- Precise Control of the Formation of a Covalent and an Ionic Bond in Carbocation-Carbanion Combination Reactions
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The electronic effect on the selectivity of covalent or ionic bond formation was examined for the reaction of Kuhn's anion 1(1-) (C67H39(1-); tris(7H-dibenzofluorenylidenemethyl)methide ion) and 1-aryl-2,3-dicyclopropylcyclopropenylium ions.The carbocation stability was progressively changed by varying the substituent on the phenyl ring, while the steric effect was kept essentially unchanged.The cations having the p-chlorophenyl (2a(1+)), phenyl (2b(1+)), m-methylphenyl (2c(1+)), or m,m'-dimethylphenyl (2d(1+)) group gave a covalent product, whereas a carbocation-carbanion salt was obtained from the cations having the p-methylphenyl (2e(1+)) or p-methoxyphenyl (2f(1+)) group.The reduction potentials Ered of the cations, as determined by cyclic voltammetry, showed that the formation of the covalent or ionic product is switched by a small difference in stability ( 0.4 kcal/mol) between 2d(1+) and 2e(1+).In chloroform, the salts 1(1-)2e(1+) and 1(1-)2f(1+) were transformed into covalent forms 1-2e and 1-2f, which can exist only in solution.When 1-(2a-d) and 1(1-)2e,f(1+) were dissolved in DMSO, equilibrium between a covalent compound and ions was established.A plot of the free energy of heterolysis ΔG0het for 1-(2a-f) against the Ered of the corresponding cations 2a-f(1+) showed that ΔG0het decreases as the cation is more stabilized.The heterolysis in DMSO was shown to be enhanced by ca. 13 kcal/mol both by the steric congestion in the covalent molecules and the stabilization of the cyclopropenylium ions by solvation.
- Takeuchi, Ken'ichi,Kitagawa, Toshikazu,Miyabo, Atsushi,Hori, Hideshi,Komatsu, Koichi
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p. 5802 - 5810
(2007/10/02)
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- Vinylcations, 39. Zinc Chloride Catalysed Addition of Hydrogen Chloride to Cyclopropylalkynes
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Zinc chloride catalysed addition of hydrogen chloride to 1-cyclopropylalkynes 5a-e (R = CH3, c-C3H5, phenyl, p-tolyl, 4-methoxyphenyl) is studied and the results are compared with those of the addition of HCl/ZnCl2 to several substituted arylalkynes 10a-h.Thus, the alkynes are reacted with HCl/ZnCl2 in dichloromethane and the reaction products are investigated also with respect to their stereochemistry.All alkynes yield predominantly the direkt hydrogen chloride addition products.The 1-cyclopropylalkynes 5a-d give (E)-1-chloro-1-cyclopropyl-1-alkenes 15, and (E)-1-chloro-2-cyclopropyl-1-(4-methoxyphenyl)ethene (16e) is obtained as the major product from 5e (R = 4-CH3OC6H4).Moreover, ring opening to homoallenyl chlorides 19 and, as a side reaction, formation of the ketones 17 and 18 by the addition of water are observed.In a secondary addition reaction, the dichlorides 20 are also obtained by homoallyl rearrangement.The arylalkynes 10a-g react preferentially with formation of (E)-1-aryl-1-chloroalkenes 21.Relative rates are obtained by inter- and intramolecular competition reactions of the alkynes 23 and 5b-e with HCl/ZnCl2 showing the order of stabilization by substituents of the intermediate vinyl cation 2 to be 4-ClC6H4 E2 mechanism.The preferential formation of the addition products E-15, E-16, and E-21 is attributed to a syn-vinyl cation ion pair and to steric approach control of the β-substituents in the vinyl cation intermediate 2.
- Hanack, Michael,Weber, Erhard
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p. 777 - 797
(2007/10/02)
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