198
K. Zeitler
followed by β-lacton formation and subsequent CO 2 extrusion
6 (Nair
et al. 2006b), the cis products have been demonstrated to stem from
a crossed benzoin-oxy-Cope cascade that leads to the identical intermediate as proposed earlier from conjugate addition (Chiang et al. 2007).
With respect to the differences in the equilibria of s-cis and s-trans conformers of different enones this mechanistic reaction pathway provides
an explanation for the varying stereochemical outcome (Chiang et al.
2007).
In addition, an umpolung reaction without participation of an aldehyde moiety has been disclosed recently (Fischer et al. 2006). Michael
acceptors (unsaturated esters, amides and nitriles) that bear pendant
leaving groups such as bromides or tosylates can undergo an organocatalytic, intramolecular β-alkylation. Carbene addition at the β-position
of the Michael acceptor and subsequent tautomerization also promotes
the generation of a homoenolate equivalent which is trapped intramolecularly under ring closure.
7
Finally, there are also some special NHC-mediated transformations
that do not completely fit into the classification, such as triazolylidenecatalyzed hydroacylations (Chan and Scheidt 2006). Aldehydes can
serve as hydride donors for activated ketones partly following a standard 1,2-addition of the NHC to the aldehyde, but instead of the usual
carbonyl umpolung a hydride (‘H-umpolung’) transfer is initiated.
A related ‘Cannizzaro-type’ transformation has been described for
indazole-derived carbene catalysts (Schmidt et al. 2007).
In conclusion, it is obvious that N-heterocyclic carbenes and their
unique, but concurrently versatile reactivity have already proven wide
applicability. The offered catalytic (and enantioselective) access to important reactive intermediates such as homoenolates or enolates as well
as their additional catalytic properties will smooth the way for mild and
sustainable synthesis of multifunctionalized compounds.
6 Conjugate or vinylogous umpolung can also lead to the formation of β-lactones; interestingly their stability is highly dependent on the substrates. For stable β-lactones please
see: Burstein et al. 2006).
7 The first step of this unprecedented umpolung reaction is identical to the standard
MBH mechanism. Only the tautomerization event differentiates between α- (enolate) and
β-functionalization (homoenolate).
K. Zeitler
followed by β-lacton formation and subsequent CO 2 extrusion
6 (Nair
et al. 2006b), the cis products have been demonstrated to stem from
a crossed benzoin-oxy-Cope cascade that leads to the identical intermediate as proposed earlier from conjugate addition (Chiang et al. 2007).
With respect to the differences in the equilibria of s-cis and s-trans conformers of different enones this mechanistic reaction pathway provides
an explanation for the varying stereochemical outcome (Chiang et al.
2007).
In addition, an umpolung reaction without participation of an aldehyde moiety has been disclosed recently (Fischer et al. 2006). Michael
acceptors (unsaturated esters, amides and nitriles) that bear pendant
leaving groups such as bromides or tosylates can undergo an organocatalytic, intramolecular β-alkylation. Carbene addition at the β-position
of the Michael acceptor and subsequent tautomerization also promotes
the generation of a homoenolate equivalent which is trapped intramolecularly under ring closure.
7
Finally, there are also some special NHC-mediated transformations
that do not completely fit into the classification, such as triazolylidenecatalyzed hydroacylations (Chan and Scheidt 2006). Aldehydes can
serve as hydride donors for activated ketones partly following a standard 1,2-addition of the NHC to the aldehyde, but instead of the usual
carbonyl umpolung a hydride (‘H-umpolung’) transfer is initiated.
A related ‘Cannizzaro-type’ transformation has been described for
indazole-derived carbene catalysts (Schmidt et al. 2007).
In conclusion, it is obvious that N-heterocyclic carbenes and their
unique, but concurrently versatile reactivity have already proven wide
applicability. The offered catalytic (and enantioselective) access to important reactive intermediates such as homoenolates or enolates as well
as their additional catalytic properties will smooth the way for mild and
sustainable synthesis of multifunctionalized compounds.
6 Conjugate or vinylogous umpolung can also lead to the formation of β-lactones; interestingly their stability is highly dependent on the substrates. For stable β-lactones please
see: Burstein et al. 2006).
7 The first step of this unprecedented umpolung reaction is identical to the standard
MBH mechanism. Only the tautomerization event differentiates between α- (enolate) and
β-functionalization (homoenolate).