Tetracarbene complex

After the initial discovery, several sugge.stions were made for the mechanism during the 1970s (Eq. 12.8). A cyclobutane metal complex was considered, but cyclobutanes were not formed in the reaction and added cyclobutane did not participate in the reaction. Pettit proposed a tetracarbene complex, for example M(=CH2)4 from ethylene, but that seemed to require an unreasonably large number of available sites on the metal. Grubbs proposed a metallocyclopentane intermediate, formed from oxidative coupling of the two alkenes, but it was... 

The generation of an arylnitrene from an arylazide catalysed by an iron-tetracarbene complex has been highlighted as an efficient method to produce arylaziridines in a single step by reaction with an olefin. ... 

Trimethylsiloxyphenyl isocyanide enters the cyclization reaction with [MCl2(NCPh)2] (M = Pt, Pd) to yield the homoleptic tetracarbenes 77 (M=Pt, Pd) (97JOM(541)51). Complex 77 (M = Pd) enters an interesting reaction with ammonia to yield the species 78 where two of this benzoxazol-2-ylidene ligands are deprotonated and become C-coordinated benzoxazole moieties, while the other two remain intact. Palladium(II) iodide in these conditions behaves differently yielding the di-Mo-cyanide complex, which in the presence of tetra- -butyl ammonium fluoride gives the dicarbene 79. 

It is interesting to note, that apart from the impressive series of transition metal complexes with monodentate NHCs, tetracarbene palladium(II) and platinum(II) as well as hexacarbene iron(III) complexes with bi- and tridentate chelating carbene ligands were prepared. 

Although arguably the most popular route to NHC is via the azolium salt, there are many other options for their synthesis. An excellent and comprehensive overview of the synthesis of NHC and some of their complexes can be found in Hahn [34]. An interesting route to tri- and tetracarbenes is provided by the template controlled cyclisation of isocyanides, a method pionered by Lappert and coworkers [35,36] (see Fignre 1.2). 

Figure 30 Iron(ll) complex with a macrocyclic tetracarbene ligand used as catalyst in the aziridination of olefins.

Meyer S, Orben CM, Demeshko S, Dechert S, Meyer F. Synthesis and characterization of di- and tetracarbene iron(II) complexes with chelating N-heterocychc carbene ligands and their apphcation in aryl Grignard-alkyl halide cross-coupling. Organometallics. 2011 30 6692—6702. 

In 2011, Cramer and Jenkins synthesised dicationic octahedral Fe" complex 10 that featured a macrocyclic tetracarbene ligand and two molecules of acetonitrile in its solid-state structure. This compound, reminiscent of an iron porphyrin, catalysed the aziridination of simple di-, tri-, and even tetrasubsti-tuted alkenes and cycloalkenes with aryl azides [eqn (7.8)]. The reaction most likely proceeded via the intermediacy of a highly reactive Fe imido species. 

Most recently, Cramer and Jenkins have developed a novel tetracarbene iron(ll) complex 8, which was readily prepared by the reaction of the macrocyclic tetraimidazolium 7 with iron(ll) iodide using lithium diisopropylamide followed by the addition of thallium hexafluorophosphate (Scheme 2.12) [18]. Employing this catalyst system, aziridination reactions of various types of substituted aliphatic alkenes including trisubstituted and tetrasubstituted ones with both electron-donating and electron-withdrawing aryl azides were achieved. One advantage of this methodology is... 

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