Regioselective B2-6 penta-iodination of the [CBH] monocarborane cluster by palladium catalysis.

Penta-iodination of the B2-6 positions of the {CB} monocarborane cluster is reported. Products of the structure [2,3,4,5,6-I-CBH-12-X] (X = H, Me, Et, Ph, Br, I) were obtained and fully characterized. X-ray crystal structures of three new compounds confirm this particular substitution pattern. The synthetic method relies on palladium catalysis/B-H activation, assisted by the C1-COOH directing group. The one-pot procedure enables penta-iodination and subsequent decarboxylation under convenient conditions. The B2-6 regioselectivity is complementary to the commonly observed reactivity of {CB} clusters, which follows the trend B12 > B7-11 > B2-6 for electrophilic substitution. Thus, for the first time upper-belt halogenation is achieved without prior modification of the lower-belt positions.