对初级同位素的结构反应效应,对环置换的α-甲氧化的质子化效应
Wing-Yin Tsang1, John P Richard
1Department of Chemistry, University of Buffalo, SUNY, Buffalo, New York 14260-3000, USA.
Journal of the American Chemical Society
|October 1, 2009
概括
这项研究研究了α-methoxystyrenes的质子化中的初级动态同位素效应 (PIEs). 在0的反应驱动力附近观察到最大的PIE,这表明转移具有对称的过渡状态.
科学领域:
- 物理有机化学 有机化学
- 化学动力学 化学动力学
- 同位素效应的影响
背景情况:
- 初级动态同位素效应 (PIEs) 提供了对反应机制和过渡状态结构的洞察.
- 质子转移反应在有机化学和催化中具有根本性的作用.
- 了解PIE中反应驱动力的作用对于力学阐明至关重要.
研究的目的:
- 用于计算由L(+) 和碳酸催化环置换的α-甲氧化的质子化 PIEs.
- 为了将PIE与反应驱动力相关联,以确定过渡状态特征.
- 为了研究量子力学道在子转移中的潜力.
主要方法:
- 使用质子核磁共振 (1H NMR) 量化初级产品同位素效应 (PIEs).
- 对各种酸催化质子反应的反应驱动力 (DeltaG(o)) 的测量.
- 对PIEs的Arrhenius型图表进行分析,以确定激活能差异和PIE限制值.
主要成果:
- 通过二乙酸 (DeltaG(o) = 1.0 kcal/mol) 对4-MeO-alpha-methoxystyrene的质子化观察到8.7的最大PIE.
- 随着反应驱动力的增加或减少,PIEs下降,最大值约为DeltaG(o) = 0.
- 计算的参数[EaH] - (EaD) = -1.2 kcal/mol和 (AH) /AD) =1.0与能量屏障上的子转移一致,没有道挖掘的证据.
结论:
- 根据PIE数据,转移的离子在酸供体和接受体之间大约处于同等距离的过渡状态.
- 这些发现与之前的研究相反,这些研究表明基于Brønsted参数的产品类过渡状态.
- 观察到的PIE支持一种反应机制,涉及水子在障碍物上的转移,而不是通过量子力学道.
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