- THIONYL CHLORIDE - A GOOD LIGAND COUPLING REAGENT
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Thionyl chloride was found to react with phenylethynyllithium and heteroaryllithium affording 1,4-diphenylbutadiyne and biheteroaryl, and the corresponding sulfoxides and sulfides, respectively.Apparently, ligand coupling proceeded within the intermediary sulfurane.The occurrence of ligand coupling was estimated from the pKa value of the carbon acid as a nucleophile.Furthermore, it was found that the presence of p-orbital, i.e., ?-bond, is preferable at the ipso carbon atom of the nucleophile for a good ligand coupling reaction, while for alkyllithiums and alkylmagnesium chloride do not give bialkyl as the main coupling product, but others, which are different from the organic lithiums or magnesium chloride used. Key words: Thionyl chloride, hypervalent, ligand coupling, sulfurane
- Oae, Shigeru,Inubushi, Yoichi,Yoshihara, Masakuni
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p. 101 - 110
(2007/10/02)
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- Studies of the Borderline between Concerted and Stepwise Mechanisms of Elimination : E1cB Elimination of Fluoren-9-ylmethyl Carboxylate Esters
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Rates of β-elimination of carboxylate leaving groups from fluoren-9-ylmethyl carboxylate esters in methanolic sodium methoxide at 25 deg C are reported.An E1cB mechanism with rate-determining formation of a carbanion intermediate is assigned on the basis of near identity of measured elimination rates and rates of carbanion formation predicted from a Taft correlation, and the similarity with elimination of 1-(1-acetoxy-1-methylethyl)indene for which the mechanism has been established by Ahlberg and Thibblin.Values of ρ=0.42 and βlg=0.27 measured for substituted benzoate leaving groups are a little larger than expected (ca. 0.24 and 0.18, respectively) and the discrepancy is tentatively ascribed to conformational enhancement of remote substituent effects, rather than to a contribution of E2 elimination.The effects of alkyl and aryl substitution α to the leaving group are discussed, especially in relation to the borderline between concerted and stepwise mechanisms.The measurements fail to confirm an earlier inference that the borderline shows a discontinuity in transition-state structure at the point of mechanistic change.
- O'Ferrall, Rory A. More,Larkin, Finbar,Walsh, Peter
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p. 1573 - 1580
(2007/10/02)
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