For a long time multi-component syntheses of heterocycles have undeniably been a domain of classical carbonyl condensation chemistry. However, the advent of transition-metal catalysis not only has fertilized strategies in heterocyclic synthesis by uni- and bimolecular transformations but the past decade has also witnessed a rapid development of transition-metal catalysis in new multi-component reactions (MCR). Expectedly, palladium catalyzed processes have received a dominant position, yet, other transition-metal complexes are catching up implying organometallic elementary steps that reach even further than cross-coupling and carbometallation. Besides domino MCRs that are purely based upon organometallic catalysis the sequential and consecutive combination with condensation, addition and cycloaddition steps opens a vast playground for the invention of new sequences in heterocyclic synthesis. This tutorial review outlines the underlying reaction based principles of transition-metal catalysis in multi-component syntheses of heterocycles, summarizes recent developments of palladium catalyzed MCR, and highlights the more recent contributions to MCR based heterocyclic synthesis by virtue of rhodium, ruthenium, and copper catalysis.

Multicomponent reactions are a valuable tool for the synthesis of functional π-electron systems. Two different approaches can be taken into account for accessing the target structures. In the more conventional scaffold approach an already existing chromophore is coupled with other components to give a complex functional π-system. Here, electronically monotonous components can also be introduced, which may exert synergistic electronic effects within the novel compound. The more demanding chromophore concept generates a complete π-electron system and a scaffold concurrently. The latter approach is particularly stimulating for methodologists since π-systems might be accessible from simple starting materials. This review encompasses the advances in the preparation of functional π-electron systems via multicomponent processes during the past few years, based both on the scaffold and chromophore concepts. Besides the synthetic strategies the most important properties, i.e. redox potentials, absorption and emission maxima or fluorescence quantum yields, of the synthesized molecules are highlighted.

The N-donor complexing ligand 2,6-bis(5-(2,2-dimethylpropyl)-1H-pyrazol-3-yl)pyridine (C5-BPP) was synthesized and screened as an extracting agent selective for trivalent actinide cations over lanthanides. C5-BPP extracts Am(III) from up to 1 mol/L HNO(3) with a separation factor over Eu(III) of approximately 100. Due to its good performance as an extracting agent, the complexation of trivalent actinides and lanthanides with C5-BPP was studied. The solid-state compounds [Ln(C5-BPP)(NO(3))(3)(DMF)] (Ln = Sm(III), Eu(III)) were synthesized, fully characterized, and compared to the solution structure of the Am(III) 1:1 complex [Am(C5-BPP)(NO(3))(3)]. The high stability constant of log β(3) = 14.8 ± 0.4 determined for the Cm(III) 1:3 complex is in line with C5-BPP's high distribution ratios for Am(III) observed in extraction experiments.

Starting from N-hexylphenothiazine, a versatile construction kit of brominated and borylated phenothiazines can be easily prepared by a sequence of bromination, bromo-lithium exchange/borylation, and Suzuki coupling. Subsequent Suzuki arylation of the building blocks gives soluble, monodisperse, and structurally well defined oligophenothiazines in good yields. The molecular weights at the peak maximum (Mp), obtained by GPC (gel permeation chromatography), and the actual molecular weights of the oligomer series, obtained by mass spectrometry, show excellent correlation. A QM/MM conformational analysis for the complete series reveals that the obvious butterfly-shaped phenothiazine structure multiplies and significantly reduces the hydrodynamic volume of the oligomers. The electronic properties (absorption and emission spectroscopy and cyclic voltammetry) give reasonable correlations with the chain length. With regard to the emission maxima, the effective conjugation length is already reached with the hexamer. Oligophenothiazines are highly fluorescent, with high fluorescence quantum yields, and are simultaneously highly electroactive, with low oxidation potentials.