Rhodium complexes catalyze oxidative coupling between salicylaldehyde and phenylacetylene via C-H bond activation

Article abstract of DOI:10.1007/s11696-017-0153-4

A coupling reaction between salicylaldehyde and phenylacetylene was catalyzed by well-defined rhodium complexes, Rh(cod)(l-amino acid) (cod is 1,5-cyclooctadiene; l-amino acid is l-proline, l-phenylalanine and l-valine), to give a flavonoid in 40-88% yield, providing a method for flavonoid synthesis. The coupling reactions catalyzed by Rh(cod)(l-amino acid)s gave higher yields than those by [Rh(cod)Cl]₂ without l-amino acid ligands. The reaction mechanism may be that l-amino acid ligands of the rhodium complexes can provide an empty track for phenylacetylene to form a ring structure that fractures to produce the aim flavonoid and Rh^IX species. Then, the active Rh^IX specie is oxidized to regenerate Rh^IIIX₃ by Cu(OAC)₂.

Full text of DOI:10.1007/s11696-017-0153-4

Chem. Pap.
DOI 10.1007/s11696-017-0153-4
SHORT COMMUNICATION
Rhodium complexes catalyze oxidative coupling
between salicylaldehyde and phenylacetylene via C–H bond
activation
Hongge Jia1 Yanan Tang2 Yongqiang Shi1 Liqun Ma1 Zijian He1
•
•
•
•
•
Weiwei Lai1 Yi Yang1 Yazhen Wang1 Yu Zang1 Shuangping Xu1
•
•
•
•
Received: 23 September 2016 / Accepted: 23 February 2017
Ó Institute of Chemistry, Slovak Academy of Sciences 2017
Abstract A coupling reaction between salicylaldehyde
and phenylacetylene was catalyzed by well-defined rho-
dium complexes, Rh(cod)(L-amino acid) (cod is 1,5-cy-
clooctadiene; L-amino acid is L-proline, L-phenylalanine
and L-valine), to give a flavonoid in 40–88% yield, pro-
viding a method for flavonoid synthesis. The coupling
reactions catalyzed by Rh(cod)(L-amino acid)s gave higher
yields than those by [Rh(cod)Cl]2 without L-amino acid
ligands. The reaction mechanism may be that L-amino acid
ligands of the rhodium complexes can provide an empty
track for phenylacetylene to form a ring structure that
fractures to produce the aim flavonoid and RhIX species.
Then, the active RhIX specie is oxidized to regenerate
RhIIIX3 by Cu(OAC)2.
Introduction
In previous studies, transition-metal catalysts have been
investigated in the polymerization of monomers and
organic synthetic reactions (Baker et al. 2012; Riener et al.
2012). In both the synthesis of active bonds or functional
groups and the asymmetric polymerization of substituted
acetylenes, transition metals have always been used
(Nishimura et al. 2012). Rh-diene complexes exhibit high
catalytic activity in the asymmetric polymerization of
various substituted acetylenes to give one-handed helical
polymers (Jia et al. 2009; Liu et al. 2010; Teraguchi et al.
2010; Liu et al. 2013; Kaneko et al. 2013; Kim et al. 2016;
Jin et al. 2016). Chiral well-defined rhodium complexes,
Rh(cod)(L-Phe) (cod is 1,5-cyclooctadiene; Phe is pheny-
lalanine) and Rh(cod)(L-Val) (Val is valine), were synthe-
sized, isolated by recrystallization, and characterized (Jia
et al. 2016). The helix-sense-selective polymerization
(HSSP) of achiral 3,4,5-trisubstituted phenylacetylene, p-
dodecyloxy-m,m-dihydroxyphenylacetylene (DoDHPA)
was carried out using the two Rh complexes as catalysts.
All these results indicate that these well-defined Rh com-
plexes serve as excellent catalysts for the HSSP of
DoDHPA (Jia et al. 2016). However, these new Rh com-
plexes were not still used for other types of reactions, i.e.,
oxidative coupling reaction between aldehyde derivatives
and phenylacetylene via C–H bond activation.
Keywords Rhodium Á Oxidation Á Arene ligands Á
Carbonylation Á Flavone
Electronic supplementary material The online version of this
article (doi:10.1007/s11696-017-0153-4) contains supplementary
material, which is available to authorized users.
& Hongge Jia
1542151015@qq.com
& Yanan Tang
1327260670@qq.com
Some functionalized arenes have been obtained by
oxidative coupling reactions with alkenes in the presence
of a Pd catalyst and a Cu/air oxidant (Sahoo et al. 2012;
Youn et al. 2011). Only a few rhodium catalysts have been
used in oxidative coupling reactions (Shimizu et al. 2008;
Kokubo et al. 1997, 1999; Huang et al. 2013; Luo et al.
2015a, b). Recently, Mikael Brasse and co-workers
described the [Cp*RhCl]2 (Cp*= C5Me5)-catalyzed
& Liqun Ma
maliqun6166@163.com
1
College of Materials Science and Engineering, Key
Laboratory of Polymeric Composition and Modification,
Qiqihar University, Wenhua Street 42, Qiqihar 161006,
China
2
College of Chemistry and Chemical Engineering, Qiqihar
University, Wenhua Street 42, Qiqihar 161006, China
123

Chem. Pap.
oxidative coupling of styrene with 2-phenylpyridine
derivatives via C–H activation (Brasse et al. 2013). Their
mechanistic study found that the turnover-limiting step is
the migratory insertion of the alkene into the Rh-C(aryl)
bond. Later, Yuki Fukui and co-workers reported that two
tunable arylative cyclizations of 1,6-enynes were catalyzed
by [Cp*RhCl]2 via C–H activation of O-substituted N-hy-
droxybenzamides (Fukui et al. 2014), and the mechanistic
investigations of these two reactions clearly indicated that
the C–H bond cleavage process was also involved in the
turnover-limiting step.
0.06 mmol) in methanol (2.00 mL). The obtained was
stirred for 1 h at room temperature. The mixture was
cooled, and then the precipitate was separated by cen-
trifugation. The final product was a yellow powder, which
was recrystallized from dichloromethane (Jia et al. 2016).
1
Yield 52.7%. H-NMR (600 MHz, CDCl3, TMS, d): 4.46,
3.75 and 3.58 (3b, 4H, CH=CH); 3.86 (m, 1H, CH2CH2-
CHCOO); 2.96 (m, 2H, NCH2CH2); 2.55, 2.48 2.36 and
2.15 (4 m, 8H, HC=CHCH2CH2); 1.95 (m, 2H, CH2CH2-
CH2CHCOO); 1.89 (b, 1H, NH); 1.73 (b, 1H, COOH); 1.65
(m, 2H, NCH2CH2CH2CH). 13C-NMR (150 MHz, CDCl3,
TMS, d): 206.96, 132.11, 63.89, 55.43, 49.94, 30.94, 29.90,
25.58. IR (KBr): 3427.87, 2930.79, 1719.43, 1619.13,
1375.66, 1282.79, 1194.70, 1085.57, 934.80, 802.01,
728.21 cm-1. Elemental analysis for C13H19NO2Rh: calcd.
C 34.57, H 5.15, N 1.55; found C 34.65, H 5.24, N 1.50
(See Figs. S4–S6 in the Supporting Information).
However, the use of Rh-diene complexes, i.e., Rh(-
cod)(L-amino acid), has not been explored in oxidative
coupling reactions. In this study, a chiral catalyst, Rh(-
cod)(L-Pro) (cod is 1,5-cyclooctadiene; L-pro is L-proline),
was synthesized and purified by recrystallization, and the
oxidative coupling reaction between salicylaldehyde and
phenylacetylene was catalyzed by the rhodium and Cu(II)
catalysts (See Scheme 1). The catalytic efficiencies and
mechanisms of these catalysts are discussed.
Rh(cod)(L-Phe): This compound was prepared and
purified by a method similar to that for Rh(cod)(L-Pro) (Jia
1
et al. 2016). The H-NMR, 13C-NMR and IR spectra of
Rh(cod)(L-Phe) see Figs. S7–S9 in the Supporting
Information).
Experimental
Rh(cod)(L-Val): This compound was prepared and
Synthesis of Rh(cod)(L-Pro), Rh(cod)(L-Val), and Rh(-
cod)(L-Phe) : The chiral Rh catalysts, Rh(cod)(L-Pro),
Rh(cod)(L-Val) and Rh(cod)(L-Phe), were synthesized
according to Scheme 2. The all reaction procedures were
conducted under dry nitrogen.
purified by a method similar to that for Rh(cod)(L-pro) (Jia
1
et al. 2016). The H-NMR, 13C-NMR and IR spectra of
Rh(cod)(L-Pro) See Figs. S10–S12 in the Supporting
Information).
Synthesis of 2-phenyl-4H-chromen-4-one (3): Rh(-
cod)(L-phenylalanine) (2.2 mg, 0.01 mmol), C5H2Ph4
(7.2 mg, 0.02 mmol) and Cu(OAc)2ÁH2O (200.0 mg,
1.00 mmol) were placed in a two-necked flask. Then, sal-
icylaldehyde (0.50 mL, 0.50 mmol), phenylacetylene
(0.50 mL, 0.50 mmol) and o-xylene (2.80 mL, 0.04 mmol)
were added, and the resulting mixture was stirred under N2
at 120 °C for 5.5 h. The raw product was extracted with
ether, and the final product was isolated by column chro-
matography on silica gel with hexane. The solid products
were recrystallized from hexane to give a yield of 88%
(white crystals). GC spectrum appeared one peak at
11.196 min. 1H-NMR (600 MHz, DMSO-d6, TMS, d):
7.62 (d, 2H, HC–C–C=O in Ph and C=CH–O), 7.61 (d, 2H,
HC–C–C=C–O in o-Ph to alkenyl), 7.51 (m, 1H, HC=C–
C–O in m-Ph to oxygen), 7.50 and 7.49 (2t, 2H, HC–C–C–
C=C–O in m-Ph to alkenyl), 7.46 (s, 1H, HC–C–C–C–
C=C–O in p-Ph to alkenyl), 7.45 (s, 1H, HC=C–C–C=O in
m-PhH to carbonyl), 7.44 (t, 1H, HC–C–O in Ph). 13C-
NMR (150 MHz, DMSO-d6, TMS, d): 73.91, 76.81, 77.02,
77.23, 81.56, 121.82, 128.45, 129.22, 132.52 ppm. IR
(KBr): 3049, 2952, 2925, 2854, 2190, 1740, 1484,
1251 cm-1 (see Figs. S13–S15). Anal Calcd for C15H10O2:
calcd. C 91.43, H 7.20, O 1.37; found C 91.08, H 6.50, O
2.42. Additional data for the products of catalysis by the
rhodium catalysts are listed in Table 1.
[Rh(cod)Cl]2: This compound was synthesized and
purified according to the literature procedure (Staubitz
et al. 2015). Yield 55%. 1H-NMR (600 MHz, CDCl3,
TMS, d): 4.42 (s, 8H, CH=CH), 2.50 (s, 8H, CHCH2CH2),
1.76 (s, 8H, CHCH2CH2). 13C-NMR (150 MHz, CDCl3,
TMS, d): 78.63, 30.85. IR (KBr): 3433, 2988, 2936, 2873,
2828, 1640, 1468, 1299, 1151, 994, 960, 867, 816, 775,
486 cm-1 (See Figs. S1–S3 in the Supporting Information).
Rh(cod)(L-Pro): A solution of L-proline (14.10 mg,
0.13 mmol) and NaOH (4.88 mg, 0.12 mmol) in H2O
(0.60 mL) was added with [Rh(cod)Cl]2 (30.00 mg,
Cu oxidant,
N
H
*
O
O
Rh
O
O
H
Rh(cod)(L-Pro)
+
H2
O
OH
N
R
*
Rh
3
1
2
O
O
-R=
:
:
-CH2C6H5
-CH(CH3)2
Rh(cod)(L-Phe)
Rh(cod)(L-Val)
Scheme 1 Oxidative coupling reaction of salicylaldehyde and
phenylacetylene using Rh(cod)(L-Pro), Rh(cod)(L-Phe) and Rh(-
cod)(L-Val) and Cu(II) as catalysts
123

Chem. Pap.
Scheme 2 Synthetic route to
Rh(cod)(L-Pro)
.
Cl
Cl
N
H
RhCl3 3H2O
L-Proline, NaOH
CH3OH, rt
*
Rh
Rh
Rh
ethanol, H2O
O
O
cod
[Rh(cod)Cl]2
Rh(cod)(L-Pro)
Rh(cod)(L-Pro) showed that the methyne peaks of the L-
proline residue were detected at d 3.86 ppm, and the
methylene peaks of the L-proline residue were detected at d
2.96, 1.95, and 1.65 ppm . The coordination of L-Phe to
rhodium is confirmed. Judging from the IR spectra, 13C-
NMR spectra, and elemental analysis results, the novel
chiral rhodium complex (Rh(cod)(L-Pro)) was synthesized .
Confirmation of 2-phenyl-4H-chromen-4-one (3): In an
initial attempt to carry out the desired coupling reaction
between salicylaldehyde (1) and phenylacetylene (2), these
reagents were treated with Rh catalysts in o-xylene at
120 °C for 5.5 h (Scheme 1). The C–H bond of the alde-
hyde in salicylaldehyde was activated by the Rh catalysts
and underwent oxidative coupling with the triple bond in
phenylacetylene to give white crystals (3), which were
Table 1 Oxidative coupling reactions between salicylaldehyde (1)
and phenylacetylene (2) using Rh(cod)(L-Phe), Rh(cod)(L-Val), Rh(-
cod)(L-Pro) and [Rh(cod)Cl]2 as catalysts
No.
Rh catalyst
Amine cocatalyst
Yield (%)
1
2
3
4
5
6
7
Rh(cod)(L-Phe)
Rh(cod)(L-Val)
Rh(cod)(L-Pro)
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
RhCl3
None
None
None
(S)-PEA
TEA
88.0
81.2
76.2
56.8
55.5
58.0
0.0
None
None
In o-xylene at 120°C for 5.5 h with [1]/[2]/Rh catalyst]/[cocata-
lyst] = 1:1:0.02:0.1 (in mmol)
1
recrystallized from hexane. In the H-NMR spectrum of
Materials
2-phenyl-4H-chromen-4-one (3) in DMSO-d6, the chemi-
cal shifts of the aromatic residue that connects the carbonyl
group occurred at d 7.62, 7.51, 7.45 and 7.44 ppm. The
aromatic peaks that connect the C=C bond were detected at
d 7.61, 7.50 and 7.66 ppm. The proton peak of the double
All the solvents were dried by standard methods and dis-
tilled over CaH2 under reduced pressure before use.
Commercially available compounds were used without
further purification. RhCl3Á3H2O and all other chemicals,
including the L-amino acids, were purchased from Shang-
hai Darui Finechem Ltd.
1
bond was located at d 7.62 ppm. Judging from the H-
NMR, 13C-NMR and IR spectra, it is concluded that the
novel compound (3) was successfully synthesized, pro-
viding a new method for the synthesis of flavonoids.
Rh catalysts possessing L-amino acids as ligands cat-
alyzing the oxidative coupling of salicylaldehyde: When
the new Rh(cod)(L-amino acid)s were used as the catalyst
in the coupling reaction between salicylaldehyde and
phenylacetylene, excellent results (Table 1, nos. 1–3;
yields of 76.2–88.0%) were obtained that were much better
than the results obtained using [Rh(cod)Cl]2/(R)-
phenylethylamine ((R)-PEA) or triethylamine (TEA) as a
catalytic system (Table 1, entries 4–6; yields of
55.5–58.0%). Why is it that the introduction of an L-amino
acid as a ligand in a Rh catalyst improves the yields so
much? These Rh complexes are very active, unstable. It is
easy to react with O atom, N atom or unsaturated bond to
form a new active Rh species. We believe that the Rh-
amino acid complex is the actual active species in the
coupling reaction. L-Amino acids can form two coordina-
tion bonds with Rh, Rh-O and Rh-N bonds. In the reaction,
Instruments
GC analysis was carried out 7280A GC System, HP-5
column (i.d. 30 m9320 lm90.25 lm).Infrared (IR) spec-
tra were recorded using a Perkin-Elmer Spectrum One B IR
spectrophotometer. Mass spectrometric analysis was per-
formed with a Thermo Q Exactive mass spectrometer
(Thermo Fisher Scientific, United States). An NMR
instrument [Bruker, Germany; 1H (600 MHz) and 13C
(150 MHz)] was used to determine the molecular structures
of the products. Elemental analysis was carried out on a
Perkin-Elmer 2400 II CHNS/O instrument.Yield = Product
weight 7 (Mole number of feeding compound 9 Product
molecular weight).
Results and discussion
´
because the Rh-N bond is active (Azpıroz et al. 2014), the
Confirmation of Rh(cod)(L-pro): We refined the rhodium
catalyst, Rh(cod)(L-Pro) by recrystallization to yield yellow
solid (Jia et al. 2016). The 1H-NMR spectrum of
O atom in the phenolic hydroxyl group of salicylaldehyde
can replace the N atom to form a new Rh–O bond. The
123

Chem. Pap.
O
original Rh–O bond in Rh(cod)(L-amino acid) may provide
an empty track for phenylacetylene. Hence, the Rh(cod)(L-
amino acid)s catalysts play an important role in the cat-
alytic process. A detailed discussion of this reaction
mechanism will be presented in a later section.
H
H2
N
R
O
*
O
OH
Rh
Rh
H
O
O
O
Rh(cod)(L-amino acid)
O
NH2
The new viewpoint that Rh–O can provide acetylene
with an empty track in Rh is supported by the following
results. Among the three types of Rh catalysts containing L-
amino acids as ligands, Rh(cod)(L-Phe) has the largest
aromatic side group. When Rh(cod)(L-Phe) was used as the
catalyst, the reaction gave the highest yield (88.0%). The
fact that L-phenylalanine, containing a large aromatic
group, can dissociate easily form Rh and provide an empty
track may be cause of this high yield.
*
R
a
Cu(OAC)2
O
H2N
R
H2N
R
O
*
*
O
O
OH
OH
L-amino acid
L-amino acid
O
O
Rh
Rh
H
O
O
In the Rh-catalyzed asymmetric polymerization of sub-
stituted phenylacetylene, a certain type of amine (such as
(R)- or (S)-PEA, TEA or L-amino acids) is necessary as a
cocatalyst (Jia et al. 2010). The amine can coordinate to the
rhodium catalyst and form an active species in the asym-
metric polymerization of the substituted phenylacetylene.
The amine ligand of the catalyst does not dissociate in the
initiation stage and remains a part of the propagating spe-
cies (Staubitz et al. 2015). However, the yield of the
oxidative coupling reaction catalyzed by [Rh(cod)Cl]2
without an amine cocatalyst was close to those with
[Rh(cod)Cl]2/(R)-PEA or TEA (Table 1, nos. 4–6).
Therefore, the amines did not play an important role in the
coupling reaction between salicylaldehyde and pheny-
lacetylene. We believe that the O atom of the phenolic
hydroxyl group in salicylaldehyde takes priority over the N
atom of the amine when coordinating to the rhodium atom.
This opinion is consistent with our later mechanistic
discussion.
c
b
Conclusions
Scheme 3 Plausible mechanism for the oxidative coupling reaction
catalyzed by Rh(cod)(L-amino acid)s
hydroxyl groups, and the carbonyl attacks the Rh atom to
give an empty track with ejection of the L-amino acid (b).
Third, the triple bond of phenylacetylene enters into the
empty track and coordinates to the rhodium catalyst to
form a ring structure (c). Then, the Rh–O bond in the ring
structure fractures to produce flavonoid (3) and RhIX
species. Finally, the active RhIX specie is oxidized to
regenerate RhIIIX3 by Cu(OAC)2 (Shimizu et al. 2008).
When only RhCl3 was used as the catalyst, the oxidative
coupling reaction was not catalyzed (Table 1, no. 7).
Comparison of this result to that obtained with [Rh(-
cod)Cl]2 indicates that a diene ligand (cod) is necessary.
The diene ligand may be retained throughout the whole
catalytic process (See Scheme 3).
Conclusions
In summary, we have demonstrated that salicylaldehyde
undergoes aldehyde C–H bond activation in the presence of
novel rhodium catalysts with different ligands to produce
2-phenyl-4H-chromen-4-one. The role of L-amino acid
ligands (L-pro, L-Phe or L-Val) in Rh complexes in oxida-
tive coupling between salicylaldehyde and phenylacetylene
via C–H bond activation was discussed in this paper. L-phe
containing a large aromatic group can dissociate easily to
form Rh and provide an empty track, which can improve
catalytic activity of Rh complexes.
Mechanism
According to the above discussions, a plausible mechanism
for the oxidative coupling reaction between salicylalde-
hyde and phenylacetylene is illustrated in Scheme 3 (Shi-
mizu et al. 2008; Kokubo et al. 1997, 1999). First, because
the O atom has stronger donating and accepting capacities
than the N atom (Luo et al. 2015a, b), the R–N bond of
Rh(cod)(L-amino acid) is replaced by the O atom of the
phenolic hydroxyl group in salicylaldehyde to produce a
rhodium(III) intermediate (a). Second, the electronegativity
of the carbonyl in salicylaldehyde is improved by the ortho
Acknowledgements This work was supported by the National Nat-
ural Science Foundation of China (U1162123) and (51103076), the
Natural Science Foundation of Heilongjiang Province of China
(LC2011C12), the Overseas Scholars Foundation of the Education
Department of Heilongjiang Province of China (1251H012) and the
New Century Excellent Talents of the Education Department of
Heilongjiang Province of China (1253-NCET-24).
123

Chem. Pap.
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