Highly efficient catalytic system for electrophilic amination of arylzinc reagents

Article abstract of DOI:10.1002/aoc.1574

The effect of catalyst on the yield of amine in the amination of three classes of arylzinc reagents with acetone oxime O-tosylate was investigated. Since they allowed the preparation of arylamines in excellent yields in the presence of a minimum amount of copper (I) or copper (II) compounds, the catalytic systems using copper (I) or copper (II) combined with a P-, N- or S-donor ligand were revealed to be the best catalysts for the electrophilic amination of arylzinc reagents with acetone oxime O-tosylate in the presence of DMPU. Copyright

Full text of DOI:10.1002/aoc.1574

Full Paper
Received: 14 August 2009
Revised: 20 September 2009
Accepted: 20 September 2009
Published online in Wiley Interscience: 6 November 2009
(
www.interscience.com) DOI 10.1002/aoc.1574
Highly efficient catalytic system
for electrophilic amination of arylzinc reagents
∗
Tahir Da s¸ kapan and Sel c¸ uk Koca
The effect of catalyst on the yield of amine in the amination of three classes of arylzinc reagents with acetone oxime O-tosylate
was investigated. Since they allowed the preparation of arylamines in excellent yields in the presence of a minimum amount of
copper (I) or copper (II) compounds, the catalytic systems using copper (I) or copper (II) combined with a P-, N- or S-donor ligand
were revealed to be the best catalysts for the electrophilic amination of arylzinc reagents with acetone oxime O-tosylate in the
presence of DMPU. Copyright ꢀc 2009 John Wiley & Sons, Ltd.
Keywords: electrophilic amination; organozinc reagents; oximes; amines; cosolvent; dipolar aprotic solvent; ligand; catalysis; copper
catalysis; C–N bond
Introduction
solvent was investigated to develop a less harmful and more
effective catalyst. For this reason, various copper (I) and copper
Organozinc compounds are an important class of main-group
organometallic reagents.
(II) compounds [CuI, CuCl, CuBr, CuCN, CuSCN, CuCl2, CuBr2, and
[
1–3]
The bond between carbon and zinc
Cu(SCN)2] were screened as catalysts and the effects of P-, S-
and N-donor ligands [Me2S, PPh3, NPh3, P(n-Bu)3] on the catalytic
activity of copper (I) and copper (II) catalysts were examined in
the electrophilic amination of bromomagnesium triarylzincate,
diarylzinc, and arylzinc chloride with 1 (Scheme 1). Compound 1
hasacovalentcharacterandthereforetheyshowmodestreactivity
towards many electrophilic reagents. This characteristic allows
them to tolerate numerous functional groups. For this reason,
we decided to focus on the preparation of primary arylamines
which are key intermediates in the synthesis of various practically
[
4]
2
is an sp -N containing amination reagent. Arylzinc chlorides, di-
[
5,6]
important organic compounds
by electrophilic amination of
arylzincs and bromomagnesium triarylzincates were prepared by
addition of 1, 2 or 3 mol equiv. of corresponding arylmagnesium
bromide in THF, respectively, to a solution of 1 mol equiv. of zinc
ordinary organozinc reagents with oximes. The reactivity of
organozinc reagents can be increased by using transition metal
catalysts. Even though copper catalysis has been known to
promote the reactions of organozinc reagents, the amination
reaction of ordinary organozinc reagents with oximes in the
presence of copper catalyses was not effective enough to provide
◦
chloride in THF at −3 C. DMPU [1,3-dimethyl-3,4,5,6-tetrahydro-
[41]
2(1H)-pyrimidinone], which is known as a green solvent,
was
used as cosolvent. It is a dipolar aprotic solvent that coordinates to
metalions, thereforeincreasingthenucleophilicityoftheircounter
ions. It can easily be removed from reaction mixture by simply
washing with water. The final product, arylamine, was isolated as
[
7]
the preparation of primary aryl amines in high yields.
Various methods have been developed for aryl C–N bond
formation.[
8–15]
These methods require harsh reaction conditions
[42]
its N-benzoyl derivative that was identified by its melting point
(
i.e. high temperatures, the usage of strong base, long reaction
and FT-IR spectrum.
times, etc.), their scope is limited and they contain numerous
synthetic steps. On the other hand, electrophilic amination of
an easily available and functional group-tolerant organometallic
Results and Discussion
[
16–20]
reagent appeared to be a viable alternative method.
Among a few electrophilic amination processes reported to
Optimal reaction conditions and the best catalytic systems were
determined using phenylzinc reagents. Phenylzinc chloride and
diphenylzinc did not react with 1 under catalyst-free conditions,
but bromomagnesium triphenylzincate reacted to give aniline in
low yield (21%) after 1.5 h.[ Treatment of bromomagnesium
triphenylzincate, diphenylzinc and phenylzinc chloride with 1 in
the presence of CuCN afforded aniline in 61, 58 and 26% yields,
respectively. When CuCN-catalyzed aminations of diphenylzinc
date,[
21–29]
there have been only four successful methods for
the preparation of primary arylamines using ordinary organozinc
reagents.[
30–33]
These methods allow the synthesis of primary
40]
arylamines in moderate to good yields. Knochel’s group has
shown that alkylzinc reagents react with functionalized arylazo
tosylates to form secondary amines in moderate to good yields.[34]
The synthesis of tertiary amines by electrophilic amination of
organozincchlorides[35] anddiorganozincreagents[36–38] hasbeen
described by Johnson’s group.
◦
and bromomagnesium triphenylzincate were carried out at 40 C,
Recently, we have developed a cosolvent-promoted elec-
trophilic amination method employing ordinary arylzinc reagents
∗
Correspondence to: Tahir Da ¸s kapan, Ankara University Science Faculty,
Department of Chemistry, 06100 Besevler, Ankara, Turkey.
E-mail: daskapan@science.ankara.edu.tr
[
39,40]
for the preparation of arylamines in high yields.
In this work,
the effect of catalyst on the yield of amine in the electrophilic
amination of three classes of ordinary arylzinc reagents with
acetone oxime O-tosylate (1) in the presence of DMPU as co-
Ankara University, Science Faculty, Department of Chemistry, 06100 Be s¸ evler,
Ankara, Turkey
Appl. Organometal. Chem. 2010, 24, 12–16
Copyright ꢀc 2009 John Wiley & Sons, Ltd.

Electrophilic amination of arylzinc reagents
THF, Catalyst,
DMPU, r.t., 1.5h
1. Concd HCl
2. PhCOCl
ArM + (CH ) C=NOTs
(CH ) C=NAr
PhCONHAr
3
2
3 2
1
M
: ZnCl, 1/2 Zn, 1/3 ZnMgBr
: Phenyl, 1-naphthyl, 4-tolyl, 4-anisyl, 3-chlorophenyl, 4-thioanisyl, 2,5-xylyl
Ar
Catalyst : CuCN, CuCl, CuBr, CuI, CuSCN,CuBr , CuCl ,Cu(SCN)
2
2
2
Cu (I)/Ligand, Cu (II)/Ligand
Ligand : Me S, n-Bu P, Ph N, Ph P
2
3
3
3
Scheme 1. Electrophilic amination of arylzinc reagents with 1.
product yield was not improved (56 and 64%, respectively).
However, when the reaction temperature was raised to 40 C for
Table 1. High-yielding electrophilic amination of Ph3ZnMgBr with 1
◦
the CuCN-catalyzed electrophilic amination of phenylzinc chloride
with 1, the yield of aniline improved to 54%.
b
1
. THF, DMPU , Cu (I or II),ligand, r.t., 1.5 h.
. Concd. HCl
a
Ph ZnMgBr + 1
PhNH2
3
2
Copper (I)-catalyzed amination of phenylzinc reagents resulted
in high yields in the presence of DMPU (Tables 1–3). The opti-
mal amount of copper (I) catalysts was determined as 5 mol% for
bromomagnesium triphenylzincate and 10 mol% for diphenylz-
inc. A 5 mol% CuCN-catalyzed amination of bromomagnesium
triphenylzincate resulted in 80% yield after 1 h. CuCN and CuBr,
which provided aniline in excellent yields, were the best copper
Entry
Catalyst, %
Ligand, %
PhNH2, %
1
2
3
4
5
6
7
8
9
CuCN, 5
–
90
75
71
75
58
74
60
91
66
84
80
64
75
81
75
82
90
83
92
82
93
91
91
87
CuCN, 2.5
CuSCN, 5
CuI, 5
–
–
–
CuI, 2.5
–
(I) catalysts for the electrophilic amination of bromomagnesium
CuCl, 5
–
triphenylzincate (Table 1, entries 1 and 8). CuCN, CuBr, CuCl and
CuSCN showed almost equal catalytic activity in the amination
of diphenylzinc with 1 and aniline was prepared in high yield by
the use one of these catalysts in 10 mol%. In the case of amina-
tion of phenylzinc chloride, the best results were accomplished
by CuCN and CuCl, as shown in entries 1 and 7 of Table 3. They
allowed preparation of aniline in high yields even when they were
used at 2.5 mol% (Table 3, entries 2, 8). The yield of aniline was
found as 80% after 1 h with 5% CuCN catalysis. CuSCN afforded
aniline in high yield when it was used in 7.5 mol%. While CuI
allowed preparation of aniline in good yield in the amination of
bromomagnesium triphenylzincate (Table 1, entry 4), it was not
so effective in the amination of diphenylzinc (Table 2, entry 6) and
phenylzinc chloride (Table 3, entries 9 and 10).
CuCl, 2.5
CuBr, 5
–
–
CuBr, 2.5
Cu(SCN)2, 2.5
CuBr2, 2.5
CuBr2, 1.25
CuCl2, 2.5
CuBr, 2.5
CuCl, 1.25
CuI, 2.5
–
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
–
–
–
–
Me2S, 6
Ph3P, 1.25
n-Bu3P, 2.5
Ph3P, 1.25
Ph3N, 1.25
n-Bu3P, 2.5
n-Bu3P, 1.25
Me2S, 6
Ph3N, 2.5
Ph3P, 2.5
n-Bu3P, 2.5
CuI, 1.25
CuI, 1.25
CuBr2, 2.5
CuBr2, 1.25
CuBr2, 2.5
CuBr2, 2.5
CuBr2, 2.5
CuCl2, 2.5
Investigationoftheeffectofcopper(II)catalysisintheamination
of phenylzinc reagents using CuCl2, CuBr2 and Cu(SCN)2 showed
that these compounds were more efficient than corresponding
copper (I) compounds in the amination of bromomagnesium
triphenylzincate (Table 1). When the yield of aniline is taken into
consideration, CuBr seems to be more effective than CuBr2 in the
aminationofbromomagnesiumtriphenylzincate(Table 1,entries8
and11).Sincetheefficiencyofacatalystismeasuredbyitsturnover
a
b
Ph3ZnMgBr : 1 = 1 : 1. DMPU : Ph3ZnMgBr = 1.5.
[
43]
number(TON=mmolofproduct/mmolofcatalyst )andtheTON
ofCuBr2 (33.2)ishigherthantheTONofCuBr(19), CuBr2 appearsto
be the preferred catalyst for the amination of bromomagnesium
triphenylzincate. Copper (II) catalysts were less efficient in the
amination of phenylzinc chloride as compared with the amination
of bromomagnesium triphenylzincate (Table 1, entries 10–13 vs
Table 3, entries 11–15). In the case of diphenylzinc, Cu(SCN)2
allowed the preparation of aniline in good yield; however, CuBr2
and CuCl2 were not effective and the yield of desired product
was low with these catalysts (Table 2, entries 9 and 10). For this
reason we planned to use a donor ligand with these two catalysts.
We thought that coordination of a ligand to copper atom in
the organocopper, which is accepted to be formed in catalytic
amount during the reaction, would allow phenyl group to be
transferred easily. The positive results (Table 2, entries 15–18)
showed that we made an accurate decision. Excellent yields of
amine were achieved by performing the reaction in the presence
ofCuBr2 –Ph3PorCuCl2 –Ph3Pcatalyticsystems(entries15and 18).
Aftertheseresultswedecidedtousealigandwithcopper(I) and
copper (II) halides in the amination of all three of phenylzinc
reagents. As previously mentioned, except for CuI, the other
copper (I) halides catalyzed the amination of three types of
phenylzinc reagents and copper (II) halides catalyzed amination
of bromomagnesium triphenylzincate and phenylzinc chloride to
afford aniline in good to high yields. Nevertheless, reducing the
amount of catalyst and/or increasing the productivity would be
an important development in terms of green chemistry. In some
cases, despite the dramatic decrease in the quantity of copper
(I) halides, serious increases were observed in the product yield
by the use of S-, N- or P-donor ligands (Table 1, entries 15, 17
and 18; Table 2, entries 12–14; Table 3, entry 16). For example,
Appl. Organometal. Chem. 2010, 24, 12–16
Copyright ꢀc 2009 John Wiley & Sons, Ltd.
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T. Da s¸ kapan and S. Koca
Table 2. High-yielding electrophilic amination of Ph2Zn with 1
Table 3. High-yielding electrophilic amination of PhZnCl with 1
b
b
1
. THF, DMPU , Cu (I or II), ligand, r.t., 1.5 h.
. Concd. HCl
1. THF, DMPU , Cu (I or II), ligand, r.t., 1.5 h.
a
a
Ph Zn + 1
PhNH2
PhZnCl + 1
PhNH2
2
2
2. Concd. HCl
Catalyst, %
CuCN, 5
Entry
Catalyst, %
Ligand, %
PhNH2, %
Entry
Ligand, %
PhNH2, %
1
2
3
4
5
6
7
8
9
CuCN, 10
CuCN, 5
–
83
66
87
70
80
62
78
77
54
51
90
78
84
86
90
83
82
90
1
–
94
80
86
69
79
75
95
87
51
47
95
71
75
70
76
68
85
88
–
2
CuCN, 2.5
CuSCN, 7.5
CuSCN, 5
CuBr, 7.5
CuBr, 5
–
CuCl, 10
–
3
–
CuCl, 2.5
CuBr, 10
–
4
–
–
–
5
–
CuI, 10
6
–
CuSCN, 10
Cu(SCN)2, 2.5
CuBr2, 2.5
CuCl2, 2.5
CuI, 2.5
–
7
CuCl, 5
–
–
8
CuCl, 2.5
CuI, 10
–
–
9
–
10
11
12
13
14
15
16
17
18
–
10
11
12
13
14
15
16
17
18
CuI, 5
–
n-Bu3P, 2.5
n-Bu3P, 1.25
Ph3N, 2.5
Me2S, 6
Ph3P, 2.5
Me2S, 6
n-Bu3P, 2.5
Ph3P, 2.5
Cu(SCN)2, 5
Cu(SCN)2, 2.5
CuCl2, 5
–
CuI, 1.25
CuCl, 2.5
CuCl, 2.5
CuCl2, 2.5
CuBr2, 2.5
CuBr2, 2.5
CuBr2, 2.5
–
–
–
CuCl2, 2.5
CuBr2, 2.5
CuI, 1,25
CuCl2, 2.5
CuBr2, 2.5
–
n-Bu3P, 1.25
n-Bu3P, 2.5
n-Bu3P, 2.5
a
b
a
b
Ph2Zn : 1 = 1 : 1. DMPU : Ph2Zn = 2.
PhZnCl:1 = 1.5 : 1. DMPU : PhZnCl = 1.5.
while CuI was the least active copper (I) catalyst in the absence of
the ligand, it became the best copper (I) catalyst for amination of
diphenylzinc with 1 by the use of 2.5 mol% n-Bu3P (Table 2, entries
was prepared in good to high yields; and (3) ligands have
an accelerating effect on the copper-catalyzed electrophilic
amination of arylzinc reagents with 1 in the presence of DMPU.
They caused a significant reduction in the quantity of copper
catalystsandaremarkableincreaseintheyieldofamine.Therefore,
the catalytic systems using copper (I) or copper (II) combined with
a P-, N-, or S-donor ligand were revealed to be the best catalysts
for the electrophilic amination of ordinary arylzinc reagents with
1 in the presence of DMPU.
1
1 and 12). Aniline was obtained in 74% yield after 30 min by the
use of CuI (1.25 mol%)–Ph3N(1.25 mol%) catalyst system in the
amination of bromomagnesium triphenylzincate. In the case of
copper (II) halide-catalyzed amination of phenylzinc chloride and
bromomagnesium triphenylzincate, ligands did not change the
quantity of catalysts in many cases but they lead to a significant
increase in the efficiency (Table 1 and Table 3).
We investigated the amination of naphtylzinc and some
functionalized arylzinc reagents with 1 using various catalytic
systems to broaden he scope of this method (Table 4). As seen,
arylamines bearing electron-donating or electron-withdrawing
groups at the ortho-, meta- or para-positions were synthesized in
good to high yields.
Experimental
General Procedures
We also conducted control experiments to investigate whether
ligands have any accelerating effect on the amination of arylzinc
reagents with 1 under copper-free conditions. For this reason we
usedPh3Pasligandandwecarriedoutthereactioninthepresence
All reactions involving organozinc reagents were performed
in flame-dried glassware with standard syringe/cannula
[44]
techniques
under an atmosphere of dry, oxygen-free argon.
Melting points were determined on a Gallencamp capillary melt-
ing point. IR spectra were recorded on a Perkin-Elmer Spectrum
◦
and in the absence of DMPU at room temperature and at 40 C.
Addition of Ph3P did not have any effect on the reaction yield
in the amination of phenylzinc chloride and diphenylzinc. In the
case of amination of bromomagnesium triphenylzincate at room
temperature, the yield of aniline increased only from 21 to 48%.
1
00 FT-IR spectrometer.
THF was freshly
distilled
from
solution
of
sodium–benzophenone under dry argon. DMPU was dis-
tilled under reduced pressure prior to use and kept over molecular
sieves (4 Å 4–8 mesh) and under argon atmosphere. Copper
(
I) and copper (II) salts were purified prior to use.[
45,46]
Conclusions
Organomagnesium bromides were prepared in THF by con-
Insummary, thiscurrentstudysuggeststhat:(1) P-, N-, andS-donor
ligands did not have any facilitating effects on the amination of
arylzinc reagents with 1 under copper free conditions; (2) it is
necessary to use DMPU in the copper-catalyzed amination of
arylzinc reagents with 1 – in this way, desired product arylamine
ventional standard methods and their concentrations were deter-
mined by the method of Watson and Eastham.[ Acetone oxime
O-tosylate 1 was prepared as described in the literature
and stored for months without any decomposition. Ligands were
commercially available and used without further purification.
47]
[
48,49]
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Copyright ꢀc 2009 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 12–16

Electrophilic amination of arylzinc reagents
Table 4. Electrophilic amination of arylzinc reagents with 1
b
1
. THF, DMPU , Cu (I or II), ligand, r.t., 1.5 h.
. Concd. HCl
a
ArM + 1
ArNH2
2
Entry
M
Ar
Catalyst, %
Ligand, %
ArNH2, %
1
2
3
4
5
6
7
8
9
ZnCl
ZnCl
ZnCl
4-CH3S-C6H4
4-CH3O-C6H4
3-Cl-C6H4
CuSCN, 7.5
CuI, 1.25
CuI, 1.25
CuCl, 2.5
CuI, 2.5
–
75
62
65
80
84
74
80
81
84
84
82
84
85
82
83
74
n-Bu3P, 1.25
n-Bu3P, 1.25
Ph3N, 2.5
n-Bu3P, 2.5
Ph3P, 2.5
n-Bu3P, 2.5
Me2S, 6
1/2 Zn
1-C10H7
1/2 Zn
2, 5 − (CH3)2-C6H3
4-CH3S-C6H4
2, 5 − (CH3)2-C6H3
3-Cl-C6H4
1/2 Zn
CuCl2, 2.5
CuCl2, 2.5
CuBr2, 2.5
CuBr2, 2.5
CuCN, 5
1/2 Zn
1/2 Zn
1/2 Zn
4-CH3O-C6H4
4-CH3-C6H4
4-CH3O-C6H4
1-C10H7
Ph3P, 2.5
–
1
1
1
1
1
1
1
0
1
2
3
4
5
6
1/3 ZnMgBr
1/3 ZnMgBr
1/3 ZnMgBr
1/3 ZnMgBr
1/3 ZnMgBr
1/3 ZnMgBr
1/3 ZnMgBr
CuCN, 5
–
CuCl, 2.5
CuBr2, 2.5
CuBr2, 2.5
CuBr2, 2.5
CuI, 2.5
Ph3N, 2.5
Me2S, 2.5
Ph3N, 2.5
Ph3P, 2.5
n-Bu3P, 2.5
3-Cl-C6H4
4-CH3O-C6H4
3-Cl-C6H4
4-CH3-C6H4
a
b
ArZnCl : 1 = 1.5 : 1, Ar2Zn : 1 = 1 : 1, Ar3ZnMgBr : 1 = 1 : 1. DMPU : ArZnCl = 1.5, DMPU : Ar2Zn = 2, DMPU : Ar3ZnMgBr = 1.5.
Copper (I or II)/Ligand-catalyzed Amination of Arylzinc
Reagents
Amination of RZnCl
A solution of ZnCl2 (0.4095 g, 3 mmol) in anhydrous THF (6 ml) was
◦
Amination of Ar3ZnMgBr
cooled to −3 C under argon atmosphere and phenylmagnesium
bromide (4.5 mmol) in THF was added dropwise by syringe. The
reaction mixture was stirred for an additional 5 min, the cooling
bath was removed and the resulting suspension was allowed to
warm to room temperature. To this mixture, copper (I or II) catalyst
A solution of ZnCl2 (0.4095 g, 3 mmol) in anhydrous THF (6 ml)
◦
was cooled to −3 C under argon atmosphere and arylmagnesium
bromide (9 mmol for Ph3ZnMgBr) in THF was added dropwise by
syringe. The reaction mixture was stirred for an additional 5 min,
the cooling bath was removed and the resulting suspension
was allowed to warm to room temperature. To this mixture,
copper (I or II) catalyst (1.25–2.5 mmol%), ligand (1.25–6 mmol%),
DMPU (1.5 equiv., 1.5 mmol) and a solution of 1 (3 mmol) in
dry THF (4 ml) were added. The reaction mixture was stirred at
room temperature for 1.5 h and then worked up by addition of
concentrated HCl with stirring at room temperature for 4 h. The
aqueous phase was washed with diethyl ether (2 × 50 ml), made
basic with concentrated NaOH and the free amine was extracted
with diethyl ether (3 × 100 ml). The organic layer was dried over
(
1.25–2.5 mmol%), ligand (1.25–2.5 mmol%), DMPU (PhZnCl 1.5
equiv., 2.25 mmol) and a solution of 1 (3 mmol) in dry THF (4 ml)
were added. The reaction was carried out and worked up in the
same way as in the previous section.
Acknowledgments
Financial support from Ankara University Research Foundation
(grant no. 06B4240002) is greatly acknowledged by the authors.
Na2SO4, the solvent was evaporated and the crude product was References
converted to its N-benzoyl derivative by reaction with benzoyl
chloride in the presence of NaOH. The product was recrystallized
from ethanol : water (4 : 1).
[
[
[
[
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◦
cooled to −3 C under argon atmosphere and phenylmagnesium
[
[
[
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bath was removed and the resulting suspension was allowed
to warm to room temperature. To this mixture, copper (I or
II) catalyst (1.25–2.5 mmol%), ligand (1.25–6 mmol%), DMPU (2
equiv., 2 mmol) and a solution of 1 (3 mmol) in dry THF (4 ml) were
added. The reaction was carried out and worked up in the same
way as in the previous section.
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Copyright ꢀc 2009 John Wiley & Sons, Ltd.
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[
[
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