Carbon−Oxygen Bond Cleavage Promoted by a Scandium Borohydride Complex

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)(OCHCH2) (1-OCHCH2) in 70% yield. The molecular structure of 1-OCHCH2 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.