Chiang Yvonne, Kresge A Jerry, Zhu Yu
Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada.
Photochem Photobiol Sci. 2002 Jan;1(1):67-70. doi: 10.1039/b107860g.
Cis and trans isomeric 5-methoxy-o-quinone alpha-phenylethides were generated by flash photolysis of o-hydroxybenzyl alcohol and o-hydroxystyrene precursors and their rates of decay were measured in aqueous solutions of perchloric acid, sodium hydroxide, and biphosphate (dihydrogen phosphate) ion and carbonate ion buffers. The data so obtained gave two parallel rate profiles displaced from one another by about an order of magnitude, the faster of which could be assigned to the cis isomer and the slower to the trans isomer. Both rate profiles showed acid catalyzed, uncatalyzed, and hydroxide ion catalyzed portions, with acid catalysis becoming saturated at the high acidity end of the range investigated. This saturation implies a pre-equilibrium mechanism involving quinone ethide protonation on carbonyl oxygen for the acid catalyzed process, and analysis of the data shows the trans isomer to be a slightly stronger base than the cis isomer. The fact that two separate rate profiles are observed implies that the cationic intermediates formed by substrate protonation in these acid catalyzed processes do not interconvert on the time scale of the quinone ethide decay reactions, and that in turn indicates that the former ethide bonds of the cationic intermediates have substantial double-bond character; a lower limit of 10 kcal mol(-1) may be estimated for the barrier to rotation about this bond.
通过对邻羟基苄醇和邻羟基苯乙烯前体进行闪光光解生成顺式和反式异构的5-甲氧基-邻醌α-苯乙基化物，并在高氯酸、氢氧化钠、磷酸二氢根离子和碳酸根离子缓冲液的水溶液中测量它们的衰减速率。所获得的数据给出了两个平行的速率曲线，彼此相差约一个数量级，其中较快的可归因于顺式异构体，较慢的可归因于反式异构体。两个速率曲线都显示出酸催化、无催化和氢氧根离子催化的部分，在研究范围的高酸度端酸催化趋于饱和。这种饱和意味着酸催化过程中涉及醌乙在羰基氧上质子化的预平衡机制，对数据的分析表明反式异构体是比顺式异构体稍强的碱。观察到两个独立的速率曲线这一事实意味着在这些酸催化过程中由底物质子化形成的阳离子中间体在醌乙衰减反应的时间尺度上不会相互转化，这反过来表明阳离子中间体的前乙键具有相当大的双键特征；围绕该键旋转的势垒下限估计为10千卡摩尔（-1）。
