Mayr Herbert, Lang Gabriele, Ofial Armin R
Department Chemie der Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13 (Haus F), D-81377 München, Germany.
J Am Chem Soc. 2002 Apr 17;124(15):4076-83. doi: 10.1021/ja0121538.
Benzhydryl cations were used as reference electrophiles to determine the hydride donor reactivities of unsaturated hydrocarbons. The kinetics of the reactions were followed by UV-vis spectroscopy and conductivity measurements, and it was found that the second-order rate constants for the hydride transfer processes were almost independent of the solvents or counterions employed. The rate constants correlate linearly with the previously published empirical electrophilicity parameters E of the benzhydrylium ions. Therefore, the linear free energy relationship log k(20 degrees C) = s(E + N) could be employed to characterize the hydride reactivities of the hydrocarbons by the nucleophilicity parameters N and s. The similarity of the slopes s for hydride donors and pi-nucleophiles allows a direct comparison of the reactivities of these different functional groups based on their nucleophilicity parameters N. Since nucleophilicity parameters of -5 < N < 0 have been found for a large variety of allylic and bisallylic hydride donors, a rule of thumb is derived that hydride transfer processes may compete with carbon-carbon bond-forming reactions when carbocations are combined with olefins of pi-nucleophilicity N < 0.
二苯甲基阳离子被用作参考亲电试剂，以确定不饱和烃的氢化物供体反应活性。通过紫外可见光谱和电导率测量跟踪反应动力学，发现氢化物转移过程的二级速率常数几乎与所用溶剂或抗衡离子无关。速率常数与先前发表的二苯甲基离子的经验亲电参数E呈线性相关。因此，线性自由能关系log k(20℃)=s(E + N)可用于通过亲核性参数N和s来表征烃类的氢化物反应活性。氢化物供体和π-亲核试剂的斜率s相似，这使得可以基于其亲核性参数N直接比较这些不同官能团的反应活性。由于已发现多种烯丙基和双烯丙基氢化物供体的亲核性参数为-5 < N < 0，因此得出一条经验法则：当碳正离子与π-亲核性N < 0的烯烃结合时，氢化物转移过程可能与碳-碳键形成反应竞争。
