188
K. Zeitler
Scheme 3. Amino acids as precursors for various triazolium ions via ‘lactam–
route’
Triazolylidene carbenes show great versatility as they not only
catalyze classical umpolung reactions, but have also been shown to be
successful catalysts for enantioselective extended umpolung reactions
(for details, please see the following sections).
Only restricted activity towards classical umpolung reactions, such
as benzoin, acyloin or Stetter reactions is known for imidazolylidene
carbenes (Marion et al. 2007; Enders et al. 2007a,b; Matsumoto and
Tomioka 2006). Imidazolium salts, which are typically prepared from
their bisimine precursors (Nolan 2006),
3 as well as benzimidazolium
salts can provide twofold steric shielding at both nitrogen atoms. Chirality can be integrated in the sterically demanding side chains that stem
from the corresponding chiral amines (Kano et al. 2005), whereas members of the saturated imidazolinylidene carbenes might also implement
chiral bisamines in their backbone (Fig. 2; Matsumoto and Tomioka
2006).
3 Only recently an additional, elegant access to unsymmetrically substituted imidazolium salts has been disclosed (Fürstner et al. 2006). Unsymmetric bisaryl-substituted
N-heterocyclic carbenes (imidazolidinium derived) can be prepared from ethyl
chlorooxoacetate (Waltman and Grubbs 2004).
K. Zeitler
Scheme 3. Amino acids as precursors for various triazolium ions via ‘lactam–
route’
Triazolylidene carbenes show great versatility as they not only
catalyze classical umpolung reactions, but have also been shown to be
successful catalysts for enantioselective extended umpolung reactions
(for details, please see the following sections).
Only restricted activity towards classical umpolung reactions, such
as benzoin, acyloin or Stetter reactions is known for imidazolylidene
carbenes (Marion et al. 2007; Enders et al. 2007a,b; Matsumoto and
Tomioka 2006). Imidazolium salts, which are typically prepared from
their bisimine precursors (Nolan 2006),
3 as well as benzimidazolium
salts can provide twofold steric shielding at both nitrogen atoms. Chirality can be integrated in the sterically demanding side chains that stem
from the corresponding chiral amines (Kano et al. 2005), whereas members of the saturated imidazolinylidene carbenes might also implement
chiral bisamines in their backbone (Fig. 2; Matsumoto and Tomioka
2006).
3 Only recently an additional, elegant access to unsymmetrically substituted imidazolium salts has been disclosed (Fürstner et al. 2006). Unsymmetric bisaryl-substituted
N-heterocyclic carbenes (imidazolidinium derived) can be prepared from ethyl
chlorooxoacetate (Waltman and Grubbs 2004).