Seven-membered heterocycle-derived carbenes

Stable free carbenes derived from four- (13), six-, or seven-membered heterocycles 14 are also known. Carbenes of type 13 can possess a phosphorus [19] or a boron atom [69] within the heterocycle. These carbenes exhibit extreme values regarding their NMR spectroscopic ( carbene-c 285 and 312.6 ppm) and structural properties (angle N-Ccarbene-N 96.72(13) and 94.0(2)°). 

The carbenes derived from six- [20-22] and seven-membered heterocycles [22-24, 70] of type 14 can be obtained both with an aliphatic or an aromatic backbone. The characteristic NMR resonances for the C2 carbon atoms in six-membered carbene heterocycles (8 236-248 ppm) [20, 21] have been recorded in the same range as the Ccarbene resonances for saturated imidazolidin-2-ylidenes of type 7. Dimerization to enetetramines occurs for some carbenes derived from a six-membered heterocycle [21] and this behavior is independent from the nature of the backbone (saturated or unsaturated), but depends strongly on the steric bulk of the N,N -substituents. 

No enetetramine formation has been observed with NHCs derived from seven-membered heterocycles. The C2 resonance in these derivatives is shifted further downfield (5 250-260 ppm) in the NMR spectra. The most striking difference in comparison to all NHCs discussed previously is the twisted, nonplanar conformation of the seven-membered heterocycle both in the azolium precursor and in the carbene ligand. The twisted conformation appears to be essential to accommodate the strain within the seven-membered heterocycle. 

N-Heterocyclic Carbenes Derived from Six- or Seven-membered Heterocycles... 

The addition of other bifunctional nucleophiles also allowed the preparation, in moderate to high yields, of the five-, seven-, and eight-membered heterocyclic carbene species 55-58 (Fig. 9) which, in some cases, were formed along with minor amounts of other derivatives [63]. Related 1,2,3-diheterocyclizations involving (ethoxy)allenylidene complexes have been described, ethanol instead of HNMe2 being released in this case [62],... 

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