1,2-Migration of the phosphano-group to the carbene center in N-phosphano functionalized N-heterocyclic carbenes has been studied by density functional theory (DFT) calculations. An intramolecular mechanism with a three-center transition state structure seems to be most plausible for the isolated carbenes, while an intermolecular pathway catalyzed by azolium salts may be preferable for a migration proceeding in the course of generating the carbenes in situ. Our calculations show that amino-substitution at the phosphorus atom and an enhanced nucleophilicity of the heterocycle scaffold facilitate the phosphorus shift. Calculated singlet-triplet energy gaps do not correlate with thermodynamic stability of the studied carbenes and their disposition toward the 1,2-rearrangement.
Keywords: DFT; carbenes; intramolecular migration; phosphanylated heterocycles.
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