Synthesis of polyfunctional secondary amines by the addition of functionalized zinc reagents to nitrosoarenes

Article abstract of DOI:10.1039/c4cc08846h

Addition of functionalized aryl, heteroaryl or adamantyl zinc reagents to various nitroso-arenes in the presence of magnesium salts and LiCl in THF produces after a reductive work-up with FeCl₂ and NaBH₄ in ethanol the corresponding polyfunctional secondary amines in high yields. This journal is

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Synthesis of polyfunctional secondary amines by the
addition of functionalized zinc reagents to nitroso-
arenes†
Cite this: DOI: 10.1039/x0xx00000x
Received 00th Month 2014,
Accepted 00th Month 2014
Vasudevan Dhayalan, Christoph Sämann and Paul Knochel*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Table 1 Synthesis of diarylamines of type 5 via the reaction of functionalized
arylzinc reagents 2aꢀj with nitrosobenzene 3a
Addition of functionalized aryl, heteroaryl or adamantyl zinc
reagents to various nitroso-arenes in the presence of magnesium
salts and LiCl in THF produces after a reductive work-up with
FeCl2 and NaBH4 in ethanol the corresponding polyfunctional
secondary amines in high yields.
Entry
Znꢀreagent
Product, Yield (%)a,b
2a,
NHPh
1
PhZnCl
5a
The preparation of arylamines is an important synthetic goal since
these compounds often have useful properties for pharmaceuticals or
material science applications.1 Transition metal catalyzed aminations
have been well studied,2 but the use of expensive and toxic metallic
catalysts reduces somewhat the utility of such synthetic methods.
Another approach has been the use of electrophilic nitrogen reagents
and their reactions with nonꢀexpensive and low toxic mainꢀgroup
: 85
FG
ZnCl
FG
NHPh
2
3
4
5
6
2b, FG = CF3
5b: 69
5c
c
2c
, FG = CO2Et
: 76
organometallics.3
A few years ago, we have reported that
2d, FG = tBu
5d: 96
5e: 70
5f: 78
functionalized arylmagnesium reagents add to nitrosoꢀarenes4 and
nitroꢀarenes.5 Although satisfactory yields were obtained, the high
reactivity of the carbonꢀmagnesium bond reduces the functional
group tolerance. Furthermore, nitrosoꢀarenes have proven to be
versatile reagents for performing nitroso aldol and related reactions.6
Herein, we wish to report a mild synthesis of diaryl or
heteroaryl(aryl)amines as well as functionalized highly sterically
hindered adamantyl(aryl)amines. Thus, the treatment of an arylzinc
2e, FG = SCH3
2f, FG = OCH3
HO
NHPh
NHPh
7
8
2g, FG = OTMS
d
5g:
5h:
89
BocO
BocO
ZnCl
derivative 2, prepared either by the direct insertion of Mg in the
7
presence of LiCl and ZnCl2 or by
a I/Mgꢀexchange with
iPrMgCl·LiCl8 followed by transmetalation with ZnCl2, with various
nitrosoꢀarenes of type 39 affords an intermediate zincated
hydroxylamine derivative 4 which after reductive workꢀup with
72
2h
O
O
9
ZnCl
OHC
NHPh
NHPh
Ar2NO 3
Mg, LiCl, ZnCl2
(1.0 equiv)
Ar1ꢀBr
Ar1ꢀZnCl—MgBrCl—LiCl
2ie
5i: 64f
THF, 0 to 25 °C
2ꢀ3 h
THF, 0 to 25 °C
1
2 (1.1 equiv)
O
O
10
ZnCl
H3COC
FeCl2 (2 equiv)
NaBH4 (1 equiv)
Ar2
f
2je
5j:
75
Ar1ꢀNHꢀAr2
up to 97% yield
Ar1
N
EtOH, 25 °C, 15 h
OZnX
a
General reaction conditions: arylzinc reagent (1.1 equiv), nitroso
b
4
5
electrophile (1.0 equiv), NaBH4 (1.0 equiv), FeCl2 (2.0 equiv). Yield of
analytically pure isolated product as determined by 1HꢀNMR analysis.
c
Scheme 1 Synthesis of polyfunctional secondary amines of type 5 via the
addition of functionalized zinc reagents of type 2 to various nitroso
compounds of type 3.
Prepared by I/Mgꢀexchange with iPrMgCl·LiCl.8 The TMSꢀgroup was
d
e
cleaved during workup and column chromatography purification. The
arylzinc reagents (2i and 2j) were prepared from the corresponding bromides
(see Supporting Information). f Obtained after removal of the ethylene glycol
group with CF3CO2H in CH2Cl2 (see Supporting Information).
Department of Chemistry, Ludwig-Maximilians-Universität München,
Butenandtstr. 5ꢀ13, 81377 Munich, Germany. Tel: +49ꢀ0)89ꢀ2180ꢀ77679,
Fax: +49ꢀ(0)89ꢀ2180ꢀ77680. Eꢀmail: paul.knochel@cup.uniꢀmuenchen.de
†Electronic Supplementary Information (ESI) available: Detailed
experimental procedures and spectroscopic data for all compounds. See
DOI: 10.1039/b000000x/
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DOI: 10.1039/C4CC08846H
Table 2 Synthesis of polyfunctional secondary amines of type 5 by the Table 2 Continued
addition of aryl and heteroaryl zinc reagents 2a-k to various nitroso
compounds 3a-g
Entry Znꢀreagent (Ar1) Electrophile (Ar2)
Product
Yield (%)a,b
EtO2C
14
2cd
3g
83
N
H
N
N
Me
5x
Me
N
Entry Znꢀreagent (Ar1) Electrophile (Ar2)
Product Yield (%)a,b
2k
3g
15
70
N
H
Me
Ph
NO
5y
1
2a
79
a
N
General reaction conditions: arylzinc reagent (1.1 equiv), nitroso
electrophile (1.0 equiv), NaBH4 (1.0 equiv), FeCl2 (2.0 equiv). Yield of
analytically pure isolated product as determined by HꢀNMR analysis. c The
TMSꢀgroup was cleaved during the workup and column chromatography
purification. d Prepared by I/Mgꢀexchange with iPrMgCl·LiCl.8
Ph
b
H
3b
3c
5k
1
NO
Me
2
2a
N
H
77
83
97
Me
5l
FeCl2 and NaBH4 in ethanol (25 oC, 15 h) produce the corresponding
secondary amines of type 5 in excellent yields (Scheme 1). A range
of functional groups have been tolerated in the starting arylzinc
reagent as shown in Table 1.
Thus, PhZnCl (1.1 equiv) prepared by the direct insertion of Mg
in the presence of LiCl and ZnCl2 reacts with nitrosobenzene 3a (1.0
CO2Et
EtO2C
NO
3
4
2a
N
H
CO2Et
CO2Et
3d
5m
5n
nꢀBu
CO2Et
NO
ZnCl
ZnCl
o
equiv) at 25 C within 2ꢀ3 h and produces after reductive workꢀup
with FeCl2 (2.0 equiv) and NaBH4 (1.0 equiv) in ethanol (25 C, 15
N
H
o
EtO2C
n-Bu
n-Bu
3e
3b
2i
2i
h) the corresponding diphenylamine 5a in 85% yield (Table 1, entry
1).10a The presence of both Mg salts and LiCl were found to be
essential for achieving a high yield. A variety of arylzinc reagents
prepared similarly were used in the addition to 3a. Both electron
withdrawing and donating groups can be attached at the aryl ring
(Table 1, entries 2ꢀ8).10bꢀg Arylzinc reagent 2c has been prepared via
an I/Mgꢀexchange,8 its reaction with nitrosobenzene (3a) furnishes
the corresponding secondary amine 5c in 76% yield (Table 1, entry
3). Although sensitive functional groups like a formyl or an acetyl
group are not tolerated, the corresponding bromoacetal (1i) or
bromoketal (1j) are readily converted to the zinc reagents (2i and 2j)
n-Bu
Ph
5
6
90
81
N
H
5o
5p
H3CO
CO2Et
ZnCl
ZnCl
3e
N
H
H3CO
2f
OCH3
H3CO
H3CO
H3CO
H3CO
CO2Et
CO2Et
7
8
3e
3e
97
7
by the insertion of Mg in the presence of LiCl and ZnCl2 . The
addition of nitrosobenzene (3a) provides after removal of the
ethylene glycol protecting group (CF3CO2H in CH2Cl2 at 25 C for
5ꢀ8 h) the secondary amines (5i and 5j) in 64-75% yield (Table 1,
N
H
OCH3
5q
o
2j
HO
ZnCl
97c
63
entries 9 and 10).
N
H
TMSO
ClZn
2g
5r
Table 3 Synthesis of tertiaryalkyl(aryl)amines 7a-d by the addition of
tertiary alkylzinc reagents 6a and 6b to nitroso compounds 3a, 3f and 3h
H3C
N
Product,Yield (%)a,b
9
3a
3d
Entry
Znꢀ reagent
Electrophile
N
N
CH3
H
5s
2k
2k
3a
1
ZnCl
CO2Et
H3C
N
H
55
N
6ac
10
7a
: 50
N
H
CO2Et
OCH3
R
5t
H3C
ZnCl
NO
N
H
N
60
67
2k
2a
11
12
H3CO
N
H
2
3
6b
6b
3a
7b
: R = H (89)
3f
5u
5v
Me2N
NO
7c: R = NMe2 (71)
3h
3f
N
H
N
N
Me
Me
Me
N
NO
7d
: R = OMe (56)
4
6b
3g
H3CO
96
a
General reaction conditions: alkylzinc reagent (1.1 equiv), nitroso
electrophile (1.0 equiv), NaBH4 (1.0 equiv), FeCl2 (2.0 equiv). Yield of
analytically pure isolated product as determined by 1HꢀNMR analysis.
13
2f
3g
b
N
H
c
5w
Prepared by transmetalation of commercially available tꢀBuMgCl with ZnCl2.
2 | J. Name., 2014, 00, 1-3
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DOI: 10.1039/C4CC08846H
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1ꢀ8).5a,10hꢀk Noteworthy, a heterocyclic zinc reagent (2k) has also
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a
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heteroaryl(aryl)ꢀamines 5s-y in 55ꢀ96% yield (Table 2, entries 9ꢀ15).
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adamantylzinc chloride (6b)13 add to various nitrosoꢀarenes under
similar reaction conditions producing otherwise difficult to prepare
tertiaryalkyl(aryl)amines 7a-d in 50ꢀ89% yield (Table 3, entries 1ꢀ
4).14
In summary, we have shown that aryl, heteroaryl or adamantyl
zinc reagents add to various nitrosoꢀarenes in the presence of Mgꢀ
salts and LiCl. Both Mg and Li salts are necessary to achieve high
yields for the synthesis of the corresponding functionalized
secondary amines. Further extensions of this work are currently
underway in our laboratories.
The research leading to these results has received funding from
the European Research Council under the European Community’s
Seventh Framework Programme (FP7/2007−2014) ERC grant
agreement no. 227763. We thank the Fonds der Chemischen
Industrie for financial support. We also thank Heraeus Holding
GmbH (Hanau), Rockwood Lithium (Frankfurt), and BASF SE
(Ludwigshafen) for the generous gift of chemicals.
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4CC08846H
TOC Graphics:
Aryl, heteroaryl or adamantyl zinc reagents add in the presence
of magnesiumꢀsalts and LiCl to various aryl or heteroaryl nitroso
arenes producing after reductive workꢀup polyfunctional
secondary amines.
4 | J. Name., 2014, 00, 1-3
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