Elimination Reactions of (E)- and (Z)-Benzaldehyde O-Benzoyloximes. Transition State Differences for the Syn- and Anti-Eliminations Forming Nitriles

Bong-Rae Cho, Hak Suk Chung, Nam Soon Cho

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Abstract

Elimination reactions of (E)- and (Z)-benzaldehyde O-benzoyloximes 1 and 2 with DBU in MeCN have been investigated kinetically. The reactions are second order and exhibit substantial values of Hammett ρ and kH/kD values, and an E2 mechanism is evident. The rate of elimination from 2 is approximately 36000 fold faster than that from 1. For reactions of 1 with DBU in MeCN, kH/kD = 3.3 ± 0.2, Hammett ρ value of 2.19 ± 0.05, β1g = -0.49 ± 0.02, ΔH‡ = 10.4 ± 0.6 kcal/mol, and ΔS‡ = -34.3 ± 2.6 eu have been determined. The corresponding values for 2 are kH/kD = 7.3 ± 0.2, ρ = 1.21 ± 0.05, β1g = -0.40 ± 0.01, ΔH‡ = 6.8 ± 0.5 kcal/mol, and ΔS‡ = -25.8 ± 1.9 eu, respectively. The results indicate that the anti-eliminations from 2 proceed via more symmetrical transition states with smaller degrees of proton transfer and Nα-OC (O)Ar bond cleavage, less negative charge development at the β-carbon, and a greater extent of triple bond formation than that for the syn-elimination.

Original languageEnglish
Pages (from-to)4685-4690
Number of pages6
JournalJournal of Organic Chemistry
Volume63
Issue number14
Publication statusPublished - 1998 Jul 10

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ASJC Scopus subject areas

  • Organic Chemistry

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