Carboranes, a class of polyhedral boron-carbon molecular clusters, are widely applied as functional building blocks in boron neutron capture therapy agents, pharmacophores, supramolecular design, nanomaterials, optoelectronics and organometallic/coordination chemistry. At present, considerable research interest is drawn to the functionalization of carboranes. Although noble transition metal catalyzed cage B-H activation has been achieved for the functionalization of carboranes, such transformation using base metal catalysis remains unexplored.
Profs. Zuowei Xie and Yangjian Quan et al. recently discovered the first example of base metal catalyzed regioselective and direct cage B-H derivatization of carboranes with alkynes (Scheme 1). After considering the following three issues: 1) the distinctive electronic properties of 3d base metals in terms of electronegativity and spin orbit coupling render cage B-H metalation quite different from those of 4d or 5d homologues; 2) the intermediate featuring B-M (M = base metal) bond is considered to be less stable, thus affecting its further transformation; and 3) the documented fast deboronation originated by Cu2+ ion ought to be suppressed, they developed a bidentate directing group guided copper catalyzed [4+2] annulation of carboranyl amides with internal alkynes, affording unprecedented C,B-substituted carborane-fused-pyridone derivatives, while the employment of terminal alkynes led to B-H/C(sp)-H dehydrocoupling products. On the basis of the control experiments and literature work, they proposed a plausible reaction mechanism (Scheme 2), which involves a Cu(III) intermediate. This work was published on Dec. 6 in Science China Chemistry.
This research was supported by grants from the Research Grants Council of The Hong Kong Special Administration Region (Project Nos. 14304115 and 14305017), CUHK Direct Grant and Incentive Grant from Faculty of Science, CUHK.
See the article: Chen Y, Au YK, Quan Y, Xie Z. Copper catalyzed/mediated direct B-H alkenylation/alkynylation in carboranes. Sci China Chem, 2019, 62: 74-79. https://doi.org/10.1007/s11426-018-9388-3