Copper-Catalyzed Imino C-N Bond Formation with Aryl Boronic Acids under Aerobic Conditions

Article abstract of DOI:10.1055/s-0035-1561382

A copper-catalyzed, ligand- and base-free C-N cross-coupling reaction of iminochromenes with arylboronic acids in a nontoxic ethanol solvent at room temperature under air is developed. The product yields are found to be good to moderate. The conservation of the sensitive imine functional group implies the mildness of this protocol.

Full text of DOI:10.1055/s-0035-1561382

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
letter
A
P. S. Mandal, A. V. Kumar
Letter
Synlett
Copper-Catalyzed Imino C–N Bond Formation with Aryl Boronic
Acids under Aerobic Conditions
Prashant S. Mandal
R1
R1
R3
HO
HO
Cu(OAc)2•H2O
A. Vijay Kumar*
R2
R2
B
EtOH, r.t., open flask
O
N
Department of Chemistry, Institute of Chemical
Technology, Matunga, Mumbai, Maharashtra,
400019, India
O
NH
vijayakki@gmail.com
R1 = aryl, CN R2 = Cl, N(Et)2, t-Bu, H, OMe
3 = CN, Br, F, Cl, CF3, OCF3, NO2, OMe, Me, CO2Et, pyridyl, vinyl, aryl
R3
31 examples
94–42% yield
R
Received: 17.10.2015
Accepted after revision: 18.01.2016
Published online: 18.02.2016
methodologies development program,15 we herein present
for the first time a copper-catalyzed cross-coupling of imi-
nochromenes with arylboronic acids under aerobic condi-
tions.
DOI: 10.1055/s-0035-1561382; Art ID: st-2015-b0822-l
Abstract A copper-catalyzed, ligand- and base-free C–N cross-cou-
pling reaction of iminochromenes with arylboronic acids in a nontoxic
ethanol solvent at room temperature under air is developed. The prod-
uct yields are found to be good to moderate. The conservation of the
sensitive imine functional group implies the mildness of this protocol
Initially, we tried the reaction of 3-phenylimino-
chromene (1a) and phenylboronic acid (2a) with
Cu(OAc)2·H2O as the catalyst in methanol at room tempera-
ture to afford product 3a in 85% yield (Table 1, entry 8). The
yield was further improved through optimization of various
solvents and copper salts. The product yields were superior
with Cu(OAc)2·H2O when compared with other copper salts
such as CuBr2, CuSO4·5H2O, CuCl2·2H2O, and anhydrous
Cu(OAc)2. The reaction was examined with different sol-
vents such as chloroform, acetonitrile, dichloromethane, di-
chloroethane, DMSO, and DMF (Table 1). Among them, eth-
anol was superior for the reaction. No significant improve-
ment of product yields was observed with various bases
and ligands. The products yields were found to be the same
under oxygen and air atmospheres. Therefore, with these
optimized conditions,16 namely Cu(OAc)2/ethanol/air, we
tested various substituted arylboronic acids for their reac-
tivity with 3-phenyliminochromene (1a).
The scope of N-arylation reaction was examined with
various aromatic boronic acids. The best yields were ob-
tained with 4-Br- (3b, 85%), 4-Cl- (3c, 82%), 4-F- (3d, 79%),
3-Me- (3e, 82%), 2-naphthyl- (3f, 83%), 1-naphthyl- (3g,
79%), 3,4-methylenedioxy- (3h, 76%), and 4-vinyl-substitut-
ed (3i, 78%) boronic acids. The yields in the case of boronic
acids with electron-donating groups such as 3-MeO (3k,
68% yield and the electron-withdrawing groups such as
4-F3CO (3j, 73%), 3- F3CO (3l, 63%), 4-EtO2C (3m, 71%), 4-F3C
(3n, 64%), 2-F3C (3o, 55%), 4-O2N (3p, 42%) were moderate.
The yield in the case of 4-cyano phenylboronic acid (3q,
74%) was slightly higher than the other electron-withdraw-
ing groups, this was an exception. Other aromatic boronic
Key words iminochromenes, copper, arylboronic acid, cross-cou-
pling, aerobic conditions
Iminochromenes1 are a class of chroman-based hetero-
cyclic motifs that show promising biological activities such
as antitumor,2 antimicrobial,3 anti-inflammatory,3c antifun-
gal,4 mitogen-activated protein kinase inhibition (MK-2),5
dynamins I and II GT Pase,6a HIV-1 integrase inhibition,6b
and cell imaging.7 Further they are also useful as fluores-
cent dyes,8 laser dyes,9 sensing thiols10 and sensing H2S.11
Knoevenagel reaction12 and multicomponent reactions13
are the existing methods for synthesizing iminochromenes.
However, the major drawbacks of these procedures are lim-
ited substrate scope, cumbersome purifications steps, and
low product yields especially for electron-deficient aromat-
ic groups, because the transamination is hampered by the
low nucleophilicity of the electron-deficient anilines.12a In
addition, the synthesis of iminocoumarins is not completely
exploited, especially the N-arylation of the imine functional
group owing to sensitivity of the imine bond, which is not
tolerated under the harsh conditions. Hence, alternative
methods for the N-arylated iminochromenes are highly de-
sirable. Over past years, the Chan–Lam–Evans reaction14
has emerged as a versatile tool for C–C and C–heteroatom
bond formation. The reaction is highly advantageous, as it
can be accomplished with relatively inexpensive copper
salts and milder conditions. In continuation to our novel
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

B
P. S. Mandal, A. V. Kumar
Letter
Synlett
Table 1 Evaluation of Optimal Reaction Conditions of N-Arylation of
and chloro substituents on the chromene ring proceeded to
react with phenylboronic acid to afford the products in
good yields. The yields are as follows: 3-cyanochromene
(3aa, 91%), 6,8-di-t-Bu (3ab, 79%), 7-(diethylamino) (3ac,
86%), 7-MeO (3ad, 81%), and 6-Cl (3ae, 80%). The reaction of
7-(diethylamino)-3-(4-nitrophenyl) iminochromene (1k)
also proceeded smoothly to afford the product 3ac in 84%
yield (Scheme 2). Studying a variety of substrates showed
that the reaction scope is not only limited to simple phenyl-
substituted chromene but also could be extended to substi-
tuted iminochromenes. In addition the single-crystal analy-
sis of the compound chromene 3l was obtained, and con-
firmed that the arylation of iminochromenes occurs at the
N atom (Figure 1).
Having studied the substrate scope, we further exam-
ined the one-pot synthesis of N-arylated iminochromene,
in which the imine formation and N-arylation are done se-
quentially in a one-pot fashion. At first the imine was pre-
pared by Knoevenagel reaction of salicylaldehyde and
malononitrile in the presence of piperidine as reported pre-
viously.12d This was followed by addition of Cu(OAc)2·H2O
and phenylboronic acid to the same flask. Unfortunately,
Iminochromene with Phenylboronic Acida
HO
OH
B
copper salt
O
N
+
solvent,
air, time
O
NH
1a
2a
3a
Entry
Copper
CuBr2
Solvent
Yield (%)b
1
2
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
CH2Cl2
CHCl3
MeCN
DCE
75
60
91
93
78c
55d
80
85
87
80
62
74
80
55
–
CuCl2·2H2O
CuSO4·5H2O
Cu(OAc)·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2
3
4
5
6
7
8
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
–
9
10
11
12
13
14
15
Ph
HO
OH
Ph
B
Cu(OAc)2•H2O
ethanol, r.t.
O
N
+
DMF
R3
O
NH
air, 24 h
R3
DMSO
EtOH
1a
2b–v
3b–v
a Reaction conditions: 1a (0.9 mmol), 2a (2.3 mmol), Cu salt (20 mol%),
solvent (5 mL), 24 h at room temperature.
b isolated yield.
2b: R3 = 4-Br-C6H4, 3b (85%)
2c: R3 = 4-Cl-C6H4, 3c (82%)
2d: R3 = 4-F-C6H4, 3d (79%)
2e: R3 = 3-Me-C6H4, 3e (82%)
2f: R3 = 2-naphthyl, 3f (83%)
2g: R3 = 1-naphthyl, 3g(79%)
2h: R3 = 3,4-(OCH2O)C6H3, 3h (76%)
2i: R3 = 4-vinylphenyl, 3i (78%)
2j: R3 = 4-F3CO-C6H4, 3j (73%)
2k: R3 = 3-MeO-C6H4, 3k (68%)
2l: R3 = 3-F3CO-C6H4, 3l (63%)
c Cu(OAc)2.H2O (10 mol%).
d Cu(OAc)2.H2O (5 mol%).
Ph
N
acids, such as 4-biphenyl (3r, 73%), 1-pyrene (3s, 73%),
3-pyridyl (3t, 73%), and non-arylboronic acids, such as
trans-2-(4-fluorophenyl)vinyl (3u, 76%) and trans-2-
phenylvinyl (3v, 72%), afforded the products in moderate
yields. Altogether a wide variety of substrates were success-
fully synthesized by the reaction of various arylboronic ac-
ids with 3-phenyliminochromene (1a, Scheme 1).
We further extended the study by subjecting different
iminochromenes with substituents at different positions on
the chromene ring for N-arylation. The reaction of imino-
chromenes bearing substituents at 3-, 6-, and 7-positions
was carried out with simple unsubstituted phenylboronic
acid (2a). The reactions proceeded smoothly and afforded
the corresponding products in high yields. The yields in the
case of 3-phenyl-substituted chromenes bearing electron-
withdrawing groups such as 3-F3C (3w, 94%) and 4-NO2 (3x,
82%) on the phenyl ring were the highest, followed by 4-Cl
(3y, 81%) and 2-F (3z, 77%). Similarly, the 3-cyano imino-
chromenes (1f–j) with tert-butyl, diethylamino, methoxy,
O
R3
2m: R3 = 4-ethoxycarbonyl-C6H4, 3m (71%)
2n: R3 = 4-F3C-C6H4, 3n (64%)
2o: R3 = 2-F3C-C6H4, 3o (55%)
2p: R3 = 4-O2N-C6H4, 3p (42%)
2q: R3 = 4-NC-C6H4, 3q (74%)
2r: R3 = 4-biphenyl, 3r (73%)
2s: R3 = 1-pyrene, 3s (73%)
2t: R3 = 3-pyridine, 3t (73%)
2u: R3 = trans-2-(4-fluorophenyl)vinyl, 3u (76%)
2v: R3 = trans-2-phenylvinyl, 3v (72%)
Scheme 1 Study of substrate scope in the N-arylation of 3-phenylimi-
nochromene with arylboronic acids. Reagents and conditions: 3-
phenyliminochromene (1a, 0.9 mmol), arylboronic acid 2b–v (2.3
mmol), Cu(OAc)2·H2O (0.18 mmol), EtOH (5 mL), 24 h at r.t.; yields in
parentheses are isolated yields.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

C
P. S. Mandal, A. V. Kumar
Letter
Synlett
R1
N
N
1. malononitrile
DABCO, EtOH
1 h, N2, r.t.
R2
HO
OH
R1
B
Cu(OAc)2•H2O
O
O
R2
O
N
+
2. Cu(OAc)2•H2O,
PhB(OH)2
EtOH, r.t.
air, 24 h
O
NH
OH
air, 24 h
1b–k
2a
3w–z, 3aa–af
4aa, 89%
1b: R1 = 3-F3C-C6H4, 3w (94%)
1c: R1 = 4-O2N-C6H4, 3x (82%)
1d: R1 = 4-Cl-C6H4, 3y (81%)
1e: R1 = 2-F-C6H4, 3z (77%)
Scheme 3 One-pot procedure: iminochromene synthesis followed by
N-arylation
R1
O
N
Thus, the telescopic procedure could be advantageously
utilized for the N-arylation of iminochromenes precluding
the isolation of iminochromene.
Ph
1f: R2 = H, 3aa (91%)
N
1g: R2 = 6,8-(t-Bu)2-C6H3, 3ab (79%)
1h: R2 = 7-Et2N, 3ac (86%)
1i: R2 = 7-MeO, 3ad (81%)
1j: R2 = 6-Cl, 3ae (80%)
R2
Finally, we propose a plausible mechanism for the N-
arylation of iminochromenes based on a preliminary study
to understand the catalytic pathway. Substrates are known
to bind to copper metal in Chan–Lam–Evans cross-coupling
reactions. We envisaged a similar type of association of
copper and iminochromene in the present system. So we
performed a control experiment in which Cu(OAc)2·H2O
and 3-phenyliminochromene (1:1 ratio) were stirred to-
gether at room temperature in ethanol for 24 hours. The
solution was analyzed by MALDI-ESI mass spectrometry.
Dimeric and monomeric copper–iminochromene complex-
es could be seen in the spectra.17 This evidence indicated
the formation of iminochromene–copper complex during
the reaction. We hypothesize that this association of the
substrate with copper metal enables a more facile reaction
(Scheme 4).
O
N
Ph
NO2
O
N
N
Ph
Scheme 2 Study of substrate scope in the N-arylation of imino-
chromenes with phenylboronic acid. Reagents and conditions: imino-
chromene 1b–k (0.9 mmol), phenylboronic acid (2a, 2.3 mmol),
Cu(OAc)2·H2O (0.18 mmol), EtOH (5 mL), 24 h at r.t., yields in parenthe-
sis are isolated yields.
In conclusion, we have developed a mild protocol for the
synthesis of N-arylated iminochromenes under Chan–Lam–
Evans conditions for the first time. The sensitive imine
functional group which is prone to hydrolysis is sustained,
and the yields of the products are good to moderate. We
have also demonstrated a one-pot procedure in which the
N-arylated iminochromenes can be synthesized. The mech-
anistic investigation revealed evidence for the copper–imi-
nochromene complex formation during the reaction. Fur-
ther efforts to study the detailed mechanism and synthetic
applications of this methodology are in progress in our
group.
Acknowledgment
Figure 1 Crystal structure of (Z)-3-phenyl-N-[3-(trifluorome-
thoxy)phenyl]-2H-chromen-2-imine
P.S.M. is grateful to UGC-BSR, Department of Chemistry, and ICT for
the research fellowship. A.V.K. is thankful to DST, Govt. of India for the
INSPIRE Faculty Award [IFA12-CH-40] and the research funding. Insti-
tute of Chemical Technology (ICT) is acknowledged for providing the
research facilities. The authors are grateful to SAIF-IITB for providing
the HRMS data.
piperidine was less effective in the reaction affording only
49% of 4aa. The yield was further improved to 89% by re-
placing piperidine with DABCO (Scheme 3).
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

D
P. S. Mandal, A. V. Kumar
Letter
Synlett
R
1a
O
NH
Cu(II)(OAc)2
O
OH
Ar
Cu(II)L(OAc)
R
B
2
HO
OH
O
NH
H2O
0.5 O2
R
N
(II)
Cu
O
Cu(I)OAc
O
O
R
N
(III)
Cu Ar
O
AcO
O
Ar
0.5 O2
H2O
N
3a
R
R
N
O
(II)
Cu
O
Cu(II)L(OAc) =
O
Scheme 4 Plausible mechanistic pathway of copper-catalyzed N-arylation of iminochromene
Supporting Information
F. A.; El-Agrody, A. M. Farmaco 2002, 57, 715. (e) El-Agrody, A.
M.; El-Latif, M. S. A.; El-Hady, N. A.; Fakary, A. H.; Bedair, A. H.
Molecules 2001, 6, 519. (f) Manrao, M. R.; Singh, B.; Sharma, J.
R.; Kalsi, P. S. J. Indian Counc. Chem. 1996, 12, 38.
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1561382.
S
u
p
p
ortioInfgrmoaitn
S
u
p
p
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To a round-bottomed flask equipped with a stir bar, the imino-
chromene (1 mmol), Cu(OAc)2·H2O (20 mol%), and arylboronic
acid (2.3 mmol) were added. EtOH (5 mL) was added, and the
reaction mixture was stirred for 24 h at r.t. After completion of
the reaction as monitored by TLC, the organic phase was con-
centrated under reduced pressure, and the solid residue was
fractioned in EtOAc (20 mL) and H2O (10 mL) thrice. The organic
phase was separated and dried on anhydrous Na2SO4. The
solvent was removed in vacuum, and the crude residue was
purified by column chromatography with EtOAc–PE to afford
the respective N-arylated chromene.
Physical and spectroscopic data given below are for compound
3a and may be taken as representative. For further details, see
the Supporting Information.
(Z)-N-3-Diphenyl-2H-chromen-2-imine (3a)
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Saubern, S.; Adams, J.; Winters, M. P.; Chan, D. M. T.; Combs, A.
Tetrahedron Lett. 1998, 39, 2941. (c) Evans, D. A.; Katz, J. L.;
West, T. R. Tetrahedron Lett. 1998, 39, 2937. (d) Qiao, J. X.; Lam,
Yellow solid (249 mg, 93% yield); mp 112–113 °C. IR (KBr): ν =
1737, 1643, 1587, 1485, 1365, 1230, 1116 cm–1 1H NMR (400
.
MHz, CDCl3): δ = 7.79–7.72 (m, 2 H), 7.46–7.30 (m, 7 H), 7.29 (s,
1 H), 7.20–7.13 (m, 2 H), 7.13–7.05 (m, 1 H), 7.02 (d, J = 8.2 Hz, 1
H). 13C NMR (100 MHz, CDCl3): δ = 152.9, 148.9, 146.6, 136.7,
133.6, 133.3, 132.1, 130.1, 129.1, 128.7, 128.4, 128.3, 128.0,
127.2, 123.9, 123.4, 122.8, 122.5, 120.4, 115.6, 115.3. ESI-HRMS:
m/z calcd for C21H16NO [M + H]+ : 298.1226; found: 298.1277.
(17) See the Supporting Information.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E