The Molecularity of the Newman−Kwart Rearrangement

详细信息    查看全文

• 作者：Matthew Burns ; Guy C. Lloyd-Jones ; Jonathan D. Moseley ; Joseph S. Renny
• 刊名：Journal of Organic Chemistry
• 出版年：2010
• 出版时间：October 1, 2010
• 年：2010
• 卷：75
• 期：19
• 页码：6347-6353
• 全文大小：871K
• 年卷期：v.75,no.19(October 1, 2010)
• ISSN：1520-6904

文摘

It was recently reported that the venerable Newman−Kwart rearrangement (1→2) proceeds via mixed first- and second-order kinetics. Prior to this, the rearrangement had been considered to proceed exclusively via an intramolecular O_Ar→S_Ar migration. A new bimolecular pathway, possibly involving an 8-membered cyclic transition state, was proposed to account for reaction rates that increased disproportionately with substrate concentration under microwave heating conditions. We report a reanalysis of the kinetics and molecularity of the rearrangement of N,N-dimethyl O-(p-nitrophenyl)thiocarbamate 1a in N,N-dimethylacetamide solvent. Using HPLC, isotopic labeling (²H, ¹⁸O, ³⁴S), and ESI-ICRMS methods, we show that there is no evidence for a bimolecular pathway en route to 2a, with near-perfect exponential decay in 1a at concentrations ranging from 0.11 to 4.70 M. Instead, it is demonstrated that under the microwave heating conditions, a delayed negative feedback signal to the microwave power balancing loop results in oscillatory reaction overheating. Due to higher tan δ in the solute, the amplitude of this oscillation increases with the concentration of 1a, and this phenomenon best accounts for the kinetic behavior previously misinterpreted as being mixed first- and second-order in nature.