Nozière Barbara, Córdova Armando
Department of Meteorology, Stockholm University, SE-106 91 Stockholm, Sweden.
J Phys Chem A. 2008 Apr 3;112(13):2827-37. doi: 10.1021/jp7096845. Epub 2008 Mar 4.
The amino acid catalyzed aldol condensation is of great interest in organic synthesis and natural environments such as atmospheric particles. However, kinetic and mechanistic information on these reactions is limited. In this work the kinetics of the aldol condensation of acetaldehyde in water and aqueous salt solutions (NaCl, CaCl2, Na2SO4, MgSO4) catalyzed by five amino acids (glycine, alanine, serine, arginine, and proline) at room temperature (295 +/- 2 K) has been studied. Monitoring the formation of three products, crotonaldehyde, 2,4-hexadienal, and 2,4,6-octatrienal, by UV-vis absorption over 200-1100 nm revealed two distinct kinetic regimes: at low amino acid concentrations (in all cases, below 0.1 M), the overall reaction was first-order with respect to acetaldehyde and kinetically limited by the formation of the enamine intermediate. At larger amino acid concentrations (at least 0.3 M), the kinetics was second order and controlled by the C-C bond-forming step. The first-order rate constants increased linearly with amino acid concentration consistent with the enamine formation. Inorganic salts further accelerated the enamine formation according to their pKb plausibly by facilitating the iminium or enamine formation. The rate constant of the C-C bond-forming step varied with the square of amino acid concentration suggesting the involvement of two amino acid molecules. Thus, the reaction proceeded via a Mannich pathway. However, the contribution of an aldol pathway, first-order in amino acid, could not be excluded. Our results show that the rate constant for the self-condensation of acetaldehyde in aqueous atmospheric aerosols (up to 10 mM of amino acids) is identical to that in sulfuric acid 10-15 M (kI approximately 10-7-10-6 s-1) clearly illustrating the potential importance of amino acid catalysis in natural environments. This work also demonstrates that under usual laboratory conditions and in natural environments aldol condensation is likely to be kinetically controlled by the enamine formation. Notably, kinetic investigations of the C-C bond-forming addition step would only be possible with high concentrations of amino acids.
氨基酸催化的羟醛缩合反应在有机合成以及大气颗粒物等自然环境中备受关注。然而，关于这些反应的动力学和机理信息有限。在本研究中，我们考察了室温（295±2 K）下，五种氨基酸（甘氨酸、丙氨酸、丝氨酸、精氨酸和脯氨酸）在水和盐水溶液（NaCl、CaCl2、Na2SO4、MgSO4）中催化乙醛羟醛缩合反应的动力学。通过紫外-可见吸收光谱监测200-1100 nm范围内巴豆醛、2,4-己二烯醛和2,4,6-辛三烯醛三种产物的生成，结果显示出两种不同的动力学模式：在低氨基酸浓度下（在所有情况下，低于0.1 M），总反应对乙醛为一级反应，动力学上受烯胺中间体形成的限制。在较高氨基酸浓度下（至少0.3 M），动力学为二级反应，由碳-碳键形成步骤控制。一级速率常数随氨基酸浓度呈线性增加，这与烯胺的形成一致。无机盐根据其pKb值，可能通过促进亚胺离子或烯胺的形成进一步加速烯胺的形成。碳-碳键形成步骤的速率常数随氨基酸浓度的平方而变化，这表明有两个氨基酸分子参与反应。因此，反应通过曼尼希途径进行。然而，不能排除氨基酸一级反应的羟醛途径的贡献。我们的结果表明，乙醛在大气水气溶胶中（高达10 mM氨基酸）的自缩合反应速率常数与在10-15 M硫酸中的相同（kI约为10-7-10-6 s-1），这清楚地说明了氨基酸催化在自然环境中的潜在重要性。这项工作还表明，在通常的实验室条件和自然环境中，羟醛缩合反应在动力学上可能受烯胺形成的控制。值得注意的是，只有在高浓度氨基酸条件下才有可能对碳-碳键形成加成步骤进行动力学研究。
