Abstract
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, X(2)B-SiX(3) (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 [Et(4)N][I(3)B-SiI(3)], now available on a 40 g scale via only two steps. Cation exchange with Li[Al(OC(CF(3))(3))(4)] affords the mixture Li[I(3)B-SiI(3)]/I(2)B-SiI(3)/LiI, serving as a synthetic equivalent of the elusive pure I(2)B-SiI(3). Its chlorine analogue Cl(2)B-SiCl(3) is accessible as a distillable liquid via treatment of [Et(4)N][I(3)B-SiI(3)] with GaCl(3). For both perhalogenated silylboranes, various Lewis base adducts Do·X(2)B-SiX(3) were obtained in excellent yields and structurally characterized by X-ray diffraction (Do = SMe(2), Py, PPh(3), IDipp; IDipp = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene). We demonstrated that Me(2)S·I(2)B-SiI(3) undergoes efficient 1,2-silaboration of the challenging, non-activated substrate ethylene at rt with 0.1 eq. BI(3) as promoter. In contrast, Li[I(3)B-SiI(3)]/I(2)B-SiI(3)/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.