Reports in the literature appear to differ on the effects of some C3 substituents on the relative efficiencies of isotope exchange in the nonidentical C2- and C6-positions catalyzed by organoiridium complexes. Controlled experiments were conducted using a set of model substrates in attempts to clarify these effects. The results clearly showed that, in common with most previous findings, alkyl substituents at C3 reduced the rate of isotope incorporation into C2 relative to C6, as expected on steric grounds. In contrast, all substituents possessing electron lone pairs resulted in a lessening of the inhibition of C2-vs-C6 labeling or promoted C2 labeling to such a degree that it became faster than that at C6. NMR measurements on equimolar mixtures of active iridium complex with selected substrates revealed that the ratios of C2- and C6-iridacycles present in solution correlated with the relative rates of ortho-deuteration in the rate studies. The results of the two studies, taken together, suggest that conventional explanations for the origin of the positive meta-effect may not be adequate for the present system. An alternative hypothesis is advanced. Copyright © 2009 John Wiley & Sons, Ltd.
对于某些 C3 取代基对
有机铱配合物催化的不同 C2 和 C6 位同位素交换相对效率的影响,文献中的报道似乎有所不同。使用一组模型基质进行对照实验,试图阐明这些影响。结果清楚地表明,与大多数先前的发现一样,C3 上的烷基取代基相对于 C6 降低了同位素并入 C2 的速率,正如空间方面所预期的那样。相反,所有具有电子孤对对的取代基都会导致C2-vs-C6标记的抑制减弱或促进C2标记达到比C6标记更快的程度。对活性
铱络合物与选定底物的等摩尔混合物进行的 NMR 测量表明,溶液中存在的 C2- 和 C6-
铱环的比例与速率研究中邻位
氘化的相对速率相关。两项研究的结果综合起来表明,对积极元效应起源的传统解释可能不足以适应当前的系统。提出了另一种假设。版权所有 © 2009 约翰·威利父子有限公司