UCSD-CNRS Joint Research Laboratory (UMI 3555), Department of Chemistry and Biochemistry, University of California San Diego , La Jolla, San Diego, California 92093-0358, United States.
J Am Chem Soc. 2018 Feb 21;140(7):2546-2554. doi: 10.1021/jacs.7b11183. Epub 2018 Feb 9.
A modular approach for the synthesis of Kekulé diradicaloids is reported. The key step is the insertion of a carbene, namely, a cyclic (alkyl)(amino)carbene (CAAC), into the C-H bonds of two terminal alkynes linked by a spacer. Subsequent hydride abstraction, followed by two-electron reduction of the corresponding bis(iminium) salts, affords the desired diradicaloids. This synthetic route readily allows for the installation of communicating spacers, featuring different degrees of aromaticity and lengths, and gives the possibility of generating unsymmetrical compounds with two different CAACs. Electron paramagnetic resonance (EPR), NMR, UV-vis, and X-ray studies in combination with quantum-chemical calculations give insight into the electronic nature of the deeply colored Kekulé diradicaloids. They feature a singlet ground state with varying degrees of diradical character in combination with small singlet/triplet gaps. Upon lengthening of the spacer, the properties of the compounds approach those of monoradicals in which steric protection of the propargyl radical moiety is necessary to inhibit decomposition pathways. Most of these diradicaloids are stable at room temperature, both in solution and in the solid state, but are highly oxygen-sensitive. They represent the first diradicaloids derived from iminium salts.
本文报道了一种 Kekulé 型自由基二聚体的模块化合成方法。关键步骤是将卡宾（即环状（烷基）（氨基）卡宾（CAAC））插入由间隔基连接的两个末端炔烃的 C-H 键中。随后进行氢原子的提取，然后对相应的双亚胺盐进行两电子还原，得到所需的自由基二聚体。这种合成路线可以轻松地安装具有不同程度芳香性和长度的连通间隔基，并有可能生成具有两个不同 CAAC 的不对称化合物。电子顺磁共振（EPR）、NMR、UV-vis 和 X 射线研究结合量子化学计算深入了解了深颜色的 Kekulé 型自由基二聚体的电子性质。它们具有单重态基态，具有不同程度的自由基特征，以及较小的单重态/三重态能隙。随着间隔基的延长，化合物的性质接近单重态自由基的性质，其中必须对丙炔基自由基部分进行空间位阻保护以抑制分解途径。这些自由基二聚体大多数在室温下无论是在溶液中还是在固态中都稳定，但对氧气非常敏感。它们是源自亚胺盐的第一批自由基二聚体。
