Tichy Shane E, Nelson Eric D, Amegayibor F Sedinam, Kenttämaa Hilkka I
Contribution from the Department of Chemistry, Purdue University, West Lafayette, IN 47907-2084, USA.
J Am Chem Soc. 2004 Oct 13;126(40):12957-67. doi: 10.1021/ja049534g.
Four pi,pi-biradicals, 2,6-dimethylenepyridinium and the novel isomers N-(3-methylenephenyl)-3-methylenepyridinium, N-phenyl-3,5-dimethylenepyridinium, and N-(3,5-dimethylenephenyl)pyridinium ions, were generated and structurally characterized in a Fourier transform ion cyclotron resonance mass spectrometer. Their gas-phase reactivity toward various reagents was compared to that of the corresponding monoradicals, 2-methylenepyridinium, N-phenyl-3-methylenepyridinium, and N-(3-methylenephenyl)pyridinium ions. The biradicals reactivity was found to reflect their predicted multiplicity. The 2,6-dimethylenepyridinium ion, the only biradical in this study predicted to have a closed-shell singlet ground state, reacts significantly faster than the other biradicals, which are predicted to have triplet ground states. In fact, this biradical reacts at a higher rate than the analogous monoradical, which suggests that to avoid the costly uncoupling of its unpaired electrons, the biradical favors ionic mechanisms over barriered radical pathways. In contrast, the second-order reaction rate constants of the isomeric biradicals with triplet ground states are well approximated by those of the analogous monoradicals, although the final reaction products are sometimes different. This difference arises from rapid radical-radical recombination of the initial monoradical reaction products. The overall reactivity toward the hydrogen-atom donors benzeneselenol and tributylgermanium hydride is significantly greater for the radicals with the charged site in the same ring system as the radical site. This finding indicates that polar effects play an important role in controlling the reactivity of pi,pi-biradicals, just as has been demonstrated for sigma,sigma-biradicals.
在傅里叶变换离子回旋共振质谱仪中生成并对四种π,π-双自由基、2,6-二亚甲基吡啶鎓以及新型异构体N-(3-亚甲基苯基)-3-亚甲基吡啶鎓、N-苯基-3,5-二亚甲基吡啶鎓和N-(3,5-二亚甲基苯基)吡啶鎓离子进行了结构表征。将它们对各种试剂的气相反应活性与相应的单自由基、2-亚甲基吡啶鎓、N-苯基-3-亚甲基吡啶鎓和N-(3-亚甲基苯基)吡啶鎓离子的反应活性进行了比较。发现双自由基的反应活性反映了它们预测的多重性。2,6-二亚甲基吡啶鎓离子是本研究中唯一预测具有闭壳单重基态的双自由基，其反应速度明显快于其他预测具有三重基态的双自由基。事实上，该双自由基的反应速率高于类似的单自由基，这表明为了避免其未成对电子的昂贵解耦，双自由基更倾向于离子机制而非有势垒的自由基途径。相比之下，具有三重基态的异构双自由基的二级反应速率常数与类似单自由基的二级反应速率常数非常接近，尽管最终反应产物有时不同。这种差异源于初始单自由基反应产物的快速自由基-自由基重组。对于自由基位点和电荷位点在同一环系中的自由基，其对氢原子供体苯硒酚和三丁基氢化锗的整体反应活性明显更高。这一发现表明，极性效应在控制π,π-双自由基的反应活性中起着重要作用，正如对σ,σ-双自由基所证明的那样。
