The role of the α-effect in determining nucleophilic reactivities: aromatic substitutions
作者:Gino Biggi、Francesco Pietra
DOI:10.1039/j29710000044
日期:——
ideas a large α-effect should have been predicted. These results imply that a high β Brønsted value for nucleophilic substitutions at unsaturated centres may not be a sufficient condition for the presence of a clear α-effect. Therefore we suggest that the occurrence of a clear α-effect requires also a particular structure of the transition state such as that possessed by nucleophilic displacements
在3:2的二恶烷水中,氯被2,4-二硝基苯中的氯定量取代为多种胺,包括“α-亲核试剂”,如甲氧基胺或肼。在所有情况下,均获得相对于两种试剂的二阶动力学。主正烷基胺,跨越近五P上的间隔ķ一个'单元,很好地适应扩展布朗斯台德方程日志ķ = 0·63P ķ一个' - 8·45,其中ķ是二阶速率系数和对ķ一个'值已在动力学介质中确定。肼和甲氧基胺仅显示正偏差。1·3 log k相对于伯正烷基胺的直线而言,单位为1个单位。有人认为,如此小的偏差的起因尚不明确,而根据目前的观点,应该已经预测出较大的α效应。这些结果表明,对于存在明显的α效应,在不饱和中心进行亲核取代的高β布朗斯台德值可能不是足够的条件。因此,我们认为,清楚的α效应的发生还需要过渡态的特定结构,例如存在清晰的α效应的羰基碳的亲核置换所具有的结构。