Selectivity Reverse of Hydrosilylation of Aryl Alkenes Realized by Pyridine N-Oxide with [PSiP] Pincer Cobalt(III) Hydride as Catalyst

Six silyl cobalt(III) hydrides 1-6 with [PSiP] pincer ligands having different substituents at the P and Si atoms ([(2-Ph₂PC₆H₄)₂MeSiCo(H)(Cl)(PMe₃)] (1), [(2-Ph₂PC₆H₄)₂HSiCo(H)(Cl)(PMe₃)] (2), [(2-Ph₂PC₆H₄)₂PhSiCo(H)(Cl)(PMe₃)] (3), [(2-iPr₂PC₆H₄)₂HSiCo(H)(Cl)(PMe₃)] (4), [(2-iPr₂PC₆H₄)₂MeSiCo(H)(Cl)(PMe₃)] (5), and [(2-iPr₂PC₆H₄)₂PhSiCo(H)(Cl)(PMe₃)] (6)) were synthesized through the reactions of the ligands (L1-L6) with CoCl(PMe₃)₃ via Si-H bond cleavage. Compounds 1-6 have catalytic activity for alkene hydrosilylation, and among them, complex 3 is the best catalyst with excellent anti-Markovnikov regioselectivity. A silyl dihydrido cobalt(III) complex 7 from the reaction of 3 with Ph₂SiH₂ was isolated, and its catalytic activity is equivalent to that of complex 3. Complex 7 and its derivatives 10-12 could also be obtained through the reactions of complexes 3, 1, 4, and 5 with NaBHEt₃. The molecular structure of 7 was indirectly verified by the structures of 10-12. To our delight, the addition of pyridine N-oxide reversed the selectivity of the reaction, from anti-Markovnikov to Markovnikov addition. At the same time, the reaction temperature was reduced from 70 to 30 °C on the premise of high yield and excellent selectivity. However, this catalytic system is only applicable to aromatic alkenes. On the basis of the experimental information, two reaction mechanisms are proposed. The molecular structures of cobalt(III) complexes 3-6 and 10-12 were determined by single crystal X-ray diffraction analysis.