Jankiewicz Bartłomiej J, Vinueza Nelson R, Kirkpatrick Lindsey M, Gallardo Vanessa A, Li Guannan, Nash John J, Kenttämaa Hilkka I
B. J. Jankiewicz, N. R. Vinueza, L. M. Kirkpatrick, V. A. Gallardo, G. Li, J. J. Nash, H. I. Kenttämaa Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907-2084, USA.
B. J. Jankiewicz Current Address: Institute of Optoelectronics, Military University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland.
J Phys Org Chem. 2013 Sep;26(9):707-714. doi: 10.1002/poc.3120. Epub 2013 Jun 5.
Reactive intermediates are key species involved in many chemical and biochemical processes. For example, carbon-centered aromatic σ,σ-biradicals formed in biological systems from naturally occurring enediyne antitumor antibiotics are responsible for the irreversible cleavage of double-stranded DNA caused by these prodrugs. However, because of their high reactivity, it is very difficult or impossible to isolate and investigate these biradicals. The aromatic σ,σ-biradical, 2,6-didehydropyridine, has been speculated for many years to be formed in certain organic reactions; however, no definitive proof of its generation has been obtained. We report here the successful generation of protonated 2,6-didehydropyridine and the examination of its chemical properties in the gas phase by using a Fourier transform ion cyclotron resonance mass spectrometer. The results suggest that a mixture of singlet (ground) state and triplet (excited) state 2,6-didehydropyridinium cations was generated. The two different states show qualitatively different reactivity, with the triplet state showing greater Brønsted acidity than that of the singlet state. The triplet state also shows much greater radical reactivity than that of the singlet state, as expected because of the coupling of the nonbonding electrons in the singlet state.
反应中间体是许多化学和生物化学过程中涉及的关键物种。例如，由天然存在的烯二炔类抗肿瘤抗生素在生物系统中形成的以碳为中心的芳香族σ,σ-双自由基，是这些前药导致双链DNA不可逆裂解的原因。然而，由于它们的高反应活性，分离和研究这些双自由基非常困难甚至不可能。多年来一直推测芳香族σ,σ-双自由基2,6-二脱氢吡啶会在某些有机反应中形成；然而，尚未获得其生成的确切证据。我们在此报告成功生成了质子化的2,6-二脱氢吡啶，并使用傅里叶变换离子回旋共振质谱仪在气相中研究了其化学性质。结果表明生成了单重态（基态）和三重态（激发态）2,6-二脱氢吡啶鎓阳离子的混合物。这两种不同状态表现出性质上不同的反应活性，三重态的布朗斯特酸度比单重态更高。正如预期的那样，由于单重态中非键合电子的耦合，三重态的自由基反应活性也比单重态高得多。
