Seven-Membered Heterocydes

The carbenes derived from six- [20-22] and seven-membered heterocydes [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 heterocydes ( 236-248 ppm) [20, 21] have been recorded in the same range as the Ccarbene resonances for saturated imidazolidin-2-ylidenes of type 1. 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 (sartirated or unsaturated), but depends strongly on the steric bulk of the /V,/V -substiments. 

Reactions of functionalized alkoxyethylenes with nucleophilic reagents in synthesis of five-, seven-membered heterocydes and macroheterocycles 06ZOR167. 

No enetetramine formation has been observed with NHCs derived from sevenmembered heterocydes. The C2 resonance in these derivatives is shifted further downfield (5 250-260 ppm) in the C 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. 

Many of these heterocydes are commerdally available. They arise from petroleum as source, except LA. Methyl-substituted seven-membered ring carbonates (7CCs), namely 4-methyl-and 5-methyl-l,3-dioxepan-2-one (R-Me7CC and p-Me7CC), have been synthesized by cydization of the corresponding a,[Pg.71]

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