Pd-Catalyzed One-Pot Synthesis of Difunctionalized o-Carboranes via Construction of B—C and B—Heteroatom Bonds^※

Icosahedral carboranes are carbon-boron molecular clusters, sharing many features with benzene such as aromaticity, high thermal and chemical stability. On the other hand, carboranes have their own unique characteristics like spherical geometry and three-dimensional electronic delocalization. These properties render carboranes unique building blocks for various applications ranging from versatile ligands to functional materials to medicine. In this regard, functionalization of carboranes, particularly regioselective functionalization of cage B-vertexes has recently received much attention. Based on our recently developed Pd-catalyzed iodine-migration on o-carborane cage, a Pd-catalyzed regioselective difunctionalization of 3-iodo-o-carborane in a one-pot manner has been achieved to afford a series of 3-alkenyl-4-Nu-o-carboranes (Nu= arylamino, alkoxyl, alkyl and arylthio) in 47%similar to 99% yields. This protocol combines the sequential activation of cage B(3)-I and B(4)-H bonds by Pd migration, as well as further Pd-catalyzed transformation of B(4)-I bond, leading to the construction of B-C and B-Heteroatom bonds. A general procedure for the synthesis of 3-alkenyl-4-Nu-o-carboranes is described as follows: to a tetrahydrofuran (THF) solution (1 mL) of NuH (1.0 mmol) was added base (1.0 mmol) at 0 degrees C under an atmosphere of dry nitrogen. The reaction mixture was stirred for another 10 min to obtain the NuM solution (NuM =ArNHMgBr, base=EtMgBr; NuM=Ar2NLi, base=(BuLi)-Bu-n). Another oven-dried Schlenk flask equipped with a stir bar was charged with 3-iodo-o-carborane (1, 27 mg, 0.1 mmol), Pd(PPh3)(4) (12 mg, 0.01 mmol), diphenylacetylene (89 mg, 0.5 mmol) and dry toluene (1 mL) under an atmosphere of dry nitrogen. The flask was closed, and stirred at 80 degrees C for 72 h. Then, the resulting solution was cooled to 0 degrees C, to which was slowly added NuM (0.15 mmol) (NuM=ArNHMgBr, Ar2NLi, (BuONa)-Bu-t and RSNa). The reaction mixture was warmed to room temperature, and stirred at 80 degrees C for 24 h. After quenching with water (1 mL) and extraction with ethyl acetate (5 mLx3), the organic portions were combined and concentrated to dryness in vacuo. The residue was subjected to flash column chromatography on silica gel (300 similar to 400 mesh) using n-hexane as eluent to give the product.