- Lawesson's reagent for direct thionation of hydroxamic acids: Substituent effects on LR reactivity
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To explore the generality and scope of direct thionation of hydroxamic acids (HAs), the reaction of various structurally diverse HAs with Lawesson's reagent was investigated. The yield of thiohydroxamic acid (THAs) is poor when HAs possess bulky acyl and/or N-substituents, acidic α-hydrogen atoms, or an N-phenyl ring. THAs yields were correlated with Brown sigma parameter. The relative rates of two subsequent processes kT2 and kR2 were also measured. Correlation was also found for methine proton chemical shifts of N-isopropyl benzothiohydroxamic acids.
- Przychodzen, Witold
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p. 676 - 684
(2007/10/03)
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- Gas-Phase Pyrolytic Reactions. Rate Data for Pyrolysis of N-t-Butylthioacetamide and N-Acetylthioacetamide: Role of Polarity of Transition State and γ-Carbonyl Group Protophilicity
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In the gas phase, both N-t-butylthioacetamide and N-acetylthioacetamide undergo unimolecular first-order elimination reactions for which log A = 11.58 s-1 and 10.64 s-1, and Ea = 16.45 kJ mol-1 and 117.15 kJ mol-1, respectively.The results are in accord with a reaction pathway involving a cyclic six-membered transition state, and show each compound to be more reactive than its oxygen-containing analogue.At 600 K, the statistically corrected reactivity ratios: t-butyl thioacetate (1)/t-butyl acetate (2); N-acetylthioacetamide (3)/diacetamide (4); and N-t-butylthioacetamide (5)/N-t-butylacetamide (6) are 83, 173, and 1 404, respectively.The above rate factors are consistent with the tenet that as Cα-X bond fission becomes less rate-contributing in these electrocyclic reactions, so attack by the C=Y bond upon the β-hydrogen atoms becomes more important.Thus, whereas t-butyl acetate at 600 K is some 68 700 times more reactive than N-t-butylacetamide, t-butyl thioacetate is only 4 060 times more reacitve than N-t-butylthioacetamide.
- Al-Awadi, Nouria A.,Al-Bashir, Rasha F.,El-Dusouqui, Osman M. E.
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p. 579 - 582
(2007/10/02)
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