Catalytic Approaches to Multicomponent Reactions: A Critical Review and Perspectives on the Roles of Catalysis.

In this review, we comprehensively describe catalyzed multicomponent reactions (MCRs) and the multiple roles of catalysis combined with key parameters to perform these transformations. Besides improving yields and shortening reaction times, catalysis is vital to achieving greener protocols and to furthering the MCR field of research. Considering that MCRs typically have two or more possible reaction pathways to explain the transformation, catalysis is essential for selecting a reaction route and avoiding byproduct formation. Key parameters, such as temperature, catalyst amounts and reagent quantities, were analyzed. Solvent effects, which are likely the most neglected topic in MCRs, as well as their combined roles with catalysis, are critically discussed. Stereocontrolled MCRs, rarely observed without the presence of a catalytic system, are also presented and discussed in this review. Perspectives on the use of catalytic systems for improved and greener MCRs are finally presented.

Review on the Ugi Multicomponent Reaction Mechanism and the Use of Fluorescent Derivatives as Functional Chromophores.

In the present mini-review we discuss the findings, controversies, and gaps observed for the Ugi four-component reaction. The Ugi multicomponent reaction, performed by mixing an aldehyde, an amine, a carboxylic acid, and an isocyanide, is among the most important isocyanide-based multicomponent reactions (MCRs), allowing multiple bond formations (C-C and C-N) in a single synthetic step. The possibility of two reaction pathways and the little understood solvent effect over this transformation renders this reaction as one of the hardest challenges to overcome. The little knowledge of the mechanism of the Ugi MCR hinders the development of new and efficient chiral catalytic systems to further the application of the derivatives obtained by enantioselective versions. The asymmetric transformation is in this context a bigger challenge, and little is known about the mechanism of these few available versions. The new trend of functional chromophore synthesis by MCRs is also highlighted, and the few examples already disclosed in the literature exemplify the huge opportunity for investigation and creative ideas using the Ugi four-component reaction.