Carboryne Insertion Goes Two Ways

Researchers in China have discovered that reactive boron-carbon clusters called carborynes, C₂B₁₀H₁₀, which are in effect three-dimensional versions of benzyne, can exist in two resonance forms that exhibit significantly different reactivity patterns (J. Am. Chem. Soc., DOI: 10.1021/ja201126h). Last year, Sunewang R. Wang, Zaozao Qiu, and Zuowei Xie at the Chinese University of Hong Kong reported that carborynes can insert into a C–C bond of aromatic rings in a [2 + 2] cycloaddition, expanding the six-membered ring of anisoles to eight-membered rings (shown, top). The team has discovered that carborynes can also insert into the C–H bond of diethyl ether (shown, bottom). After conducting a set of kinetics experiments, the team concluded that carborynes are best described as resonance hybrids with bonding and biradical forms: The bonding form reacts with aromatics to give cycloaddition products, whereas the biradical form undergoes C–H insertions with aliphatic ethers. The pair of reactions serve as a new strategy for generating functionalized carboranes, the researchers note, which feature extensively in the synthesis of polymers, ceramics, catalysts, and radiopharmaceuticals.