Recently, Wen-Xiong Zhang group at College of Chemistry and Molecular Engineering (CCME) of Peking University published a communication entitled “Selective Cleavage of the Strong or Weak C–C Bonds in Biphenylene Enabled by Rare-Earth Metals” in Journal of American Chemical Society (Miaomiao Zhu, Zhengqi Chai, Ze-Jie Lv, Tianyu Li, Wei Liu, Junnian Wei, and Wen-Xiong Zhang*, J. Am. Chem. Soc.2023,DOI: 10.1021/jacs.3c01466).
Selective cleavage of inert C−C bond is one of the most direct and efficient approaches to construct organic compounds. Due to the thermodynamic and kinetic disadvantages, the selective cleavage of inert C−C bond is of great challenge. Regarding the substrates that contain both strained C−C bond and aromatic C−C bond, the selective cleavage of the aromatic C−C bond rather than the strained C−C bond is very challenging. As a model molecule for the study of C–C bond activation, biphenylene possesses two types of C−C bonds, including the ring strained Caryl–Caryl (C1–C7) bond and the normal aromatic C–C bonds. Thus, C–C bond cleavage always takes place typicallyat the weakest C1–C7 σ-bond affording 9-metallafluorene in the reaction between transition-metal species and biphenylene. New C–C bond activation modes of biphenylene were recently discovered usingthe emerging low-valence aluminum species. Despite these advancements, the controllable cleavage of different C–C bonds in biphenylene with high selectivity is still unknown. Compared with transition metals or main group metals, the research on C−C bond activation promoted by rare-earth metals is scarce.
Fig. 1 Rare-earth metal promoted C–C bond cleavage of biphenylene
In their previousresearch, they found that the strong N≡N (J. Am. Chem. Soc. 2019, 141, 8773.) and C≡C (J. Am. Chem. Soc. 2019, 141, 20547;J. Am. Chem. Soc. 2021, 143, 9151.) bonds could be reduced and activated through the combination of potassium graphite (KC8) and rare-earth chloride precursors bearing amidinate/Cp* (Cp* = C5Me5) ligands. Based on these, they explored the reaction of KC8, rare-earth chloride precursors and biphenylene (Fig. 1). The one-pot reaction of scandium chloride 1-Sc, biphenylene and KC8 was conducted to provide 9-scandafluorene 2a-Sc. To explore the formation mechanism of 2a-Sc, the critical intermediate biphenylene dianionic scandium 3a-Sc was obtained at low temperature(Fig. 2a). Considering that the interaction between the free K+ ion and the reduced biphenylene unit may affect the selectivity of the reaction, complex 3b-Sc was synthesized by the encapsulation of the K+ ion of 3a-Sc by 18-crown-6 (18-c-6).3b-Sc could transform to 2b-Sc and 4b-Sc at elevated temperatures. The conversion from biphenylene to benzopentalene dianion is a novel C−C bond activation mode of biphenylene. Since rare-earth metals involved chemical transformations often exhibit the effect of ionic radius, the C–C bond cleavage of biphenylene promoted by lutetium with larger ionic radius than scandium was then investigated.Similar to thereaction of 1-Sc with biphenylene, the reaction of lutetium chloride 1-Lu, biphenylene, KC8 with 18-c-6 or crypt-222 (crypt) was conducted to provide benzopentalene dianioniclutetium 4b-Luor 4c-Luexclusively (Fig. 2b). Similar to the preparation of 3a-Sc, biphenylene dianionic lutetium3a-Lu was synthesized. Subsequently, 18-c-6 or crypt coordinated biphenylene dianionic lutetium complexes 3b-Lu and 3c-Lu were prepared by the addition of 18-c-6 or crypt to 3a-Lu. In an effort to clarify the selectivity, DFT calculations were applied. The formation of 4b-Lu starts with the all-carbon 6π 1,5-electrocyclic ring-closure generating IM1. The following 4π electrocyclic ring-opening and 1,5-hydrogen migration provide the final product 4b-Lu. As for scandium, 3a'-Sc prefers togo through 4π electrocyclic ring-opening to provide 2a'-Sc. The computational results indicate that both the metal centers and the chelating reagents play an important role in the selectivity. The different selectivity of scandium and lutetium is probably due to the different polarized and ionicnature of the RE−C σ bond. Besides, ion radii of Lu3+(0.977 Å) and Sc3+(0.870 Å) mayalsoplay an important role.
Fig.2 Scandium or lutetium promoted C–C bond cleavage of biphenylene
In summary, this work realizes highly selective C–C bond activation of biphenylene promoted by rare-earth metals and provides new ideas to study metal-promoted inert C–C bond activation. Miaomiao Zhu, a Ph.D. candidate at Peking University; Zhengqi Chai, a Ph.D. candidate at Peking University; and Ze-Jie Lv, a Ph.D. alumnus at Peking University, are the co-first authors of this study. The corresponding author is Prof. Wen-Xiong Zhang. The work was supported by the Natural Science Foundation of China,the National Key R&D Program 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˗RareEarth Metals (CCREM).
Original link: https://pubs.acs.org/doi/10.1021/jacs.3c01466