129 research outputs found
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 100ā000 were achieved, at a catalyst loading of only 3 ppm. Remarkably, the most active catalyst system was able to achieve a TON of 340ā000, 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 Ī±-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 bisĀ(iminium)
salts [derived from cyclic (alkyl)Ā(amino)Ācarbenes] bridged by various
spacers. These compounds show characteristic intervalence charge transfer
(IV-CT) bands in the near-infrared (NIR). Cyclic voltammetry, EPR,
IR, UVāvis, 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)Ā(amino)Ācarbenes
(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 KekuleĢ Diradicaloids Derived from Cyclic (Alkyl)(amino)carbenes
A modular approach for the synthesis
of KekuleĢ diradicaloids
is reported. The key step is the insertion of a carbene, namely, a
cyclic (alkyl)Ā(amino)Ācarbene (CAAC), into the CāH bonds of
two terminal alkynes linked by a spacer. Subsequent hydride abstraction,
followed by two-electron reduction of the corresponding bisĀ(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, UVāvis, and X-ray studies in combination
with quantum-chemical calculations give insight into the electronic
nature of the deeply colored KekuleĢ 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 KekuleĢ Diradicaloids Derived from Cyclic (Alkyl)(amino)carbenes
A modular approach for the synthesis
of KekuleĢ diradicaloids
is reported. The key step is the insertion of a carbene, namely, a
cyclic (alkyl)Ā(amino)Ācarbene (CAAC), into the CāH bonds of
two terminal alkynes linked by a spacer. Subsequent hydride abstraction,
followed by two-electron reduction of the corresponding bisĀ(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, UVāvis, and X-ray studies in combination
with quantum-chemical calculations give insight into the electronic
nature of the deeply colored KekuleĢ 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
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