Halogen exchange at boron in nido-C4B2 carboranes

The Pd(PPh3)4-catalyzed reaction of 1-iodo-6-phenylethynyl-2,3,4,5-tetraethyl-2,3,4,5-tetracarba-nido-hexaborane(6) (1a) with in situ generated arylzinc reagents (from arylbromides) leads to 1-bromo-6-phenylethynyl-2,3,4,5-tetraethyl-2,3,4,5-tetracarba-nido-hexaborane(6) (2a) and a trace amount of nido-(EtC)4(BCtriple bond; length of mdashCPh)2 (3a). 3a is separately synthesized by a Pd-catalyzed Negishi-type coupling reaction of 1a with phenylethynyl-zincchloride. This is confirmed by the Pd-catalyzed reactions of 1-iodo-6-tert-butyl-ethynyl-2,3,4,5-tetraethyl-2,3,4,5-tetracarba-nido-hexaborane(6) (1b) with n-BuBr and PhZnCl, respectively, which produce the brominated carborane 2b and nido-(EtC)4(BCtriple bond; length of mdashCtBu)2 (3b). With a dialkynylzinc reagent, a similar reaction of 1-iodo-6-p-tolylethynyl-2,3,4,5-tetraethyl-2,3,4,5-tetracarba-nido-hexaborane(6) (1c) gives the linked compound 4, and a very small amount of ZnI2(OPPh3)2 is formed. The selective basal halogen exchange of 1,6-diiodo-2,3,4,5-tetraethyl-2,3,4,5-tetracarba-nido-hexaborane(6) (5) with an excess of AgF leads to 1-iodo-6-fluoro-2,3,4,5-tetraethyl-2,3,4,5-tetracarba-nido-hexa-borane(6) (6). The composition of the products follows from NMR and MS data and a single crystal X-ray analysis of 2a. The X-ray structure of ZnI2(OPPh3)2 is reported.