Oxyhalogen-Sulfur Chemistry: Kinetics and Mechanism of Oxidation of N-Acetyl Homocysteine Thiolactone by Acidified Bromate and Aqueous Bromine

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• 作者：Wilbes Mbiya ; Boyoung Choi ; Bice S. Martincigh ; Moshood K. Morakinyo ; Reuben H. Simoyi
• 刊名：Journal of Physical Chemistry A
• 出版年：2013
• 出版时间：December 12, 2013
• 年：2013
• 卷：117
• 期：49
• 页码：13059-13069
• 全文大小：569K
• 年卷期：v.117,no.49(December 12, 2013)
• ISSN：1520-5215

文摘

N-Acetyl homocysteine thiolactone (NAHT), medically known as citiolone, can be used as a mucolytic agent and for the treatment of certain hepatic disorders. We have studied the kinetics and mechanisms of its oxidation by acidic bromate and aqueous bromine. In acidic bromate conditions the reaction is characterized by a very short induction period followed by a sudden and rapid formation of bromine and N-acetyl homocysteine sulfonic acid. The stoichiometry of the bromate鈥揘AHT reaction was deduced to be: BrO₃^{鈥?/sup> + H₂O + CH₃CONHCHCH₂CH₂SCO 鈫?CH₃CONHCHCH₂CH₂(SO₃H)COOH + Br鈥?/sup> (S1) while in excess bromate it was deduced to be: 6BrO₃鈥?/sup> + 5CH₃CONHCHCH₂CH₂SCO + 6H+ 鈫?3Br₂ + 5CH₃CONHCHCH₂CH₂(SO₃H)COOH + 2H₂O (S2). For the reaction of NAHT with bromine, a 3:1 stoichiometric ratio of bromine to NAHT was obtained: 3Br₂ + CH₃CONHCHCH₂CH₂SCO + 4H₂O 鈫?6Br鈥?/sup> + CH₃CONHCHCH₂CH₂(SO₃H)COOH + 6H+ (S3). Oxidation occurred only on the sulfur center where it was oxidized to the sulfonic acid. No sulfate formation was observed. The mechanism involved an initial oxidation to a relatively stable sulfoxide without ring-opening. Further oxidation of the sulfoxide involved two pathways: one which involved intermediate formation of an unstable sulfone and the other involves ring-opening coupled with oxidation through to the sulfonic acid. There was oligooscillatory production of aqueous bromine. Bromide produced in S1 reacts with excess bromate to produce aqueous bromine. The special stability associated with the sulfoxide allowed it to coexist with aqueous bromine since its further oxidation to the sulfone was not as facile. The direct reaction of aqueous bromine with NAHT was fast with an estimated lower limit bimolecular rate constant of 2.94 卤 0.03 脳 102 M鈥? s鈥?.}