NHCs – Highly Versatile Organocatalysts
189
Fig. 2. Chiral imidazolium precatalysts
Flexible steric bulk is a characteristic of a new class of imidazolium
salts derived from bisoxazolines (IBiox) (Glorius et al. 2002; Altenhoff
et al. 2004), which were tested for the organocatalytic synthesis of butyrolactones (Burstein and Glorius 2004).
Only recently, based on earlier reports on achiral examples (Alcarazo
et al. 2005; Burstein et al. 2005), have optically active imidazopyridinium salts been prepared (Schmidt and Movassaghi 2007).
So far, chiral imidazolium precatalysts have been used successfully
for kinetic resolutions of racemic secondary alcohols via enantioselective acylation (Kano et al. 2005; Suzuki et al. 2004).
Apart form a great number of chiral NHC carbenes that have been
used as ligands in enantioselective transition-metal catalysis (Gade and
Bellemin-Laponnanz 2007), some less usual heterazolium salts have
been tested in organocatalytic transformations. A planar-chiral thiazolium salt (Pesch et al. 2004) and a rotaxane-derived precatalyst were
reported (Tachibana et al. 2004), as well as catalytically active peptides containing an unnatural thiazolium-substituted alanine amino acid
(Fig. 3; Mennen et al. 2005a,b).
Because of the growing importance of sustainable synthetic methods
on the one hand and the need for simple reaction procedures that allow
Fig. 3. Structurally unique chiral heterazolium salts