NHCs – Highly Versatile Organocatalysts
195
The ring opening reaction of N-tosylaziridines with certain silylated
nucleophiles via a similar activation has been reported recently (Wu
et al. 2006).
A NHC-catalyzed aza–Morita–Baylis–Hillman reaction (aza–MBH)
following a standard nucleophile-mediated MBH mechanism has been
disclosed very recently (He et al. 2007). Although combined with a preceding equilibrium for the reversible formation of imine–carbene
adducts this reaction has similarities with phosphines and their organocatalytic reactivity (Methot and Roush 2004).
Classical umpolung reactions such as Stetter reactions (Christmann
2005), benzoin reactions and nucleophilic aromatic substitution (S N Ar)
(Suzuki et al. 2003) have already become important tools in organic
synthesis as exemplified by their application in the syntheses of several natural product (Nicolaou et al. 2007; Harrington and Tius 2001).
Significant progress has been reported for enantioselective crossed acyloin formations (Takikawa et al. 2006; Enders et al. 2006), but also new
applications, such as an elegant desymmetrization reaction (Liu and Rovis 2006), have been described recently. An interesting domino reaction
involving a combination of amine and carbene catalysis allows simplified one-pot access to β-substituted ester derivatives with high enantioselectivity (Zhao et Córdova 2007). The aerobic ring opening of Ntosylaziridines with aldehydes is proposed to proceed via an alkylated
‘Breslow intermediate’ which is subsequently attacked by dioxygene as
the electrophile (Liu et al. 2006).
5
Moreover, the growing group of extended umpolung reactions is attracting considerable attention. This integration of functionalized aldehyde substrates in umpolung reactions (Reynolds et al. 2004; Chow and
Bode 2004), and hence their cooperative interaction to yield new, defunctionalized intermediates has already been applied in a remarkable
number of stereoselective variations (Scheme 7).
In general, all extended umpolung reactions deal with aldehyde substrates that bear somehow reducible side chains prone to subsequent
interaction at the stage of the common so-called ‘Breslow intermediate’
(Zeitler 2005). Their subsequent participation leads—depending on the
5 For two further aziridine opening reactions please refer to transesterification and
silicon activation section above.
195
The ring opening reaction of N-tosylaziridines with certain silylated
nucleophiles via a similar activation has been reported recently (Wu
et al. 2006).
A NHC-catalyzed aza–Morita–Baylis–Hillman reaction (aza–MBH)
following a standard nucleophile-mediated MBH mechanism has been
disclosed very recently (He et al. 2007). Although combined with a preceding equilibrium for the reversible formation of imine–carbene
adducts this reaction has similarities with phosphines and their organocatalytic reactivity (Methot and Roush 2004).
Classical umpolung reactions such as Stetter reactions (Christmann
2005), benzoin reactions and nucleophilic aromatic substitution (S N Ar)
(Suzuki et al. 2003) have already become important tools in organic
synthesis as exemplified by their application in the syntheses of several natural product (Nicolaou et al. 2007; Harrington and Tius 2001).
Significant progress has been reported for enantioselective crossed acyloin formations (Takikawa et al. 2006; Enders et al. 2006), but also new
applications, such as an elegant desymmetrization reaction (Liu and Rovis 2006), have been described recently. An interesting domino reaction
involving a combination of amine and carbene catalysis allows simplified one-pot access to β-substituted ester derivatives with high enantioselectivity (Zhao et Córdova 2007). The aerobic ring opening of Ntosylaziridines with aldehydes is proposed to proceed via an alkylated
‘Breslow intermediate’ which is subsequently attacked by dioxygene as
the electrophile (Liu et al. 2006).
5
Moreover, the growing group of extended umpolung reactions is attracting considerable attention. This integration of functionalized aldehyde substrates in umpolung reactions (Reynolds et al. 2004; Chow and
Bode 2004), and hence their cooperative interaction to yield new, defunctionalized intermediates has already been applied in a remarkable
number of stereoselective variations (Scheme 7).
In general, all extended umpolung reactions deal with aldehyde substrates that bear somehow reducible side chains prone to subsequent
interaction at the stage of the common so-called ‘Breslow intermediate’
(Zeitler 2005). Their subsequent participation leads—depending on the
5 For two further aziridine opening reactions please refer to transesterification and
silicon activation section above.