O‑Carboranylene versus Phenylene Backbones in Cyclization Reactions of 1,2 Diketones with Hydrosilanes

The difference in reactivity of 1,2-dibenzoyl-o-carborane (2) and its analogue with an aromatic backbone and two C6F5 electron withdrawing groups, C6H4(CO­(C6F5))2 (5), in cyclization reactions with R3SiH (R = Et, Ph (a)) with or without a catalytic amount of B­(C6F5)3 was studied. In contrast to the reaction of 2 with R3SiH, which directly leads to the cyclic hydrofurane products 3 and 4, 5 and R3SiH do not react without a catalytic amount of B­(C6F5)3. However, in the presence of a catalytic amount of B­(C6F5)3, 5 reacts with R3SiH forming through hydrofurane-type products 9, 9a, isobenzofuran 6. 2, on the other hand, reacts with R3SiH in the presence of a catalytic amount of B­(C6F5)3 giving stable products 10, 10a. 10, 10a in the presence of B­(C6F5)3 transform over time to a mixture of diastereomers 3′, 3a′. The mechanisms leading to these reactions are proposed on the basis of experimental and computational investigation.