A New Insight into Using Chlorine Leaving Group and Nucleophile Carbon Kinetic Isotope Effects To Determine Substituent Effects on the Structure of SN2 Transition States

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• 作者：Kenneth C. Westaway ; Yao-ren Fang ; Susanna MacMillar ; Olle Matsson ; Raymond A. Poirier ; Shahidul M. Islam
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：August 23, 2007
• 年：2007
• 卷：111
• 期：33
• 页码：8110 - 8120
• 全文大小：102K
• 年卷期：v.111,no.33(August 23, 2007)
• ISSN：1520-5215

文摘

Chlorine leaving group k³⁵/k³⁷, nucleophile carbon k¹¹/k¹⁴, and secondary MG SRC="/images/gifchars/alpha.gif" BORDER=0>-deuterium [(k_H/k_D)_{mages/gifchars/alpha.gif" BORDER=0>}] kinetic isotopeeffects (KIEs) have been measured for the S_N2 reactions between para-substituted benzyl chlorides andtetrabutylammonium cyanide in tetrahydrofuran at 20 mages/entities/deg.gif">C to determine whether these isotope effects can beused to determine the substituent effect on the structure of the transition state. The secondary mages/gifchars/alpha.gif" BORDER=0>-deuteriumKIEs indicate that the transition states for these reactions are unsymmetric. The theoretical calculations at theB3LYP/aug-cc-pVDZ level of theory support this conclusion; i.e., they suggest that the transition states forthese reactions are unsymmetric with a long NC-C_{mages/gifchars/alpha.gif" BORDER=0>} and reasonably short C_{mages/gifchars/alpha.gif" BORDER=0>}-Cl bonds. The chlorine isotopeeffects suggest that these KIEs can be used to determine the substituent effects on transition state structurewith the KIE decreasing when a more electron-withdrawing para-substituent is present. This conclusion issupported by theoretical calculations. The nucleophile carbon k¹¹/k¹⁴ KIEs for these reactions, however, donot change significantly with substituent and, therefore, do not appear to be useful for determining how theNC-C_{mages/gifchars/alpha.gif" BORDER=0>} transition-state bond changes with substituent. The theoretical calculations indicate that the NC-C_{mages/gifchars/alpha.gif" BORDER=0>}bond also shortens as a more electron-withdrawing substituent is placed on the benzene ring of the substratebut that the changes in the NC-C_{mages/gifchars/alpha.gif" BORDER=0>} transition-state bond with substituent are very small and may not bemeasurable. The results also show that using leaving group and nucleophile carbon KIEs to determine thesubstituent effect on transition-state structure is more complicated than previously thought. The implicationof using both chlorine leaving group and nucleophile carbon KIEs to determine the substituent effect ontransition-state structure is discussed.