Recently, Wen-Xiong Zhang’s group in College of Chemistry and Molecular Engineering (CCME) of Peking University published a research article entitled “Selective Coupling of Lanthanide Metallacyclopropenes and Nitriles via Azametallacyclopentadiene and η2-Pyrimidine Metallacycle” in Journal of American Chemical Society (Ze-Jie Lv, Zhengqi Chai, Miaomiao Zhu, Junnian Wei, and Wen-Xiong Zhang*, J. Am. Chem. Soc. 2021, DOI: 10.1021/jacs.1c03604.), which was selected as a supplementary cover.
The synthesis, structural characterization, and reactivity study of metallacycles, the key intermediates in organic synthesis and catalytic reactions, have always been targeted by organometallic and coordination chemists. Although transition metallacycles have been widely studied in the last decades, rare-earth metallacycles are rarely reported owing to the lack of efficient synthetic routes. Prof. Zhang’s group has long been engaged in the synthesis, structural characterization, bonding mode and reaction chemistry of novel rare-earth metallacycles. In 2019, they successfully synthesized and isolated the first well-defined scandacyclopropenes, which showed Hückel aromaticity (Ze-Jie Lv, Zhe Huang, Jinghang Shen, Wen-Xiong Zhang*, and Zhenfeng Xi, J. Am. Chem. Soc. 2019, 141, 20547−20555). Based on this, they systematically examined the selective coupling reaction between lanthanide metallacyclopropenes and nitriles (Fig. 1).
Fig. 1 The selective coupling between lanthanide metallacyclopropenes and nitriles.
Nitriles are key synthons of N-heterocycles, and their reaction with metallacyclepropenes is one of the most important reactivities of metallacyclopropenes. However, the large steric hindrance on the metal center results in the limited reactivity of the scandacyclopropenes. Since lanthanides possess larger ionic radii than scandium, they subsequently synthesized two lanthanide metallacyclopropenes: lutetacyclopropene 2a and dysprosacyclopropene 2b. The selective coupling of 2a and three molecules of PhCN provides an unexpected fused lutetacycle 3a with one 1,6-dihydropyrimidine ring (Fig. 2a). Mechanistic studies by in situ NMR spectra and DFT calculations reveal that the formation of 3a proceeds through four key steps: the insertion of the first PhCN into 2a giving azalutetacyclopentadiene IM1, the insertion of the second PhCN into the Lu−N bond of IM1 providing an unsymmetric seven-membered lutetacycle IM2’, the intramolecular 6π electrocyclization of IM2’ furnishing a highly strained η2-pyrimidine metallacycle IM3’, and the insertion of the third PhCN into the Lu−Csp3 bond of IM3’ affording 3a (Fig. 2b). Furthermore, they successfully isolated azalutetacyclopentadiene 3 (i.e. IM1) through the reaction between 2a and one equiv of PhCN at low temperature (Fig. 2c), and obtained the η2-pyrimidine dysprosacycle 4 (i.e. the analog of IM3’) from 2b and two equivalents of PhCN (Fig. 2d). The isolation and characterization of 3 and 4 not only suggest the mechanism of 6π electrocyclization of diazametallacycloheptatriene IM2’, but also offer a new insight into metal-catalyzed [2+2+2]-cycloaddition. The reactivity chemistry of lutetacyclopropene 2a is significantly different from these metallacyclopropenes of scandium and other metals. Theoretical investigations imply the more polarized and ionic nature of the Lu−Csp2 bonds in lutetacyclopropene than the Sc−Csp2 bonds in scandacyclopropene, and this indicates the higher reactivity of lutetacyclopropene. Besides, ion radii of Lu3+ (0.977 Å) and Sc3+ (0.870 Å) may also play an important role in reactivity.
Fig. 2 The synthesis, structure, and reaction toward nitriles of metallacyclopropenes of lutetium and dysprosium.
In summary, this work realizes the synthesis and characterization of the first azametallacyclopentadiene and η2-pyrimidine metallacycle of rare-earth metals. It provides a new 6π electrocyclization mechanism for metal-catalyzed [2+2+2]-cycloaddition. The first author and corresponding author of this work are Ze-Jie Lv and Wen-Xiong Zhang, respectively. The work was supported by the Natural Science Foundation of China and Beijing National Laboratory for Molecular Sciences. Special thanks to Professor Zhenfeng Xi for his support to the research group of Chemistry of Carbon˗Rare-Earth Metals (CCREM).
Link: https://pubs.acs.org/doi/10.1021/jacs.1c03604.
