Amine catalysis in the vinylic substitution of α-methylthio-α-arylmethylene Meldrum's acids and its absence in the substitution of methyl β-iodo-α-nitrocinnamate by amines
作者:Michal Beit-Yannai、Xin Chen、Zvi Rappoport
DOI:10.1039/b103486n
日期:——
Substitution of the iodine of (E)- and (Z)-methyl β-iodo-α-nitrocinnamates (5) by amines gives identical (Z)-enamines with aniline (Ani) and piperidine (Pip). No amine catalysis was observed with Pip, Ani, morpholine (Mor), or p-MeOC6H4NHMe (MMA) in MeCN nor with Pip or Mor in EtOH: kPip/kMor = 115â138 (MeCN), 3.3â6.9 (EtOH); kMeCN/kEtOH = 25.5 ± 2.2 (Pip), 0.79â1.16 (Mor); k(Z)-5/k(E)-5 = 1.3â2.9 (13.5 with MMA in MeCN). Replacement of the MeS group in six α-methylthio-α-arylmethylene
Meldrum's acid (6-X) by Pip resulted in amine catalysis in MeCN and EtOH. In EtOH, the p-anisyl derivative (6-MeO) and in MeCN 6-MeO, 6-Me and 6-H displayed second order catalysis in Pip. Other 6-X compounds show orders between one and two in Pip with amine catalyzed (k3B)/non-catalyzed (k2) rate coefficient ratios of 281â731 (EtOH) and 504â635 (MeCN) at 30 °C. kMeCN/kEtOH = 3.0â4.9. In MeCN ÎHâ¡Â = â0.8 to â5.9 kcal molâ1 and ÎSâ¡Â = â50
to â72 e.u. An intermediate zwitterion, 3a, is formed in all cases. For system 5 the rate of Iâ expulsion from 3a exceeds its deprotonation rate, and the observed rate coefficient is composite: kobs = k1k2/kâ1 in MeCN (k1 = rate coefficient of nucleophilic attack) but kobs = k1 in EtOH. In MeCN the deprotonation is faster than the expulsion rate of MeSâ, and more so for 6-X with X = p-Br, p-CF3, m,mâ²-(CF3)2. Different electrophilicities of 6-X, different extents of hydrogen bonding, steric and electronic effects account for the kinetic differences.
用胺取代(E)-和(Z)-甲基δ-碘±-硝基肉桂酸盐 (5) 的碘,可得到与苯胺 (Ani) 和哌啶 (Pip) 相同的 (Z)- 烯胺。在 MeCN 中,Pip、Ani、吗啉 (Mor) 或 p-MeOC6H4NHMe (MMA);在 EtOH 中,Pip 或 Mor 均未发生胺催化作用:kPip/kMor = 115â138 (MeCN), 3.3â6.9(EtOH);kMeCN/kEtOH = 25.5 ±Â 2.2(Pip),0.79â1.16(Mor);k(Z)-5/k(E)-5 = 1.3â2.9 (MMA 在 MeCN 中为 13.5)。在 MeCN 和 EtOH 中,用 Pip 取代 6 个δ-甲硫基-δ-芳基亚甲基梅氏酸(6-X)中的 MeS 基团会产生胺催化作用。在 EtOH 中,对甲氧基苯甲酰衍生物(6-MeO)和在 MeCN 中,6-MeO、6-Me 和 6-H 在 Pip 中显示出二阶催化作用。其他 6-X 化合物在 Pip 中的催化顺序介于一阶和二阶之间,30°C 时,胺催化(k3B)/非催化(k2)速率系数比分别为 281â731 (EtOH)和 504â635 (MeCN),kMeCN/kEtOH=3.0â4.9。在MeCN中,δHÂ=Â0.8至Â5.9 kcal molÂ1,δSÂ=Â50至Â72 e.u。对于体系 5,Iâ从 3a 中排出的速率超过了它的去质子化速率,观察到的速率系数是复合的:在 MeCN 中 kobs = k1k2/kâ1 (k1 = 亲核攻击速率系数),但在 EtOH 中 kobs = k1。在 MeCN 中,去质子化的速度快于 MeSâ 的排出速度,对于 X = p-Br、p-CF3、m,mâ²-(CF3)2 的 6-X 来说更是如此。6-X 的不同亲电性、不同程度的氢键、立体效应和电子效应是造成动力学差异的原因。