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Radical Reductive Elimination from Tetrabenzyluranium Mediated by an Iminoquinone Ligand

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journal contribution
posted on 28.04.2014, 00:00 by Ellen M. Matson, Sebastian M. Franke, Nickolas H. Anderson, Timothy D. Cook, Phillip E. Fanwick, Suzanne C. Bart
Reductive elimination from U­(CH2Ph)4 (1-Ph) mediated by 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)­imino]­quinone (dippiq) was observed, resulting in the formation of (dippap)2U­(CH2Ph)2(THF)2 (2) (dippap = 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)­amido]­phenolate) and bibenzyl. The crossover experiment with U­(CD2C6D5)4 showed formation of bibenzyl-d7, indicating that reductive elimination occurs in a stepwise fashion via benzyl radical extrusion, presumably through an iminosemiquinone tris­(benzyl) intermediate, (dippisq)­U­(CH2Ph)3. Synthesis of this intermediate was attempted by addition of the iminoquinone ligand to UI3(THF)4 to form (dippisq)­UI3 (3), followed by alkylation with 3 equiv of benzylpotassium. However, this only resulted in the isolation of 2. Reduction of 3 with KC8 afforded the amidophenolate diiodide species (dippap)­UI2(THF)2 (4), maintaining the tetravalent oxidation state of the uranium and reducing the ligand. Attempts at the formation of 2 via addition of 2 equiv of benzylpotassium to 4 resulted in decomposition. The uranium mono­(alkyl) (dippap)­UI­(CH2Ph)­(THF)2 (5) was observed upon addition of 1 equiv of benzylpotassium to 4. All products have been characterized by 1H NMR and electronic absorption spectroscopy. X-ray crystallography was employed to ascertain ligand reduction in 2, 3, and 5.