One-Electron Reduction of Aromatic Ketones by Low-Valent Lanthanides. Isolation, Structural Characterization, and Reactivity of Lanthanide Ketyl Complexes

Abstract

In this paper we report on the isolation, structural characterization, and reactivity of a series of lanthanide ketyl complexes, which are generated by reactions of benzophenone and fluorenone with several different types of lanthanide reducing agents including Ln(OAr)2(L)x (Ar = C6H2-tBu2-2,6-Me-4; 1a: Ln = Sm, L = THF, x = 3; 1b: Ln = Yb, L = THF, x = 3; 1c: Ln = Sm, L = HMPA, x = 2; 1d: Ln = Yb, L = HMPA, x = 2), (C5Me5)2Ln(THF)2 (Ln = Sm, Yb), (C5Me5)Sm(OAr)(HMPA)2, Sm(N(SiMe3)2)2(THF)2, and Ln/HMPA (Ln = Sm, Yb) (HMPA = hexamethylphosphoric triamide). Reactions of 1a−d with 1 equiv of fluorenone in THF afforded the corresponding ketyl complexes Ln(OAr)2(ketyl)(L)2 (3a−d) in 85−90% isolated yields. Hydrolysis of 3a (Ln = Sm, L = THF) gave the corresponding pinacol-coupling product, 1,2-bis(biphenyl-2,2‘-diyl)ethane-1,2-diol (4), while air oxidation of 3a yielded fluorenone almost quantitatively. Reaction of 3a with 1 equiv of 1a followed by hydrolysis afforded fluorenol quantitatively. When 3a was dissolved in hexane/ether, pinacol-coupling of the ketyl unit occurred to give the OEt2-coordinated pinacolate complex [Sm(OAr)2(OEt2)]2[μ-pinacolate] (5a, pinacolate = 1,2-bis(biphenyl-2,2‘-diyl)ethane-1,2-diolate). Dissolving 5a in THF regenerated 3a via C−C bond cleavage of the pinacolate unit, showing that the pinacol-coupling process was reversible. Addition of 2 equiv of HMPA (per Sm) to a THF solution of 3a or 5a gave the corresponding HMPA-coordinated ketyl complex 3c. Complex 3c was more stable than 3a, and no reaction was observed when 3c was treated similarly with hexane/ether. Reactions of fluorenone with (C5Me5)2Ln(THF)2 (Ln = Sm, Yb) and (C5Me5)Sm(OAr)(HMPA)2 gave the corresponding ketyl complexes (C5Me5)2Ln(ketyl)(THF) (3e: Ln = Sm, 3f: Ln = Yb) and (C5Me5)Sm(OAr)(ketyl)(HMPA) (3i), respectively. In contrast, the similar reaction of Sm(N(SiMe3)2)2(THF)2 with fluorenone in THF yielded the pinacolate complex [Sm(N(SiMe3)2)2(THF)]2[μ-pinacolate] (5b) as the only isolable product, although the formation of a ketyl species was evident in THF solution. Reaction of 4 equiv of HMPA with 5b in THF gave a mixture of Sm(N(SiMe3)2)2(ketyl)(HMPA)2 (3k) and [Sm(N(SiMe3)2)2(HMPA)]2[μ-pinacolate] (5c), while reactions of 5b with 4 equiv of ArOH (Ar = C6H2-tBu2-2,6-Me-4) in THF and THF/HMPA produced the corresponding ArO-ligated ketyl complexes 3a and 3c, respectively. A variable-temperature UV−vis spectroscopic study in toluene derived a dissociation enthalpy of 11 kcal/mol for 5b. Reactions of metallic Ln (Ln = Sm, Yb) with 3 equiv of fluorenone and 3 equiv of HMPA in THF yielded the corresponding tris(ketyl)metal complexes Ln(ketyl)3(HMPA)3 (7a: Ln = Sm, 7b: Ln = Yb). Hydrolysis of 7a,b afforded the pinacol 4, while reaction of 7a with 0.5 equiv of 4 or benzopinacol produced the fluorenoxide/pinacolate complex 6. Reaction of 4 with Sm(N(SiMe3)2)3 in THF/HMPA also afforded 6. In contrast to the reactions of fluorenone, the similar reactions of the above reducing agents with benzophenone did not afford a structurally characterizable ketyl species, and in the case of 1d, the corresponding hydrogen radical abstraction product Yb(OCHPh2)2(OAr)(HMPA)2 (2) was isolated.