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 - with [PSiP] pincer ligands having different substituents at the P and Si atoms ([(2-PhPCH)MeSiCo(H)(Cl)(PMe)] (), [(2-PhPCH)HSiCo(H)(Cl)(PMe)] (), [(2-PhPCH)PhSiCo(H)(Cl)(PMe)] (), [(2-PrPCH)HSiCo(H)(Cl)(PMe)] (), [(2-PrPCH)MeSiCo(H)(Cl)(PMe)] (), and [(2-PrPCH)PhSiCo(H)(Cl)(PMe)] ()) were synthesized through the reactions of the ligands (-) with CoCl(PMe) via Si-H bond cleavage. Compounds - have catalytic activity for alkene hydrosilylation, and among them, complex is the best catalyst with excellent -Markovnikov regioselectivity. A silyl dihydrido cobalt(III) complex from the reaction of with PhSiH was isolated, and its catalytic activity is equivalent to that of complex . Complex and its derivatives - could also be obtained through the reactions of complexes , , , and with NaBHEt. The molecular structure of was indirectly verified by the structures of -. To our delight, the addition of pyridine N-oxide reversed the selectivity of the reaction, from -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 - and - were determined by single crystal X-ray diffraction analysis.