Intramolecular Pericyclic Re-hydroboration via Six-Membered Transition State: Synthesis of Fused Borolo[3,2-b]siloles

We experimentally demonstrate a new type of the intramolecular reaction between non-activated alkyne units and the dialkylboryl group (9-BBN), which was previously only hypothesized and studied on a calculational level. The reaction described here can formally be classified as a 1,2-hydroboration reaction, but, in contrast to the classical mechanism via a cyclic four-membered transition state, the reaction proceeds by a pericyclic mechanism involving a six-membered transition state. In practice, the reaction allows the synthesis of a new class of the borolenes fused with silole or dihydrosilole units. Thus, the heating (125 °C) of 2,3-dihydrosiloles or 2,3-dihydrosilolo[2,3-b]silole bearing 9-BBN and propargyl or dimethylpropargyl groups in positions 4 and 5, respectively, affords, in high to moderate yields, di- and tricyclic compounds containing a borolo[3,2-b]silole unit. In all cases, the reactions are accompanied by an unprecedented dehydroboration of the 9-BBN fragment, leading, in the final products, to a cyclooct-4-enyl group on the boron atom. The effects of the substituents on the rate and the selectivity of the reaction are discussed. The structures of all the new compounds have been determined from HRMS together with multinuclear 1D and 2D NMR spectroscopy, the results of which are in excellent agreement with DFT calculated chemical shifts.