- Kinetics and mechanism of the protonation of 6-(triphenylphosphonio-3′ -cyclopentadienyl)-2,3,5-trihalocyclohexa-2,5-diene-1,4-dione by trifluoroacetic acid in aprotic media
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6-(Triphenylphosphonio-3′-cyclopentadienyl)-2,3,5-trihalocyclohexa-2, 5-diene-1,4-diones (2) are a new class of phosphorus containing dyes that behave as weak bases. Mono-protonation occurs at the cyclopentadiene ring leading to three isomers as assessed by NMR. The kinetics of the protonation were studied by the stopped-flow technique in aprotic media. The activation energies and deuterium isotope effects suggest that the rate-limiting step is the addition of trifluoroacetic acid to the quinone ring rather than direct protona-tion of the cyclopentadiene ring.
- Perez-Pla,Housseini, Saad,Palou,Hall
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p. 561 - 574
(2007/10/03)
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- Kinetics and Mechanism of the Addition of Triphenylphosphoniocyclopentadienide to Tetrahalo-p-benzoquinones. Part 2. Reaction with Bromanil and Iodanil
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The reaction of tetrahalo-p-benzoquinones with triphenylphosphoniocyclopentadienide yields 6-(triphenylphosphino-3'-cyclopentadienyl)-2,3,5-trihalocyclohexa-2,5-diene-1,4-dione (4a-d), a new class of zwitterionic dyes containing phosphorus.The rate-limiting step has been found to be the addition of the ylide to the quinone through a highly polar betaine intermediate.The elimination of hydrogen halide from the betaine, is of the E2 or E1cB type for the bromanil system and of the E1 type for iodanil.SCF-AM1 calculations suggest that the E1cB path is energetically favoured relative to the E1 elimination path.
- Valero, Rosa,Pla, Francisco Perez,Palou, Juan,Hall, C. Dennis,Speers, Peter
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p. 425 - 434
(2007/10/02)
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- Kinetics and Mechanism of The Addition of Triphenylphosphoniocyclopentadienide to Tetrachloro-p-benzoquinone
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The kinetics and mechanism of the reaction of triphenylphosphoniocyclopentadienide 1 with tetrachloro-p-benzoquinone (chloranil 2a) to yield 2,3,5-trichloro-6-(3-triphenylphosphoranylidenecyclopenta-1,4-dienyl)-p-benzoquinone 3 in dichloromethane solution are reported.The rate limiting step was found to be electrophilic attack of tetrachloro-p-benzoquinone on the aromatic cyclopentadiene ring of the ylide.A thermochemical study of the system showed a small enthalpy and a large negative entropy of activation in agreement with the existence of a highly polar betaineas an intermediate in the reaction.The proposed mechanism involves bimolecular transfer of a proton to a molecule of ylide.No evidence was found either for the formation of a ?-complex or for acid catalysis of the reaction.
- Pla, Francisco Perez,Palou, Juan,Valero, Rosa,Hall, C. Dennis,Speers, Peter
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p. 1925 - 1932
(2007/10/02)
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- THE KINETICS AND MECHANISM OF THE REACTION OF ONIUM CYCLOPENTADIENYLIDES WITH TETRAHALO-p-BENZOQUINONES
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The reaction of triphenylphosphonium cyclopentadienylide (1) with halogen-substituted p-benzoquinones (4) is shown to give a new class of dipolar (zwitterionic) dyes (5) containing phosphorus.The general structure of these molecules has been investigated by a combination of mass spectrometry and multinuclear (1H, 13C and 31P) nmr using the specialist techniques of DEPT spectroscopy, homonuclear (COSY) and heteronuclear 2-D nmr.In addition, stopped-flow (uv/visible) techniques have been used to study the kinetics of the reactions and hence demonstrate that the rate-limiting step is nucleophilic addition of the ylid nucleophile to the quinone, followed by a rapid loss of halide ion.This mechanism follows the classical pattern associated with nucleophilic aromatic substitution in activated aryl halides. - Key words: dipolar dyes containing phosphorus.
- Hall, C. Dennis,Speers, Peter,Valero, Rosa,Pla, Francesco Perez,Denney, Donald B.
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p. 249 - 254
(2007/10/02)
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