Carbon−Oxygen Bond Cleavage Promoted by a Scandium Borohydride Complex

Abstract

Treatment of (Nacnac)ScCl2(THF) (Nacnac- = ArNC(CH3)CHC(CH3)NAr, Ar = 2,6-(CH(CH3)2)2C6H3) with KNHAr in THF affords in 83% yield the chloro−anilide complex (Nacnac)ScCl(NHAr)(THF) (1-Cl(THF)). 1-Cl(THF) was fully characterized, and a single-crystal X-ray diffraction study reveals the complex to be a five-coordinate scandium complex containing a bound THF ligand. The complex 1-Cl(THF) reacts cleanly with NaBHEt3 in toluene to yield the triethylborohydride adduct (Nacnac)Sc(NHAr)(HBEt3) (1-HBEt3) in 78% yield. The molecular structure of 1-HBEt3 indicated a bridged hydride ligand confined between the scandium and boron atoms. The coordination environment of scandium was that of an octahedron, with the two remaining coordination sites being occupied by one hydrogen from each methylene group on the borane interacting with the scandium center through agostic interactions. Variable-temperature 11B NMR spectroscopy indicated a doublet with JBH = 53 Hz (70 °C). Borane extrusion in the complex 1-HBEt3 in Et2O affords the ethoxide (Nacnac)Sc(NHAr)(OEt) (1-OEt), which was structurally characterized. Tetrahydrofuran also reacts cleanly with 1-HBEt3 to yield the enolate complex (Nacnac)Sc(NHAr)(OCHCH2) (1-OCHCH2) in 70% yield. The molecular structure of 1-OCHCH2 was also determined by single-crystal X-ray diffraction methods. In the absence of Lewis bases such as Et2O or THF, 1-HBEt3 was found to thermally decompose to a mixture of products. When 1 equiv of benzophenone was added to 1-HBEt3, the putative hydride “1-H” was trapped to yield the diphenylmethoxide complex (Nacnac)Sc(NHAr)(OCHPh2) (1-OCHPh2) in 80% yield. As an alternative to 1-HBEt3 cleaving C−O bonds, the stable tert-butyl complex (Nacnac)Sc(NHAr)(tBu) (1-tBu), prepared in 94% yield from 1-Cl(THF) and solid LitBu, was also found to react with Et2O to yield 1-OEt. The complex 1-tBu was fully characterized, and the molecular structure was established by single-crystal X-ray diffraction methods. 1-tBu was thermally stable in the absence of Et2O and THF.