Copper-Mediated CC Cross-Coupling Reaction of Monocarba-closo-dodecaborate Anion for the Synthesis of Functional Molecules

a counterion for the isolation of reactive cations, and for activation of catalysts in organic synthesis. Its unique electronic and molecular structures render 1 as an attractive starting material for the construction of various functional molecules and bioactive/pharmaceutical building blocks. However, progress in the synthesis of such materials is hindered by the lack of methods for the functionalization of 1. In particular, no efficient general methods are available for introduction of aryl and sp/sp-hybridized carbon centers at the carbon vertex of C1-carborane. A cross-coupling reaction is an obvious method for the introduction of aryl groups and sp/sp-hybridized carbon centers at the carbon vertex of C1-carborane derivatives, but has not yet been successful. 10] The intrinsic difficulty appears to be associated mainly with the sterically hindered and hypervalent nature of the vertex carbon atom in 1. We report herein a general and efficient copper-mediated C C cross-coupling reaction of 1 under palladium catalysis that yields a variety of C-functionalized C1-carborane derivatives and provides a basic architecture for pharmacophores and functional materials. The reaction is demonstrated not only for the parent C1-carborane but also its B-halogenated derivatives. To demonstrate utility of the method and the resulting new class of compounds, we describe the synthesis of multivalent anions and disclose preliminary results of the biological activity and liquid crystal behavior of selected derivatives. We commenced our studies by screening for a suitable metal species at the C1-carborane carbon vertex for crosscoupling reaction with aryl iodide 2a under palladium catalysis (Table 1). Reactions of various zinc carborate derivatives 1a (i.e. Negishi-type reaction), including zincate complexes, were unsuccessful (Table 1, entry 1). The Suzuki–Miyaura-type cross-coupling with pinacol borate 1b was also examined under a variety of conditions, but failed to give the desired product, affording only halogenated C1carboranes, with most of the pinacol borate moiety remaining intact (Table 1, entry 2). These results imply that the nature of the vertex carbon atom with respect to transmetalation is quite different from that of typical aryl or alkyl carbon atoms. After extensive screening of other systems, we found that a combination of copper(I) carborate 1c and a palladium catalyst promoted the coupling reaction with 2a at room temperature to give the desired product 3a (Table 1, Scheme 1. C1-carborane (1).