Silver-catalyzed cross-coupling of propargylic alcohols with isocyanides: An atom-economical synthesis of 2,3-ALLENAMIDES

Article abstract of DOI:10.1002/chem.201304255

Cross-coupling reactions between propargylic alcohols and isocyanides, by means of silver catalysis, have been described. This new reaction is both atom and step efficient and is applicable to a broad scope of substrates, allowing the synthesis of a range of synthetically valuable 2,3-allenamides in moderate to excellent yields. Making allenamides: Cross-coupling reactions between propargylic alcohols and isocyanides, by means of silver catalysis, are described. This new reaction is both atom and step efficient and is applicable to a broad scope of substrates, allowing the synthesis of a range of synthetically valuable 2,3-allenamides in moderate to excellent yields (see scheme). Copyright

Full text of DOI:10.1002/chem.201304255

DOI: 10.1002/chem.201304255
Communication
&
Cross-Coupling Chemistry
Silver-Catalyzed Cross-Coupling of Propargylic Alcohols with
Isocyanides: An Atom-Economical Synthesis of 2,3-Allenamides
Jianquan Liu,[a] Zhenhua Liu,[a] Nannan Wu,[a] Peiqiu Liao,[a] and Xihe Bi*[a, b]
Abstract: Cross-coupling reactions between propargylic
alcohols and isocyanides, by means of silver catalysis,
have been described. This new reaction is both atom and
step efficient and is applicable to a broad scope of sub-
strates, allowing the synthesis of a range of synthetically
valuable 2,3-allenamides in moderate to excellent yields.
Figure 1. Transition-metal-catalyzed CÀC bond formation at the b-carbon
atom of propargylic alcohols along with oxygen transposition.
Transition-metal-catalyzed CÀC bond formation, by the cross-
coupling reaction of two components, has displayed a funda-
a-hydroxy allenes (Figure 1).[5] The difficulty in developing new
mental role in modern organic synthesis. Enormous research
efforts devoted to this area in the past decades has led to
a myriad of CÀC bond forming reactions.[1] However, the devel-
CÀC coupling methods that involve the oxygen transposition
of propargylic alcohols is due to the difficulty of finding both
opment of chemoselective cross-coupling reactions that use
simple starting materials remains an important research chal-
lenge. Propargylic alcohols are very useful bifunctional building
blocks and the close proximity of the hydroxyl group to the
CꢀC bond provides these molecules with the potential to un-
dergo reactions that are not possible with the corresponding
alkynes.[2] The transition-metal-catalyzed CÀC bond formation
at the a- or b-carbon atom of propargylic alcohols, along with
oxygen transposition by cross-coupling with a coupling part-
ner, is especially striking because it constitutes a powerful
strategy towards oxygen-functionalized unsaturated com-
pounds.[3–5] However, such a useful synthetic strategy remains
underexploited so far. Several kinds of a-CÀC coupling and
1,3-oxygen transposition of propargylic alcohols with coupling
partners, including aldehydes, imines, allyl carbonates, and dia-
ryliodonium salts, have been achieved by the Trost and Gaunt
groups, allowing otherwise inaccessible a-functionalized
enones to be synthesized.[3] By comparison, reports regarding
b-CÀC coupling and oxygen transposition of propargylic alco-
hols are rare, and are limited to 1) ruthenium-catalyzed cross-
coupling of propargylic alcohols with an alkyne or alkene spe-
cies to give enones;[4] and 2) a rhodium-catalyzed tandem reac-
tion between propargylic alcohols and diazoacetates to afford
an appropriate coupling partner and a catalyst.
Isocyanides are versatile reagents and are analogous to
carbon monoxide (CO) in carbene character.[6] The strategic use
of isocyanides in forming CÀC bonds and assembling complex
molecules by insertion into carbon–metal bonds has become
an active research field in recent years.[7] However, the cou-
pling of isocyanides and terminal alkynes to form CÀC bonds
is scarcely reported.[8] Reports of silver-catalyzed alkyne-involv-
ing organic reactions have been rapidly increasing in the past
decade.[9] One of the advantages of silver catalysis is that oxi-
dative coupling of terminal alkynes is skillfully avoided.[10] As
part of our efforts towards developing novel transition-metal-
catalyzed organic reactions,[11] we recently, and almost at the
same time as Lei and co-workers,[12a] reported the first silver-
catalyzed tandem coupling and cyclization of terminal alkynes
with isocyanides, leading to oligosubstituted pyrroles.[12] We
envisaged, thereafter, that utilization of propargylic alcohols,
instead of simple alkynes, in this silver-catalyzed protocol
might lead to a different reaction pattern because of the close
proximity of a highly reactive hydroxyl group to the alkyne
unit.[2] Delightfully, our experimental results confirmed this hy-
pothesis and disclosed an unprecedented b-CÀC coupling and
oxygen transposition of propargylic alcohols with isocyanides,
thereby providing a novel route to a variety of synthetically
valuable 2,3-allenamides (Figure 1).[13] To the best of our knowl-
edge, this represents the first example of a CÀC coupling reac-
tion between propargylic alcohols and isocyanides.[2,6,14]
Herein, we communicate our results regarding this exciting
transformation.
[a] J. Liu, Z. Liu, N. Wu, Dr. P. Liao, Prof. X. Bi
Department of Chemistry
Northeast Normal University
Changchun 130024 (China)
E-mail: bixh507@nenu.edu.cn
[b] Prof. X. Bi
Our initial efforts focused on the optimization of reaction
conditions by using the reaction of propargylic alcohol 1a and
tosylmethyl isocyanide (TosMIC) 2a as a model (Table 1). A
range of silver salts were firstly screened in 1,4-dioxane at
State Key Laboratory of Elemento-Organic Chemistry, Nankai University
Tianjin, 300071 (China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304255.
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Table 1. Screening of reaction conditions.[a]
Entry
Ag Source
Solvent
T [oC]
Yield [%][b]
1
2
3
4
5
6
7
8
AgOTf
Ag2SO4
AgOAc
AgF
AgNO3
AgBF4
Ag3PO4
Ag2CO3
AgOAc
AgOAc
AgOAc
AgOAc
–
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
CH3CN
DMF
CH2Cl2
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
80
80
80
80
80
80
80
80
80
80
80
80
80
40
25
0
0
92
73
0
0
0
0
42
0
0
0
0
51
39
9
10
11
12
13
14
15
AgOAc
AgOAc
[a] Reactions were carried out on a scale of 0.5 mmol of 1a and 0.6 mmol
of 2a, in the presence of silver salts (20 mol%), in 2 mL of solvent under
ambient atmosphere for 0.5 h. [b] Yield of isolated product.
808C. It was found that both AgOAc (Ac=acetyl) and AgF are
effective catalysts; the former affording high yields (up to
92%, entries 3 and 4). The other silver salts that were screened
did not result in desired allene product 3a (entries 1, 2, and 5–
8). Notably, no by-product was observed, from the homocou-
pling of the terminal alkyne, in the presence of AgOAc and
AgF. The solvent also exhibited significant influence on this re-
action. For example, whereas the use of acetonitrile (CH3CN)
proceeded well, albeit with a lower product yield of 42%
(entry 9), dimethylformamide (DMF) and methylene dichloride
(CH2Cl2) were ineffective (entries 10 and 11). In a control experi-
ment, the necessity of a silver catalyst was demonstrated
(entry 13). When the reaction temperature was lowered to 40
and then 258C, a gradual decrease in product yield, from 51 to
39%, was observed (entries 14 and 15). Therefore, the condi-
tions listed in entry 3, being optimal, were selected for further
investigations.
Scheme 1. Synthesis of trisubstituted allenes. Ts=tosyl.
affording the desired products in good to high yields (3e–3g).
The presence of alkyl substituents, such as n-butyl and benzyl,
had no effect on the outcome of the reaction (3h, 3i). More-
over, several functional groups, including ether, cyclopropyl,
and trifluoromethyl groups, were introduced into the allene
products by choosing an appropriate substrate, resulting in
good to excellent yields of the functionalized allenes (3j–3m).
Although trifluoromethylated allenes are useful for the synthe-
sis of fluorinated organic compounds, only a limited number
of protocols are available in the literature for their prepara-
tion.[15] Furthermore, bisalkyl-substituted and cyclic propargylic
alcohols were also applicable to this silver-catalyzed procedure
with good to moderate yields (3n–3p). The results shown in
Scheme 1 demonstrate the potency of this new reaction for
the synthesis of functionalized trisubstituted allenes.[14d]
The reaction scope for the synthesis of trisubstituted allenes
was evaluated with respect to both the isocyanide and the ter-
tiary propargylic alcohol (Scheme 1). The first experiments
were conducted with tertiary propargylic alcohol 1b and
a series of differently substituted isocyanides. A remarkable
effect on the outcome of the reaction was observed, for exam-
ple, TosMIC, 2a, and benzyl isocyanide 2b underwent clean
conversion into the corresponding 2,3-allenamides (3b and 3c,
respectively) in excellent yields, unlike the reaction of 1-tosy-
lethyl isocyanide 2c, which did not afford desired product 3d.
As a result, convenient and inexpensive TosMIC was selected
as the isocyanide component for screening different tertiary
propargylic alcohols. The results showed that various tertiary
propargylic alcohols are suitable reaction partners for TosMIC
2a in the formation of trisubstituted allenes (3e–3p). Bis-
(hetero)aryl-substituted propargylic alcohols reacted smoothly,
Next, the scope of this reaction, with respect to the secon-
dary propargylic alcohols, was explored. Contrary to the syn-
thesis of trisubstituted allenes, silver carbonate (Ag2CO3)
showed slightly better catalytic performance than silver acetate
(AgOAc). As shown in Scheme 2, even though the substrates
included a variety of aryl and fused-aryl propargylic alcohols,
transformations that afforded the corresponding disubstituted
allenes in good to excellent yields (5a–5l) were achieved. Im-
portantly, substrates with both electron-donating and electron-
withdrawing groups underwent transformation into the de-
sired products. It is noteworthy that substituents at different
positions on the arene group (para, meta, or ortho positions)
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Control experiments were carried out to provide insights
into the reaction pathway (Scheme 3). 18O-Labeled substrate
[18O]-4j was first subjected to the standard conditions, giving
Scheme 2. Synthesis of disubstituted allenes.
did not affect the reaction efficiency significantly, but more im-
portantly, that the halo-substituted aryl propargylic alcohols
were tolerated well, thus leading to halo-substituted products
5 f, 5g, and 5i, which could be used in further modifications.[16]
Moreover, the propargylic alcohol containing an additional
double bond was easily accommodated into the allene scaffold
without any difficulties (5l). This 1,n-allenene structural motif
could be useful in transition-metal-catalyzed cycloisomeriza-
tions.[17] Notably, cyclization of the allenamides was not ob-
served under these silver-catalyzed reaction conditions, dem-
onstrating the stability of the products.[18]
Scheme 3. Mechanistic investigations.
To investigate the synthetic potential of this method, the re-
action of propargylic alcohol 4a with TosMIC 2a was carried
out on a gram scale. To our delight, corresponding allene 5a
was obtained in 88% yield. Furthermore, good conversion of
5a into 4-bromopyrrol-2-one 6a was achieved under copper-
mediated conditions [Eq. (1)].[13b]
71% yield of [18O]-5j [Scheme 3, Eq. (1)]. However, [18O]-5j was
not detected by HRMS when the reaction of 4j and 2a was
carried out in the presence of external H218O (2.0 equivalents).
Evidently, the transfer of a hydroxyl group from the propargylic
alcohol to the isocyanide part of the product is an intramolec-
ular process. In the absence of Ag2CO3, silver acetylide [Ag]-1a
reacted with 2a and afforded the allene product in 68% yield
within a shorter time [Scheme 3, Eq. (2)], suggesting that silver
acetylide is the intermediate initially formed during the reac-
tion. Interestingly, when two equivalents of deuterium oxide
(D2O) were added to the reaction of [Ag]-1a with 2a, deuteri-
um was exclusively incorporated (25% deuterium content) into
the methylene moiety of product [D]-3b [Scheme 3, Eq. (3)]. A
similar result was obtained when the deuterated substrate [D]-
1a was used [Scheme 3, Eq. (4)]. Intriguingly, in both experi-
ments, the presence of deuterium was not observed at the al-
lenic hydrogen position (see the Supporting Information for
1H NMR spectra). Finally, it was demonstrated that it is necessa-
ry for the reaction to be open to air; only a trace amount of
allene 3b was produced under an atmosphere of argon, in-
stead a silver mirror was observed [Scheme 3, Eq. (5)].
The practicality of this silver-catalyzed coupling of propargyl-
ic alcohols with isocyanides was further investigated by the
direct synthesis of allenes starting from ketones and aldehydes,
without the isolation of propargylic alcohols. Delightfully, three
representative allenes (3b, 3h, and 5a) were prepared in good
yields by using this protocol. This procedure dramatically re-
duces cost and waste, making it suitable for a large range of
applications.
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Following these preliminary mechanistic studies, a plausible
reaction pathway is tentatively outlined in Scheme 4. The initial
step is formation of silver acetylide intermediate A from the in-
Acknowledgements
This work was supported by NSFC (21172029, 21202016,
21372038) and NCET-13-0714.
Keywords: 2,3-allenamides · cross-coupling · isocyanides ·
propargylic alcohols · silver catalysis
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Scheme 4. A plausible reaction pathway.
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another molecule of 4a to yield aminal adduct C,[20] which is
ultimately converted through intramolecular hydride transfer
into complex D.[21] Next, an intramolecular rearrangement
ensues to yield silver acetylide intermediate E, which is then
subjected to 1,1-insertion of the isocyanide 2a, resulting in
complex F. Finally, a sequential imide addition and hydride
transfer takes place to furnish intermediate 5a’, with the re-
generation of complex D for the next reaction. Product 5a re-
sults through an amide–hydroxyl imide tautomerism of 5a’.
This reaction pathway is in agreement with the observations in
the mechanistic studies (Scheme 3), except for the role of di-
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In conclusion, an unprecedented cross-coupling of propar-
gylic alcohols with isocyanides by means of silver catalysis has
been described. This reaction is a unique type of b-CÀC cou-
pling and oxygen transposition of propargylic alcohols that
provides a novel route to a variety of synthetically valuable
2,3-allenamides in moderate to excellent yields. The increased
accessibility of these highly functionalized products provides
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Received: October 31, 2013
Published online on January 29, 2014
Chem. Eur. J. 2014, 20, 2154 – 2158
www.chemeurj.org
2158
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