2-(Pentafluorophenyl)imidazolidines

Introduction

N-Heterocyclic carbenes (NHCs) have emerged as a unique class of organocatalysts and as ligands for transition metals.[1] The high reactivity of free NHCs, which makes these compounds very useful, translates into their sensitivity towards air and moisture. That is why NHCs are generally prepared in situ by deprotonation of azolium salts with strong non-nucleophilic bases [e.g., t-BuOK or KN(SiMe₃)₂]. The use of strong bases which are incompatible with many functional groups and the formation of typical byproducts (e.g., inorganic salts and alcohols) limit the scope of this method. An alternative approach to NHCs includes thermolysis of imidazolium-2-carboxylates, 2-(trichloromethyl)imidazolidines, silver–NHC com­plexes and other reagents. However, many of these reagents are poorly soluble. Moreover, they are commonly synthesized from NHCs. In 2004, Waymouth, Hedrick, and co-workers reported the synthesis of 2-(pentafluorophenyl)imidazolidines as useful and readily available NHC-transfer reagents (Scheme [1]).[2] The advantages of pentafluorophenyl adducts are: (a) high stability in air; (b) good solubility in common organic solvents; (с) thermolysis generally occurs under mild heating; (d) the only byproduct of the thermolysis is pentafluorobenzene; (e) straightforward synthesis. 2-(Pentafluorophenyl)imidazolidines can be prepared from structurally diverse ­diamines and commercially available pentafluorobenzaldehyde (Scheme [1]).

Scheme 1 Synthesis and thermolysis of 2-(pentafluoro­phenyl)imidazolidines[2] [3]

• References

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