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Influence of Cu(II) Ions on the Mechanism of the Ring Transformation of S ‐(2‐Oxotetrahydrofuran‐3‐yl)‐ N ‐(4‐methoxyphenyl)isothiouronium Bromide
Author(s) -
Váňa Jiří,
Sedlák Miloš,
Hanusek Jiří
Publication year - 2013
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.20761
Subject(s) - chemistry , bromide , copper , intramolecular force , alkoxide , medicinal chemistry , aqueous solution , ion , inorganic chemistry , stereochemistry , catalysis , organic chemistry
The effect of additional Cu(II) ions on the rate of transformation of S ‐(2‐oxotetrahydrofuran‐3‐yl)‐ N ‐(4‐methoxyphenyl)isothiouronium bromide ( 1 ) into 5‐(2‐hydroxyethyl)‐2‐[(4‐methoxyphenyl)imino]‐1,3‐thiazolidin‐4‐one ( 2 ) has been studied in aqueous buffer solutions. The reaction acceleration in acetate buffers is caused by the formation of a relatively weakly bonded complex ( K c = 600 L·mol −1 ) of substrate with copper(II) acetate in which the Cu(II) ion acts as a Lewis acid coordinating the carbonyl oxygen and facilitating the intramolecular attack, leading to the formation of intermediate T ± . The formation of the complex of copper(II) acetate with free isothiourea in the fast preequilibrium (K c ) is followed by the rate‐limiting transformation (k Cu ) of this complex. At the high concentrations of the acetate anions, the reaction is retarded by the competitive reaction of these ions with copper(II) acetate to give an unreactive complex [Cu(OAc) 4 ] 2− . The influence of Cu(II) ions on the stability of reaction intermediates and the leaving group ability of the alkoxide‐leaving group compared to the Cu(II)‐uncatalyzed reaction is also discussed.