Carbon−Oxygen Bond Cleavage Promoted by a Scandium Borohydride Complex

• Published in: Organometallics Vol. 22; no. 23; pp. 4705 - 4714
• Main Authors: Basuli, Falguni, Tomaszewski, John, Huffman, John C, Mindiola, Daniel J
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 10.11.2003; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Inorganic & Nuclear; Chemistry, Organic; Physical Sciences; Science & Technology; BENZENE ACTIVATION; HYDRIDE COMPLEX; DIALKYLSCANDIUM COMPLEXES; HYDROCARBON ACTIVATION; TRANSITION-METAL; CHEMISTRY; BETA-DIKETIMINATO LIGANDS; C-H ACTIVATION; REVERSIBLE 1,2-RH-ELIMINATION; STRUCTURAL CHARACTERIZATION
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om030374b

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-HBEt 3 ) in 78% yield. The molecular structure of 1-HBEt 3 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 J BH = 53 Hz (70 °C). Borane extrusion in the complex 1-HBEt 3 in Et2O affords the ethoxide (Nacnac)Sc(NHAr)(OEt) (1-OEt), which was structurally characterized. Tetrahydrofuran also reacts cleanly with 1-HBEt 3 to yield the enolate complex (Nacnac)Sc(NHAr)(OCHCH2) (1-OCH  CH 2 ) in 70% yield. The molecular structure of 1-OCH  CH 2 was also determined by single-crystal X-ray diffraction methods. In the absence of Lewis bases such as Et2O or THF, 1-HBEt 3 was found to thermally decompose to a mixture of products. When 1 equiv of benzophenone was added to 1-HBEt 3 , the putative hydride “1-H” was trapped to yield the diphenylmethoxide complex (Nacnac)Sc(NHAr)(OCHPh2) (1-OCHPh 2 ) in 80% yield. As an alternative to 1-HBEt 3 cleaving C−O bonds, the stable tert-butyl complex (Nacnac)Sc(NHAr)(tBu) (1- t Bu), 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- t Bu was fully characterized, and the molecular structure was established by single-crystal X-ray diffraction methods. 1- t Bu was thermally stable in the absence of Et2O and THF.