Improved Synthesis of Unsymmetrical N ‐Aryl‐ N′ ‐alkylpyridyl ß‐Diketimines Using Molecular Sieves and their Lithium Complexes

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 H2L (L = MeC(NDipp)CHC(Me)NCH2CH2-3-(1-R-C8H4N), R = CH2-(2-C4H7O), L1; R = (CH2)2OMe, L2; Dipp = 2,6-iPr2C6H3) 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(CH2SiMe3)3(THF)2 generated the corresponding unsymmetric N,N,C,O-tetradentate β-diketiminato rare-earth metal monoalkyl complexes LRE(CH2SiMe3) (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 sp2 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.

Ligand Degradation Study of Unsymmetrical β-Diketiminato Copper Dioxygen Adducts: The Length Chelating Arm Effect

An investigation on the reactivity of O₂ binding to unsymmetrical β-diketiminato copper(I) complexes by spectroscopic and titration analysis was performed. The length of chelating pyridyl arms (pyridylmethyl arm vs pyridylethyl arm) leads to the formation of mono- or di-nuclear copper-dioxygen species at -80 °C. The pyridylmethyl arm adduct (L₁CuO₂) forms mononuclear copper-oxygen species and shows ligand degradation, resulting in the formation of (2E,3Z)-N-(2,6-diisopropylphenyl)-4-(((E)-pyridin-2-ylmethylene)amino)pent-3-en-2-imine, which slowly converts to its cyclization isomer 1-(2,6-diisopropylphenyl)-4,6-dimethyl-2-(pyridin-2-yl)-1,2-dihydropyrimidine after addition of NH₄OH at room temperature. On the other hand, the pyridylethyl arm adduct [(L₂Cu)₂(μ-O)₂] forms dinuclear species at -80 °C and does not show any ligand degradation product. Instead, free ligand formation was observed after the addition of NH₄OH. These experimental observations and product analysis results indicate that the chelating length of pyridyl arms governs the Cu/O₂ binding ratio and the ligand degradation behavior.

Nitric oxide generation study of unsymmetrical β-diketiminato copper(II) nitrite complexes

The β-diketiminato copper(II) L1CuCl−L4CuCl and their nitrite complexes L1Cu(O2N) and L2Cu(O2N) has been synthesized and characterized. The X-ray structure of the L1CuCl−L4CuCl complexes clearly indicates towards the mononuclear structure with...

Synthetic Factors Governing Access to Tris(β-diketimine) Cyclophanes versus Tripodal Tri-β-aminoenones

Tris(β-diketimine) cyclophanes are an important ligand class for investigating cooperative multimetallic interactions of bioinorganic clusters. Discussed herein are the synthetic factors governing access to tris(β-diketimine) cyclophanes versus tripodal tri-β-aminoenones. Cyclophanes bearing Me, Et, and MeO cap substituents and β-Me, Et, or Ph arm substituents are obtained, and a modified condensation method produced α-Me β-Me cyclophane. These operationally simple procedures produce the ligands in gram quantities and in 22-94% yields.