Synthesis of chiral amino acid anilides by ligand-free copper-catalyzed selective N-arylation of amino acid amides

Article abstract of DOI:10.1002/adsc.201200752

An atom-economic, practical and cost-effective protocol for synthesis of chiral amino acid anilides via ligand-free copper-catalyzed selective C-N cross coupling of chiral amino acid amides and aryl halides, hetereoaryl halides and a vinyl bromide has been developed. No racemization occurred during the C-N coupling. A plausible mechanism is proposed. Copyright

Full text of DOI:10.1002/adsc.201200752

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DOI: 10.1002/adsc.201200752
Synthesis of Chiral Amino Acid Anilides by Ligand-Free Copper-
Catalyzed Selective N-Arylation of Amino Acid Amides
Junyu Dong,a Yan Wang,a Qinjie Xiang,a Xirui Lv,a Wen Weng,b and Qingle Zenga,*
a
Institute of Green Catalysis and Synthesis, College of Materials and Chemistry & Chemical Engineering, Chengdu
University of Technology, Chengdu 610059, Peopleꢀs Republic of China
Fax : (+86)-28-8407-9074; e-mail: qinglezeng@hotmail.com
Department of Chemistry and Environmental Science, Zhangzhou Normal University, Zhangzhou 363000, Peopleꢀs
b
Republic of China
Received: August 23, 2012; Revised: December 13, 2012; Published online: February 22, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200752.
Abstract: An atom-economic, practical and cost-ef-
fective protocol for synthesis of chiral amino acid
anilides via ligand-free copper-catalyzed selective
C N cross coupling of chiral amino acid amides
and aryl halides, hetereoaryl halides and a vinyl
bromide has been developed. No racemization oc-
synthesized via hydrolysis of the optically active b-
lactam obtained from a chiral auxiliary-mediated
asymmetric induction.[10] All of these approaches in-
volve multiple step synthetic routes, thus resulting in
low atom economy, high cost, tedious work-up and
potential environmental pollution.
As to the synthesis of racemic amino acid anilides,
there are some other methods,[11] but they need multi-
step synthetic routes or sometimes rare reactants.
Since chiral amino acid anilides are a type of basic
and important compound extensively used in medici-
nal chemical synthesis, chiral catalysis and organic
synthesis, there is an important need to search for an
efficient, atom-economic, inexpensive synthetic proto-
col of chiral amino acid anilides.
À
À
curred during the C N coupling. A plausible mech-
anism is proposed.
Keywords: amino acid amides; amino acid anilides;
chiral pool; copper; cross-coupling
Chiral amino acid anilides are a subclass of amino
Based on Coreyꢀs anti-synthetic analysis of the mo-
À
acid derivatives and have extensive and important ap- lecular structures, direct disconnection the C N bond
plications. They are essential core structures in of chiral amino acid anilides gives chiral amino acid
a number of bioactive compounds,[1] such as clinical amides and aryl halides, which means C N cross-cou-
À
antiarrythmic agent tocainide,[1a] histone deacetylases pling is another potential method for their synthesis.
(HDACs) inhibitors,[1b] pH-controlled light-activated
But there are two amino groups in an amino acid
reagents for cancer therapy,[1c] cold menthol receptor amide molecule, so control of the selective C N cross
antagonists,[1d] and antimalarial agents.[1e] Chiral coupling of these compounds is unavoidable. To the
amino acid anilides are also found as chiral ligands[2] best of our knowledge, there is no report about con-
À
À
used in catalytic asymmetric Mannich-type reaction- trol of the selective C N cross-coupling of chiral
s,[2a] ruthenium(II)-catalyzed asymmetric transfer hy- amino acid amides and aryl halides to synthesize
drogenation of ketones[2b,c] and chiral organocatalysts chiral amino acid anilides. Related to this, Cesati III
for asymmetric aldol reactions.[3] Besides, they are reported the copper-catalyzed C N coupling of amino
À
used as organic synthesis intermediates[4] and utilized acid amides and protected amino acid amides and
in the synthesis of peptides and peptidomimetics.[5]
vinyl iodides to synthesize amino acid-derived enam-
Chiral amino acid anilides are typically synthesized ides.[13] And Buchwald reported the selective C N
via three-step synthetic routes. The first step is protec- cross-coupling of aniline amino groups and aromatic
tion of l-, d- and dl-amino acids with CbzCl,[2c,6] amides of ortho-, meta- and para-aminobenzamides.[14]
À
Boc2O,[1b,4b,d,7] phthalic anhydride[8] or ethyl thioltri-
Chiral a-amino acid amides (i.e., 2-alkyl-2-amino
fluoroacetate.[9] The second step is condensation of N- acetamides) with aliphatic amino groups and amide
protected amino acids and anilines in the presence of amino groups in the specific steric structure are differ-
carboxylic activating agents. The last step is removal ent from these substrates investigated by Buchwald.
of the protecting groups to furnish the chiral amino Given the versatility of palladium catalysts and our
À
acid anilides. In addition, phenylalanine anilide was experience of palladium-catalyzed C N cross-cou-
692
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Adv. Synth. Catal. 2013, 355, 692 – 696

Synthesis of Chiral Amino Acid Anilides by Ligand-Free Copper-Catalyzed Selective N-Arylation
pling,[15] firstly we tried the palladium-catalyzed C N
À
cross-coupling of phenylalaninamide and bromoben-
zene under various reaction conditions, but only start-
ing materials were recovered. The reason perhaps is
that formation of the especially stable five-member
palladium-diamino complex, whose analogues are re-
ported as being very stable,[16] prevents the formation
À
Scheme 1. Ligand-free copper-catalyzed selective C N
cross-coupling of amide groups of amino acid amides and
aryl halides.
À
of the C N bond from bromobenzene and phenyl-
A
À
Copper-catalyzed Ullmann-type C N coupling has detailed information may be found in the Supporting
a history of more than one hundred years, and as Information), finally we achieved the optimized reac-
À
a cost-effective and versatile C N coupling, it is still tion conditions: 5 mol% copper(I) iodide, K2CO3
actively used in the field of organic synthesis.[17] The (2 equivalents), l-phenylalaninamide (1.2 mmol), bro-
vinyl halides have some similar catalytic performance mobenzene (1 mmol) in toluene (6 mL) at 1108C for
[18]
À
in the copper-catalyzed C N coupling. Inspired by 24 h. Under the optimized reaction conditions, l-phe-
Cesati IIIꢀs research,[13b] we tried the CuI-catalyzed nylalanine anilide 3a was obtained exclusively.
C N coupling of bromobenzene and phenylalanin-
amide with N,N’-dimethylethylenediamine as ligand CuI-catalyzed C N cross-couplings of various aryl
and K2CO3 as base. Fortunately, this reaction pro- halides and l-phenylalaninamide were examined.
ceeded in toluene at 1108C for 24 h and afforded only Since bromobenzene produced 3a a high yield of up
one new compound with 83% yield, which was veri- to 86% (Table 1, entry 1), more aryl bromides were
À
With the optimized reaction conditions at hand,
À
U
À
fied to be l-phenylalanine anilide [i.e., (S)-2-amino- evaluated: the C N cross-coupling of para-bromoto-
N,3-diphenylpropanamide] by spectral analysis and luene and l-phenylalaninamide produced 3b with ap-
comparison with the literature data.[4d,5b]
proximately the same yield (Table 1, entry 2). Perhaps
Encouraged by our former study on ligand-free due to the steric hindrance, ortho-bromotoluene af-
À
À
CuI-catalyzed cascade C S/C N coupling to synthe- forded 3d with lower yield than para- and meta-bro-
size phenanthiazines,[19] we tried ligand-free copper motoluene (Table 1, entries 4 vs. 2 and 3). The elec-
iodide as catalyst for this reaction. Interestingly, in tron property of the substituent group on the aryl bro-
the C N cross-coupling of l-phenylalaninamide and
bromobenzene, the simple, ligand-free copper catalyt-
À
À
Table 1. Copper-catalyzed C N coupling of aryl halides and
ic system also afforded l-phenylalanine anilide with
l-phenylalaninamide.[a]
a marginally higher yield of 86%. That the two amino
groups of the amino acid amides act as ligand to coor-
dinate with copper could be the reason for this
ligand-free catalysis. And thus additional ligands may
influence this catalytic reaction and result in lower
yields (see the Supporting Information). The ligand-
free copper-catalyzed C N coupling of the amide
amino groups of phenylalaninamide is different from
the classical copper-catalyzed C N coupling of
amides, which gave no yield when suitable 1,2-di-
Entry
ArX
Product
Yield [%]
À
1
2
3
4
5
6
7
8
PhBr
3a
3b
3c
3d
3e
3f
3g
3h
3a
3b
3f
3i
3a
3f
3i
3j
3k
86
84
76
55
77
81
94
56
96
93
93
76
21
57
65
42
80
À
4-Me-C6H4Br
3-Me-C6H4Br
2-Me-C6H4Br
3-MeO-C6H4Br
3-Br-C6H4CN
4-NO2-C6H4Br
4-F-C6H4Br
PhI
ACHTUNGTRENNUNG
amine ligand was not added.[20]
Since most of l-, d- and dl-form amino acid
amides are commercially available and inexpensive,
this protocol has the advantage of being direct (only
one-step), atom-economic, ligand-free, practical and
cost-effective.
Our research on the ligand-free copper-catalyzed
selective C N cross-coupling of amide amino groups
of amino acid amides and aryl halides for the synthe-
sis of chiral amino acid anilides will be described in
detail below (Scheme 1).
Firstly, we took l-phenylalaninamide and bromo-
benzene as the model substrates for the copper-cata-
lyzed C N cross-coupling to optimize the reaction
conditions. After examining a series of copper sour-
ces, ligands, bases, solvents and temperatures (more
9
10
11
12
13
14
15
16
17
4-Me-C6H4I
3-I-C6H4CN
4-F-C6H4I
À
PhCl
4-NO2-C6H4Cl
2-Br-pyridine
2-Br-thiophene
b-Br-styrene
[a]
À
Reaction conditions: l-amino acid amide (1.2 mmol), aryl
halide (1.0 mmol), CuI (0.05 mmol), K2CO3 (2.0 mmol)
and toluene (6 mL) for 24 h at 1108C.
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Junyu Dong et al.
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mide obviously influences the efficiency of this CuI-
Heteroaryl bromides, 2-bromopyridine and 2-bro-
catalyzed N-arylation of the amide amino group of l- mothiophene, also coupled with phenylalaninamide,
phenylalaninamide. As a substrate with an electron- but afforded the corresponding product 3i and 3j with
donating group, coupling of 3-bromoanisole afforded moderate yields (Table 1, entries 15-16).
3e with a good yield of 77% (Table 1, entry 5). As
As a vinyl halide with less activity than vinyl io-
a substrate with an electron-withdrawing group, 3- dides, b-bromostyrene was examined under our cata-
bromobenzonitrile afforded the coupling product 3f lytic conditions and produced phenylalanine 2-phenyl-
with a little better yield of 81% than 3-bromoanisole ethenaminide 3k with good yield (Table 1, entry 17).
(Table 1, entry 6). 4-Bromonitrobenzene with a strong
Once aryl halides had been evaluated, the CuI-cata-
À
electron-withdrawing nitro group afforded 3g with lyzed C N cross-couplings of various amino acid
a much higher yield of up to 94% (Table 1, entry 7). amides and bromobenzene or iodobenzene were in-
Perhaps the fluoro group of 4-bromofluorobenzene vestigated (Table 2). Coupling of l-phenylalaninamide
took part in certain side reactions under the reaction with PhBr and PhI afforded (S)-N-(2-amino-3-phenyl-
conditions and produced by-products, so 3h was ob- propanoyl)benzamide 3a with 86% and 96% yield, re-
tained with a moderate yield (Table 1, entry 8).
spectively (Table 2, entry 1). dl-Phenylalaninamide
Aryl iodides have much higher reactivity than the afforded nearly the same yield as the l-isomer
corresponding aryl bromides, i.e., all of the aryl io- (Table 2, entry 2).
dides afforded much higher yields than the aryl bro-
mides and chlorides. Furthermore, except for 1- during the C N cross-coupling, the enantiomeric
fluoro-4-iodobenzene (Table 1, entry 12), other tested excess
In order to verify whether racemization occurred
À
of
the
product
(S)-N-(2-amino-3-
aryl iodides including iodobenzene, para-tolyl iodide phenylpropan
N
and 3-iodobenzonitrile, all afforded the targeted chiral HPLC on a Daicel Chiralcel OD-H column.
chiral amino acid anilides with more than 90% yields The chiral HPLC results demonstrated the ee value of
(Table 1, entries 9–11).
(S)-N-(2-amino-3-phenylpropanoyl)benzamide 3a was
However, aryl chlorides are poorer substrates for as high as 99% (Table 2, entry 1), which means that
CuI-catalyzed N-arylation of l-phenylalaninamide. no racemization occurred during the coupling. Phe-
Chlorobenzene only gave a 21% yield (Table 1, nylglycinamide, which is more apt to racemize, gave
entry 13); and the activated substrate para-chloroni- a partially racemized product at 1108C. But at
trobenzene afforded
entry 14).
a
moderate yield (Table 1, a lower temperature of 908C, 99% ee of phenylgly-
[a]
À
Table 2. Copper-catalyzed C N coupling of aryl bromides or aryl iodides and amino acid amides.
Entry
Amino acid amide
Amino acid anilide
Yield [%][b] (PhBr)
Yield [%][b] (PhI)
1
2
l-phenylalaninamide
dl-phenylalaninamide
l-alaninamide hydrochloride
l-valinamide hydrochloride
l-leucinamide
l-phenylglycinamide
l-prolinamide
l-ttryptophanamide
3a
86[c]
84
62
68
73
72
54
45
0
96
(Æ)-3a
N/A[d]
3[e]
4[e]
5
3l
81
82
3m
3n
3o
3p
3q
3r
3s
3a
86
6[e]
7
90 (86)[f]
76
8
N/A[d]
9[e]
10[e]
11[g]
l-serinamide hydrochloride
l-threoninamide hydrochloride
l-phenylalaninamide
0
0
92
0
83
[a]
Reaction conditions: l-amino acid amide (1.2 mmol), bromobenzene or iodobenzene (1.0 mmol), CuI (0.05 mmol), K2CO3
(2.0 mmol), and toluene (6 mL) for 24 h at 1108C, unless otherwise noted.
Isolated yields with bromobenzene and with iodobenzene as the substrates, respectively.
Enantiomeric excess as checked by chiral HPLC on a Daicel Chiralcel OD-H is 99% ee.
N/A means that no experiment was done, and so no yield is reported here.
[b]
[c]
[d]
[e]
[f]
K2CO3 (3.0 mmol) was added.
99% ee and 86% yield were obtained at 908C for 24 h (Chiral HPLC trace is given in the Supporting Information).
Ten times the amount of the reactants of entry 1 were used, and the reaction was performed under the same conditions.
[g]
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Synthesis of Chiral Amino Acid Anilides by Ligand-Free Copper-Catalyzed Selective N-Arylation
cine anilide 3o was obtained with 86% yield (Table 2,
entry 6).
Prolinamide gave relatively lower yields than other
amino acid amides, such as phenylalaninamide,
alaninACHTUNGTRENNUNGamide, valinamide, leucinamide, phenylglycina-
mide (Table 2, entry 7 vs. entries 1–6). Two possible
reasons are that prolinamide has a secondary amino
group and is apt to form a more stable copper com-
plex.
Probably due to the effect of the indolyl group,
tryptophanamide gave a moderate yield when it react-
ed with bromobenzene (Table 2, entry 8).
Neither bromobenzene nor iodobenzene coupled
with serinamide or threoninamide (Table 2, entry 9
and 10). The reason probably is that the two amino
acid amides have two amino groups and one hydroxy
group in the right positions to act as tridentate ligands
to undergo firm and stable coordination with copper,
and the resulting copper complexes are too stable to
react with bromobenzene or iodobenzene.
Iodobenzene and amino acid amides afforded the
corresponding amino acid anilides (3a and 3l to 3p)
with much better yields than bromobenzene (Table 2,
entries 1 and 3 to 7).
Scheme 2. A plausible mechanism for the CuI-catalyzed se-
lective C N cross-coupling of amino acid amides and iodo-
benzene.
À
Furthermore, the reaction could be scaled up to tion takes place to afford a new dicoordinated cop-
a gram scale without any problem (Table 2, entry 11). per(I) complex 7. Another amino acid amide mole-
As we known, copper can promote the N-arylation cule 1 drives the product 3 out of the catalytic cycle
of both amines and amides.[17] But why does the and forms the next copper(I) complex 4, which begins
copper catalyst selectively accomplish N-arylation of a new catalytic cycle.
amide amino groups of amino acid amides? The
In summary, we have discovered a direct, practical,
reason may be attributed in the stronger electron-do- cost-effective and atom-economic protocol for chemi-
À
nating property of the aliphatic amino groups and the cally selective copper-catalyzed selective C N cou-
stronger acidity of the amide amino groups. On the pling of chiral amino acid amides and aryl halides, he-
one hand, the aliphatic amino groups have a stronger tereoaryl halides and a vinyl bromide to synthesize
electron-donating property and thus stronger coordi- chiral amino acid anilides, which are common core
nating ability with copper than the amide amino structures for a number of biologically important mol-
groups. On the other hand, the amide amino groups ecules, chiral catalysts, organocatalysts and organic
possess stronger acidity than the aliphatic amino synthetic intermediates. Furthermore, amino acid
groups. Therefore the aliphatic amino group could amides with 99% ee were obtained, which means that
À
not be deprotonated in the presence of free amide no racemization occurred during the C N coupling. A
À
groups, and the amide amino groups were deprotonat- mechanism of CuI-catalyzed selective C N cross-cou-
ed and then reacted with aryl halides. Both of the rea- pling of the amide amino groups of chiral amino acid
sons results in the sole formation of amino acid ani- amides and iodobenzene was proposed.
lides.
A proposed mechanism is shown in Scheme 2. As
a start, a copper(I) iodide molecule encounters a sub-
strate amino acid amide molecule 1 under the reac-
Experimental Section
tion conditions, and then a copper(I) complex 4 of the
amino acid amide is formed, analogues of which have
reported.[21] The amide amino group of the complex 4
is facilely deprotonated by a K2CO3 molecule to pro-
duce a new copper(I) complex 5 with a s bond be-
tween copper and the amide amino group.[21a] And at
the same time KI and KHCO3 are released. When the
copper(I) complex 5 meets a PhI molecule, oxidative
Typical Procedure for the CuI-Catalyzed Coupling
Reaction of Amino Acid Amides with Aryl Halides
An oven-dried test tube with a stir bar was charged with l-
phenylalaninamide (1.2 mmol), K2CO3 (2.0 mmol), and CuI
(0.05 mmol). The test tube was sealed with a rubber stopper
and evacuated and refilled with argon for three times.
Under an argon atmosphere, iodobenzene (1.0 mmol) and
toluene (6 mL) were added by a syringe (if the aryl halides
are solid at room temperature, they were added at the same
addition occurs and a tetracoordinated copperACTHNUGTRNEUNG(III)
complex 6 is obtained.[22] And then reductive elimina- time with CuI and K2CO3). The tube was put into an oil
Adv. Synth. Catal. 2013, 355, 692 – 696
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asc.wiley-vch.de
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Junyu Dong et al.
COMMUNICATIONS
bath preheated at 1108C and the mixture kept stirring at
that temperature for 24 h. The reaction mixture was then
cooled to room temperature, quenched with water, and ex-
tracted with ethyl acetate (20 mL) for three times. The or-
ganic layers were combined, dried over anhydrous sodium
sulfate, and filtered. The filtrate was condensed under re-
duced pressure. The residue was purified by silica gel
column chromatography with a solution of petroleum ether
and ethyl acetate (1/5 to 2/1) to afford l-phenylalanine ani-
lide.
Assem, A. Natarajan, A. K. Yudin, J. Am. Chem. Soc.
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Procedures and analytical data for all compounds are
given in the Supporting Information.
[8] J. S. Wixey, B. D. Ward, Chem. Commun. 2011, 47,
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[9] E. E. Schallenberg, M. Calvin, J. Am. Chem. Soc. 1955,
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Acknowledgements
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We thank the Science and Technology Bureau of Sichuan
(Grant No. 2011HH0016), Incubation Program for Excellent
Innovation Team of Chengdu University of Technology (No.
HY0084), Opening Fund of State Key Laboratory of Geoha-
zard Prevention and Geoenvironment Protection (No.
SKLGP2012K005), the Education Bureau of Fujian Prov-
ince of China (No. JK2011030) and the Natural Science
Foundation of Fujian Province of China (No. 2012J06005)
for financial support. We appreciate sincerely Mr. Allan Che-
lashaw for his warm-hearted help.
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