Transition Metal-Free Approach to Propynenitriles and 3-Chloropropenenitriles

Article abstract of DOI:10.1002/adsc.201501103

A transition metal-free, facile and efficient one-pot protocol for the synthesis of propynenitriles from readily available 3-chloropropenals is disclosed. The reaction conditions have also been optimized for the exclusive formation and isolation of 3-chloropropenenitriles which are important building blocks in general and are intermediates in the synthesis of propynenitriles. The hallmark of the methodology is the use of non-toxic reagents, milder, metal-free and economically benign reaction conditions avoiding a harsh dehydration step while achieving excellent yields.

Full text of DOI:10.1002/adsc.201501103

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DOI: 10.1002/adsc.201501103
Very Important Publication
Transition Metal-Free Approach to Propynenitriles and
3-Chloropropenenitriles
Pawan K. Sharma,a,* Sita Ram,a and Navneet Chandaka
a
Department of Chemistry, Kurukshetra University, Kurukshetra – 136119, India
Fax: (+91)-01744-238-277; e-mail: pksharma@kuk.ac.in
Received: December 3, 2015; Revised: January 6, 2016; Published online: March 8, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201501103.
Abstract: A transition metal-free, facile and effi-
cient one-pot protocol for the synthesis of propyne-
nitriles from readily available 3-chloropropenals is
disclosed. The reaction conditions have also been
optimized for the exclusive formation and isolation
of 3-chloropropenenitriles which are important
building blocks in general and are intermediates in
the synthesis of propynenitriles. The hallmark of
the methodology is the use of non-toxic reagents,
milder, metal-free and economically benign reac-
tion conditions avoiding a harsh dehydration step
while achieving excellent yields.
synthesis of a plethora of heterocyclic motifs associat-
ed with interesting chemical and biological properties
(Figure 1).[1] Furthermore, the chemistry of variously
substituted acetylenes has undergone a renaissance in
the past decade as the carbon-carbon triple bond of
alkynes is one of the constitutional functional groups
in organic chemistry exhibiting fundamental reactions
of synthetic utility.[2] On the other hand, the cyano
group has always been envisaged as a fountainhead of
functionalization in organic medicinal chemistry due
to its facile transformation to functional groups of
medicinal significance such as amines, amides, ami-
dines, aldehydes, ketones, carboxylic acids, esters etc.,
many of which have already been used as its bioisos-
teres. The combination of these two versatile func-
tionalities, viz. alkyne and cyano, in alkynenitriles
make these as much sought after intermediates of po-
tential synthetic as well as medicinal importance.
Keywords: aqueous ammonia; chloropropeneni-
triles; iodine; propynenitriles; transition metal-free
conditions
Surprisingly, there is only one method available in
literature for the synthesis of 3-chloropropenenitriles
Propynenitriles 1 as well as 3-chloropropenenitriles 2 2 through a three-step process utilizing acetophenones
are versatile synthons in organic chemistry for the as starting material [Eq. (1), Scheme 1].[3] There have
Figure 1. Synthetic applications of propynenitriles and 3-chloropropenenitriles.
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Transition Metal-Free Approach to Propynenitriles
Scheme 1. Synthetic routes for 3-chloropropenenitriles and propynenitriles.
been slight variations to this three-step process by offered any explanation or clue for negating the obvi-
way of changing the dehydration reagent from tri- ous choice of acetophenones. It was interesting as
chlorophosphate to acetic anhydride,[4] attempting it well as intriguing on one hand to ponder upon the
as a one-pot process[5] or achieving the synthesis by choice of authors not to include acetophenones in the
a continuous flow method.[6] A fundamental strategy study, on the other hand it was tempting to find an
employed widely for the preparation of alkynenitriles answer to this question by investigating the fate of
1 involves introduction of a cyano group to the termi- acetophenones under the reaction conditions used by
nal alkyne via metal acetylide and using cyanating Togo and co-workers[15] for converting propiophe-
agents such as cyanogen bromide,[7] cyanogen nones to 5.
iodide,[8] cuprous cyanide,[9] 1-cyanobenzotriazole,[10]
In our investigation for extending the scope of the
phenyl cyanate,[11] 1-cyanoimidazole,[12] and 2-pyridyl above methodology to acetophenones, we treated ace-
cyanate[13] [Eq. (2), Scheme 1]. The transformation of tophenone under their reaction conditions.[15] Un-
a functional group such as aldehyde, alcohol, amide, fortunately, the expected 3-chloropropenitrile was not
azide, etc. already present in the alkyne into nitrile is obtained as a single or major isolable product, instead
the other important strategy to prepare alkynenitriles a gummy mass indicating a mixture of products on
[Eq. (2), Scheme 1].[14] However, all these protocols TLC was obtained. Repeated purification of the mix-
often require harsh reaction conditions, toxic reagents ture by column chromatography led to the isolation
such as dehydrating and cyanating agents and are nei- of a tiny amount of the predominant product as 3-
ther economic nor eco-friendly. Herein, we report chloropropenenitrile 2a (Scheme 3).
a simple, facile and transition metal-free approach for
Isolation of an insignificant amount of 3-chloropro-
the synthesis of both propynenitriles 1 and 3-chloro- penenitrile 2a, in line with the conversion of propio-
propenenitriles 2 starting from 3-chloropropenals 3 phenones to 5 (Scheme 2), reaffirmed our notion that
[Eq. (3), Scheme 1].
this methodology is also applicable to acetophenones
Recently, Togo and co-workers[15] reported a simple but the presence of a proton, rather than a methyl
one-pot conversion of propiophenones 4 to the corre- group, at the a-position to nitrile is responsible for
sponding propenenitriles (b-chloro-a-methylcinnamo- side reactions leading to the mixture of products. It
nitriles) 5 (Scheme 2). Surprisingly, the authors nei- also confirmed that formation of a mixture of prod-
ther included any acetophenone (a simple acetyl de- ucts and isolation of the required 3-chloropropeneni-
rivative other than propiophenone) in the study nor triles in a very poor yield might have forced Togo and
Scheme 2.
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Pawan K. Sharma et al.
Scheme 3.
co-workers[15] to limit their investigations to propio- is worth mentioning that controlled addition of a solu-
phenones. In our next attempt to explore the possibil- tion of iodine (1 equiv.) in THF over a period of 10–
ity of driving the reaction in favour of exclusive for- 12 h at À158C could control the reaction to some
mation of 3-chloropropenenitrile 2a, we first isolated extent and led to isolation of 3-chloropropenenitrile 2
3-chloropropenal 3a, an intermediate in line with the as a single product in all cases except for p-nitro (3g,
conversion of propiophenones to b-chloro-a-methyl- entry 7) and m-nitro (3j, entry 10) substituents. These
cinnamonitriles, and treated it with iodine (1 equiv.) observations led us to conclude that electronic effects
and aqueous ammonia in THF which again led to the play a great role in controlling the reaction course
formation of a mixture of products with 3-chloro-3- thus giving a mixture of products in the case of sub-
phenylpropenenitrile 2a as the predominating product strates having electron-withdrawing groups. As the
on TLC; this reinforced our belief that it is the nitro group is the strongest electron-withdrawing
second step in the process which is responsible for the group, the reaction could not be controlled even at
side reactions when propiophenone is replaced by À158C in the case of substrates 3g and 3j.
acetophenone. Next, we envisaged to investigate the
Considering the significance of electronic effects,
effect of substituents and temperature in order to ex- next we investigated the effect of different solvents
plore the possibility of driving the reaction towards on this reaction. As the nitro substituent was found to
exclusive conversion of 3-chloropropenal 3a to 2a be the most difficult for controlling the reaction, a sol-
(Table 1). In general, a mixture of products was ob- vent study was carried out taking 3-chloro-3-(4-nitro-
tained in most cases, however, to our surprise, exclu- phenyl)propenal 3g as the starting material. Accord-
sive formation of the 3-chloropropenenitriles was ob- ingly, the reaction of 3g with molecular iodine
served in the case of 3-chloro-3-phenylpropenals (1 equiv.) and aqueous ammonia at room temperature
having p-methoxy (3c, entry 3) or p-fluoro (3d, for 3 h was carried out in various solvents which are
entry 4) substituents. Lowering the reaction tempera- compatible for using the iodine/aqueous ammonia
ture to À158C resulted in the exclusive formation of system (see Table S1 in the Supporting Information).
3-chloropropenentirile in another case involving a p- Although a mixture of products was observed on
bromo substituent (3f, entry 6) while a mixture of TLC in THF, ethyl acetate, acetonitrile, DMSO and
products was obtained in all other cases (Table 1). It 1,4-dioxane, it was satisfying to note that no side reac-
tions were observed while using chloroform (entry 7
of Table S1, Supporting Information) or dichlorome-
Table 1. Substituent and temperature study.
thane (entry 8 of Table S1, Supporting Information)
as solvent leading to the exclusive formation of 2g.
More importantly, the reaction was completely repro-
ducible in both these solvents and there was no need
for controlling either the temperature or the rate of
addition of iodine. Reaction in p-xylene (entry 4 of
Table S1, Supportting Information) also controlled
the side reactions as evident from TLC but the reac-
tion did not go to completion even after 10 h at room
temperature. These observations led us to conclude
that solvents having low water miscibility were effi-
cient in controlling the side reactions. Optimization of
reaction time led us to use dichloromethane as sol-
vent as the reaction was completed in 45 min
(entry 10 of Table S1, Supporting Information) and
pure 2g could be isolated in 87% yield. The reaction
under these conditions can also be monitored visually
as consumption of iodine, indicated by decolouriza-
tion of the original violet colour, signifies the com-
Entry 3, R
Product (Yield) at:
r.t./3 h
À158C/3 h À158C/10–12 h
1
2
3
4
5
6
7
8
9
10
3a, H
3b, p-CH3
mixture[a] mixture[a] 2a (78%)
mixture[a] mixture[a] 2b (81%)
3c, p-OCH3 2c (89%) 2c (91%) 2c (90%)
3d, p-F
3e, p-Cl
3f, p-Br
3g, p-NO2
3h, p-Ph
3h, m-Br
3j, m-NO2
2d (78%) 2d (80%) 2d (80%)
mixture[a] mixture[a] 2e (77%)
mixture[a] 2f
2f (75%)
mixture[a] mixture[a] mixture[a]
mixture[a] mixture[a] 2h (83%)
mixture[a] mixture[a] 2i (82%)
mixture[a] mixture[a] mixture[a]
[a]
As visible on TLC, mixture could not be identified.
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Transition Metal-Free Approach to Propynenitriles
Table 2. Substrate scope for 3-chloropropenenitriles.
ly focused on examining the mechanism of reaction of
3-chloropropenals with molecular iodine and aqueous
ammonia in THF as solvent leading to the formation
of a mixture of products, it was intriguing to know the
fate of 3-chloropropenals either with molecular iodine
under neutral conditions or with molecular iodine
under ammonia-free basic conditions (to avoid the
formation of aldimine and hence the nitrile) by em-
ploying sodium hydroxide, sodium bicarbonate, tri-
ethylamine, piperidine and pyridine. Either of these
conditions failed to give a reaction which led us to
conclude that 3-chloropropenal is first converted to 3-
chloropropenenitrile followed by abstraction of the a-
proton to the nitrile under basic conditions leading to
the formation of a mixture of products. Next we en-
visaged to react 3-chloropropenenitrile 2a with molec-
ular iodine alone as well as with molecular iodine
under basic conditions using different bases including
aqueous ammonia (Table 3). Again, a mixture of
products (entry 1) was formed in the presence of
iodine and aqueous ammonia. Surprisingly, a single
product was seen on TLC plate when 2a was treated
with molecular iodine and aqueous sodium hydroxide
(1 equiv.) at room temperature for 3 h leading to com-
plete consumption of reactant 2a as evident from
TLC of the reaction mixture (entry 2). The single
product formed was isolated and identified as 3-phe-
nylpropynenitrile 1a on the basis of its spectral data.
3-Phenylpropynenitrile 1a was also formed as a single
product when 2a was treated with molecular iodine
[a]
2 equiv. of iodine were used.
plete conversion of chloropropenal 3g to chloro- and aqueous sodium bicarbonate but the reaction was
acrylonitrile 2g.
incomplete after 3 h (entry 3). No reaction occurred
Having the optimized reaction conditions in hand,
we explored the substrate scope and successfully syn-
thesized seventeen differently substituted 3-chloro-
acrylonitriles (Table 2). It is pertinent to mention here
that only Z-isomers of alkenes were obtained except
for the methyl ketones substituted with 2-chlorophen-
yl 2l, 2-methoxyphenyl 2m and 1-naphthyl 2n groups
where both E and Z forms were obtained in 54:46,
25:75 and 43:57 ratios, respectively, as measured by
1H NMR. Two compounds, 2p and 2q, having double
chloropropenenitrile functionalities were found to be
too unstable to be isolated and characterized although
they could be further transformed to propynenitriles
1p and 1q by a one-pot methodology (see Table 4).
Successful development of this new methodology
for the conversion of acetophenones to chloroacrylo-
nitriles via chloropropenals allows us to reflect upon
the difference in behaviour of propiophenones and
acetophenones and also paves our way to synthesize
propynenitriles. It seems that under basic conditions,
a trigger for the side reactions is the abstraction of an
alkene proton either at the propenal stage or at the
propenenitrile stage that is more likely in polar water-
miscible solvents than in relatively non-polar water-
immiscible solvents. Furthermore, as we were precise-
Table 3. Reaction of 3-chloro-3-phenylpropenenitrile (3a)
with different bases.
Entry Reagents
Time Product(s)
1.
2.
3.
4.
5.
6.
I2 (1 equiv.)+aq. ammo-
nia
3 h
3 h
3 h
3 h
3 h
3 h
mixture
I2 (1 equiv.)+aq. NaOH
(1 equiv.)
I2 (1 equiv.)+aq.
NaHCO3 (1 equiv.)
I2 (1 equiv.)+TEA
(1 equiv.)
I2 (1 equiv.)+pyridine
(1 equiv.)
I2 (1 equiv.)+piperidine
(1 equiv.)
1a
1a, incomplete
reaction
no reaction
no reaction
no reaction
7.
8.
9.
aq. ammonia
aq. NaOH (1 equiv.)
aq. NaOH (1 equiv.)
3 h
3 h
15 min 1a
mixture
1a
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Pawan K. Sharma et al.
when TEA, pyridine or piperidine were used along pynenitrile from 3-chloropropenals, we synthesized
with molecular iodine (entries 4, 5 and 6). Next we seventeen differently substituted propynenitriles
carried out the reaction of 2a with aqueous ammonia (Table 4). It is noteworthy to mention here that when
without using iodine and the same mixture of prod- differently substituted arylchloropropenenitriles were
ucts (as in case of entry 1) was obtained (entry 7) but subjected under the basic conditions, only Z-isomers
using aqueous NaOH solution alone again led to the of alkenes underwent elimination reaction leading to
formation of 1a exclusively (entry 8). The reaction the formation of corresponding propynenitriles while
time was then optimized and it was found that reac- E-isomers were not converted to propynenitriles and
tion takes just 15 min for complete conversion of 3- remained as such. Since replacing the aromatic ring
chloropropenenitrile to propynenitrile. These results by o-Cl-phenyl, o-OCH3-phenyl and 1-naphthyl led to
validated our assumption that the reaction of 3- formation of both E and Z isomers of chloropropene-
chloro-3-phenylpropenal (3a) with I2 and NH3 first nitriles, mixtures of propynenitriles and unreacted E-
leads to the formation of 3-chloro-3-phenylpropeneni- isomer of chloropropenenitrile were obtained in all
trile (2a) which further reacts under the reaction con- these three cases. The mechanism of elimination reac-
ditions to give a mixture of products including 3-phe- tion leading to the formation of propynenitriles 1 is
nylpropynenitrile (1a). Thus, in this study, we success- given in the Supporting Information (Scheme S1).
fully identified another component of the mixture of
In summary, we have established an efficient one-
products obtained on treatment of 3-phenyl-3-chloro- pot protocol to synthesize propynenitriles from 3-
propenal with molecular iodine and aqueous ammo- chloropropenals via 3-chloropropenenitriles employ-
nia in THF and also developed the methodology for ing mild aqueous reaction conditions. The methodolo-
the preparation of propynenitriles 1.
gy is featured by use of non-toxic reagents, milder,
Finally, the two-step conversion of 3-chloroprope- metal-free and economically benign aqueous reaction
nals to propynenitrile was achieved in a one-pot conditions avoiding a harsh dehydration step while
manner by concentrating the reaction mixture achieving excellent yields. Furthermore, the chloro-
through evaporation of the solvent of the first step on propenenitrile as well as propynenitriles (except 1l,
a rotary evaporator and treating the residual mass as 1m and 1n which were separated from their corre-
such with aqueous NaOH in THF. After optimization sponding E-chloropropenenitriles left unreacted by
of the protocol for the one-pot preparation of pro- chromatography) obtained after simple work-up are
sufficiently pure thus negating the need for flash chro-
matography.
Table 4. One-pot methodology for propynenitriles.
Experimental Section
Typical Experimental Procedure for the Conversion
of b-Chloropropenals 3 into 3-Chloropropenenitriles
2
To a solution of 3-chloropropenal (3, 1.00 mmol) in 10 mL
of dichloromethane under stirring was added molecular
iodine (1.00 mmol) in one lot. 2 mL of 30% aqueous ammo-
nia solution were then added and reaction mixture was al-
lowed to stir further. The violet colour of iodine started to
fade and the solution became almost colourless in 45 min in-
dicating complete consumption of iodine. Aqueous sodium
thiosulfate solution was then added in order to neutralize
excess iodine if any, and the organic layer was separated,
washed 2–3 times with water, dried (fused CaCl2) and finally
dichloromethane was evaporated under reduced pressure to
afford pure b-chlorocinnamonitrile 2.
Typical Experimental Procedure for One-Pot
Conversion of b-Chloropropenals 3 into
Propynenitriles 1
To a stirring solution of 3-chloropropenal (3, 1.00 mmol) in
10 mL of dichloromethane was added molecular iodine
(1.00 mmol) followed by addition of 2 mL of 30% aqueous
ammonia solution. The reaction mixture was allowed to stir
[a]
2 equiv. of iodine were used.
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Transition Metal-Free Approach to Propynenitriles
further for 45 min when whole solution became almost col-
ourless showing the complete consumption of iodine. Excess
of iodine was neutralized with aqueous sodium thiosulfate
solution and the organic layer was evaporated under re-
duced pressure. THF was then added to dissolve the solid
mass left followed by addition of aqueous NaOH solution.
The reaction mixture was further allowed to stir for 15 min.
when the TLC showed complete consumption of reactant.
The whole reaction mixture was then added to water and
the resulting solid was filtered off, dried and recrystallized
from a mixture of ethyl acetate and petroleum ether.
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