Synthesis of perhalogenated silylboranes (X = Cl, I) and their application in regiodivergent alkene silaboration

Silaboration of olefins is a synthetically valuable and atom-economic mode of functionalization; however, it typically requires transition-metal catalysis. We have overcome this requirement by using highly reactive perhalogenated silylboranes, X2B–SiX3 (X = Cl, I), for which we herein report a straightforward synthesis, a full characterization, and their key properties. Access to this compound class was enabled by substantial improvement in the synthesis protocol for our previously published compound [Et4N][I3B–SiI3], now available on a 40 g scale via only two steps. Cation exchange with Li[Al(OC(CF3)3)4] affords the mixture Li[I3B–SiI3]/I2B–SiI3/LiI, serving as a synthetic equivalent of the elusive pure I2B–SiI3. Its chlorine analogue Cl2B–SiCl3 is accessible as a distillable liquid via treatment of [Et4N][I3B–SiI3] with GaCl3. For both perhalogenated silylboranes, various Lewis base adducts Do·X2B–SiX3 were obtained in excellent yields and structurally characterized by X-ray diffraction (Do = SMe2, Py, PPh3, IDipp; IDipp = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene). We demonstrated that Me2S·I2B–SiI3 undergoes efficient 1,2-silaboration of the challenging, non-activated substrate ethylene at rt with 0.1 eq. BI3 as promoter. In contrast, Li[I3B–SiI3]/I2B–SiI3/LiI effects a quantitative, unprecedented 1,1-silaboration of cyclohexene at rt. This remarkable reactivity switch was elucidated by experimental and quantum-chemical studies of the underlying steric and electronic factors.