expensive phosphoranes as the stoichiometric olefinating
reagent and the production of an equal amount of the
phosphine oxide as waste. Here we report a new strategy
for the synthesis of allenoates from alkynals by means of
N-heterocyclic carbene (NHC) catalysis.
Scheme 1. NHC-Catalyzed Reaction Patterns of Alkynals
The use of NHCs for catalysis has emerged as an area of
intense investigations in the past few years.8,9 The majority
of these catalytic reactions take advantage of the polarity
reversal (“umpolung”) process of the aldehyde function-
ality. For example, in the presence of a catalytic amount of
NHC, aldehydes can serve as precursors for acyl anion
equivalents, such as those in benzoin reactions and Stetter
reactions.10 Enals have also been demonstrated as con-
venient precursors for homoenolates, resulting in the
development of numerous processes with new bond
formation in the β position.11 In contrast to the large
number of publications on nonconjugated aldehydes and
enals, reports on the use of alkynals to generate the
corresponding allenolates are scarce (I in Scheme 1).12
These isolated examples share a common feature, i.e.,
trapping the allenolate I with a proton to form the acti-
vated unsaturated acyl species II for further reactions with
different nucleophiles.
(8) For leading reviews on NHC organocatalysis, see: (a) Enders, D.;
Balensiefer, T. Acc. Chem. Res. 2004, 37, 534. (b) Enders, D.; Niemeier,
We envisioned that allenolate I, which is difficult to
generate by other strategies,13 can be further utilized to
expand the scope of NHC catalysis. For example, we
hypothesized that if a leaving group is present in the γ
position, the negative charge in the β position of I can
trigger an elimination of the leaving group, forming a
cumulative allenol III (Scheme 1, path 2). In the presence
of a suitable nucleophile, III can be converted to activated
allene products. The overall reaction is redox-neutral.14
This process not only provides a new access to activated
allenes but also represents a new platform for the discovery
of other NHC-catalyzed reactions with alkynals. Also, the
formation of allenoates can give useful insight into the
question of whether an allenic species is involved in NHC-
catalyzed reactions of alkynals.15
O.; Henseler, A. Chem. Rev. 2007, 107, 5606. (c) Marion, N.; Dıez-
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(12) To our knowledge, there are isolated examples on NHC-
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