Cho Bong Rae, Cho Nam Soon, Lee Sang Kook
Department of Chemistry, Korea University, Seoul 136-701, Korea.
J Org Chem. 1997 Apr 4;62(7):2230-2233. doi: 10.1021/jo9618637.
Elimination reactions of (E)- and (Z)-benzaldehyde O-pivaloyloximes 1 and 2 with DBU in MeCN have been investigated kinetically. The reactions are second order and exhibit substantial values of Hammett rho and k(H)/k(D) values, and an E2 mechanism is evident. The rate of elimination from 2 is approximately 20 000-fold faster than that from 1. For reactions of 1 with DBU in MeCN, a Hammett rho value of 2.4 +/- 0.1, k(H)/k(D) = 2.7 +/- 0.3, DeltaH() = 12.5 +/- 0.2 kcal/mol, and DeltaS() = -31.0 +/- 0.6 eu have been determined. The corresponding values for 2 are rho = 1.4 +/- 0.1, k(H)/k(D) = 7.8 +/- 0.3, DeltaH() = 8.8 +/- 0.1 kcal/mol, and DeltaS() = -23.6 +/- 0.4 eu, respectively. The results indicate that the nitrile-forming anti eliminations from 2 proceed via a more symmetrical transition state with a smaller degree of proton transfer, less negative charge development at the beta-carbon, and greater extent of triple-bond formation than that for the syn elimination.
对（E）-和（Z）-苯甲醛O-新戊酰肟1和2在乙腈中与DBU的消除反应进行了动力学研究。这些反应是二级反应，具有较大的哈米特ρ值和k(H)/k(D)值，E2机理明显。2的消除速率比1快约20000倍。对于1与DBU在乙腈中的反应，已确定哈米特ρ值为2.4±0.1，k(H)/k(D)=2.7±0.3，ΔH() = 12.5±0.2 kcal/mol，以及ΔS() = -31.0±0.6 eu。2的相应值分别为ρ = 1.4±0.1，k(H)/k(D)=7.8±0.3，ΔH() = 8.8±0.1 kcal/mol，以及ΔS() = -23.6±0.4 eu。结果表明，与顺式消除相比，2形成腈的反式消除通过更对称的过渡态进行，质子转移程度较小，β-碳上负电荷发展较少，三键形成程度更大。
