N -heterocyclic carbene-catalyzed internal redox reaction of alkynals: An efficient synthesis of allenoates

Article abstract of DOI:10.1021/ol300111m

An efficient N-heterocyclic carbene (NHC)-catalyzed internal redox reaction of alkynals that bear a γ leaving group has been developed. This process provides a new access to a range of allenoates in good yields. Preliminary results demonstrate that the en

Full text of DOI:10.1021/ol300111m

ORGANIC
LETTERS
2012
Vol. 14, No. 6
1398–1401
N-Heterocyclic Carbene-Catalyzed
Internal Redox Reaction of Alkynals: An
Efficient Synthesis of Allenoates
Yu-Ming Zhao,† Yik Tam,† Yu-Jie Wang,‡ Zigang Li,‡ and Jianwei Sun*,†
Department of Chemistry, The Hong Kong University of Science and Technology, Clear
Water Bay, Kowloon, Hong Kong SAR, China, and Laboratory of Chemical Genomics,
School of Chemical Biology and Biotechnology, Shenzhen Graduate School of Peking
University, Shenzhen 518055, China
sunjw@ust.hk
Received January 16, 2012
ABSTRACT
An efficient N-heterocyclic carbene (NHC)-catalyzed internal redox reaction of alkynals that bear a γ leaving group has been developed. This
process provides a new access to a range of allenoates in good yields. Preliminary results demonstrate that the enantioselective variant can also
be achieved.
Allenes represent an important structural motif that
widely occurs in natural products and synthetic molecules
with significant biological activity.1 They also serve as
versatile building blocks in organic synthesis.2 Among
them, allenoates have been demonstrated as a family of
particularly useful synthetic intermediates due to their easy
access to a variety of heterocycles and chiral molecules.3
Although several methods are available for the synthesis
of allenoates, such as base-catalyzed isomerization of
3-alkynoates,4 transition-metal-catalyzed carbonylation
of propargylic carbonates and tosylates,5 etc.,6 the most
general and widely employed method is the olefination of
ketenes, which can be generated in situ from acyl
chlorides.7 Drawbacks of this method include the use of
† The Hong Kong University of Science and Technology.
‡ Shenzhen Graduate School of Peking University.
(1) For some leading references, see: (a) The Chemistry of the Allenes;
Landor, S. R., Ed.; Academic Press: London, 1982. (b) Modern Allene
Chemistry; Krause, N., Hashimi, A. S. K., Eds.; Wiley-VCH: Weinheim,
2004. (c) Hoffmann-Roder, A.; Krause, N. Angew. Chem., Int. Ed. 2004, 43,
1196.
(4) For seminal work, see: (a) Eglinton, G.; Jones, E. R. H.;
Mansfield, G. H.; Whiting, M. C. J. Chem. Soc. 1954, 3197. For a
recent example, see: (b) Liu, H.; Leow, D.; Huang, K.; Tan, C. J. Am.
Chem. Soc. 2009, 131, 7212.
€
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731. (b) Marshall, J. A.; Wolf, M. A.; Wallace, E. M. J. Org. Chem. 1997,
62, 367.
(6) For examples of other methods, see: (a) Lee, P. H.; Mo, J.; Kang,
D.; Com, D.; Park, C.; Lee, C.; Jung, Y. M.; Hwang, H. J. Org. Chem.
2011, 76, 312. (b) Bhowmick, M.; Lepore, S. Org. Lett. 2010, 12, 5078. (c)
Gray, P. J.; Motherwell, W. B.; Sheppard, T. D.; Whitehead, A. J.
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13, 3257. (b) Lang, R. W.; Hansen, H. Helv. Chim. Acta 1980, 63, 438.
For recent examples using this strategy for allenoate synthesis, see: (c)
Fujiwara, Y.; Sun, J.; Fu, G. C. Chem. Sci. 2011, 2, 2196. (d) Chen, B.;
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Dinesh, C. U.; Nandanan, E.; Khan, F. A. Chem. Rev. 2000, 100, 3067.
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Luo, T.; Quan, J.; Wang, G.; Yang, Z. Org. Lett. 2007, 9, 4057. (b) Fang,
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r
10.1021/ol300111m
Published on Web 02/21/2012
2012 American Chemical Society

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
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(11) For reviews on homoenolates generated by NHC-catalyzed
reactions of enals, see ref 8lÀ8m.
(12) To our knowledge, there are isolated examples on NHC-
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(b) Kaeobamrung, J.; Mahatthananchai, J.; Zheng, P.; Bode, J. W.
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mechanism of NHC-catalyzed reactions of alkynals, see: Mahatthananchai,
J.; Zheng, P.; Bode, J. W. Angew. Chem., Int. Ed. 2011, 50, 1673.
(16) For examples using thiazolidenes as effective catalysts, see: (a)
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formation. For examples of normal allenolate reactivity, see: (a)
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A brief experimental survey of different leaving groups,
e.g. OTs, OMe, and OAc, reveals that the methyl carbo-
nate group (OCO2Me) is superior. Thus, we used alkynal
1, which bears a methyl carbonate group in the γ position,
for our further optimization (Table 1). To our delight, the
15). The use of ethanol instead of methanol as solvent results
in the formation of the corresponding ethyl allenoate
product, albeit in low yield (entry 16).17 Finally, although
the reaction proceeds faster at 40 °C than that at rt, the
product is obtained in slightly diminished yield (entry 17),
presumably due to the increased side reactions or decom-
position of the allenoate at an elevated temperature.
The internal redox reaction proceeds efficiently with an
array of alkynals (Table 2). Thus, cyclic and acyclic
allenoate products are all obtained in good yields. The
catalytic process is also compatible with a variety of func-
tional groups, including cyanides and ethers (entries 4, 5). It
is noteworthy that steric hindrance in the γposition does not
affect the reaction efficiency (entry 2). Moreover, alkynals
with aryl substitutions in the γ position are compatible with
the reaction conditions (entry 3).
Table 1. Effect of Reaction Parameters on the NHC-Catalyzed
Reaction of Alkynal 1
While the reactions of alkynals with a quaternary γ
carbon center proceed to form allenoates as the sole product,
the reaction of alkynals with a tertiary γ carbon center may
give a mixture of two isomers, i.e., allenoate 6 and alkynoate
7 (Table 3), with the allenoate as the major product. The
ratio of the allenoate and alkynoate products varies with
different substituents at the γ position. For example, mod-
erate allenoate/alkynoate ratios are observed for substrates
with phenyl-, phenylethyl-, n-decyl-, and 9-decenyl substi-
tutions (entries 1À4), but improved ratios can be obtained
with sterically more demanding substituents (entries 5, 6).
In fact, with tert-butyl substitution, essentially only the
allenoate product is formed. Finally, all these reactions
proceed with high efficiency in terms of overall yield.
To gain more insight into the reaction mechanism, we con-
ducted the reaction of alkynal 8 in CD3OD as solvent under
otherwise identical conditions (eq 1). We observed almost com-
plete deuterium incorporation for the methyl group of the
allenoate product 9 based on 1H NMR. Also, there is signifi-
cant deuterium incorporation at the R position of the allenoate
product, which is consistent with the proposed mechanism
(vide infra), in which a Breslow intermediate is involved.18
a Determined by 1H NMR analysis with dibromomethane as an
internal standard. b Ethyl allenoate was observed in 35% yield.
reaction proceeds smoothly in the presence of 20 mol % of
thiazolium salt B and 20 mol % of NaOMe in methanol at
rt, providing the desired allenoate 2 in 77% yield (Table 1,
entry 1).16 In the absence of B, no desired 2 is formed,
indicating that the reaction is catalyzed by NHC instead of
a base (entry 2). NHCs derived from other precatalysts, e.
g. thiazoliums A, C, triazoliums D, E, imidazolium F, and
dihydroimidazolium G, proved to be less effective (entries
3À8). With regard to the choice of base, NaOMe is better
than other bases, such as DBU, Cs2CO3, KOtBu, and
KHMDS (entries 9À12). While 20 mol % of NaOMe is
sufficient to promote the reaction with high efficiency,
excess base is detrimental to the product yield (entry 13). It
isworthnotingthatthe useofmethanol assolventiscritical
to the success of the reaction. In fact, when DCM is used as
solvent, either with 3.0 equiv of MeOH or without MeOH,
the reaction gives a trace of desired product 2 (entries 14,
A plausible reaction mechanism is depicted in Scheme 2.
The catalytic cycle begins with nucleophilic addition of
the NHC to the carbonyl group of alkynal 3, which is
predominantly in its trimer form in methanol.19 Next,
electron-pushing from the nitrogen lone pair of the result-
ing Breslow intermediate 10, along the conjugated π-
system, triggers the elimination of methyl carbonate group
to form cumulative allenol 11. Subsequent steps involve
tautomerization to form allene 12 and then nucleophilic
acyl substitution by MeOÀ to form the observed allenoate
4 and regenerate the NHC catalyst.
(18) Breslow, R. J. Am. Chem. Soc. 1958, 80, 3719.
(17) We believe that the allenic acyl intermediates, e.g., 12 in
Scheme 2, are quite unstable. Tapping with MeOH is relatively faster
than that with EtOH.
(19) On the basis of 1H and 13C NMR spectra, alkynal 3 exists as a
monomer in CDCl3. In contrast, it exists as a trimer in CD3OD.
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Org. Lett., Vol. 14, No. 6, 2012

Table 2. NHC-Catalyzed Reactions of Alkynals: Scope of
Alkynals with a Quaternary γ Carbon
Table 3. NHC-Catalyzed Reactions of Alkynals: Scope of
Alkynals with a Tertiary γ Carbon
a Ratio was determined by 1H NMR. b Isolated total yield of
allenoate and alkynoate. c The starting alkynal is a mixture of two
diastereomers (dr = 3.9:1), and the dr of the allene product is ∼1:1.
a Yield of purified product.
We have begun to develop an enantioselective variant of this
process. We anticipated that this might be challenging, due to
issues such as potential generation of mixtures of E/Z isomers
of the key intermediate 11 and the difficulty in controlling facial
selectivity of the proton transfer in the tautomerization to 12.
Yet we were pleased to find that the chiral NHC generated
from precatalyst H can catalyze the synthesis of an allenoate
with promising enantioselectivity (30% ee; eq 2).20,21
Scheme 2. Proposed Mechanism
In summary, we have developed an efficient internal
redox process of alkynals, which is now added to the small
family of known reactions of alkynals with NHC catalysis.
This first successful exploitation of an allenolate I inter-
mediate for allenoate formation provides a versatile plat-
form for further development of new NHC-catalyzed
reactions of alkynals. Without the use of transition metals
and the production of phosphine oxide waste, this process
serves as a green alternative for the synthesis of allenoates.
Preliminary studies have also shown that an enantioselec-
tive variant can be achieved.
Acknowledgment. Support has been provided by HKUST
and Hong Kong RGC (SBI11SC02, DAG11SC01). We
thank Profs. W.-M. Dai, R. K. Haynes, and G. Jia for
generously sharing their chemicals.
(20) The NHC derived from precatalyst H has been employed in a
range of catalytic asymmetric processes. For some selected examples,
see: (a) Kerr, M. S.; Rovis, T. J. Am. Chem. Soc. 2004, 126, 8876.
(b) Reynolds, N. T.; Rovis, T. J. Am. Chem. Soc. 2005, 127, 16406.
(c) Ozboya, K. E.; Rovis, T. Chem. Sci. 2011, 2, 1835.
(21) For examples of enantioselective synthesis of allenoates, see: (a)
Li, C.; Wang, X.; Sun, X.; Tang, Y.; Zheng, J.; Xu, Z.; Zhou, Y.; Dai, L.
J. Am. Chem. Soc. 2007, 129, 1494. (b) Zhang, W.; Zheng, S.; Liu, N.;
Werness, J. B.; Guzei, I. A.; Tang, W. J. Am. Chem. Soc. 2010, 132, 3664.
(c) Li, C.; Zhu, B.; Ye, L.; Jing, Q.; Sun, X.; Tang, Y.; Shen, Q.
Tetrahedron 2007, 63, 8046. (d) Reference 4.
Note Added after ASAP Publication. Missing references
20 and 21 were replace on February 23, 2012.
Supporting Information Available. Experimental proce-
dures and compound characterization. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 6, 2012
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