Au-Catalyzed Stereoselective Ritter Reaction of Haloalkynes with Nitriles for (Z)-β-Halogenated Enamides

Article abstract of DOI:10.1002/ejoc.201901318

An efficient and stereoselective protocol has been developed for the synthesis of (Z)-β-halogenated enamide through gold catalyzed Ritter reaction. In the presence of 2 mol% BrettPhosAuCl and 2 mol% AgNTf₂, a broad range of nitriles smoothly underwent Ritter reaction with aromatic, vinylic or aliphatic haloalkynes to give structurally diverse (Z)-β-halogenated enamides in excellent to good yields.

Full text of DOI:10.1002/ejoc.201901318

DOI: 10.1002/ejoc.201901318
Communication
Gold Catalysis | Very Important Paper |
Au-Catalyzed Stereoselective Ritter Reaction of Haloalkynes
with Nitriles for (Z)-ꢀ-Halogenated Enamides
Congrong Liu*[a] and Fulai Yang*[b]
Abstract: An efficient and stereoselective protocol has been nitriles smoothly underwent Ritter reaction with aromatic, vin-
developed for the synthesis of (Z)-ꢀ-halogenated enamide ylic or aliphatic haloalkynes to give structurally diverse (Z)-ꢀ-
through gold catalyzed Ritter reaction. In the presence of halogenated enamides in excellent to good yields.
2 mol% BrettPhosAuCl and 2 mol% AgNTf2, a broad range of
Introduction
are no examples describing the reaction of nitriles with halo-
alkynes. Herein, we report an efficient and facile method for the
synthesis of ꢀ-halogenated enamides via Au-catalyzed stereo-
selective Ritter reaction of nitriles with haloalkynes.
Enamides, endowed with a fine reactivity and balance of stabil-
ity, have emerged frequently as nucleophiles in a manifold of
organic reactions.[1] In this respect, ꢀ-halogenated enamides are
valuable building blocks in organic synthesis.[2,3] They not only
constitute versatile intermediates in the construction of biologi-
cally active natural products but also serve as key precursors in
the synthesis of a variety of pharmaceuticals. For example, they
readily undergo addition reactions and cross-coupling reactions
and are useful precursors of heterocycles. So developing effi-
cient synthetic approaches to construct differently substituted
ꢀ-halogenated enamides is one of the most exciting topics in
organic synthesis.[4,5] Recently, Jiang and co-workers reported
palladium-catalyzed dehydrogenative aminohalogenation of
alkenes with molecular oxygen as the sole oxidant.[5d] In 2014,
Zhang et al. described an alkyne aminohalogenation enabled
by DBU-activated N-haloimides.[4i] In spite of the versatility and
efficiency of this transformation in this area, significant limita-
tions still exist. For instance, the nitrogen sources have been
limited to aromatic amines,[5e,6] carbamates,[7] sulfonamides[8]
and amides[9] so far. Simple nitriles have not yet been success-
fully used as the nitrogen source in the synthesis of ꢀ-halogen-
ated enamides.
Results and Discussion
We commenced our study by investigating the Au-catalyzed
Ritter reaction of phenylethynyl chloride (1a) and acetonitrile
(2a) at 50°C (Table 1). Initially, we treated phenylethynyl chlor-
ide with acetonitrile and 2 mol% IPrAuNTf2 without adding wa-
ter, no desired product (Z)-3aa was found. We adjusted the
amount of water and the yield reached 79 % when 1.0 eq water
was added (Table 1, entry 4). The use of Ph3PAuOTf, Et3PAuNTf2,
L1AuNTf2 and Ph3PAuNTf2 as the catalyst led to decreased effi-
ciency (Table 1, entries 6–9). Because LAuCl is rock stable and
inactive in AuI, the labile counterion must be added. The combi-
nation of BrettPhosAuCl and AgNTf2 resulted in 85 % yield
(Table 1, entry 13). However, replacing NTf2– with other counter
anions led to decreased yields (Table 1, entries 15–17). Increas-
ing the reaction temperature the yield was decreased slightly
(Table 1, entry 18). In addition, a gram-scale synthesis of (Z)-
3aa (1.68 g, 86 % yield) was successfully performed according
to this protocol.
After establishing the optimized conditions (Table 1, entry
9), we evaluated the substrate scope of this transformation. The
optimized reaction conditions were found to be suitable to a
broad range of haloalkynes (Table 2). A number of arylethynyl
chloride bearing either electron-donating groups or electron-
withdrawing groups on the ortho-, meta- or para-positions of
the aromatic rings were transformed into their corresponding
(Z)-ꢀ-halogenated enamides at 50 °C in excellent to good yields
(Table 2, entries 1–10). Subsequently, 2-(2-chloroethynyl)-
naphthalene reacted efficiently to afford (Z)-ꢀ-chlorogenated
enamides 3ka in an excellent 95 % yield (Table entry 11). In
addition, vinylic or aliphatic chloroalkynes also participated well
in this reaction to afford the corresponding products 3la, 3ma
and 3na in 80 %, 82 % and 79 % yields, respectively (Table 2,
entries 12–14). Notably, bromoalkyne can also undergo the
The Ritter reaction, in which nitriles function as a nitrogen
source, is a simple and one-pot synthetic method for preparing
N-acyl-protected amines.[10] The classical Ritter reaction em-
ploys alcohols and alkenes as starting materials.[11] Recent
years, carboxylic acids were also applied in the Ritter reaction,
which was first described by Minakata et al.[12] Although Ritter
reaction has versatile applications in organic chemistry, there
[a] School of Environment Engineering, Nanjing Institute of Technology
1 Hongjingdadao, Nanjing, Jiangsu 211167, China
E-mail: congrong@njit.edu.cn
[b] Department State Key Laboratory of Natural Medicines,
Department of Organic Chemistry, China Pharmaceutical University,
Nanjing, 210009, P. R. China
E-mail: Fly1986@mail.ustc.edu.cn
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201901318.
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Table 1. Initial reaction conditions optimization.[a]
Table 2. Gold catalyzed Ritter reaction of haloalkyne 1 with acetonitrile 2a.[a]
Yield [%][b]
Entry
[Au] (2 mol-%)
MX (2 mol-%)
Yield [%][b]
Entry
1, R
X
3
1[c]
2[d]
3[e]
4
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
0
1
2
3
4
5
6
7
8
1a, C6H5
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Br
Br
Br
Br
I
3aa
3ba
3ca
3da
3ea
3fa
3ga
3ha
3ia
3ja
3ka
3la
3ma
3na
3oa
3pa
3qa
3ra
3sa
85
69
61
80
90
86
92
83
80
70
95
80
82
79
68
77
84
60
0
22
41
79
67
29
38
47
60
80
63
70
85
75
13
76
68
79
1b, 2-Cl C6H4
1c, 2-Br C6H4
1d, 2-MeC6H4
1e, 4-Me C6H4
1f, 4-MeO C6H4
1g, 4-Ph C6H4
1h, 4- BrC6H4
1i, 3,5-Cl2C6H3
1j, 2,4,6-Me3 C6H2
1k, 2-naphthyl
1l, 1-cyclohexenyl
1m, BnOCOCH=CH
1n, cyclopropyl
1o, C6H5
5[f]
6
IPrAuNTf2
Ph3PAuOTf
Ph3PAu NTf2
Et3PAuNTf2
L1AuNTf2
JohnphosAuCl
IMesAuCl
tBuXPhosAuCl
BrettPhosAuCl
IPrClAuCl
BrettPhosAuCl
BrettPhosAuCl
BrettPhosAuCl
BrettPhosAuCl
7
8
9
9
10
11
12
13
14
15
16
17
18[g]
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
NaBARF
AgOTf
10
11
12
13
14
15
16
17
18
19
1p, 4-Ph C6H5
1q, 2-naphthyl
1r, 1-cyclohexenyl
1s, C6H5
AgSbF6
AgNTf2
[a] Reaction conditions: Chloroalkyne 1 (0.40 mmol), MeCN 2a (0.50 mL),
water 0.4 mmol, BrettphosAuCl (2 mol-%), AgNTf2 (2 mol-%), 50 °C. [b] Iso-
lated yield.
philicity of nitrile. Similarly, we also found that 2-methylbenzo-
nitrile afforded the corresponding compound (3aj) in a low
yield with 69 % (Table 3, entry 2). This may be due to the steric
hindrance of ortho-position (Table 3, entry 9).
On the basis of our results and previous relevant mechanistic
studies,[10d, 11, 12] a mechanism for the gold-catalyzed Ritter re-
action of haloalkynes with nitriles was proposed, as shown in
Scheme 1. The gold-activated haloalkyne 4 or its polarized reso-
nance structure 4′ is attacked by the nitrogen atom of nitrile
from the direction of small steric hindrance. The alkenyl gold
Table 3. Gold catalyzed Ritter reaction of phenylethynyl chloride 1a with
nitrile 2a.[a]
[a] Reaction conditions: phenylethynyl chloride 1a (0.4 mmol), MeCN 2a
(0.5 mL), water 0.4 mmol, [Au] (2 mol-%), MX (2 mol-%), 50 °C. [b] Isolated
yield. [c] No water added. [d] Water 0.1 mmol. [e] Water 0.2 mmol. [f] Water
0.5 mmol. [g] 70 °C.
Ritter reaction, albeit less efficient than its chloro counterpart
(Table 2, entries 15–18). However, the corresponding iodo-
alkyne led to no corresponding product 3sa (Table 2, entry 16).
To further determine the scope of the procedure, the next
various nitriles were investigated, and the results are shown
in Table 3. Considering that other nitriles are not as cheap as
acetonitrile, we used 1,2-dichloroethane as solvent to reduce
the amount of nitrile to 0.1 mL. Table 3 showed that most
nitriles could react with phenylethynyl chloride (1a) to afford
the corresponding (Z)-ꢀ-chlorogenated enamides 3 with excel-
lent to good yields. It was found that 2-bromoacetonitrile af-
forded the corresponding (Z)-ꢀ-chlorogenated enamide (3ac) in
a slightly low yield with 75 % (Table 3, entry 2). It may be that
the electron-withdrawing effect of bromine reduces the nucleo-
Entry
2, R
3
Yield [%][b]
1
2
3
4
5
6
7
8
9
2b, C2H5
2c, BrCH2
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
88 %
75 %
83 %
87 %
81 %
90 %
92 %
84 %
69 %
2d, ClCH2CH2
2e, BrCH2CH2
2f, iPr
2g, cyclohexyl
2h, C6H5
2i, 4-BrC6H4
2j, 2-CH3C6H4
3aj
[a] Reaction conditions: phenylethynyl chloride 1a (0.40 mmol), nitrile 2
(0.10 mL), water 0.4 mmol, 1,2-dichloroethane (0.4 mL), BrettphosAuCl (2 mol-
%), AgNTf2(2 mol-%), 50 °C. [b] Isolated yield.
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phenylethynyl chloride 1a (0.40 mmol) in DCE (0.40 mL) were added
BrettPhosAuCl (6.24 mg, 0.0080 mmol) and AgNTf2 (3.11 mg,
0.0080 mmol) subsequently. The mixture was stired at 50 °C for
10 min. Nitrile/H2O (0.11 mL, 14:1) was then added dropwise over
20 minutes. The resulting mixture was stirred at 50 °C for 23.5 h.
The mixture was cooled to room temperature, and purified by silica
gel column chromatography, eluting with petroleum ether/ethyl
acetate (10:1 to 4:1), to give ꢀ-chlorogenated enamides 3.
intermediate 5 is generated, then followed a hydrolysis and pro-
ton transfer to afford (Z)-ꢀ-halogenated enamide 3 and regen-
erate the Au(I) catalyst. We believe the terminal halo group is
critical for the Ritter reaction of haloalkynes with nitriles. As the
inductively electron-withdrawing nature of X makes the alkyne
gold complex 4/4′ more polarized and hence more reactive to
react with nitriles.
Acknowledgments
We are grateful for the financial support from the National Nat-
ural Science Foundation of China (No. 21502182, 21202154).
Keywords: Halogenated enamide · Ritter reaction · Gold
catalysis · Stereoselectivity · Chloroalkyne
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Conclusions
In conclusion, we developed an efficient and stereoselective
protocol for the synthesis (Z)-ꢀ-halogenated enamide via gold
catalyzed Ritter reaction. In the presence of 2 mol-% Brett-
PhosAuCl and 2 mol-% AgNTf2, a variety of (Z)-ꢀ-halogenated
enamide bearing different functional groups can be prepared
in excellent to good yields. The current study is focusing on
the synthesis of biologically important N-containing molecules
using this method.
Experimental Section
General Information: 1H and 13C NMR Spectra were recorded on
a Bruker AC-500 FT spectrometer (500 MHz and 100 MHz, respec-
tively) using tetramethylsilane as internal reference. Chemical shifts
(δ) and coupling constants (J) were expressed in ppm and Hz,
respectively. IR spectra were recorded on a Perkin-Elmer 2000 FTIR
spectrometer. High resolution mass spectra were recorded on a
LC-TOF spectrometer (Micromass). the UV detection was monitored
at 254 nm. Melting points were uncorrected.
General Procedure for the Gold Catalyzed Ritter Reaction of
Haloalkynes with Acetonitrile (Table 2): To
a solution of
haloalkyne 1 (0.40 mmol) in acetonitrile (0. 14 mL) were added
BrettPhosAuCl (6.24 mg, 0.0080 mmol) and AgNTf2 (3.11 mg,
0.0080 mmol) subsequently. The mixture was stired at 50 °C for
10 min. MeCN/H2O (0.37 mL, 50:1) was then added dropwise over
20 minutes. The resulting mixture was stirred at 50 °C for 23.5 h.
The mixture was cooled to room temperature, and purified by silica
gel column chromatography, eluting with petroleum ether/ethyl
acetate (10:1 to 4:1), to give ꢀ-halogenated enamides 3.
General Procedure for the Gold Catalyzed Ritter Reaction of
Phenylethynyl Chloride with Nitriles (Table 3): To a solution of
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Received: September 4, 2019
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Gold Catalysis
C. Liu,* F. Yang* .................................. 1–5
Au-Catalyzed Stereoselective Ritter
Reaction of Haloalkynes with
Nitriles for (Z)-ꢀ-Halogenated Enam-
ides
An efficient and stereoselective proto- nitriles smoothly underwent Ritter re-
col has been developed for the synthe- action with aromatic, vinylic or ali-
sis (Z)-ꢀ-Halogenated enamide via phatic haloalkynes to give structurally
gold catalyzed Ritter reaction. In the diverse (Z)-ꢀ-Halogenated enamides in
presence of 2 mol-% BrettPhosAuCl excellent to good yields.
and 2 mol-% AgNTf2, a broad range of
DOI: 10.1002/ejoc.201901318
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