Cyclic Alkyl(aryl)boranes for 1,1-Carboboration of Monoalkynyltin Compounds

Two cyclic alkyl(aryl)boranes, a 1-bora-indane derivative 1, and a tricyclic derivative 4, containing the boron atom in a six-membered ring, were structurally characterized by NMR techniques in solution. The solid-state structure of the 1-bora-indane 1 was determined by X-ray crystallography. The reactivity of these cyclic alkyl(aryl)boranes towards monoalkynyltin compounds, Me₃Sn–C≡C–Me and Me₃Sn–C≡C–Fc (Fc = ferrocenyl), was studied using multinuclear magnetic resonance methods (¹H, ¹¹B, ¹³C, ¹¹⁹Sn NMR). Novel alkenylboranes were formed by 1,1-carboboration reactions. This process involves an expansion of both five- and six-membered rings. Insertion into the respective B–C(aryl) bond was preferred with high selectivity. In the case of the six-membered ring in 4, the ring expansion to seven-membered rings proved to be readily reversible, and the thermodynamically stable reaction products were formed by ring contraction and concomitant transfer of the exocyclic B-ⁿPr group.