Oxidative coupling of benzylamines to imines by molecular oxygen catalyzed by cobalt(II) β-tetrakis(trifluoromethyl)-mesotetraphenylporphyrin

Article abstract of DOI:10.1021/jo5017212

Oxidative coupling of benzylamines to imines by molecular oxygen is efficiently realized in the presence of very low catalyst loadings of Co(II) β-tetrakis-(trifluoromethyl)-meso-tetraphenylporphyrin. Due to the effect of four β-CF₃ groups, the catalyst shows good selectivity and very high turnover number. The reaction is easily scaled up and may provide a convenient way to prepare many imines in large scale.

Full text of DOI:10.1021/jo5017212

Note
pubs.acs.org/joc
Oxidative Coupling of Benzylamines to Imines by Molecular Oxygen
Catalyzed by Cobalt(II) β‑Tetrakis(trifluoromethyl)-meso-
tetraphenylporphyrin
Shuai Zhao, Chao Liu, Yong Guo, Ji-Chang Xiao, and Qing-Yun Chen*
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling
Road, 200032 Shanghai, China
S
* Supporting Information
ABSTRACT: Oxidative coupling of benzylamines to imines
by molecular oxygen is efficiently realized in the presence of
very low catalyst loadings of Co(II) β-tetrakis-
(trifluoromethyl)-meso-tetraphenylporphyrin. Due to the effect
of four β-CF3 groups, the catalyst shows good selectivity and
very high turnover number. The reaction is easily scaled up
and may provide a convenient way to prepare many imines in large scale.
On the other hand, imines are very useful and important
intermediates in pharmaceutical, biological, and chemical
syntheses.5 Many synthetic methods were developed for
efficient synthesis of various imines.6 Among them, oxidation
of primary amines to imines is a fundamental and practical
approach. However, comparatively little attention has been
devoted to it probably due to the product selectivity issue.
Although much progress has been made in this area in recent
years by using green oxidants with different catalysts, such as
Au,7 Ru,8 Cu,9 Fe,10 Co,11 AIBN,12 photocatalyst,13 manganese
metalloporphyrins,14 etc., it is always desired to further improve
the catalytic efficiency and easily scale up the reaction in order
to obtain the product in large quantity. During our ongoing
efforts in the synthesis (Scheme 1) and application of
fluoroporphyrins, it was found that cobalt(II) β-tetrakis-
(trifluoromethyl)-meso-tetraphenylporphyrin (CoTPP(CF3)4)
showed very good catalytic selectivity and activity on the
oxidative coupling of benzylamines to imines with environ-
mentally friendly oxidant molecular oxygen, and the reaction
could be easily scaled up. Herein we present the results.
Free-base porphyrin H2TPP(CF3)4 was conveniently pre-
pared by palladium-catalyzed trifluoromethylation of
CuTPPBr4 and subsequent demetalation according to our
previous report.4d Insertion of Co(II) and Mn(III) into
H2TPP(CF3)4 afforded the catalysts used in our following
study, CoTPP(CF3)4 and manganese(III) β-tetrakis-
(trifluoromethyl)-meso-tetraphenylporphyrin chloride
(MnTPP(CF3)4), respectively. Benzylamine 1a was chosen as
the model substrate. Our study was conducted with the
oxidative coupling of 1a with catalytic amounts of MnTPP-
(CF3)4 or CoTPP(CF3)4 (0.01 mol %) under oxygen
atmosphere. The results are summarized in Table 1.
orphyrins, due to the unique 18π-electron macrocycle,
Phave emerged as useful ligands for transition metal
catalysts in organic synthesis in a variety of atom/group
transfer reactions, including oxygen, carbene, and nitrene
transfer reactions.1 As model catalysts of cytochrome P450,2
metalloporphyrins were extensively studied and used as efficient
catalysts in practical aerobic oxidation reactions of various
organic compounds,1e−g such as oxidation of olefins to
epoxides, cycloalkanes to alcohols or ketones, alcohols to
aldehydes, etc. Though great progress has been achieved, the
desire to further improve the catalytic efficiency remains. Low
catalytic efficiency is mainly due to the rapid destruction by self-
oxidation or formation of inactive μ-oxo dimers. Introduction
of electron-withdrawing groups into the periphery of the
porphyrin macrocycle has been proven to be a good solution
because it is possible to avoid catalyst self-degradation or the
formation of inactive μ-oxo dimers by creating a cage around
the metal-oxo intermediate.1f Moreover, the electron-with-
drawing groups may enhance the electrophilicity of an active
metal-oxo intermediate and subsequently improve the catalytic
efficiency.1f,3
A number of porphyrins with steric and electron-withdrawing
groups including halogens, CN, NO2, CF3 groups, etc. have
been reported, and some of them have been used as good
catalysts for various oxidation reactions.1f Among them, the
CF3 group shows unique influence on the porphyrin macro-
cycle due to its steric and electronic effects.4 We found that β-
tetrakis(trifluoromethyl)-meso-tetraphenylporphyrin (H2TPP-
(CF3)4) with a severely distorted macrocycle induced by the
bulky CF3 groups is easily reduced to 20π-electron β-
tetrakis(trifluoromethyl)-meso-tetraphenylporphyrins but is rel-
atively hard to be oxidized due to the four strong electron-
withdrawing CF3 groups.4e In view of these great effects, we
supposed that H2TPP(CF3)4 may have potential applications in
catalytic oxidation reactions.
Received: July 29, 2014
© XXXX American Chemical Society
A
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The Journal of Organic Chemistry
Note
Scheme 1. Synthesis of the Catalysts
a
Table 1. Oxidative Coupling of Benzylamines to Imines Catalyzed by Metalloporphyrin under Various Conditions
b
entry
catalyst
solvent
dioxane
t (h)
T (°C)
conversion (%)
yield (%)
1
2
3
4
5
MnTPP(CF3)4
CoTPP(CF3)4
MnTPP
3
3
3
3
3
6
66
7
3
3
3
3
4
4
4
3
3
3
130
130
130
130
130
reflux
80
67
65
66
65
0
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
hexane
MeCN
toluene
DMF
100
100
35
none
0
MnTPP(Br)4
MnTPP(CF3)4
MnTPP(CF3)4
MnTPP(CF3)4
MnTPP(CF3)4
MnTPP(CF3)4
MnTPP(CF3)4
MnTPP(CF3)4
MnTPP(CF3)4
MnTPP(CF3)4
MnTPP(CF3)4
MnTPP(CF3)4
MnTPP(CF3)4
CoTPP(CF3)4
29
9
c
6
9
7
8
55
55
0
130
130
130
130
130
130
130
130
130
130
130
100
49
9
47
36
55
0
10
11
12
13
14
15
16
17
18
36
60
100
100
83
DCE
0
DME
47
0
DG
100
18
EtOH
18
83
92
d
d
dioxane/H2O
83
dioxane/H2O
94
a
The reactions were carried out with benzylamine 1a (535 mg, 5 mmol) and catalyst (5 × 10−4 mmol) in solvent (5 mL) under O2 atmosphere of 6
b
c
d
atm. Conversion of benzylamine and yield of product were determined by GC-MS. O2 (1 atm). Dioxane/H2O = 5:1 v/v.
Among several β-sterically hindered metalloporphyrin
catalysts screened, both CoTPP(CF3)4 and MnTPP(CF3)4
were highly selective catalysts for the oxidative coupling
reaction, affording the desired imine with >98% selectivity
and moderate conversion (Table 1, entries 1 and 2). As a
comparison, without a catalyst or with Mn(III) meso-
tetraphenylporphyrin chloride (MnTPP) as catalyst under the
similar conditions, no desired product was observed despite
complete conversion of the starting benzylamine 1a (Table 1,
entries 3 and 4). Also, Mn(III) β-tetrakisbromo-meso-
tetraphenylporphyrin chloride (MnTPPBr4) afforded lower
conversion and selectivity, indicating the important role of CF3
groups (Table 1, entry 5). According to GC-MS analysis, the
possible byproducts were PhCOOH, PhCN, and PhCONH2.
These byproducts may induce and accelerate the degradation of
desired imine formed in the reaction. As a result, without a
catalyst or with a poor catalyst, the byproducts were formed at
the beginning of the reaction and destroyed the desired imine
immediately (Table 1, entries 3 and 4). However, with a good
catalyst, only the desired imine was produced and its
decomposition was avoided (Table 1, entries 1, 2, and 5).
The oxygen pressure of 6 atm seems necessary to the reaction
because reduced pressure resulted in lower conversion and
yield (Table 1, entry 6). Elevated reaction temperatures were
beneficial to the formation of the oxidative coupling product
because lower reaction temperatures significantly slowed the
reaction rate. Both the conversion and yield were reduced to
55% with a reaction temperature of 80 °C and even prolonged
reaction time of 66 h (Table 1, entry 7). Reaction time was
another crucial factor. Prolonged reaction time did not further
B
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Note
Scheme 2. Scale-up Experiments of the Oxidative Coupling Reactions of Benzylamine
a
Table 2. Oxidative Coupling of Various Benzylamines to Imines Catalyzed by CoTPP(CF3)4
a
b
Amine (5 mmol), catalyst (5 × 10−4 mmol), O2 (6 atm), dioxane/H2O (10 mL/2 mL), 130 °C, 3 h. Isolated yields based on the starting primary
amines used; GC yields in parentheses.
improve yield of the desired imine but may lead to
decomposition of the desired product (Table 1, entry 8).
During the first 3 h, only the conversion of amine to imine was
observed; no decomposition of the desired imine occurred.
However, after about 3 h, decomposition of the desired imine
started. It seems that this result is not related to the
concentration of the desired imine because the reaction
shows similar results on a large scale (1 mol) under solventless
conditions (Scheme 2). Reduced reaction time led to decreased
conversion of the starting benzylamine and yield of the desired
imine. Among various solvents screened, such as hexane,
acetonitrile, toluene, DMF, dichloromethane, DME, DG, and
ethanol, dioxane stood out to be the best one (Table 1, entries
9−16). To our delight, when a small amount of water was used
as cosolvent (dioxane/H2O = 5:1 v/v), the yield of desired
imine increased up to 83% for MnTPP(CF3)4 catalyst and 92%
for CoTPP(CF3)4 catalyst (Table 1, entries 17 and 18). All of
these systematic studies indicated that a combination of 6 atm
oxygen in dioxane/H2O (5:1 v/v) at 130 °C for 3 h with a
catalytic amount of CoTPP(CF3)4 was the most effective
system for the oxidative coupling of benzylamines to imines.
With the optimized reaction conditions in hand (Table 1,
entry 18), we examined the CoTPP(CF3)4-catalyzed oxidative
coupling of various benzylamines. The results are summarized
in Table 2. In general, a variety of substituted benzylamines
with either electron-withdrawing or -donating groups were
suitable substrates in the reaction, affording the desired imines
in good yields. The electronic properties of the substituents on
the aryl ring have little influence on the efficiency of the
oxidation reactions (Table 2, entries 2−6 and 8). It was found
C
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Note
Scheme 3. Proposed Mechanism of the Oxidative Coupling of Benzylamines to Imines
that the reaction is sensitive to the steric hindrance of the
substituents on the aryl ring. Lower yield of the desired imine
was obtained for ortho-methyl benzylamine (Table 2, entry 7).
It should be mentioned that heteroaromatic amine 1k, which
might deactivate the catalyst due to the coordination with metal
ion of the catalyst, can also be applied to the reaction, providing
a moderate yield of the desired imine (Table 2, entry 11).
However, aliphatic amines are not suitable substrates for the
reaction, resulting in complex reaction mixtures.
catalyst self-degradation and the formation of an inactive μ-oxo
dimer. At the same time, this structure might be beneficial for
the high selectivity of the reaction. In addition, the strong
electron-withdrawing abilities of four CF3 groups might
increase the reactivity of the key active metal-oxo intermediate
B, resulting in rapid formation of the desired product.
In conclusion, an efficient and practical method for oxidative
coupling of benzylamines to imines by molecular oxygen
catalyzed by Co(II) β-tetrakis(trifluoromethyl)-meso-tetraphe-
nylporphyrin has been developed. The reaction shows good
selectivity and high turnover number, which might be due to
the effect of the four bulky electron-withdrawing β-CF3 groups
on the catalyst. In addition, the reaction is easily scaled up and
leads to a high yield of the desired imine, which makes it
applicable for large-scale preparation.
To demonstrate the applicability of this method, the
oxidative coupling of benzylamine 1a was conducted on a 1
mol (107 g) scale (Scheme 2). The reaction proceeded
smoothly and provided the desired imine in good isolated yield
of 81%. It should be noted that a quite low catalyst loading of
10 ppm was used in this case, and the mole turnover number
(TON) and mole turnover frequency (TOF) are as high as 40
500 and 13 500 h−1, respectively, indicating the high efficiency
and stability of the catalyst. When the catalyst loading was
further reduced to 1 ppm, the oxidative coupling reaction
afforded a lower yield of the desired imine with a very high
TON and TOF of 277 000 and 92 300 h−1, respectively. It is
worth mentioning that the reaction showed a high selectivity
and resulted in only the desired imine besides a bit of
unconsumed starting benzylamine. Moreover, for large-scale
preparation, no solvent is needed for the reaction if a liquid
amine is used.
EXPERIMENTAL SECTION
■
General Remarks. NMR spectra were recorded on a NMR
spectrometer at 400 MHz for 1H and 376 MHz for 19F NMR spectra.
All spectra were obtained at room temperature in CDCl3. Chemical
shifts are reported in parts per million (ppm) relative to
tetramethylsilane (TMS) as an internal standard (δTMS = 0 ppm) for
1H NMR spectra and CFCl3 as an external standard (negative for
upfield) for 19F NMR spectra. Thin-layer chromatography (TLC)
analysis was performed on a silica gel plate and flash column
chromatography over silica gel (mesh 300−400). All solvents and
chemicals were reagent grade, purchased commercially, and used
without further purification if not noted. The catalyst CoTPP(CF3)4
was readily prepared according to reported procedures with slight
modifications.4
Procedures for the Synthesis of CoTPP(CF3)4. Synthesis of
CuTPP(CF3)4.4d FSO2CF2COOMe (1.94 g, 10.1 mmol, 20 equiv) was
added at room temperature to a mixture of CuTPPBr4 (500 mg, 0.5
mmol, 1 equiv), Pd(dba)2 (14 mg, 5 mol %), and CuI (1.92 g, 10.1
mmol, 20 equiv) in dry DMF (100 mL). The reaction mixture was
stirred at 100 °C under nitrogen for 4 h. After being cooled to room
temperature, the reaction mixture was diluted with CH2Cl2 and filtered
through a pad of Celite. The filtrate was washed three times with
water, dried over anhydrous Na2SO4, and evaporated to dryness. The
resulting solid was purified by flash chromatography using petroleum
ether/dichloromethane (10:1) as the eluent to give the target product
CuTPP(CF3)4 (408 mg, 86%) as a dark green solid.
On the basis of above experiments and previous reports,6,15
a
plausible reaction mechanism for the oxidative coupling of
benzylamine with CoTPP(CF3)4 as catalyst has been proposed
as shown in Scheme 3. CoTPP(CF3)4 reacts with oxygen under
reaction conditions to generate the active intermediate B, which
coordinates with benzylamine to form intermediate C. It is
subsequently converted to a Schiff base intermediate D.
Hydrolysis of this imine intermediate D induced by H2O
with the elimination of a molecule of NH3 results in the
formation of benzaldehyde E, which reacted with another
benzylamine molecule to generate the desired imine product
with the release of a molecule of H2O.
The key step might be the activation of molecule oxygen
with CoTPP(CF3)4. It is easier for Co(II) than Mn(III) in the
catalyst to react with molecular oxygen to form active
intermediate B.15 As a result, CoTPP(CF3)4 shows higher
catalytic activation. The high efficiency and selectivity of
CoTPP(CF3)4 catalyst might arise from the effect of the four
bulky and electron-withdrawing β-CF3 groups. As shown in
Scheme 3, based on the previous reports,4 a proposed structure
of key intermediate B demonstrates a saddle conformation due
to the steric hindrance of four CF3 groups, which creates a cage
around the active metal-oxo intermediate B and prevents
Demetalation of CuTPP(CF3)4.4a,d CuTPP(CF3)4 (400 mg, 0.43
mmol) was dissolved in CH2Cl2 (40 mL). H2SO4/CF3COOH (1:10, 4
mL) was added dropwise, the resulting dark brown mixture stirred at
room temperature for about 4 h, until TLC indicated that the substrate
disappeared completely. Then the mixture was poured into brine (80
mL) and neutralized with NaHCO3. The organic layer was washed
with water and brine, passed through dry silica gel, and evaporated to
dryness. The resulting residue was purified by flash chromatography
using petroleum ether/dichloromethane (1:2) as the eluent to give the
desired product H2TPP(CF3)4 (275 mg, 70%) as a brown solid.
D
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The Journal of Organic Chemistry
Note
Insertion of Cobalt(II) into H2TPP(CF3)4.4c H2TPP(CF3)4 (500 mg,
0.564 mmol, 1 equiv) was dissolved in DMF (100 mL) and heated to
reflux, then Co(OAc)2·4H2O (0.7 g, 2.82 mmol, 5 equiv) was added.
The reaction mixture was stirred at reflux for 30 min. After being
cooled to room temperature, the reaction mixture was diluted with
CH2Cl2 (200 mL) and washed three times with water. The organic
layer was separated, dried with anhydrous Na2SO4, and evaporated to
dryness. The resulting residue was purified by flash chromatography
using petroleum ether/dichloromethane (5:1) as the eluent to give the
desired product CoTPP(CF3)4 (483 mg, 91%) as a dark green solid.
H2TPP(CF3)4: 1H NMR (400 MHz, C6D6) δ 8.12−8.14 (m, 8H, Ph-
ortho), 8.01 (s, 4H, β-H), 8.42−8.51 (m, 12H, Ph-meta, para), −1.41
(s, 2H, N−H); 19F NMR (376 MHz, C6D6) δ −49.27 (s, 12F, β-CF3);
MS (MALDI) m/z = 887.6 (M + H)+.4a,d
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Copies of H and 19F NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: chenqy@sioc.ac.cn.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
CoTPP(CF3)4: 1H NMR (400 MHz, CDCl3) δ 15.44 (br s, 4H, β-H),
13.56 (br s, 8H, Ph-ortho), 9.51−9.57 (m, 12H, Ph-meta, para); 19F
NMR (376 MHz, CDCl3) δ −53.41 (br s, 12F, β-CF3); HRMS
(MALDI) calcd for C48H24CoF12N4 (M+) 943.1136, found 943.1145.4c
General Procedure for the Synthesis of 2a−2m. A solution of
benzylamine 1a (535 mg, 5 mmol, 1 equiv) and CoTPP(CF3)4 (0.5
mg, 10−4 equiv) in dioxane/H2O (12 mL, 5:1 v/v) was stirred in a 100
mL autoclave under oxygen (6 atm) at 130 °C for 3 h. After being
cooled to room temperature, the reaction mixture was evaporated to
dryness directly and the resulting residue was purified by flash column
chromatography (eluent, PE/EA/NEt3 = 100:10:1) to give the desired
target product 2a.
■
Thanks for the financial support from the Natural Science
Foundation of China (21032006, 21302207) and the 973
Program of China (2012CB821600).
REFERENCES
■
(1) For representative reviews, see: (a) Handbook of Porphyrin
Science, Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Scientific:
River Edge, NJ, 2010; Vols. 10 and 21. (b) Lu, H. J.; Zhang, X. P.
Chem. Soc. Rev. 2011, 40, 1899−1909. (c) Hu, Y.; Zhang, X. P. Top.
Curr. Chem. 2012, 327, 147−162. (d) Caselli, S. A.; Gallo, E. Dalton
Trans. 2009, 5434−5443. (e) Che, C. M.; Huang, J. S. Chem. Commun.
2009, 3996−4015. (f) Dolphin, D.; Traylor, T. G.; Xie, L. Y. Acc.
Chem. Res. 1997, 30, 251−259. (g) Mansuy, D. Coord. Chem. Rev.
1993, 125, 129−141.
(2) Meunier, B.; de Visser, S. P.; Shaik, S. Chem. Rev. 2004, 104,
3947−3980.
(3) Ochsenbein, P.; Mandon, D.; Fischer, J.; Weiss, R.; Austin, R.;
Jayaraj, K.; Gold, A.; Terner, J.; Bill, E.; Muther, M.; Trautwein, A. X.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1437−1439.
1
Imine 2a: yellow oil (423 mg, 87% yield); H NMR (400 MHz,
CDCl3) δ 8.41 (s, 1H), 7.79−7.81 (m, 2H), 7.44−7.25 (m, 8H), 4.81
(s, 2H); MS (EI) m/z = 195.1 (M+).9f
Imine 2b: yellow oil (485 mg, 84% yield); H NMR (400 MHz,
1
CDCl3) δ 8.35 (s, 1H), 7.78 (dd, J = 5.6, 8.4 Hz, 2H), 7.30 (dd, J =
5.6, 8.4 Hz, 2H), 7.11 (t, J = 8.8 Hz, 2H), 7.03 (t, J = 8.8 Hz, 2H), 4.74
(s, 2H); 19F NMR (376 MHz, CDCl3) δ −109.22 (s, 1F), −115.99 (s,
1F); MS (EI) m/z = 231.1 (M+).9f
1
Imine 2c: yellow oil (552 mg, 84% yield); H NMR (400 MHz,
(4) (a) Terazono, Y.; Dolphin, D. J. Org. Chem. 2003, 68, 1892−
1900. (b) Terazono, Y.; Dolphin, D. H. Inorg. Chim. Acta 2003, 346,
261−264. (c) Terazono, Y.; Patrick, B.; Dolphin, D. H. Inorg. Chim.
Acta 2003, 346, 265−269. (d) Liu, C.; Chen, Q. Y. Eur. J. Org. Chem.
2005, 3680−3686. (e) Liu, C.; Shen, D. M.; Chen, Q. Y. J. Am. Chem.
Soc. 2007, 129, 5814−5815.
CDCl3) δ 8.33 (s, 1H), 7.71 (d, J = 8.4 Hz, 2H), 7.39 (d, J = 8.4 Hz,
2H), 7.32 (d, J = 8.4 Hz, 2H), 7.27 (d, J = 8.4 Hz, 2H), 4.76 (s, 2H);
MS (EI) m/z = 263.0 (M+).9f
1
Imine 2d: yellow oil (585 mg, 89% yield); H NMR (400 MHz,
CDCl3) δ 8.34 (s, 1H), 7.82 (m, 1H), 7.62−7.64 (m, 1H), 7.40−7.43
(m, 1H), 7.33−7.38 (m, 2H), 7.21−7.31 (m, 3H), 4.78 (s, 2H); MS
(EI) m/z = 263.0 (M+).9f
(5) (a) Murahashi, S. I. Angew. Chem., Int. Ed. Engl. 1995, 34, 2443−
2465. (b) Adams, J. P. J. Chem. Soc., Perkin Trans. 1 2000, 125−139.
1
Imine 2e: yellow oil (750 mg, 85% yield); H NMR (400 MHz,
(c) Samec, J. S. M.; Ell, A. H.; Backvall, J. E. Chem.Eur. J. 2005, 11,
̈
CDCl3) δ 8.33 (s, 1H), 7.64 (d, J = 8.4 Hz, 2H), 7.56 (d, J = 8.4 Hz,
2H), 7.47 (d, J = 8.4 Hz, 2H), 7.21 (d, J = 8.4 Hz, 2H), 4.75 (s, 2H);
MS (EI) m/z = 352.9 (M+).16
2327−2334. (d) Enders, D.; Shilvock, J. P. Chem. Soc. Rev. 2000, 29,
359−373. (e) Kobayashi, S.; Mori, Y.; Fossey, J. S.; Salter, M. M.
Chem. Rev. 2011, 111, 2626−2704.
1
Imine 2f: yellow oil (473 mg, 85% yield); H NMR (400 MHz,
(6) For representative reviews, see: (a) Patil, R. D.; Adimurthy, S.
Asian J. Org. Chem. 2013, 2, 726−744. (b) Largeron, M. Eur. J. Org.
Chem. 2013, 2013, 5225−5235.
CDCl3) δ 8.36 (s, 1H), 7.69 (d, J = 8.4 Hz, 2H), 7.24 (d, J = 4.4 Hz,
2H), 7.22 (d, J = 4.4 Hz, 2H), 7.17 (d, J = 8.4 Hz, 2H), 4.79 (s, 2H),
2.40 (s, 3H), 2.36 (s, 3H); MS (EI) m/z = 223.1 (M+).9f
1
(7) (a) Neeli, C. K. P.; Ganji, S.; Ganjala, V. S. P.; Kamaraju, S. R. R.;
Burri, D. R. RSC Adv. 2014, 4, 14128−14135. (b) So, M. H.; Liu, Y.;
Ho, C. M.; Che, C. M. Chem.Asian J. 2009, 4, 1551−1561. (c) Zhu,
B.; Angelici, R. J. Chem. Commun. 2007, 2157−2159.
(8) (a) He, L. P.; Chen, T.; Gong, D.; Lai, Z. P.; Huang, K. W.
Organometallics 2012, 31, 5208−5211. (b) Prades, A.; Peris, E.;
Albrecht, M. Organometallics 2011, 30, 1162−1167.
Imine 2g: yellow oil (396 mg, 71% yield); H NMR (400 MHz,
CDCl3) δ 8.68 (s, 1H), 7.94 (d, J = 7.5 Hz, 1H), 7.31−7.18 (m, 7H),
4.84 (s, 2H), 2.52 (s, 3H), 2.40 (s, 3H); MS (EI) m/z = 223.1 (M+).9f
1
Imine 2h: yellow oil (580 mg, 91% yield); H NMR (400 MHz,
CDCl3) δ 8.28 (s, 1H), 7.70 (d, J = 8.8 Hz, 2H), 7.24 (d, J = 8.8 Hz,
2H), 6.86−6.92 (m, 4H), 4.71 (s, 2H), 3.81 (s, 3H), 3.77 (s, 3H); MS
(EI) m/z = 255.1 (M+).9f
1
(9) (a) Largeron, M.; Fleury, M.-B. Angew. Chem., Int. Ed. 2012, 51,
5409−5412. (b) Largeron, M.; Fleury, M.-B. Science 2013, 339, 43−44.
(c) Patil, R. D.; Adimurthy, S. Adv. Synth. Catal. 2011, 353, 1695−
1700. (d) Patil, R. D.; Adimurthy, S. RSC Adv. 2012, 2, 5119−5122.
(e) Hu, Z.; Kerton, F. M. Org. Biomol. Chem. 2012, 10, 1618−1624.
(f) Huang, B.; Tian, H. W.; Lin, S. S.; Xie, M. H.; Yu, X. C.; Xu, Q.
Tetrahedron Lett. 2013, 54, 2861−2864.
Imine 2i: yellow oil (472 mg, 74% yield); H NMR (400 MHz,
CDCl3) δ 8.36 (s, 1H), 7.41 (s, 1H), 7.34−7.26 (m, 3H), 7.00 (dq, J =
1.6, 7.6 Hz, 1H), 6.95−6.92 (m, 2H), 6.83 (dd, J = 2.4, 8.0 Hz, 1H),
4.81 (s, 2H), 3.85 (s, 3H), 3.82 (s, 3H); MS (EI) m/z = 255.1 (M+).9f
1
Imine 2j: white solid (946 mg, 81% yield); H NMR (400 MHz,
CDCl3) δ 8.56 (s, 1H), 8.27 (s, 2H), 7.97 (s, 1H), 7.85 (s, 2H), 7.82
(s, 1H), 4.98 (s, 2H); 19F NMR (376 MHz, CDCl3) δ −62.99 (s, 6F),
−63.15 (s, 6F); MS (EI) m/z = 467.1 (M+).17
(10) Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H. ChemCatChem
2010, 2, 1438−1443.
1
Imine 2k: yellow oil (269 mg, 52% yield); H NMR (400 MHz,
(11) Maruyama, K.; Kusukawa, T.; Highchi, Y.; Nishinaga, A. Chem.
Lett. 1991, 1093−1096.
CDCl3) δ 8.42 (s, 1H), 7.42 (d, J = 5.2, 1H), 7.33 (dd, J = 0.8, 3.6,
1H), 7.24 (dd, J = 1.2, 5.2, 1H), 7.07 (dd, J = 3.6, 4.8, 1H), 7.34−7.26
(m, 3H), 7.00 (dq, J = 1.6, 7.6 Hz, 1H), 6.98−7.01 (m, 2H), 4.95 (s,
2H); MS (EI) m/z = 207.0 (M+).12
(12) Liu, L.; Wang, Z.; Fu, X.; Yan, C.-H. Org. Lett. 2012, 14, 5692−
5695.
E
dx.doi.org/10.1021/jo5017212 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
(13) (a) Lang, X.; Ji, H.; Chen, C.; Ma, W.; Zhao, J. Angew. Chem.,
Int. Ed. 2011, 50, 3934−3937. (b) Lang, X.; Ma, W.; Zhao, Y.; Chen,
C.; Ji, H.; Zhao, J. Chem.Eur. J. 2012, 18, 2624−2631. (c) Furukawa,
S.; Ohno, Y.; Shishido, T.; Teramura, K.; Tanaka, T. ACS Catal. 2011,
1, 1150−1153. (d) Su, F.; Mathew, S. C.; Mohlmann, L.; Antonietti,
M.; Wang, X.; Blechert, S. Angew. Chem., Int. Ed. 2011, 50, 657−660.
(e) Park, J. H.; Ko, K. C.; Kim, E.; Park, N.; Ko, J. H.; Ryu, D. H.; Ahn,
T. K.; Lee, J. Y.; Son, S. U. Org. Lett. 2012, 14, 5502−5505.
(14) (a) Zhou, X. T.; Ren, Q. G.; Ji, H. B. Tetrahedron Lett. 2012, 53,
3369−3373. (b) Yuan, Q. L.; Zhou, X. T.; Ji, H. B. Catal. Commun.
2010, 12, 202−206.
(15) Guo, C. C.; Chu, M. F.; Liu, Q.; Liu, Y.; Duo, D. C.; Liu, X. Q.
Appl. Catal., A 2003, 246, 303−309 and the references cited therein.
(16) Jiang, G. X.; Chen, J.; Huang, J. S.; Che, C. M. Org. Lett. 2009,
11, 4567−4571.
(17) Glink, P. T.; Horn, M.; Ihringer, J.; Stoddart, J. F. Chem.Eur. J.
2003, 9, 4046−4054.
F
dx.doi.org/10.1021/jo5017212 | J. Org. Chem. XXXX, XXX, XXX−XXX