Saturated NHC-Stabilized Borenium, Boronium, Hydride-Bridged Boron Cations, and a Bora-Acyl Chloride

This study investigates the reactivity of 5-SIDipp-boranes and haloboranes (5-SIDipp = 1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene) with Lewis acids (AlCl3 and GaCl3). The reaction between 5-SIDipp·BH3 and GaCl3 resulted in the formation of 3c-2e hydride-bridged boron cations (1). Treatment of AlCl3 to the mixture of 5-SIDipp and BHCl2·SMe2 produced a tetra-coordinate boronium cation of the composition 5-SIDipp → B(H)Cl+ ← SMe2 (2). The SMe2 moiety remains coordinated to the boron center throughout the reaction, playing a crucial role in the isolation of compound 2. The interaction of 5-SIDipp·BCl3 with AlCl3 led to the formation of a tri-coordinate borenium cation (4). However, the formation of 4 was always accompanied by the generation of a bora-acyl chloride (B(═O)Cl) derivative (5).