Coupling of Azomethine Ylides with Nitrilium Derivatives of closo-Decaborate Clusters: A Synthetic and Theoretical Study

The azomethine ylides p-R3C5H4N+CH−COC6H4R2-p (3 a: R3=H, R2=H, X=Br; 3 b: R3=H, R2=Me, X=I; 3 c: R3=H, R2=OMe, X=I; 3 d: R3=H, R2=F, X=I; 3 e: R3=Me, R2=Me, X=Br) react with the nitrile functionality of the closo-decaborate clusters [Bun4N][B10H9(NCR1)] (1 a: R1=Me; 1 b: R1=Et; 1 c: R1=Ph) in a CH3NO2 solution under mild conditions (20–25 °C, 2 min) to afford selectively products of the nucleophilic addition (ca. quantitative yields based on NMR analysis in [D6]DMSO, 71–87 % yield of isolated products). These products are the borylated enamino ketones as the salts bearing exclusively a tetrabutylammonium cation [Bun4N][B10H9{NCR1=C(N+C5H4R3-p)COC6H4R2-p}] (4 a–h,k–n) or the mixed salts [Bun4N]1-x[p-R3C5H4N+CH2COC6H4R2-p]x[B10H9{NCR1=C(N+C5H4R3-p)COC6H4R2-p}] (4 i, 4 j, and 4 o). This reaction represents the first example of the selective nucleophilic addition of pyridinium azomethine ylides to the nitrile group and the first example of new CC bond formation in the reaction of nitrilium derivatives of boron clusters with any nucleophiles so far tested. Compounds 4 a–h,k–n were characterized by ICP-MS, high resolution ESI-MS, molar conductivity, and IR, 1H, 13C{1H}, and 11B{1H} NMR spectroscopy. The structures of 4 a and 4 b were determined by a single-crystal X-ray analysis. Theoretical calculations at the DFT level allowed the interpretation of the observed reaction selectivity, which is driven mostly by thermodynamic rather than kinetic factors, and steric repulsion between the boron cluster and the azomethine ylide molecule plays a crucial role. The activation of the nitrile group upon binding to the boron cluster is explained in terms of the electrostatic arguments. The mechanisms of the nucleophilic addition and the hypothetical cycloaddition between the azomethine ylide and nitriles were investigated in detail.