Reactivity of lanthanocene amide complexes toward ketenes: unprecedented organolanthanide-induced conjugate electrophilic addition of ketenes to arenes

This paper presents some unusual types of reactions of lanthanocene amide complexes with ketenes, and demonstrates that these reactions are dependent on the nature of amide ligands and ketenes as well as the stoichiometric ratio under the conditions involved. The reaction of [{Cp(2)LnNiPr(2)}(2)] with four equivalents of Ph(2)CCO in toluene affords the unexpected enolization dearomatization products [Cp(2)Ln(OC{2,5-C(6)H(5)(==CPhCONiPr(2)-4)}==CPh(2))] (Ln = Yb (1 a), Er (1 b)) in good yields, representing an unprecedented conjugate electrophilic addition to a non-coordinated benzenoid nucleus. Treatment of [{Cp(2)LnNiPr(2)}(2)] with four equivalents of PhEtCCO under the same conditions gives the unexpected enolization dearomatization/rearomatization products [{Cp(2)Ln(OC{C(6)H(4)(p-CHEtCONiPr(2))}==CEtPh)}(2)] (Ln = Yb (2 a), Er (2 b), Dy (2 c)). However, reaction of [{Cp(2)YbNiPr(2)}(2)] with PhEtCCO in THF forms only the mono-insertion product [Cp(2)Yb{OC(NiPr(2))==CEtPh}](THF) (3). Hydrolysis of 2 afforded aryl ketone PhEtCHCOC(6)H(4)(p-CHEtCONiPr(2)) (4) and the overall formation of aryl ketone 4 provides an alternative route to the acylation of aromatic compounds. Moreover, reaction of [{Cp(2)LnNHPh}(2)] with excess of PhEtCCO or Ph(2)CCO in toluene affords only the products from a formal insertion of the C==C bond of the ketene into the N--H bond, [(Cp(2)Ln{OC(CHEtPh)NPh})(2)] (Ln = Yb (5 a), Y (5 b)) or [(Cp(2)Er{OC(CHPh(2))NPh})(2)] (6), respectively, indicating that an isomerization involving a 1,3-hydrogen shift occurs more easily than the conjugate electrophilic addition reaction, along with the initial amide attack on the ketene carbonyl carbon. [{Cp(2)ErNHEt}(2)] reacts with an excess of PhEtCCO to give [(Cp(2)Er{PhEtCHCON(Et)COCEtPh})(2)] (7), revealing another unique pattern of double-insertion of ketenes into the metal-ligand bond without bond formation between two ketene molecules. All complexes were characterized by elemental analysis and by their spectroscopic properties. The structures of complexes 1 b, 2 a, 2 b, 5 a, 5 b, 6, and 7 were also determined through X-ray single-crystal diffraction analysis.