Synthesis and catalytic activity of tetradentate β-diketiminato rare-earth metal monoalkyl complexes in tandem Oppenauer oxidation and cross-aldol condensation.

A series of unsymmetric tetradentate β-diketiminato rare-earth metal monoalkyl complexes were synthesized, and their catalytic behavior has been well developed. Indole-incorporated β-diketiminato proligands H 2 L (L = MeC(NDipp)CHC(Me)NCH 2 CH 2 -3-(1-R-C 8 H 4 N), R = CH 2 -(2-C 4 H 7 O), L1; R = (CH 2 ) 2 OMe, L2; Dipp = 2,6- i Pr 2 C 6 H 3 ) were prepared by the reaction of an arylamino-enone with 1-substituted-tryptamine in good yields. Treatment of the proligands with the rare-earth metal trialkyl complexes RE(CH 2 SiMe 3 ) 3 (THF) 2 generated the corresponding unsymmetric N , N , C , O -tetradentate β-diketiminato rare-earth metal monoalkyl complexes LRE(CH 2 SiMe 3 ) (L1, RE = Y (1a), Gd (1b), Yb (1c), Lu (1d); L2, RE = Y (2a), Gd (2b), Yb (2c), and Lu(2d)). During the process, the activation of the sp 2 C-H bond at the 2-position of the indole ring led to the formation of an unprecedented β-diketiminato dianion L2-, bonding to the rare-earth metal ions in a chelating N , N , C , O -tetradentate manner. Further studies indicated that these tetradentate rare-earth metal complexes could initiate the Oppenauer oxidation of secondary alcohols into the corresponding ketones in high yields. In the case of primary alcohols, a tandem Oppenauer oxidation and cross-aldol condensation occurred unexpectedly. Various α-mono-substituted benzylidene acetones, α,α'-bis-substituted benzylidene acetones and cyclohexanones were obtained under mild conditions only by controlling the molar ratio of alcohols to ketones. Notably, all these alkenylation ketones exhibited exclusive E configuration.