Trans-spanned palladium catalyst for mild and efficient amination of aryl halides with benzophenone imine

Article abstract of DOI:10.1055/s-2008-1032142

A new protocol for palladium-catalyzed Buchwald-Hartwig amination of aryl chlorides and bromides with benzophenone imine as ammonia surrogate is described. The suggested reaction conditions are mild and may therefore be applied to a variety of sensitive starting materials. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1032142

PAPER
537
Trans-Spanned Palladium Catalyst for Mild and Efficient Amination of Aryl
Halides with Benzophenone Imine
New
P
alladium Ca
l
talyst
f
g
or
E
fficient
Amination of
A
ry
G
l
H
alides rossman,a Karola Rueck-Braun,b Dmitri Gelman*a
a
Department of Organic Chemistry, The Hebrew University of Jerusalem, Givat Ram, 91904 Jerusalem, Israel
E-mail: dgelman@chem.ch.huji.ac.il
b
Technische Universität Berlin, Institut für Chemie, Straße des 17. Juni 135, 10623 Berlin, Germany
Received 30 September 2007; revised 22 November 2007
yielding and hardly reproducible (15–30% based on the
commercial 1,8-dinitroanthraquinone). In addition, it is
accompanied by laborious purification workup.9 Consid-
ering this, palladium-catalyzed amination of 1,8-dihalot-
Abstract: A new protocol for palladium-catalyzed Buchwald–
Hartwig amination of aryl chlorides and bromides with benzophe-
none imine as ammonia surrogate is described. The suggested reac-
tion conditions are mild and may therefore be applied to a variety of
sensitive starting materials.
riptycene – a readily available starting material10b,11
–
appears as the most straightforward route. In addition, the
cross-coupling strategy allows more synthetic flexibility
for the modification of the frame.
Key words: trans-spanning ligands, aminations, palladium-cata-
lyzed, triptycene
Unfortunately, our first experiments aiming at a double
amination of 1,8-dibromotriptycene10b by different am-
monia surrogates using published protocols encountered
unexpected difficulties – unreasonably incomplete con-
version, high percentage of hydrogenolyzed by-products,
and formation of unidentified substances were observed
when we used LiHMDS or NaHMDS as ammonia equiv-
alents and commercially available catalysts.7e,12
The N-aryl moiety represents an important motif in natu-
ral products,1 biologically relevant compounds,2 poly-
mers,3 and materials.4 Classical methods for the synthesis
of N-aryl compounds proved to be very limited in selec-
tivity and scope.5 However, during the past decade, great
progress has been made in the development of the palladi-
um-catalyzed amination of aryl halides and pseudoha-
lides.6 A wide range of transition-metal complexes have
been shown to successfully catalyze arylation of primary
and secondary amines, as well as of ammonia and ammo-
nia equivalents.7
At this point, we performed a series of experiments in or-
der to discover optimal reaction conditions for the desired
transformation. At the first stage, optimization experi-
ments were focused on the following variables: i) ammo-
nia surrogates (freshly prepared vs. commercially
available LiHMDS and NaHMDS salts, NaNH2, LiNH2
and benzophenone imine/base), and ii) solvents (toluene,
dioxane, and THF) in the presence of three different cata-
lysts – Pd2(dba)3/Xantphos (4),13a–c Pd2(dba)3/X-Phos,14
and Pd2(dba)3/2-dicyclohexylphosphinobiphenyl.7e On
the whole, we found that bidentate Xantphos (4) shows
superior reactivity over the monodentate ligands and ‘op-
timizable’ results were obtained when benzophenone
During the course of our studies focusing on the develop-
ment of a new family of bis-imine binucleating ligands,8
we became interested in the preparation of triptycene-1,8-
diamine as a key intermediate. Both catalytic and noncat-
alytic strategies toward the target were considered
(Scheme 1).
The noncatalytic route requires the synthesis of an-
thracene-1,8-diamine as a precursor – a valuable, known,
but somewhat problematic compound. Its synthesis is low
Buchwald–Hartwig
amination
Diels–Alder
reaction
NH
HN
PG
PG
H2N
Hal
H2N
Hal
binucleating
ligands
Scheme 1 Possible syntheses of 1,8-diaminotriptycenes
SYNTHESIS 2008, No. 4, pp 0537–0542
x
x
.
x
x
.2
0
0
8
Advanced online publication: 31.01.2008
DOI: 10.1055/s-2008-1032142; Art ID: T15107SS
© Georg Thieme Verlag Stuttgart · New York

538
O. Grossman et al.
PAPER
Table 1 Initial Optimization Experiments
[Pd]/L (2 mol%), benzophenone
imine (2.5 equiv), NaOt-Bu (4 equiv)
toluene, 100 °C, 24 h
N
Br
Br
Ph2C
N
CPh2
Ph2P
PPh2
PPh2
PPh2
PPh2
Fe
O
Ph2P
(i-Pr)2P
P(i-Pr)2
BINAP (1)
DPPF (2)
Xantphos (4)
1,8-bisdiisopropyl-
phosphinotriptycene (3)
Entry
Pd precursor
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd(OAc)2
Pd(OAc)2
Ligand
Conversion (%)a
Yield (%)a
n/a
1
2
3
4
5
6
1
2
3
4
3
4
10
–
n/a
99
99
90
27
70b
93b
n/a
n/a
a Based on crude NMR analysis; n/a = not applicable.
b Missing percentage refers to monosubstituted and dehalogenated species.
imine/t-BuONa in toluene was used as an ammonia zophenone imine adduct after 24 hours at 100 °C
source.
(Table 2, entry 1). Of course, there was no need for high
palladium loading in the small-scale reactions.
At the second stage, we refined the reaction conditions by
choosing the right catalyst precursor/bidentate ligand Encouraged by these results, we decided to test the cata-
combination – PdCl2, Pd(OAc)2, and Pd2(dba)3 in combi- lyst on other substrates. It was found that, the transforma-
nation with the bidentate BINAP (1) (original Buchwald tion was successful with aryl chlorides, bromides, and
protocol),7a DPPF (2),13d Xantphos (4), and 1,8-bisdiiso- functionalized and functionless aromatic and heteroaro-
propylphosphinotriptycene (3) were tested (Table 1). The matic compounds (the results are shown in Table 2). In
last trans-spanning ligand has been used by us in different addition to the already described dibromotriptycene, other
transformations previously.10b,11
fascinating compounds can be efficiently aminated by
benzophenone imine in the presence of the Pd2(dba)3/3
catalyst. Entries 2–5, for example, represent important
building blocks for construction of sensors (anthracene-
We found that only Pd2(dba)3/Xantphos (4) and the anal-
ogous Pd2(dba)3/1,8-bisdiisopropylphosphinotriptycene
(3) led to a complete conversion of the starting 1,8-dibro-
motriptycene. However, the latter, demonstrated some-
what cleaner and faster reaction toward the desired
bisaminated product. In addition, unlike Xantphos (4), al-
most complete conversion (up to 90%) was achieved in
1,8-diamine),15
biologically
active
compounds
(thiophene-2,5-diamine),16 and molecular switches (in-
dole derivatives).17 It is worth noting that their synthesis
using other methods is complicated.
the presence of catalyst derived from Pd(OAc)2 and 1,8- Surprisingly, the suggested catalyst can tolerate function-
bisdiisopropylphosphinotriptycene (3). Therefore, it was al groups containing active hydrogens. So, unlike the orig-
decided to employ this catalyst in the desired transforma- inal Buchwald protocol7a unprotected anilines form
tion.
NH
We were glad to observe that under the chosen conditions
(2 mol% of Pd2(dba)3, 4 mol% of the ligand, 2.3:1 ben-
zophenone imine/1,8-dibromotriptycene ratio, and t-
BuONa in toluene as a base) the reaction was perfectly
scalable (up to 5 g) and yielded 87% of the desired ben-
Br
PhNH2
Ph
Ph
slow
fast
Scheme 2 Selective amination of bromobenzene with benzophe-
none imine
Synthesis 2008, No. 4, 537–542 © Thieme Stuttgart · New York

PAPER
New Palladium Catalyst for Efficient Amination of Aryl Halides
539
Table 2 Representative Amination Results
Entry
Substrate
Conditions
Product
Yield (%)a Yield after
hydrolysis (%)b
Pd2(dba)3/3 (2 mol%),
t-BuONa: 5 g scale
1
87
72
Ph
Ph
N
N
Ph
Ph
Br
X
Br
Ph
Ph
Ph
N
X
Pd2(dba)3/3 (2 mol%),
t-BuONa: 3 g scale
N
Ph
2
3
X = Br
X = Cl
93
87
80
78
S
S
Pd(OAc)2/3 (3 mol%),
t-BuONa
N
N
Ph
N
Ph
Br
Br
4
77
n/ac
Ph
Ph
Ph
Ph
Br
N
N
Pd2(dba)3/3 (0.5 mol%),
t-BuONa
5
99
89
O
O
N
Br
Pd(OAc)2/3 (1.5 mol%),
t-BuONa
Ph
6
7
87
84
n/a
87
Ph
H2N
H2N
H2N
N
Br
Ph
Pd(OAc)2/3 (1.5 mol%),
t-BuONa
Ph
H2N
Cl
Ph
Pd(OAc)2/3 (1.5 mol%),
K3PO4
N
Ph
8
99
97
Cl
N
Pd(OAc)2/3 (1.5 mol%),
K3PO4
Ph
9
10
11
12
99
82
92
81
n/a
n/a
n/a
n/a
Ph
Cl
N
Pd(OAc)2/3 (1.5 mol%),
K3PO4
Ph
N
Ph
N
N
Br
Pd(OAc)2/3 (1.5 mol%),
K3PO4
Ph
Ph
N
N
N
N
Ph
N
O2N
O2N
Br
Pd(OAc)2/3 (1.5 mol%),
K3PO4
Ph
MeO
MeO
a Isolated yield (average of two runs).
b n/a = not applicable.
c The diamine is unstable under the deprotection conditions.
Synthesis 2008, No. 4, 537–542 © Thieme Stuttgart · New York

540
O. Grossman et al.
PAPER
Amination of Aryl Halides; Typical Large-Scale Procedure
benzophenone imine adducts in very good yields under
our reaction conditions (Scheme 2, and Table 2, entries
6,7) and this is in sharp contrast to the vast majority of
known catalysts. Although a control experiment between
bromobenzene and aniline resulted in the formation of
diphenylamine, the reaction was very slow under the
present reaction conditions, which explains the selectivity
toward benzophenone imine coupling even in the pres-
ence of the unprotected aniline group. Unfortunately,
when chloro compounds were used instead of their bromo
equivalents, the yield dropped owing to more pronounced
self-coupling.
Pd2(dba)3 (163.3 mg, 0.17 mmol, 2 mol%), 3 (173.0 mg, 0.34
mmol), 1,8-dibromoanthracene (3 g, 8.91 mmol) and t-BuONa (2.5
g, 26.73 mmol, 3 equiv) were placed in a dry 50 mL Schlenk tube
equipped with a stir bar under 99.999% N2. Upon dissolution and
after stirring for 30 min in anhyd toluene (20 mL), benzophenone
imine (3.45 mL, 20.5 mmol, 2.3 equiv) was injected at once and the
sealed tube was thermostated at 100 °C for 24 h. After cooling to
r.t., the mixture was washed with H2O (15 mL), and brine (15 mL),
and dried (MgSO4). After evaporation under reduced pressure,
MeOH (10 mL) was added and left aside for 2 h to give the precip-
itated product in at least 95% purity, which was isolated by filtra-
tion.
Amination of Aryl Halides; Typical Small-Scale Procedure
Pd(OAc)2 (4.1 mg, 0.02 mmol), 3 (9.0 mg, 0.02 mmol), and t-BuO-
Na (235.0 mg, 2.42 mmol) were placed in a dry single-use test tube
equipped with a stir bar under 99.999% N2. Upon dissolution and
stirring for 30 min in anhyd toluene (4 mL), 1-chloronaphthalene
(170 mL, 1.20 mmol) and benzophenone imine (250 mL, 1.43 mmol)
were injected at once and the sealed tube was thermostated at
100 °C for 24 h. After cooling to r.t., the mixture was washed with
H2O (5 mL) and brine (5 mL), and dried (MgSO4). After evapora-
tion under reduced pressure, the product was isolated by flash chro-
matography.
In addition, we found that in many cases the use of the
costly and air-sensitive Pd(0) precursor is not necessary,
and it can be replaced by the more convenient Pd(OAc)2
without affecting the performance of the catalyst (Table 2,
entries 4, 6–12). In other simpler cases (Table 2, entries
8–12), t-BuONa can also be replaced with the less de-
manding K3PO4. The last result is important because po-
tentially this solves the problem of hydrolyzable and base-
sensitive compounds.
Finally, it is interesting to note that, we did not observe
significant difference either in the reactivity or selectivity
of aryl bromides and aryl chlorides under our reaction
conditions (with the exception 4-haloanilines – in these
cases, the difference in chemoselectivity is likely due to
the difference in pKa) – decrease in reaction temperature
slows down the reactions of both chloro- and bromo-
arenes. This fact, apparently, supports our previous hy-
pothesis that reductive elimination but not oxidative
addition is the rate determining step for our trans-spanned
catalyst.11
N1,N8-Bis(diphenylmethylene)triptycene-1,8-diamine (Table 2,
Entry 1)
Yellow solid; mp 290 °C (dec.).
1H NMR (400 MHz, CDCl3): d = 7.83 (4 H, d, J = 7.3 Hz), (2 H, t,
J = 7.3 Hz), 7.36 (2 H, t, J = 5.78 Hz), 7.28–7.24 (4 H, m, Ar),
7.15–7.11 (6 H, m, Ar), 7.07–7.05 (4 H, m, Ar), 7.04–6.96 (4 H, m,
Ar), 6.69 (2 H, t, J = 7.6 Hz), 6.12 (2 H, d, J = 7.8 Hz), 6.04 (1 H,
s), 5.35 (1 H, s).
13C NMR (400 MHz, CDCl3): d = 168.8, 146.5, 146.4, 144.5, 139.2,
136.3, 135.9, 130.5, 129.4, 129.0, 128.3, 128.1, 127.8, 124.7, 124.0,
123.3, 118.6, 116.9, 54.5, 43.8.
In conclusion, our intention to synthesize a particular sub-
strate disclosed a very useful modification of the
Buchwald–Hartwig protocol for the amination of aryl
chlorides and bromides with benzophenone imine as am-
monia surrogate. Our mild protocol allows very efficient
amination of substrates of different steric and electronic
demands (including base sensitive) without a need for
protection. In many cases simple and cheap catalyst pre-
cursors and bases may be used.
Anal. Calcd for C46H32N2: C, 90.16; H, 5.26. Found: C, 89.94; H,
4.98.
N1,N8-Bis(diphenylmethylene)anthracene-1,8-diamine (Table
2, Entry 2)
Orange solid; mp 197 °C.
1H NMR (400 MHz, CDCl3): d = 8.73 (1 H, s), 8.27 (1 H, s), 7.80
(4 H, d, J = 7.7 Hz), 7.61 (2 H, d, J = 8.5 Hz), 7.42 (2 H, t, J = 7.3
Hz), 7.24–7.05 (16 H, m, Ar), 6.59 (2 H, d, J = 6.9 Hz).
13C NMR (400 MHz, CDCl3): d = 168.8, 147.9, 139.2, 136.3, 132.1,
130.5, 129.1, 128.6, 128.5, 128.1, 127.7, 125.7, 125.5, 125.3, 123.1,
119.4, 113.0.
Pd(OAc)2
(98%),
tris(dibenzylideneacetone)dipalladium(0)
[Pd2(dba)3], K3PO4, t-BuONa, benzophenone imine (97%), and all
starting aryl halides are commercially available or were prepared
using published procedures. All reagents were used without further
purification. Ligand 3 was synthesized according to the previously
published procedure.10b All reagents were weighed and handled in
air. All reactions were carried out under N2 (99.999% pure) in oven-
dried glassware. Flash column chromatography was performed with
ultra pure silica gel (230–400 mesh). 1H NMR and 13C NMR spectra
were recorded on a Bruker 300 MHz or Bruker 400 MHz instru-
ment. The chemical shifts are reported in ppm relative to the resid-
ual deuterated solvent or the internal standard tetramethylsilane.
Yields refer to isolated yields of compounds greater than 95% puri-
ty as determined 1H NMR analysis. Yields reported in Table 2 are
for an average of two runs. Entries 3, 8, 9, and 10 in Table 2 are
known compounds. Spectroscopic data of the known compounds
match with the data reported in the corresponding references.
Anal. Calcd for C40H28N2: C, 89.52; H, 5.26. Found: C, 89.84; H,
5.03.
Anthracene-1,8-diamine18 (Table 2, Entry 3)
Yellow solid; mp 138 °C.
1H NMR (300 MHz, CDCl3): d = 8.89 (1 H, s), 8.21 (1 H, s), 7.33–
7.20 (4 H, m, Ar), 6.66 (2 H, dd, J = 6.2, 1.9 Hz), 5.85 (4 H, s, NH2).
N2,N5-Bis(diphenylmethylene)thiophene-2,5-diamine (Table 2,
Entry 4)
Yellow solid; mp 111 °C.
1H NMR (400 MHz, CDCl3): d = 7.67 (4 H, d, J = 7.0 Hz), 7.58–
7.46 (2 H, m, Ar), 7.39–7.33 (10 H, m, Ar), 7.11 (4 H, d, J = 7.2
Hz), 6.90 (2 H, s).
13C NMR (400 MHz, CDCl3): d = 161.6, 149.8, 139.7, 135.9, 130.2,
129.4, 129.2, 128.7, 128.6, 128.1, 125.4.
Synthesis 2008, No. 4, 537–542 © Thieme Stuttgart · New York

PAPER
New Palladium Catalyst for Efficient Amination of Aryl Halides
541
Anal. Calcd for C30H22N2S: C, 81.41; H, 5.01. Found: C, 81.70; H,
4.91.
1H NMR (300 MHz, CDCl3): d = 9.17 (1 H, dd, J = 2.7, 0.6 Hz),
8.25 (1 H, dd, J = 8.8, 2.7 Hz), 7.82 (1 H, d, J = 8.2 Hz), 7.51–7.37
(9 H, m, Ar), 6.70 (1 H, dd, J = 8.8, 0.6 Hz).
1-(6-Amino-1,2-dimethyl-1H-indol-3-yl)ethanone (Table 2,
Entry 5)
Yellow solid; mp 112 °C.
1H NMR (300 MHz, CDCl3): d = 7.75 (1 H, d, J = 8.4 Hz), 6.70 (1
H, dd, J = 8.5, 2.1 Hz), 6.61 (1 H, d, J = 1.7 Hz), 3.60 (3 H, s), 2.71
(3 H, s), 2.62 (3 H, s).
13C NMR (300 MHz, CDCl3): d = 171.3, 168.1, 146.7, 145.3, 140.0,
133.4, 132.7, 132.4, 130.0, 129.6, 128.3, 128.2, 115.0, 106.9.
MS (EI, 70 eV): m/z = 303 [M+], 226 [M+ – Ph].
Anal. Calcd for C18H13N3O2: C, 71.28, H, 4.32. Found: C, 71.15, H,
4.12.
13C NMR (300 MHz, CDCl3): d = 194.4, 143.3, 142.2, 138.0, 121.4,
N-(Diphenylmethylene)-5-methoxypyridin-2-amine (Table 2,
Entry 12)
Yellow solid; mp 159 °C.
119.2, 114.1, 111.9, 95.3, 31.5, 29.3, 12.5.
MS (EI, 70 eV): m/z = 202 [M+], 186 [M+ – NH2].
Anal. Calcd for C12H14N2O: C, 71.26; H, 6.98. Found: C, 71.70; H,
6.91.
1H NMR (400 MHz, CDCl3): d = 7.77 (2 H, d, J = 7.2 Hz), 7.68 (1
H, d, J = 2.6 Hz), 7.47–7.28 (6 H, m, Ar), 7.14–7.12 (2 H, m, Ar),
7.00 (1 H, dd, J = 8.7, 2.2 Hz), 3.86 (3 H, s).
13C NMR (400 MHz, CDCl3): d = 169.5, 160.5, 152.5, 141.2, 139.5,
139.2, 136.1, 132.7, 130.9, 129.4, 129.3, 128.9, 128.6, 128.3, 128.2,
127.9, 110.1, 53.4.
N1-(Diphenylmethylene)benzene-1,4-diamine (Table 2, Entry 6)
Yellow solid; mp 171 °C.
1H NMR (400 MHz, CDCl3): d = 7.74 (2 H, d, J = 7.0 Hz), 7.41–
7.16 (8 H, m, Ar), 6.62 (2 H, d, J = 8.7 Hz), 6.52 (2 H, d, J = 8.6
Hz), 3.50 (2 H, NH2).
13C NMR (400 MHz, CDCl3): d = 167.0, 142.8, 142.4, 140.3, 137.0,
130.3, 129.6, 129.1, 128.4, 128.1, 128.0, 122.9, 115.3.
MS (EI, 70 eV): m/z = 288 [M+], 211 [M+ – Ph].
Anal. Calcd for C19H16N2O: C, 79.14, H, 5.59. Found: C, 78.83, H,
5.13.
MS (EI, 70 eV): m/z = 271 [M+], 194 [M+ – Ph].
Acknowledgment
Anal. Calcd for C19H16N2: C, 83.79; H, 5.92. Found: C, 84.02; H,
5.91.
We thank the German-Israeli Foundation for Scientific Research
and Development (Grant No. 894/05) for financial support.
N4-(Diphenylmethylene)-2,6-dimethylbenzene-1,4-diamine
(Table 2, Entry 7)
White solid; mp 163 °C.
References
1H NMR (400 MHz, CDCl3): d = 7.74 (2 H, dd, J = 6.9, 1.6 Hz),
7.45 (3 H, m, Ar), 7.31 (3 H, m, Ar), 7.18 (2 H, dd, J = 4.3, 2.0 Hz),
6.42 (1 H, s), 3.41 (2 H, s, NH2), 2.05 (6 H, s).
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13C NMR (400 MHz, CDCl3): d = 166.1, 141.9, 140.6, 138.9, 137.2,
(2) (a) Chakraborti, D.; Colis, L.; Schneider, R.; Basu, A. K.
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York, 1992, 4th ed.. (b) Mitchell, H.; Leblanc, Y. J. Org.
Chem. 1994, 59, 682. (c) Banfi, A.; Bartoletti, M.; Bellora,
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130.1, 129.6, 129.1, 128.2, 128.1, 128.0, 122.2, 121.8, 17.6.
MS (EI, 70 eV): m/z = 300 [M+], 223 [M+ – NH2 – C].
Anal. Calcd for C21H20N2: C, 83.96; H, 6.71. Found: C, 84.08; H,
6.99.
Naphthalen-1-amine19 (Table 2, Entry 8)
Yellow solid; mp 195 °C (Lit.19 mp 203 °C).
1H NMR (400 MHz, CDCl3): d = 7.77 (2 H, d, J = 7.2 Hz), 7.68 (2
H, d, J = 2.6 Hz), 7.47–7.28 (2 H, m, Ar), 7.00 (1 H, dd, J = 8.7, 2.2
Hz), 3.86 (2 H, NH2).
p-Toluidine20 (Table 2, Entry 9)
Yellow solid; mp 39–42 °C (Lit.20 mp 46–48 °C).
1H NMR (400 MHz, CDCl3): d = 7.02 (2 H, d, J = 8.1 Hz), 6.66 (2
H, d, J = 8.3 Hz), 3.52 (2 H, s, NH2), 2.28 (3 H, s).
N-(Diphenylmethylene)pyridin-3-amine21 (Table 2, Entry 10)
Yellow solid; mp 98 °C (Lit.21 mp 105 °C).
1H NMR (400 MHz, CDCl3): d = 8.20 (1 H, d, J = 4.4 Hz), 8.06 (1
H, d, J = 1.7 Hz), 7.80 (2 H, d, J = 7.6 Hz), 7.57 (1 H, t, J = 8.7 Hz),
7.47 (2 H, t, J = 7.7 Hz), 7.31 (3 H, d, J = 6.5 Hz), 7.14 (2 H, d,
J = 7.5 Hz), 7.10–7.05 (2 H, m, Ar).
(6) (a) Hartwig, J. F. In Handbook of Organopalladium
Chemistry for Organic Synthesis, Vol. 1; Negishi, E. I., Ed.;
Wiley-Interscience: New York, 2002, 1051. (b) Hartwig, J.
F. In Modern Arene Chemistry; Astruc, C., Ed.; Wiley-VCH:
Weinheim, 2002, 107. (c) Muci, A. R.; Buchwald, S. L. Top.
Curr. Chem. 2002, 219, 131.
N5-(Diphenylmethylene)-5-nitropyridin-2-amine (Table 2,
Entry 11)
Yellow solid; mp 78–81 °C.
Synthesis 2008, No. 4, 537–542 © Thieme Stuttgart · New York

542
O. Grossman et al.
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