Reactions of carbonyl compounds in basic solutions. Part 24.1 The mechanism of the base-catalysed ring fission of substituted benzocyclobutene-1,2-diones
作者:Keith Bowden、M. Vahid Horri
DOI:10.1039/a606310a
日期:——
The rate coefficients for the base-catalysed ring fission of a
series
of substituted benzocyclobutenediones to give the corresponding
2-formylbenzoic acids have been determined in water at 25.0 and
60.0 °C. The effects of 4-substituents and
4,5-di-substituents on the rates have been correlated using a modified
Hammett equation to give a reaction constant, ρ, equal to
ca. 3.6 at 25.0 °C. The activation
parameters have been calculated. The effect of solvent composition on
the rates has been studied. The kinetic solvent isotope effect, product
composition and enrichment in 18O-enriched water have
also been studied. All the evidence indicates a mechanistic pathway
which proceeds by rapid reversible addition of hydroxide anion to the
dione, followed by intramolecular nucleophilic attack on the second
carbonyl group and formation of a carbanionic intermediate.
在25.0和60.0°C下,测定了水溶液中一系列取代苯并环丁二烯二酮在碱催化开环反应中生成相应2-甲酰基苯甲酸的速率系数。使用改进的哈米特方程,对4-取代基和4,5-二取代基对速率的影响进行了关联,计算出反应常数ρ在25.0°C时约为3.6。计算了活化参数。研究了溶剂组成对速率的影响。还研究了动力学溶剂同位素效应、产物组成和在18O富集水中的富集情况。所有证据表明,反应机制是通过羟基离子对二酮的快速可逆加成,随后分子内亲核攻击第二个羰基,形成碳负离子中间体。