applied in the racemisation of (S)-1-phenylethanol. In this case the racemisation does not proceed via an oxidation/reduction pathway but via elimination of the hydroxy group and its readdition. This mechanism, however, is not characteristic for all racemisation reactions with zeolite Beta. When 4-tert-butyl cyclohexanone is reduced with this catalyst, a classical MPV reaction takes place exclusively. This demonstrates that zeolite Beta has a substrate dependent reaction pathway.
- Klomp, Dirk,Maschmeyer, Thomas,Hanefeld, Ulf,Peters, Joop A.
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p. 2088 - 2093
(2007/10/03)
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- Stereospecific oxygen rearrangement in the reduction of optically pure methyl mandelate to phenylethanol isomers
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The reduction of methyl (S)-(+)-mandelate, 1, produces the expected 2-phenylethanol, 3, and the unexpected optically pure 1-phenylethanol, 6, by a stereospecific oxygen atom metathesis; which occurs through a styrene oxide intermediate, whose concentration varies with solvent polarity.
- Angelis,Smonou
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p. 488 - 490
(2007/10/03)
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- Formation of cyclopent[a]indene and acenaphthylene from allyl esters of biphenyl mono- and di-carboxylic acids and from biphenyl dicarboxylic anhydrides on flash vacuum pyrolysis at 1000-1100°C
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Flash vacuum pyrolysis at 1000-1100°C of the allyl esters of the three isomeric biphenylcarboxylic acids, of the allyl esters of the 12 biphenyldicarboxylic acids and of the three biphenyldicarboxylic anhydrides gave pyrolysates which were examined by 1H n.m.r. spectroscopy at temperatures below -50°C. In all cases the spectra showed the presence of cyclopent[a]indene and acenaphthylene together with other products. Possible mechanisms for these ring contraction and cyclization processes are discussed and the results of pyrolyses of [2,3-13C2]biphenyl-2,3-dicarboxylic anhydride, and [3,4-13C2]-and (2-2H1)-biphenyl-3,4-dicarboxylic anhydrides are reported.
- Bapat, Jayant B.,Brown, Roger F.C.,Bulmer, Glenn H.,Childs, Trevor,Coulston, Karen J.,Eastwood, Frank W.,Taylor, Dennis K.
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p. 1159 - 1182
(2007/10/03)
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- On the Thermal Cycloisomerization of 1-Vinylnaphthalene to Acenaphthene. A Mechanistic D-Labeling Study
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Unlabeled 1-vinylnaphthalene (1) as well as positionally D-labeled 1 have been prepared and subjected to gas phase pyrolysis at low partial pressures of the educt compounds in nitrogen.At 700 deg C, 1 rearranges exclusively to acenaphthene (2).Under the conditions applied a significant H-D-exchange in the 1-vinylnaphthalene as well as in the formed acenaphthene takes place.The results obtained in this way and those resulting from pyrolysis in hydrogen (instead of nitrogen) show that despite the very high degree of dilution the high selectivity of cycloisomerization from 1 to 2 is firstly controlled by H-atom driven radical chain processes, in which the cyclization of the 2-naphthylethylradical dominates the reaction course.
- Zimmermann, Gerhard,Ondruschka, Bernd,Nuechter, Matthias,Kopinke, Frank-Dieter,Remmler, Matthias
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p. 415 - 420
(2007/10/02)
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- Rearrangements Accompanying the Fragmentation of Ionized 1-Phenylalkan-1-ols
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Some aspects of the fragmentation sequence of 1-phenylalkan-1-ols(C6H5CH(OH)R), which consists of the loss of R(.) followed by the elimination of CO and subsequently of H2, are discussed.Labelling studies and collision activation data of reference compounds allow a mechanism to be proposed for this rearrangement.
- Budzikiewicz, H.,Drabner, G.,Hammes, Ch.
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p. 1326 - 1328
(2007/10/02)
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- Intrinsic Deuterium Isotope Effects of Deuteriated tert-Butyl Groups on the 13C NMR Spectra of Aromatic Compounds
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The 3Δ-deuterium isotope effects of partially and fully deuteriated tert-butyl groups on the 13C NMR spectra of tert-butylbenzene and derivatives are discussed in detail.It is shown that they correlate with the chemical shift of C-1 of the aromatic ring.It has been demonstrated that when deuterium is replaced with some other substituents, the SCS values of these substituents show a parallel behaviour to the deuterium isotope effects.It is concluded that, for the compounds studied, deuterium isotope effects and substituent chemical shifts can be described on a common basis.
- Balzer, Hartmut H.,Berger, Stefan
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p. 437 - 442
(2007/10/02)
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- The Ei Reaction of Substituted threo- and erythro- 1-Phenylethyl Phenyl Sulfoxides
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Substituted (RS,SR)-1-phenylethyl phenyl sulfoxides (threo) (XC6H4S(O)CH(CH3)C6H4Y) and some substituted (RR,SS)-sulfoxides (erythro) were prepared and kinetic investigation for the thermal decomposition was carried out at 80.0, 90.0, and 100.0 deg C in dioxane.Hammett plots for threo-XXC6H4S(O)CH(CH3)C6H5 gave positive ρ-values (ρx=0.60-0.64 at three temoeratures), while those for threo- and erythro-C6H5S(O)CH(CH3)C6H4Y showed V-shape lines with bottoms at the m-OCH3 substituent though the effects of the substituents were small.Meanwhile, large kinetic isotope effects for threo- and erythro-C6H5S(O)CH(O)CH(CH3)C6H4Y (Y=H, p-OMe, m-Cl) (kH/kD=4-6) were observed at all temperatures.The activation energies were in the range of 104-121 kJ mol-1 for all sulfoxides, while the activation entropies were relatively large (7-37 JK-1mol-1) and were correlated with Hammett ?-values to give small negative trend.Reactions of all erythro-isomers examined were 2-3 times faster than those of corresponding threo-isomers.From these results, it is suggested that the pyrolysis of 1-arylethyl aryl sulfoxides proceeds via a concerted mechanism in which the transition state is variable from an Ei'1-like to a conjugated one.In the latter transition state, conjugation of the phenyl group bearing the electron-withdrawing substituent with developing ?-bond electron acidifies the β-proton.
- Yoshimura, Toshiaki,Tsukurimichi, Eiichi,Iizuka, Yukihiko,Mizuno, Hironobu,Isaji, Hiroshi,Shimasaki, Choichiro
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p. 1891 - 1899
(2007/10/02)
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- The effect of the nature of the amine leaving group on the nature of the E2 transition state for the reaction of 1-phenylethylammonium ions with sodium ethoxide in ethanol
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To investigate the effect of the leaving group on the elimination reaction of 1-phenylethylammonium ions with sodium ethoxide in ethanol at 60 deg C, the reaction of seven different quaternary ammonium salts and their β-deuterated analogues with trimethylamine, N-methylpiperidine, N-methyldiethylamine, triethylamine, N,N-dimethylbenzylamine, tripropylamine, and N,N-diethylbenzylamine as leaving groups has been studied.In all cases the elimination, which was shown to proceed via the concerted E2 process, was accompained by competing substitution reactions.Although a significant depedence of the rate of the elimination process on the nature of the leaving group was noted, there was not any linear correlation with the basicity of the amine leaving group.The primary hydrogen-deuterium kinetic isotope effect for the elimination process, (kH/kD)E, was found to increase initially with an increase of reaction rate, kHE, for substrates containing the leaving groups trimethylamine, N-methylpiperidine, N-methyldiethylamine, triethylamine, and N,N-dimethylbenzylamine; i.e., (kH/kD)E=5.03, 5.26, 5.40, 5.83, and 5.85, respectively.A further increase in rate, using substrates with tripropylamine and N,N-diethylbenzylamine as leaving groups resulted in a decrease of the magnitude of the hydrogen-deuterium isotope effect; i.e., (kH/kD)E= 5.42 and 4.67, respectively.It is concluded that steric effects mainly determine leaving group ability.As well, it is concluded that the leaving group ability of the amine determines the structure of the E2 transition state.For the reaction of the poorer leaving groups, trimethylamine, N-methylpiperidine, and N-methyldiethylamine, the proton is more than one-half transferred at the transition state while for reaction involving the two best leaving groups, tripropylamine and N,N-diethylbenzylamine, the Cβ-H bond is less than one-half broken at the transition state.The conclusions are considered in the light of the More O'Ferrall-Jencks potential energy surface diagram.Key words: elimination mechanism, transition state, isotope effects, leaving group, quaternary salts.
- Smith, Peter James,Amin, Md.
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p. 1457 - 1467
(2007/10/02)
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- Gas-Phase Pyrolysis of Phenylcyclopropane
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The gas-phase pyrolysis of phenylcyclopropane (5) has been ivestigated in a quartz tubular reactor between 560 and 680 deg C and a contact time of about 1 s.Main reaction is the thermal isomerization to allylbenzene (4) as well as to cis-(1b) and trans--β-methylstyrene (3b).Besides 1, 3, and 4, indene (8) is formed, which is characterized by a statistical distribution of the H atom over the three positions of the five-membered ring.To distinguish between the two possible mechanisms of the isomerization (opening of the cyclopropane ring C-C bond dissociation/ 1,3-diyl formation and 1,2-C,H retro-insertion/ethylcarbene formation), the D distribution in 1 and 3 was carefully determined by 1H-NMR spectroscopy.The results show that the isomerization is initiated by generation of a 1,3-diradical. - Keywords: Isotopic labeling/ Isomerization/ Phenylcyclopropane/ Pyrolysis
- Kopinke, Frank-Dieter,Zimmermann, Gerhard,Aust, Janette,Scherzer, Klaus
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p. 721 - 726
(2007/10/02)
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- Palladium(II)-Catalyzed Acetalization of Terminal Olefins Bearing Electron-Withdrawing Substituents with Optically Active Diols
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Terminal olefins bearing electron-withdrawing substituents such as CH2=CHCOR (R=Ph, Me, t-Bu), CH2=CHCOOMe, and CH2=CHCN are regioselectively acetalized at the terminal carbon (C1) by diols in the presence of PdCl2 (0.1 equiv) and CuCl (1 equiv) in DME at 50 deg C under an atmosphere of O2 (1 atm).The use of optically active (R,R)-2,4-pentanediol (4) gives homochiral cyclic acetals of aldehyde precursors in good yields.The acetalization of CH2=CHCOR is accompanied by the formation of Michael-type adducts such as 3a (R=Ph).However, of importance is that their formation can be prevented by the use of Na2HPO4 as an additive.Although in an early stage of the reaction of CD2=CHPh with 4, a statistical d scrambling of the starting olefin occurs, no such scrambling is observed with CD2=CHCOPh.Additionally, the acetalization of CD2=CHCOPh with 4 results in 1,2 deuterium migration, together with 25percent d loss.These results are accounted for by the reaction pathways involving oxypalladation, Pd-H elimination, and subsequent ring closure giving enol ether.A catalytic cycle involving the oxygenation of Pd-H species with molecular oxygen is proposed.
- Hosokawa, Takahiro,Ohta, Toshiyuki,Kanayama, Satoshi,Murahashi, Shun-Ichi
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p. 1758 - 1764
(2007/10/02)
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- Isotope effects in nucleophilic substitution reactions. V. The mechanism of the decomposition of 1-phenylethyldimethylphenylammonium halides in chloroform
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Secondary α and β hydrogen-deuterium kinetic isotope effects have been used together to show that the SN reaction between 1-phenylethyldimethylammonium ion and bromide or iodide in chloroform occurs by way of an SN2 mechanism within a triple ion in spite of the fact that it reacts faster than the primary substrate, benzyldimethylphenylammonium bromide.The very loose transition state and steric effects in the ground state appear to be responsible for the unusually fast SN2 reactions between 1-phenylethyldimethylphenylammonium ion and halide ions in chloroform.
- Joly, Helen Alma,Westaway, Kenneth Charles
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p. 1206 - 1214
(2007/10/02)
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