Enyo Tomonori, Arai Nobuaki, Nakane Norio, Nicolaides Athanassios, Tomioka Hideo
Chemistry Department for Materials, Faculty of Engineering, Mie University, Tsu, Mie 514-8507 Japan.
J Org Chem. 2005 Sep 16;70(19):7744-54. doi: 10.1021/jo0512204.
[reaction: see text] Computations find that o-phenylene(halo)carbenonitrenes 2-XN, X = F, Cl, Br, have quinoidal singlet biradical ground states such as the parent o-phenylenecarbenonitrene (2-HN). Compared to the parent 2-HN, halogen substitution stabilizes the A'' states relative to the A' ones. Halogen substitution also affects the barrier and exothermicity of the ring-opening reaction (to form unsaturated nitriles 4-XN, X = F, Cl, Br), but it has a smaller effect on the ring-closing reaction (to form benzo(aza)cyclobutadiene 3-XN, X = F, Cl, Br). Attempts to generate and observe the o-phenylene(halo)carbenonitrenes 2-XN, X = F, Cl, Br, using matrix isolation spectroscopy under conditions similar to those of the successful observation of 2-HN failed. Instead, the observed photoproducts were a mixture of 3-XN and 4-XN. In each case, the major product of the mixture appears to be the thermodynamically more stable one. In the case of X = Br, the observed mixture contains an additional component that is postulated to be Z-6-BrN. o-Phenylenechlorocarbenocarbene is also computed to have a quinoidal singlet biradical ground state and relatively stabilized A'' excited states. Attempts to generate the biscarbene under matrix isolation conditions led to the detection of benzochlorocyclobutadiene (3-ClC), small amounts of the ring-open product (dienediyne 4-ClC), and cycloalkyne 5-ClC. Computations suggest that the formation of 5-ClC implies the generation of Z-6-ClC, which is analogous to the formation of Z-6-BrN from 2-BrN.
[反应：见正文] 计算结果表明，邻亚苯基（卤代）碳烯腈2-XN（X = F、Cl、Br）具有醌型单重态双自由基基态，例如母体邻亚苯基碳烯腈（2-HN）。与母体2-HN相比，卤素取代使A''态相对于A'态更稳定。卤素取代还影响开环反应（形成不饱和腈4-XN，X = F、Cl、Br）的势垒和放热，但对闭环反应（形成苯并（氮杂）环丁二烯3-XN，X = F、Cl、Br）的影响较小。在与成功观测2-HN相似的条件下，尝试使用基质隔离光谱法生成并观测邻亚苯基（卤代）碳烯腈2-XN（X = F、Cl、Br）未成功。相反，观测到的光产物是3-XN和4-XN的混合物。在每种情况下，混合物的主要产物似乎是热力学上更稳定的产物。在X = Br的情况下，观测到的混合物包含一种额外的成分，推测为Z-6-BrN。邻亚苯基氯代碳烯也被计算具有醌型单重态双自由基基态和相对稳定的A''激发态。在基质隔离条件下尝试生成双碳烯导致检测到苯并氯环丁二烯（3-ClC）、少量的开环产物（二烯二炔4-ClC）和环炔烃5-ClC。计算表明，5-ClC的形成意味着Z-6-ClC的生成，这与由2-BrN形成Z-6-BrN类似。
