Elimination reactions of (Z)-thiophene- and (Z)-furan-2-carbaldehyde O- benzoyloximes. Effect of β-aryl group upon the nitrile-forming anti transition state

Elimination reactions of (Z)-thiophene- and (Z)-furan-2-carbaldehyde 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 k(H)/k(D) values, and an E2 mechanism is evident. The relative rates of elimination from (Z)-ArCH=NOC(O)C₆H₄Y are 1/1.1/0.6 for Ar = phenyl/thienyl/furyl, respectively. For reactions of 1 with DBU in MeCN, k(H)/k(D) = 8.2 ± 0.1, Hammett ρ = 1.22 ± 0.19, β(1g) = -0.43 ± 0.01, ΔH(+) = 5.9 ± 0.1 kcal/mol, and ΔS(+) = -28.5 ± 0.3 eu have been determined. The corresponding values for 2 are k(H)/k(D) = 8.8 ± 0.2, ρ = 1.87 ± 0.05, β(1g) = -0.55 ± 0.10, ΔH(+) = 6.5 ± 0.1 kcal/mol, and ΔS(+) = -29.0 ± 1.5 eu. The k(H)/k(D), Hammett ρ, and |β(1g)| values increase as the β-aryl group is changed in the order phenyl < thienyl < furyl. The results indicate that the transition state structure for the nitrile-forming elimination changes slightly toward product-like with the change of the β-aryl group.