engleski

Kinetic and computational study of the base promoted mechanism of N−benzoilethylpyridinium−4−aldoxime rearrangement.

Our last research revealed that N-benzoylethylpyridinium-4-aldoxime chloride (BEPA4-Cl) was subject to decomposition in alkaline media with the generation of phenyl vinyl ketone and pyridine-4-aldoxime.[1] In addition, BEPA4-Cl underwent rearrangement reaction in highly alkaline solutions with the formation of the corresponding O-alkylated pyridine-4-aldoxime ether which was successfully isolated and characterized.[2] In the present work, in order to elucidate the mechanism of BEPA4-Cl base induced transformations in aqueous alkaline solutions, we have combined the experimental results obtained by kinetic and NMR measurements with theoretical calculations. NMR analysis of buffered aqueous mixtures confirmed that phenyl vinyl ketone and pyridine-4-aldoxime were the sole products in the pH range 7.5­10.5, while the pyridine oxime ether was formed at pH above 10.5. The combined kinetic and computational studies have shown two important aspects of mechanism. First, that only the oximate form of BEPA4-Cl is susceptible to further ionization and consequent β-elimination. Second, that the formation of pyridine oxime ether proceeds via tandem β-elimination/Michael addition reactions where β-elimination is the rate determining process. The deduced kinetic and thermodynamic activation parameters at 25 C and I=0.1 M confirmed that there is no change in mechanism of β-elimination within the investigated pH range. [1] B. Foretić, I. Picek, V. Damjanović, D. Cvijanović, D. Milić, J. Mol. Struct. 1019 (2012) 196–205. [2] I. Picek, J. Plavec, P. Šket, N. Burger, B. Foretić, Neočekivana izomerizacija 1-benzoil-etilpiridinij-4-aldoksim-iona, XXII. Hrvatski skup kemičara i kemijskih inženjera - Knjiga sažetaka / V. Tomašić, K. Maduna Valkaj (ur.), Zagreb : HDKI ; HKD, 2011. 169-169.

pyridine-oxime ether

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