Determining the Transition-State Structure for Different SN2 Reactions Using Experimental Nucleophile Carbon and Secondary α-Deuterium Kinetic Isotope Effects and Theory

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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：2008
• 出版时间：October 16, 2008
• 年：2008
• 卷：112
• 期：41
• 页码：10264-10273
• 全文大小：130K
• 年卷期：v.112,no.41(October 16, 2008)
• ISSN：1520-5215

文摘

Nucleophile ¹¹C/¹⁴C [k¹¹/k¹⁴] and secondary α-deuterium [(k_H/k_D)_α] kinetic isotope effects (KIEs) were measured for the S_N2 reactions between tetrabutylammonium cyanide and ethyl iodide, bromide, chloride, and tosylate in anhydrous DMSO at 20 °C to determine whether these isotope effects can be used to determine the structure of S_N2 transition states. Interpreting the experimental KIEs in the usual fashion (i.e., that a smaller nucleophile KIE indicates the Nu−C_α transition state bond is shorter and a smaller (k_H/k_D)_α is found when the Nu−LG distance in the transition state is shorter) suggests that the transition state is tighter with a slightly shorter NC−C_α bond and a much shorter C_α−LG bond when the substrate has a poorer halogen leaving group. Theoretical calculations at the B3LYP/aug-cc-pVDZ level of theory support this conclusion. The results show that the experimental nucleophile ¹¹C/¹⁴C KIEs can be used to determine transition-state structure in different reactions and that the usual method of interpreting these KIEs is correct. The magnitude of the experimental secondary α-deuterium KIE is related to the nucleophile−leaving group distance in the S_N2 transition state (R_TS) for reactions with a halogen leaving group. Unfortunately, the calculated and experimental (k_H/k_D)_αʼs change oppositely with leaving group ability. However, the calculated (k_H/k_D)_αʼs duplicate both the trend in the KIE with leaving group ability and the magnitude of the (k_H/k_D)_αʼs for the ethyl halide reactions when different scale factors are used for the high and the low energy vibrations. This suggests it is critical that different scaling factors for the low and high energy vibrations be used if one wishes to duplicate experimental (k_H/k_D)_αʼs. Finally, neither the experimental nor the theoretical secondary α-deuterium KIEs for the ethyl tosylate reaction fit the trend found for the reactions with a halogen leaving group. This presumably is found because of the bulky (sterically hindered) leaving group in the tosylate reaction. From every prospective, the tosylate reaction is too different from the halogen reactions to be compared.