Free Energy Relationships for Reactions of Substituted Benzhydryliunn Ions: From Enthalpy over Entropy to Diffusion Control
Free Energy Relationships for Reactions of Substituted Benzhydryliunn Ions: From Enthalpy over Entropy to Diffusion Control
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DOI：
10.1021/ja306522b
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发表时间：
2012-08-22
影响因子：
15
通讯作者：
Mayr, Herbert
中科院分区：
文献类型：
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作者：
Ammer, Johannes;Nolte, Christoph;Mayr, Herbert
用激光闪光照射苯基三芳基膦盐Ar2CH-Par3+X-，测定了在大量亲核试剂存在下，不同给体和受体取代的苯基离子Ar2CH+与pi-亲核试剂在CH2Cl2中反应的二级速率常数k(2)。这种方法使我们能够研究扩散极限以下的快速反应，包括高活性的苯并氢离子与间氟和对三氟甲基取代基的反应。本文测定的速率常数与相关文献数据进行了关联分析，得到了受体取代苯肼离子的亲电性参数E，其定义为线性自由能关系log k(2)(20℃)=S(N)(N+E)。新的相关分析还得到了18个pi亲核试剂的N和S(N)参数，这些参数以前只被模糊地表征过。对数k(2)与E的关联性远远超出了反应的活化热？H-双匕首外推至？H-双匕首=0的范围，表明从热控到熵控制的变化不会引起线性自由能关系的弯曲，这是补偿效应的一个新的表现。当接近扩散极限时，只有k(2)>；10(8)M~(-1)S(-1)的关联线才发生扁平。
Second-order rate constants k(2) for the reactions of various donor- and acceptor substituted benzhydrylium ions Ar2CH+ with pi-nucleophiles in CH2Cl2 were determined by laser flash irradiation of benzhydryl triarylphosphonium salts Ar2CH-PAr3+X- in the presence of a large excess of the nucleophiles. This method allowed us to investigate fast reactions up to the diffusional limit including reactions of highly reactive benzhydrylium ions with m-fluoro and p-(trifluoromethyl) substituents. The rate constants determined in this work and relevant literature data were jointly subjected to a correlation analysis to derive the electrophilicity parameters E. for acceptor-substituted benzhydrylium ions, as defined by the linear free energy relationship log k(2)(20 degrees C) = s(N)(N + E). The new correlation analysis also leads to the N and s(N) parameters of 18 pi-nucleophiles, which have only vaguely been characterized previously. The correlations of log k(2) versus E are linear well beyond the range where the activation enthalpies Delta H-double dagger of the reactions are extrapolated to reach the value of Delta H-double dagger = 0, showing that the change from enthalpy control to entropy control does not cause a bend in the linear free energy relationship, a novel manifestation of the compensation effect A flattening of the correlation lines only occurs for k(2) > 10(8) M-1 s(-1) when the diffusion limit is approached.