Triphenylsilyl as a protecting group in the synthesis of 1,12-heterodisubstituted p-carboranes

A triphenylsilyl group is used as an auxiliary in the synthesis of heterodi substituted p-carboranes via triphenylsilyl-p-carborane (1). The preparatio n of 1 is statistical, but with recovery of the starting p-carborane, the e ffective conversion to 1 is about 90%. Carborane 1 has been easily converte d to its Lithium and copper derivatives, which were reacted with a range of electrophiles including alkyl halides: an aryl iodide, an acetylene bromid e, and a sulfenyl chloride. The derivatives of I. are crystalline and UV ac tive, which facilitates their isolation and purification, The Ph3Si group i s efficiently removed with fluoride to give monosubstituted p-carboranes I, which upon further nucleophilic substitution yield p-carboranes II. The yi elds of heterodisubstituted products UC are higher than for direct, "statis tical" syntheses. For example, 12-pentyl-1,12-dicarba-closo-dodecaborane-1- carboxylic acid was synthesized from p-carborane via 1 in 62% overall yield , a considerable improvement over the 44% yield obtained by direct methods.