Palladium/nickel-cocatalyzed cycloaddition of 1,3-dehydro-o-carborane with alkynes. Facile synthesis of C,B-substituted carboranes |
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Authors: | Qiu Zaozao Xie Zuowei |
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Institution: | Department of Chemistry and Center of Novel Functional Molecules, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China. |
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Abstract: | o-Carboryne (1,2-dehydro-o-carborane) has been reported as a very reactive intermediate and regarded as a three-dimensional relative of benzyne, whereas the 1,3-dehydro-o-carborane has remained elusive. In this article, we present the preparation of 1,3-dehydro-o-carborane from 3-iodo-1-lithio-o-carborane mediated by palladium(0). This reactive intermediate can be trapped by alkynes via Pd/Ni-cocatalyzed 2 + 2 + 2] cycloaddition reaction, leading to the formation of C,B-substituted-o-carborane derivatives. The possible reaction mechanism involving the formation of metal-1,3-dehydro-o-carborane followed by stepwise insertions of 2 equiv of alkyne and reductive elimination is proposed, and the relative reactivity of M-C versus M-B bond in metal-1,3-dehydro-o-carborane complexes is also discussed. This work offers a new methodology for B-functionalization of carboranes and demonstrates that metal-1,3-dehydro-o-carborane can be viewed as a new kind of boron nucleophile. |
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