Recent Developments in the Synthesis of β-Diketones

Apart from being one of the most important intermediates in chemical synthesis, broadly used in the formation of C-C bonds among other processes, the β-dicarbonyl structure is present in a huge number of biologically and pharmaceutically active compounds. In fact, mainly derived from the well-known antioxidant capability associated with the corresponding enol tautomer, β-diketones are valuable compounds in the treatment of many pathological disorders, such as cardiovascular and liver diseases, hypertension, obesity, diabetes, neurological disorders, inflammation, skin diseases, fibrosis, or arthritis; therefore, the synthesis of these structures is an area of overwhelming interest for organic chemists. This paper is devoted to the advances achieved in the last ten years for the preparation of 1,3-diketones, using different chemical (Claisen, hydration of alkynones, decarboxylative coupling) or catalytic (biocatalysis, organocatalytic, metal-based catalysis) methodologies: Additionally, the preparation of branched β-dicarbonyl compounds by means of α-functionalization of non-substituted 1,3-diketones are also discussed.

Charge Neutral [Cu2L2] and [Pd2L2] Metallocycles: Self-Assembly, Aggregation, and Catalysis

A range of morphologically distinct metallosupramolecular Cu(II) and Pd(II) complexes has been constructed, based on the tritopic ligand 1,1',1″-(benzene-1,3,5-triyl)tris(4,4-dimethylpentane-1,3-dione) (H₃L). By control of the reaction conditions, it is possible to generate distinct coordination assemblies possessing either macrocyclic or polymeric structures and more importantly distinct activity in catalysis of the Suzuki-Miyaura cross-coupling.

1,3-Diketone Calix[4]arene Derivatives-A New Type of Versatile Ligands for Metal Complexes and Nanoparticles

The present review is aimed at highlighting outlooks for cyclophanic 1,3-diketones as a new type of versatile ligands and building blocks of the nanomaterial for sensing and bioimaging. Thus, the main synthetic routes for achieving the structural diversity of cyclophanic 1,3-diketones are discussed. The structural diversity is demonstrated by variation of both cyclophanic backbones (calix[4]arene, calix[4]resorcinarene and thiacalix[4]arene) and embedding of different substituents onto lower or upper macrocyclic rims. The structural features of the cyclophanic 1,3-diketones are correlated with their ability to form lanthanide complexes exhibiting both lanthanide-centered luminescence and magnetic relaxivity parameters convenient for contrast effect in magnetic resonance imaging (MRI). The revealed structure–property relationships and the applicability of facile one-pot transformation of the complexes to hydrophilic nanoparticles demonstrates the advantages of 1,3-diketone calix[4]arene ligands and their complexes in developing of nanomaterials for sensing and bioimaging.

A simple synthetic approach to enhance the thermal luminescence sensitivity of Tb3+ complexes with thiacalix[4]arene derivatives through upper-rim bromination

The present work for the first time reports an application of the thiacalix[4]arene scaffold for the preparation of Tb3+ complexes possessing high thermal luminescence sensitivity in the physiological temperature range of 20-50 °C. Non-substituted thiacalix[4]arenes form luminescent complexes with Tb3+ ions, but they do not reveal any meaningful thermal sensitivity. To solve this problem, an upper-rim bromination of thiacalix[4]arenes, as well as distal bromination along with the embedding of two 1,3-diketone substituents are proposed as new simple synthetic approaches to enhance the thermal luminescence sensitivity of the Tb3+ complexes. A combination of mass spectrometry, NMR, UV-Vis and luminescence spectroscopy with quantum chemical calculations reveals a dimeric structure of the complexes formed by thiacalix[4]arenes with Tb3+ ions in DMF solutions. The steady-state luminescence of the Tb3+ complexes has demonstrated more than one order higher thermal sensitivity for the complexes of bromo-substituted ligands in comparison with the non-substituted thiacalix[4]arenes. The reasons for such behavior are discussed. The results highlight new opportunities for the thiacalix[4]arene platform for controlling ligand-to-metal energy transfer in terbium complexes and tuning their thermo-responsive luminescence properties.

Synthesis of novel polycarbonyl Schiff bases by ring-opening reaction of ethyl 5-acyl-4-pyrone-2-carboxylates with primary mono- and diamines

An approach for the introduction of the tricarbonyl moiety into aromatic, heterocyclic, and aliphatic amines with the use of acylpyrones has been developed for the synthesis and the design of novel polycarbonyl Schiff base ligands, including salphen structures. This Michael addition-ring-opening reaction proceeds under mild conditions (stirring at 0-20 °C) via the attack at the C-6 position of the pyrone ring in good to high yields (up to 99%) with excellent selectivity. The products can be easily isolated by crystallization without the use of chromatography. The scope of the reaction, tautomeric equilibrium of open-chain products, and their cyclization into pyridone structures were investigated.

Structural and photophysical properties of Tb3+-tetra-1,3-diketonate complexes controlled by calix[4]arene-tetrathiacalix[4]arene scaffolds

The present work highlights the key aspects of the influence of calix[4]arene and tetrathiacalix[4]arene scaffolds on the structural and photophysical properties of Tb3+ complexes with tetra-1,3-diketone derivatives of the macrocycles in DMF solutions. The equilibrium forms of Tb3+ complexes with unsubstituted and functionalized by acetylacetonyl groups at the upper rim of calix[4]arenes and thiacalix[4]arenes are revealed from UV-, NMR, MALDI TOF mass spectroscopy, quantum-chemical calculations at the DFT level and luminescence spectroscopy data. In alkaline DMF solutions, the ligands form predominantly 1 : 1 complexes with Tb3+ ions. However, the replacement of a calix[4]arene-scaffold by a thiacalix[4]arene-scaffold in the tetra-1,3-diketone derivatives shifts the equilibrium forms of Tb3+ complexes from monomeric to the dimeric ones. DFT calculations in combination with experimental data reveal the most reliable structures of complexes. The quantitative analysis of the photophysical parameters in correlation with the structural features of the complexes highlights the specific inner-sphere environment of Tb3+ ions in the dimeric complexes with the thiacalix[4]arene derivatives as a reason for greater sensitization of Tb3+-centered luminescence.

A fresh look at the structural diversity of dibenzoylmethanide complexes of lanthanides

Synthetic methods for dibenzoylmethanide lanthanide complexes as well as heterolanthanide ones were systematized revealing several species.

Hydration number: crucial role in nuclear magnetic relaxivity of Gd(III) chelate-based nanoparticles

Today, nanostructure-based contrast agents (CA) are emerging in the field of magnetic resonance imaging (MRI). Their sensitivity is reported as greatly improved in comparison to commercially used chelate-based ones. The present work is aimed at revealing the factors governing the efficiency of longitudinal magnetic relaxivity (r₁) in aqueous colloids of core-shell Gd(III)-based nanoparticles. We report for the first time on hydration number (q) of gadolinium(III) as a substantial factor in controlling r₁ values of polyelectrolyte-stabilized nanoparticles built from water insoluble complexes of Gd(III). The use of specific complex structure enables to reveal the impact of the inner-sphere hydration number on both r₁ values for the Gd(III)-based nanoparticles and the photophysical properties of their luminescent Tb(III) and Eu(III) counterparts. The low hydration of TTA-based Gd(III) complexes (q ≈ 1) agrees well with the poor relaxivity values (r₁ = 2.82 mM^-1s^-1 and r₂ = 3.95 mM^-1s^-1), while these values tend to increase substantially (r₁ = 12.41 mM^-1s^-1, r₂ = 14.36 mM^-1s^-1) for aqueous Gd(III)-based colloids, when macrocyclic 1,3-diketonate is applied as the ligand (q ≈ 3). The regularities obtained in this work are fundamental in understanding the efficiency of MRI probes in the fast growing field of nanoparticulate contrast agents.