The combination of photoredox and transition metal catalysis, which is termed metallaphotoredox catalysis, is a powerful platform for building complex molecules under mild conditions. In particular, metallaphotoredox-catalyzed multicomponent coupling reactions,...
The key reactive intermediate of copper(I)-catalyzed alkyne semihydrogenations is a vinylcopper(I) complex. This intermediate can be exploited as a starting point for a variety of trapping reactions. In this manner, an alkyne semihydrogenation can be turned into a dihydrogen-mediated coupling reaction. Therefore, the development of copper-catalyzed (transfer) hydrogenation reactions is closely intertwined with the corresponding reductive trapping reactions. This short review highlights and conceptualizes the results in this area so far, with H2-mediated carbon–carbon and carbon–heteroatom bond-forming reactions emerging under both a transfer hydrogenation setting as well as with the direct use of H2. In all cases, highly selective catalysts are required that give rise to atom-economic multicomponent coupling reactions with rapidly rising molecular complexity. The coupling reactions are put into perspective by presenting the corresponding (transfer) hydrogenation processes first.1 Introduction: H2-Mediated C–C Bond-Forming Reactions2 Accessing Copper(I) Hydride Complexes as Key Reagents for Coupling Reactions; Requirements for Successful Trapping Reactions 3 Homogeneous Copper-Catalyzed Transfer Hydrogenations4 Trapping of Reactive Intermediates of Alkyne Transfer Semihydrogenation Reactions: First Steps Towards Hydrogenative Alkyne Functionalizations 5 Copper(I)-Catalyzed Alkyne Semihydrogenations6 Copper(I)-Catalyzed H2-Mediated Alkyne Functionalizations; Trapping of Reactive Intermediates from Catalytic Hydrogenations6.1 A Detour: Copper(I)-Catalyzed Allylic Reductions, Catalytic Generation of Hydride Nucleophiles from H2
6.2 Trapping with Allylic Electrophiles: A Copper(I)-Catalyzed Hydroallylation Reaction of Alkynes 6.3 Trapping with Aryl Iodides7 Conclusion
Multicomponent reactions (MCRs) constitute a powerful synthetic tool to generate a large number of small molecules with high atom economy, which thus can efficiently expand the chemical space with molecular diversity and complexity. Aryne-based MCRs offer versatile possibilities to construct functionalized arenes and benzo-fused heterocycles. Because of their electrophilic nature, arynes couple with a broad range of nucleophiles. Thus, a variety of aryne-based MCRs have been developed, the representative of which are summarized in this account.1 Introduction2 Aryne-Based Multicomponent Reactions2.1 Trapping with Isocyanides2.2 Trapping with Imines2.3 Trapping with Amines2.4 Insertion into π-Bonds2.5 Trapping with Ethers and Thioethers2.6 Trapping with Carbanions2.7 Transition-Metal-Catalyzed Approaches3 Strategies Based on Hexadehydro Diels–Alder Reaction3.1 Dihalogenation3.2 Halohydroxylation and Haloacylation3.3 Amides and Imides3.4 Quinazolines3.5 Benzocyclobutene-1,2-diimines and 3H-Indole-3-imines3.6 Other MCRs of Arynes and Isocyanides4 Conclusion
A highly dehydrated silica gel, when exposed to Ti(NMe2)4, gives a fast, high-yielding catalyst for multicomponent coupling of an amine, alkyne, and isonitrile to form 1,3-dimine tautomers, iminoamination.
