Sominsky Lena, Rozental Esther, Gottlieb Hugo, Gedanken Aharon, Hoz Shmaryahu
Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel 52900.
J Org Chem. 2004 Mar 5;69(5):1492-6. doi: 10.1021/jo035251f.
When a solution of a carbonyl compound in alcohol (primary or secondary) is heated to ca. 300 degrees C, a disproportionation reaction, in which a carbonyl compound is reduced to the corresponding alcohol and the alcohol is oxidized to the corresponding ketone, takes place. This uncatalyzed variation of the Meerwein-Ponndorf-Oppenauer-Verley reaction gives, in certain cases, e.g., reduction of acetophenone or benzaldehyde by i-PrOH, almost quantitative yields. Yields are higher with secondary alcohols such as i-PrOH than with a primary alcohol such as EtOH. The reactions were also performed in a flow system by passing at a slow rate the same solutions through a glass or a metal coil heated to elevated temperatures. Ab initio calculations performed at the B3LYP/6-31G* level show that thermodynamically i-PrOH is a more potent reducing agent than EtOH by ca. 4 kcal/mol. The computations also show that in cases of aromatic carbonyl compounds, part of the deriving force is obtained from the entropy change of the reaction. The major contributor to the high yield, however, is the excess alcohol used, which shifts the equilibrium to the right. Calculated entropy of activation as well as isotopic H/D labeling suggest a cyclic transition state.
当羰基化合物在醇（伯醇或仲醇）中的溶液加热到约300℃时，会发生歧化反应，即羰基化合物被还原为相应的醇，而醇被氧化为相应的酮。这种未催化的迈尔外因-庞多夫-奥彭瑙尔-韦利反应在某些情况下，例如用异丙醇还原苯乙酮或苯甲醛时，能给出几乎定量的产率。与伯醇（如乙醇）相比，使用仲醇（如异丙醇）时产率更高。这些反应也在流动体系中进行，即将相同的溶液以缓慢的速率通过加热到高温的玻璃或金属盘管。在B3LYP/6 - 31G*水平上进行的从头算计算表明，从热力学角度看，异丙醇作为还原剂比乙醇的效力约高4千卡/摩尔。计算还表明，对于芳香族羰基化合物，部分驱动力来自反应的熵变。然而，高产率的主要贡献因素是使用了过量的醇，这使平衡向右移动。计算得到的活化熵以及同位素H/D标记表明存在一个环状过渡态。
