A versatile and practical microwave-assisted synthesis of sterically hindered N-arylpiperazines

Article abstract of DOI:10.1021/jo101478c

A wide-ranging and practical synthesis of structurally diverse, sterically hindered N-arylpiperazines from 2,2′-(4-nitrophenylsulfonylazanediyl) bis(ethane-2,1-diyl) bis(4-nitrobenzenesulfonate) and substituted anilines has been achieved using microwave i

Full text of DOI:10.1021/jo101478c

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routes for the synthesis of N-arylpiperazines with bulky aro-
A Versatile and Practical Microwave-Assisted
Synthesis of Sterically Hindered N-Arylpiperazines
matic substituents are extremely scarce. For example, the
reaction of isopropylaniline and N,N-bis(2-chloroethyl)amine
provides 1-(2-isopropylphenyl)piperazine in 21% yield.6 This
compound can also be prepared from a 3-substituted 2-oxazo-
lidinone precursor in 36% yield.7 Alternatively, the synthesis of
the same compound has been reported using a Pd catalyst, but
no yield is included. Under the same reaction conditions, the
less hindered N-(2-ethylphenyl)piperazine was prepared in
56% yield.8 To date, an efficient and practical method for pre-
paring structurally diverse, sterically demanding N-arylpiper-
azines (e.g., 1-(2-tert-butylphenyl)piperazine, 1-(2-iodophenyl)-
piperazine, and 1-(2,6-diisopropyl phenyl)piperazine) is lacking.
Our interest in developing novel ligands for G-protein
coupled receptors (GPCRs) required the incorporation of
substituted N-arylpiperazines such as 1-(2-isopropylphenyl)-
piperazine and 1-(2-tert-butylphenyl)piperazine as key frag-
ments in the target molecules. Thus, the goal of the present
work was to design an efficient synthetic route which pro-
vided ready access to a series of substituted N-arylpiper-
azines with a wide range of steric hindrance. A review of the
literature revealed a general paucity of systematic studies
involving the preparation of these important intermedi-
ates. Thus, we set out to systematically investigate leaving/
protecting groups, reaction temperature (microwave assisted),
bases, solvents, and deprotecting conditions. The protection of
the amine during the reaction was hypothesized to lead to
improved yields by limiting possible side reactions.
Rong Gao and Daniel J. Canney*
Temple University School of Pharmacy, Department of
Pharmaceutical Sciences, 3307 North Broad Street,
Philadelphia, Pennsylvania 19140
canney@temple.edu
Received July 27, 2010
A wide-ranging and practical synthesis of structurally diverse,
sterically hindered N-arylpiperazines from 2,20-(4-nitro-
phenylsulfonylazanediyl) bis(ethane-2,1-diyl) bis(4-nitro-
benzenesulfonate) and substituted anilines has been achieved
using microwave irradiation in acetonitrile followed by
deprotection with PhSH.
Several approaches were considered as potential means of
improving the scope and yields of previously reported routes
to the target compounds. Microwave irradiation has been
reported in aniline alkylation reactions involving N,N-bis(2-
bromoethyl)amines under aqueous conditions.9 A small
series of unhindered N-arylpiperazines were synthesized in
low to moderate yields using a modified domestic microwave
oven.10 Hence, microwave irradiation was investigated here
for the preparation of sterically demanding arylpiperazines.
Replacing the halides of starting materials with better leav-
ing groups and protecting the amine with same group has
been considered as an approach to improve reaction yields.
Employment of the tosyl and mesyl groups has been reported
to give 37% and 50% (prior to deprotection) yields, respec-
tively, suggesting that these groups are not sufficient given
the poor nucleophilicity of the aniline nitrogen.11 Moreover,
these leaving/protecting groups are not ideal due to the harsh
N-Arylpiperazines are a class of heterocyclic compounds
that are important intermediates in organic synthesis and are
commonly found as fragments in natural products, receptor
ligands, and in many pharmacologically active molecules.1
There are several well-established syntheses of N-arylpiper-
azines that include the reaction of anilines with N,N-bis-
(2-chloroethyl)amine reported by Prelog.2 Modifications of
this method provided some improvements to the reaction
but were largely restricted to unhindered substituents.3
Aromatic substitution of electron-poor aryl halides by
unsubstituted piperazines represents an alternative approach,4
and catalysts have been developed to facilitate this type of
coupling reaction.5 Limitations to the currently available
methods include low to moderate yields and restrictions on
the type of aromatic substituents that are tolerated. Efficient
(1) (a) Sun, D. Q.; Wang, Z. L.; Powers, J. P. J. Med. Chem. 2008, 18,
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Commun. 1998, 28, 1175. (b) Jaisinghani, H. G.; Khadilkar, B. M. Tetra-
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1992, 3185. (b) Smid, P.; Coolen, H.; Keizer, H. G.; Kruse, C. G. J. Med.
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DOI: 10.1021/jo101478c
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Published on Web 09/23/2010
J. Org. Chem. 2010, 75, 7451–7453 7451
2010 American Chemical Society

Gao and Canney
JOCNote
TABLE 1. Optimization of the Coupling Reaction To Give 3-7
TABLE 2. Optimization of the Deprotection Reaction To Give 9
deprotecting agent
base
solvent
T (oC) yield (%)
HSCH2COOH
HSCH2COOH
HSCH2COOH
PhSH
PhSH
PhSH
K2CO3
DIPEA
LiOH
K2CO3
K2CO3
DMF
DMF
DMF
DMF
DMF
24
24
24
24
60
50
N.A.a
N.A.a
N.A.a
45b
32b
K2CO3 CH3CN, 2% DMSO
86b
aNo product is formed on the basis of LC-MS analysis. bIsolated
yield.
pressure. The preliminary results of these reactions indicate
that CH3CN gave superior yields. Finally, to explore the
effect of temperature, the reaction was carried out under
varying temperatures for 1 h (Table 1). When R2 = t-Bu or
acetamide, a temperature of 160 °C or above provided yields
of 80% or higher. When R2 = pyrrole and morpholine,
higher temperatures (175 °C) were required to provide high
yields. Thus, 175 °C was chosen for future reactions involving
nosyl-protected diethanolamine and inexpensive, substituted
anilines in CH3CN to prepare nosyl-protected substituted
N-arylpiperazines in moderate to high yields. Purification of
the protected intermediates are straightforward and involves
either recrystallization or passage through a pad of silica
precluding the need for column chromatography.
In an effort to optimize the recovery of the deprotected
N-arylpiperazines, several deprotection conditions were
tested.14 The use of HSCH2COOH in the presence of base
(K2CO3, DIPEA, LiOH) provided none of the deprotected
amine. PhSH in the presence of K2CO3 afforded low yields of
the amine when attempted in DMF presumably due to the
formation of 4-phenyl thioether.15 The use of PhSH with
K2CO3 in CH3CN and 2% DMSO gave the highest yield of
the methods tested (Table 2). The unpleasant smell of PhSH can
be easily removed by addition of concentrated NaOH solu-
tion followed by extraction with organic solvent. Thus, nosyl-
protected piperazines are stable intermediates that are easily
deprotected and may be used to prepare structurally diverse,
synthetically important target molecules.
aNo product is formed on the basis of LC-MS analysis. bIsolated
yield.
conditions required for deprotection.12 Thus, in our hands,
the nosyl group was shown to be a superior leaving/protect-
ing group and was employed in future reactions to improve
the yield.
The starting materials utilized in our initial efforts to
investigate the use of several bases were 1 and 2-tert-butyl
aniline since both are commercially available and the tosyl
group is both a reliable leaving group and a stable protecting
group (Table 1). Several bases (Et3N, pyridine, DMAP, N,N-
diisopropylethylamine) were screened under the conditions
outlined in Table 1 in a microwave reactor (1 h at 160 °C).
The use of DIPEA (N,N-diisopropylethylamine) in CH3CN
(acetonitrile) under these conditions provided the highest
yield (40%), and DIPEA was used in subsequent reactions.
Changing the leaving/protecting group from tosyl to nosyl
resulted in a dramatic improvement in yield (91%), and the
nosyl group was used to explore the suitability of several
solvents. Importantly, the nosylated starting material is
easily synthesized in bulk quantities (40 g in our hands)
and purified by recrystallization precluding the need for
column chromatography.13 Due to the susceptibility of some
starting materials to hydrolysis, polar aprotic solvents were
used here instead of water. Acetonitrile (CH3CN), THF, and
1,4-dioxane were chosen because of their excellent solubiliz-
ing properties, the ability to reach high temperatures under
pressure, and the relative ease of removal under reduced
To explore the efficiency and scope of the approach, a
broad range of sterically hindered, electronically and lipo-
philically diverse anilines were reacted with 2. The results of
those efforts are summarized in Table 3 and illustrate the
general applicability of the method and the moderate to high
yields of the target N-arylpiperazines obtained. Further-
more, a diverse series of aniline substrates have been utlized
including those containing substituents with considerable
steric hindrance (e.g., 8, 9, 11, 13-16), ethers (16), halogens
(14), esters (19), amides (12), and heterocycles (15, 18).
(14) (a) Kumari, N.; Reddy, B. G.; Vankar, Y. D. Eur. J. Org. Chem.
2009, 160. (b) Crestey, F.; Witt, M.; Jaroszewski, H. J. J. Org. Chem. 2009,
74, 5652. (c) Lencina, C. L.; Dassonville-Klimpt, A.; Sonnet, P. J. Org. Chem.
2008, 19, 1689. (d) Fukuyama, T.; Jow, C-K; Cheung, M. Tetrahedron Lett.
1995, 36, 6373.
(12) Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synth-
esis, 3rd ed.; John Wiley & Sons: New York, 1999; pp 603-615.
(13) (a) Stones, G.; Tripoli, R.; Gibson, C. L. Org. Biomol. Chem 2008, 6,
374. (b) Xu, M.-H.; Lin, J.; Pu, L. J. Am. Chem. Soc. 2002, 124, 14239.
(15) Wuts, P. G.; Northuis, J. M. Tetrahedron Lett. 1998, 39, 3889.
7452 J. Org. Chem. Vol. 75, No. 21, 2010

Gao and Canney
JOCNote
TABLE 3. Microwave-Assisted Synthesis of N-Arylpiperazines 8-19
Experimental Section
General Procedure for the Synthesis of Protected N-Arylpiper-
azines (3-7). The reactions were performed in a CEM micro-
wave reaction system operated at 175 °C for 1 h. The nosyl-
protected diethanolamine (660 mg, 0.99 mmol), the representa-
tive aniline (1.2 mmol), DIPEA (516 mg, 4.0 mmol), and
CH3CN (3 mL) were mixed in a microwave reaction vial
(10 mL) fitted with a no-invasive vial cap. The reaction vials
containing the mixture were reacted in the microwave for 1 h at
175 °C. The typical reaction temperature-time profile is shown
in the Supporting Information. After 1 h, the solvent was
removed under reduced pressure. The residue was dissolved in
dichloromethane (DCM) and washed with HCl (10%, 3 ꢀ
30 mL) and saturated NaHCO3 (40 mL). The organic phase
was dried over MgSO4 and concentrated in vacuo to afford the
crude product. This crude product was purified by passage
through a pad of silica (hexanes/DCM = 1:4, silica pad thick-
ness: 10 cm, diameter: 4 cm) and used in subsequent reactions.
General Procedure for the Synthesis of Unprotected N-Arylpi-
perazines 8-19. The general procedure described above for
the synthesis of protected N-arylpiperazines was used to prepare
the corresponding nosyl-protected precursors for 8-19. The
deprotection of the precursors to the target amines is described
below.
Potassium carbonate (3.52 g, 25.47 mmol) was added to a mixture
of acetonitrile and dimethyl sulfoxide (CH3CN/DMSO = 49:1) and
heated to 50 °C. Thiophenol (2.34 g, 21.23 mmol) was added
dropwise via syringe to the mixture with stirring. After 30 min, a
solution of the nosyl-protected N-arylpiperazine (2.12 mmol) in
CH3CN and DMSO (CH3CN/DMSO = 49:1) was added drop-
wise. The reaction mixture was stirred for 3 h, quenched with excess
NaOH solution (40%; also removed the unpleasant smell of PhSH),
and concentrated under reduced pressure. The residue was extracted
with DCM (5ꢀ30 mL), and the organic phase was dried over
MgSO4 and concentrated in vacuo to give a crude oil. The oil was
purified by reversed-phase chromatography (CH3CN in H2O, gra-
dient from 1%∼100% with 0.1% formic acid) to afford the formic
acid salt of the desired piperazine. The salt was dissolved in DCM
and washed with saturated NaHCO3 solution and the organic phase
concentrated in vacuo to provide the target piperazines in moderate
to high yields (see Table 3 and the Supporting Information).
aIsolated yield. bYield calculated on the basis of compound 2.
Importantly, the short duration of the reaction provides
rapid access to a wide variety of interesting piperazines in
1 day, making it very practical for use in organic synthesis,
parallel synthesis, and combinatory chemistry.
In summary, N-arylpiperazines are important intermedi-
ates in organic synthesis and are commonly used fragments
in the drug discovery process. Currently available synthetic
routes to these important heterocyclic compounds have
limited scope, and yields are often highly dependent on
substituents and substitution patterns. The method de-
scribed herein provides an improved, general, and practical
procedure for the synthesis of sterically hindered N-arylpi-
perazines from inexpensive, commercially available anilines
and nosyl-protected diethanolamine in CH3CN using micro-
wave irradiation. The wide acceptance and improved access
to microwave reactors in recent years will make this ap-
proach available to chemists in a variety of research settings.
In our hands, these structurally diverse heterocycles are
readily coupled with other fragments to afford novel piper-
azine-based ligands for GPCRs. Screening of those com-
pounds is underway and the results of those efforts will be
reported elsewhere.
Acknowledgment. We are grateful to Temple University
(R.G., University Fellowship; D.J.C., Grant-in-Aid of Research)
and the Dean’s Office, School of Pharmacy, for generous
financial support and to Dr. Abou-Gharbia and Rogelio
Martinez (Moulder Center for Drug Discovery Research)
for access to instrumentation and helpful discussions.
Supporting Information Available: General experimental
methods, temperature-time profile, compound characteriza-
tion data, and copies of spectra for the products 3-19. This
material is available free of charge via the Internet at http://
pubs.acs.org.
J. Org. Chem. Vol. 75, No. 21, 2010 7453