Generalization of the copper to late-transition-metal transmetallation to carbenes beyond N-heterocyclic carbenes

The authors gratefully acknowledge the Royal Society (University Research Fellowship to C.S.J.C.), the EPSRC (EP/J500549/1) and the DOE (DE-FG02-13ER16370) for financial support.Carbene transition-metal complexes have become a prevalent family of catalysts enabling numerous organic transformations. Their facile synthetic access is a matter of great importance. To this end, the CuI-NHC transfer methodology has emerged as a powerful alternative presenting attractive advantages over other methods. Herein, we report the remarkable ability of copper to transfer not only NHCs but also other types of carbenes such as abnormal NHCs (aNHCs), cyclic (alkyl)(amino)carbenes (CAACs), and mesoionic carbenes (MICs) to various transition metal precursors.PostprintPeer reviewe

Cyclic Alkyl Amino Carbene (CAAC) Ruthenium Complexes as Remarkably Active Catalysts for Ethenolysis

An expanded family of ruthenium-based metathesis catalysts bearing cyclic alkyl amino carbene (CAAC) ligands was prepared. These catalysts exhibited exceptional activity in the ethenolysis of the seed-oil derivative methyl oleate. In many cases, catalyst turnover numbers (TONs) of more than 100 000 were achieved, at a catalyst loading of only 3 ppm. Remarkably, the most active catalyst system was able to achieve a TON of 340 000, at a catalyst loading of only 1 ppm. This is the first time a series of metathesis catalysts has exhibited such high performance in cross-metathesis reactions employing ethylene gas, with activities sufficient to render ethenolysis applicable to the industrial-scale production of linear α-olefins (LAOs) and other terminal-olefin products

Organic Mixed Valence Compounds Derived from Cyclic (Alkyl)(amino)carbenes

Readily available room temperature stable organic mixed valence compounds are prepared by one-electron reduction of cyclic bis­(iminium) salts [derived from cyclic (alkyl)­(amino)­carbenes] bridged by various spacers. These compounds show characteristic intervalence charge transfer (IV-CT) bands in the near-infrared (NIR). Cyclic voltammetry, EPR, IR, UV–vis, and X-ray studies, as well as DFT calculations, show that, depending on the nature of the spacer, these mixed valence compounds range from class III to class II

Crystalline Monomeric Allenyl/Propargyl Radical

Reduction of alkynyl iminium salts derived from cyclic (alkyl)­(amino)­carbenes (CAACs) affords propargyl/allenyl radicals. Depending on the nature of the CAAC and alkyne substituents, these radicals can irreversibly dimerize, exist as monomers in solution but dimerize in the solid state, or can even remain monomeric as solids. The first characterization of an allenyl radical by single crystal X-ray crystallography is reported

Modular Approach to Kekulé Diradicaloids Derived from Cyclic (Alkyl)(amino)carbenes

A modular approach for the synthesis of Kekulé diradicaloids is reported. The key step is the insertion of a carbene, namely, a cyclic (alkyl)­(amino)­carbene (CAAC), into the C–H bonds of two terminal alkynes linked by a spacer. Subsequent hydride abstraction, followed by two-electron reduction of the corresponding bis­(iminium) salts, affords the desired diradicaloids. This synthetic route readily allows for the installation of communicating spacers, featuring different degrees of aromaticity and lengths, and gives the possibility of generating unsymmetrical compounds with two different CAACs. Electron paramagnetic resonance (EPR), NMR, UV–vis, and X-ray studies in combination with quantum-chemical calculations give insight into the electronic nature of the deeply colored Kekulé diradicaloids. They feature a singlet ground state with varying degrees of diradical character in combination with small singlet/triplet gaps. Upon lengthening of the spacer, the properties of the compounds approach those of monoradicals in which steric protection of the propargyl radical moiety is necessary to inhibit decomposition pathways. Most of these diradicaloids are stable at room temperature, both in solution and in the solid state, but are highly oxygen-sensitive. They represent the first diradicaloids derived from iminium salts

Modular Approach to Kekulé Diradicaloids Derived from Cyclic (Alkyl)(amino)carbenes

A modular approach for the synthesis of Kekulé diradicaloids is reported. The key step is the insertion of a carbene, namely, a cyclic (alkyl)­(amino)­carbene (CAAC), into the C–H bonds of two terminal alkynes linked by a spacer. Subsequent hydride abstraction, followed by two-electron reduction of the corresponding bis­(iminium) salts, affords the desired diradicaloids. This synthetic route readily allows for the installation of communicating spacers, featuring different degrees of aromaticity and lengths, and gives the possibility of generating unsymmetrical compounds with two different CAACs. Electron paramagnetic resonance (EPR), NMR, UV–vis, and X-ray studies in combination with quantum-chemical calculations give insight into the electronic nature of the deeply colored Kekulé diradicaloids. They feature a singlet ground state with varying degrees of diradical character in combination with small singlet/triplet gaps. Upon lengthening of the spacer, the properties of the compounds approach those of monoradicals in which steric protection of the propargyl radical moiety is necessary to inhibit decomposition pathways. Most of these diradicaloids are stable at room temperature, both in solution and in the solid state, but are highly oxygen-sensitive. They represent the first diradicaloids derived from iminium salts