Cyclometalation of anthyridine-based ligands with dirhodium acetates: Structure and catalytic activity

Article abstract of DOI:10.1021/om400508a

Coordination of 2,8-Ar₂-5-phenylanthyridines (Ar = 2-thienyl, 1a; Ar = 2-ClC₆H₄-, 1b) with dirhodium tetraacetate yielded the cyclometalated complexes [Rh₂(OAc)₃(metalated- 1a)] (3a) and [Rh₂(OAc)₃(metalated-1b)] (3b), respectively. Under acidic conditions, cleavage of the Rh-C bond in 3a,b took place to give the corresponding coordination complexes 4a,b. Treatment of 3a,b with PPh₃ led to the phosphine-cyclometalated species [Rh ₂(OAc)₂{P,C-(C₆H₄)PPh ₂}(metalated-1a)] (5a) and [Rh₂(OAc)₂{P,C- (C₆H₄)PPh₂}(metalated-1b)] (5b), respectively. These new dirhodium complexes have been structurally characterized by NMR spectroscopy, and some representative compounds were also analyzed by X-ray methods. The use of these newly prepared dirhodium complexes as catalysts for the allylic oxidation of cyclohexenes was investigated.

Full text of DOI:10.1021/om400508a

Article
pubs.acs.org/Organometallics
Cyclometalation of Anthyridine-Based Ligands with Dirhodium
Acetates: Structure and Catalytic Activity
Da-wei Huang, Ying-Hao Lo, Yi-Hung Liu, Shie-Ming Peng, and Shiuh-Tzung Liu*
Department of Chemistry, National Taiwan University, Taipei, Taiwan 106
S
* Supporting Information
ABSTRACT: Coordination of 2,8-Ar2-5-phenylanthyridines
(Ar = 2-thienyl, 1a; Ar = 2-ClC6H4−, 1b) with dirhodium
tetraacetate yielded the cyclometalated complexes [Rh2(OAc)3-
(metalated-1a)] (3a) and [Rh2(OAc)3(metalated-1b)] (3b),
respectively. Under acidic conditions, cleavage of the Rh−C
bond in 3a,b took place to give the corresponding coordination
complexes 4a,b. Treatment of 3a,b with PPh3 led to the
phosphine-cyclometalated species [Rh2(OAc)2{P,C-(C6H4)PPh2}-
(metalated-1a)] (5a) and [Rh2(OAc)2{P,C-(C6H4)PPh2}-
(metalated-1b)] (5b), respectively. These new dirhodium
complexes have been structurally characterized by NMR spectros-
copy, and some representative compounds were also analyzed
by X-ray methods. The use of these newly prepared dirhodium complexes as catalysts for the allylic oxidation of cyclohexenes was
investigated.
INTRODUCTION
■
Cyclometalated dirhodium acetato complexes constitute a class
of interesting organometallic molecules in both bioinorganic1
and catalytic processes.2 These complexes are generally
prepared via the ortho metalation of an aromatic ring from
the donor system (Scheme 1). It is generally accepted that the
axial coordination position of the dinuclear cage is the active
(2) with methyl aryl ketones proceeded smoothly to give the
desired compounds in good yields, and these compounds were
characterized by NMR spectroscopy.
With the anthyridine ligands in hands, we first examined the
coordination of 1a,b toward dirhodium(II) tetraacetate.
Thermal reaction of 1a with an equimolar amount of
[Rh2(CH3COO)4] in chloroform provided the rhodium
complex 3a exclusively (Scheme 2): i.e., no other metal
site for C−H activation or catalysis.2b,3 However, metalation
takes place at the equatorial position in all of the reported
dirhodium acetato species (Scheme 1),4−7 presumably due to
the ring strain of the resulting metallacycles (five- vs four-membered
ring). In this report, we demonstrate the cyclometalation of
dirhodium acetato complexes with anthyridines 1a,b occurring at
the axial position through the ligand design. Furthermore, the
catalytic activity of these complexes is investigated.
complex was formed. The diamagnetic dirhodium(II) complex
3a shows nine sets of signals for thienyl and anthyridinyl
RESULTS AND DISCUSSION
protons (Table 1), revealing that the C2 symmetry of the ligand
1a is no longer maintained in the complex. The 1H NMR signal
corresponding to the C-1 position of the thienyl ring
disappears, and the 13C NMR signal corresponding to the
C-1 carbon appears as a doublet with JRh−C = 29.4 Hz, clearly
indicating the metalation taking place at that position. The two
sets of signals from the methyl groups of acetate ligands in 3a
indicate the dissymmetry introduced by the ligand 1a.
In order to unambiguously characterize this compound, the
crystal structure of 3a was determined. The molecular structure
of 3a is depicted in Figure 1, whereas selected bonding
parameters are summarized in Table 2. The two rhodium atoms
■
Dirhodium Complexes 3a and 4a. In order to achieve
cyclometalation at the axial position with the dirhodium acetato
complex, the ligands must have an accessible C−H group near
that position. In an earlier finding, we learned that C−H
activation does not occur in the complexation of 2,7-diaryl-1,8-
naphthyridine with Rh2(OAc)4, presumably due to steric
hindrance. Thus, we thought that ligands 1a,b are possible
candidates for this investigation. The nitrogen donors in the
anthyridine rings of 1a,b are capable of binding with Rh2(OAc)4 at
equatorial positions as a bridging ligand,8 whereas the ortho
hydrogens of the side-arm aryl rings are readily available for
cyclometalation. Ligands 1a,b were prepared by a double-
Friedlander reaction according to the literature procedure
Received: June 3, 2013
̈
(eq 1).9 Reaction of 2,6-diamino-3,5-pyridinedicarboxaldehyde
© XXXX American Chemical Society
A
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Scheme 1. C−H Activation with Dirhodium Acetate Complexes
Scheme 2
1
Table 1. H NMR Shifts (ppm) of Ligand 1a and the Related Complexes 3a and 4a
compd
1H NMR shifts of thienyl rings
1H NMR shifts of anthyridine
7.99 (d), 7.71 (d)
1a
3a
4a
7.86 (d), 7.57 (d), 7.16 (m)
8.29 (d, H-2), 7.63(d, H-8), 7.62 (t, H-9), 7.49 (d, H-10), 7.32 (d, H-3)
8.46 (d, H-4), 8.14 (d, H-5), 7.92 (d, H-7), 7.78 (d, H-6)
8.43 (d, H-4), 8.14 (d, H-5), 8.26 (d, H-6), 8.09 (d, H-7)
8.28 (d, H-1), 8.19 (d, H-10), 8.17 (d, H-3), 7.87 (d, H-8), 7.63 (t, H-2), 7.35 (t, H-9)
are bridged by the three acetate groups and two nitrogen
donors of the anthyridine fragment in 1a. One of the thienyl
groups in 1a occupies the axial site trans to the Rh−Rh bond
with a Rh(1)−C(7) distance of 2.064(6) Å. The C(7)−Rh(1)−
Rh(2) angle is 170.2(2)°, consistent with the metalation at the
axial position of the dirhodium system. The Rh−Rh bond
distance of 2.4399(6) Å for 3a is similar to those found in the
equatorial-metalated dirhodium complexes with a rhodium−
rhodium single bond,5d indicating no significant influence of
this axial ligand. The Rh−N and Rh−O distances are all within
the normal range for this type of compound. The lengths of
Rh−O trans to nitrogen donors are slightly longer than those
trans to oxygen donors (bridging acetates). It is noteworthy
that there is no coordinating ligand on the other axial position
of the dirhodium core. The distance between Rh(2) and S(2) is
4.336 Å, indicating that there is no coordination of the sulfur
donor toward the metal center.
It has been demonstrated that the cyclometalation of
dirhodium complexes with thienylphosphines is a reversible
process, as evidenced by the H/D exchange study.5d Treatment
B
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Figure 1. ORTEP plot of the crystal structure of 3a (30% probability
ellipsoids).
Figure 2. ORTEP plot of complex 3b (30% probability ellipsoids).
Table 2. Selected Bond Distances (Å) and Bond Angles
(deg) for 3a
Table 3. Selected Bond Distances (Å) and Bond Angles
(deg) for 3b
Rh(1)−N(1)
Rh(1)−C(7)
Rh(1)−O(1)
Rh(1)−O(5)
2.011(5)
Rh(2)−N(2)
Rh(1)−Rh(2)
Rh(1)−O(3)
Rh(2)−O(2)
2.008(5)
2.4399(6)
2.074(4)
2.051(4)
2.035(4)
175.84(18)
175.97(16)
127.3(6)
91.69(17)
90.20(18)
Rh(1)−N(1)
Rh(1)−C(7)
Rh(1)−O(1)
Rh(1)−O(5)
1.991(5)
2.023(5)
2.046(4)
2.039(4)
2.048(4)
171.4(2)
173.79(16)
174.05(15)
91.58(17)
91.42(18)
Rh(2)−N(2)
Rh(1)−Rh(2)
Rh(1)−O(3)
Rh(2)−O(2)
2.001(4)
2.4363(6)
2.056(4)
2.034(4)
2.030(4)
177.04(16)
174.17(15)
126.1(5)
92.94(17)
90.08(17)
2.064(6)
2.045(4)
2.035(4)
Rh(2)−O(4)
2.058(4)
Rh(2)−O(6)
Rh(2)−O(4)
Rh(2)−O(6)
C(7)−Rh(1)−Rh(2)
N(2)−Rh(2)−O(4)
O(6)−Rh(2)−O(2)
N(1)−Rh(1)−O(1)
N(1)−Rh(1)−O(5)
170.23(18)
175.34(18)
174.20(16)
91.45(18)
89.44(17)
N(1)−Rh(1)−O(3)
O(5)−Rh(1)−O(1)
O(2)−C(1)−O(1)
N(2)−Rh(2)−O(2)
N(2)−Rh(2)−O(6)
C(7)−Rh(1)−Rh(2)
N(2)−Rh(2)−O(4)
O(6)−Rh(2)−O(2)
N(1)−Rh(1)−O(1)
N(1)−Rh(1)−O(5)
N(1)−Rh(1)−O(3)
O(5)−Rh(1)−O(1)
O(2)−C(1)−O(1)
N(2)−Rh(2)−O(2)
N(2)−Rh(2)−O(6)
of 3a with ammonium hexafluorophosphate readily caused the
cleavage of the Rh−C bond to yield 4a, which was identified
by spectroscopic characterization. Partial 1H NMR data are
summarized in Table 1. The shift at δ 8.28 comes from the
C-1 proton, which was confirmed by a deuterium-labeling
experiment. This cleavage of the Rh−C bond is quite similar to
those in dirhodium complexes cyclometalated at equatorial
positions.5
Dirhodium Complexes 3b and 4b. The coordination
chemistry of ligand 1b toward dirhodium(II) tetraacetate is
quite similar to that of 1a. Substitution of Rh2(OAc)4 with an
equimolar amount of 1b gave 3b as the exclusive product,
which was isolated in 86% yield. Recrystallization from a
CHCl3/hexane solution gave 3b as a dark red crystalline solid.
NMR experiments and an X-ray crystal structure determination
support the structure assignment for complex 3b. The 13C
NMR signal corresponding to the ortho position of the
“ClC6H4” group appears to be a doublet at δ 174.4 with
dirhodium unit is occupied by the carbon donor with an Rh(1)−
C(7) distance of 2.023(5) Å. The Rh(1)−Rh(2)−C(7) angle,
171.4(2)°, deviates from linearity most likely due to the strain of
the resulting cyclometalated ring. Other bond distances and angles
are in the normal ranges.
Under acidic conditions, cleavage of the Rh−C bond in
complex 3b took place to give 4b, which is similar to the case
for 3a. However, isolation of a pure form of 4b was not
possible, due to its rapid conversion back to 3b. Structural
1
assignment to 4b was achieved by H NMR spectroscopy.
Substitution of PPh3 with 3. Reaction of 3a with 1 equiv
of triphenylphosphine in chloroform at room temperature yield-
ed the phosphine-substituted complex 5a exclusively (eq 2).
J
Rh−C = 30 Hz, clearly suggesting bonding between the rhodium
1
and carbon centers. H NMR data are also consistent with the
proposed structure (Experimental Section).
The structure of dirhodium complex 3b is depicted in Figure 2,
while selected bond distances and angles are presented in Table 3.
It consists of one dirhodium unit with a Rh−Rh bond length of
2.4363(6) Å. The four equatorial sites are occupied by anthyridinyl
nitrogen donors, acting as a N∼N bridging ligand, and three
bridging acetate groups. One of the axial positions of the
The formation of 5a is corroborated by its NMR spectrum and
crystallography. Its 31P NMR spectrum shows a doublet at δ 30.2
with JRh−P = 164 Hz, indicating a single isomeric form of the
C
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product. The 13C NMR spectrum of 5a exhibits two doublets at δ
191.3 (JRh−C = 37.5 Hz) and 134.2 (JRh−C = 23.5 Hz), which are
assigned to the carbons of Cthienyl−Rh and Cphenyl−Rh,
respectively. The absence of a phosphorus−carbon coupling
constant (JP−C) for the carbon shift of Cthienyl−Rh clearly suggusts
that the coodination of phosphorus occurs on the other rhodium
metal center, which is further confirmed by the crystallographic
analysis. Concerning the regioslectivity of substitution, it appears
that the substitution of triphenylphosphine takes place on the
rhodium center away from that with the metalated aryl group,
presumably due to steric hindrance.
Rh(2)−O(2) bond (2.121(3) Å, trans to the phosphorus
donor), which is due to the trans influence of M−C versus
M−P bonds. Other structural parameters are within the normal
range.
Similarly, complex 3b reacted with triphenylphosphine to
give 5b exclusively. The structure of this complex has been
characterized by NMR spectroscopy and elemental analysis
(Experimental Section). The 31P NMR spectrum of 5b shows a
doublet at δ 34.6 with JRh−P = 163 Hz, which is comparable to
that of 5a. The carbon shifts of CCl‑Ph−Rh and Cphenyl−Rh in
the 13C NMR of 5b appear at δ 177.4 (JRh−C = 26.9 Hz) and
163.3 (JRh−C = 23.5 Hz) ppm, respectively. All these data,
similar to those of 5a, are characteristic for the cyclometalated
species.
Single crystals of complex 5a suitable for X-ray analysis were
obtained from a CH2Cl2/hexane solution. The crystallographic
analysis confirms the structure of 5a (Figure 3). Important
Catalytic Allylic Oxidation. Dirhodium complexes have
been reported to have catalytic activity in the allylic oxidation of
olefins with peroxide under mild conditions.10,11 However, the
ligands have significant influence on the activity. In this context,
Doyle and co-workers demonstrated dirhodium tetracaprolac-
tamate [Rh2(Cap)4] to be an excellent catalyst for allylic
oxidation.11 Thus, with these dirhodium complexes in hand,
their catalytic activities toward the oxidation of cyclohexenes
were investigated. We would particularly like to examine the
axial ligand effect on the catalysis, since the axial position in
complexes 3−5 is capped with a carbon donor.
Figure 3. Molecular structure of 5a (30% probability ellipsoids).
bond distances and angles are given in Table 4. In the structure
the two rhodium atoms are bridged by two acetate ligands, by a
Table 4. Selected Bond Distances (Å) and Bond Angles
(deg) for 5a
We first examined the oxidation of 1-phenylcyclohexene with
TBHP as a model reaction for surveying reaction parameters
such as catalysts, bases, oxidants, and solvents (eq 3). The
Rh(1)−N(1)
Rh(1)−C(7)
2.005(4)
Rh(1)−Rh(2)
Rh(2)−N(2)
2.5102(5)
2.031(4)
2.000(4)
Rh(1)−O(1)
2.210(3)
Rh(2)−O(2)
2.121(3)
Rh(1)−O(3)
2.070(3)
Rh(2)−O(4)
2.072(3)
Rh(1)−C(32)
2.017(4)
Rh(2)−P(1)
2.1895(12)
171.08(13)
178.35(9)
88.62(3)
C(7)−Rh(1)−Rh(2)
N(1)−Rh(1)−O(3)
O(1)−Rh(1)−O(3)
C(32)−Rh(1)−O(1)
C(32)−Rh(1)−N(1)
165.63(13)
175.49(13)
90.43(12)
178.71(15)
89.91(16)
N(2)−Rh(2)−O(4)
P(1)−Rh(2)−O(2)
P(1)−Rh(2)−Rh(1)
P(1)−Rh(2)−N(2)
C(32)−Rh(1)−C(7)
typical experimental conditions are as follows: a mixture of
1-phenylcyclohexene (0.32 mmol), K2CO3 (0.16 mmol),
rhodium complex (3.2 × 10−4 mmol), and t-BuOOH (1.58 mmol)
in a solvent (2 mL) was stirred at ambient temperature for 10 h. The
results are summarized in Table 5.
With complex 3a as the catalyst, it appears that a number of
bases do assist the reactions, but sodium carbonate gives the
best result (Table 5, entries 1−4). Using a solvent such as
dichloromethane is important in obtaining the best results. The
precatalyst 3a shows a poor activity in other organic solvents
(Table 5, entries 8−10). As for the oxidant, TBHP appears to
be the best reagent. It is noted that the reactions using H2O2
and K2S2O8 as the oxidants did not perform well, presumably
due to the solubility in dichloromethane.
92.46(10)
91.91(18)
PPh3 metalated in one phenyl ring, and by two nitrogen donors
of the anthyridinyl ligand. The Rh−Rh bond distance is
2.5102(5) Å, slightly longer than those in 3a,b, which compares
well to those observed in the related complexes. The
conformation of the metalated phosphine appears to be in an
“envelope” form, which is normal in these types of cyclo-
metalated dirhodium complexes. It is noted that cyclo-
metalations of thienyl and phenyl rings occur at the same
rhodium metal center. The Rh(1)−C(7) bond trans to the
Rh−Rh bond, 2.000(4) Å, is quite similar to that trans to the
oxygen donor, Rh(1)−C(32) = 2.017(4) Å, showing a small
effect of the trans influence of the metal−metal bond on this
structural parameter. The Rh(1)−O(1) bond distance
(2.210(3) Å, trans to the carbon donor) is longer than the
Catalyst screening suggests that both complexes 3a and 5a
give rise to the best catalysts for the oxidation of 1-phenyl-
cyclohexene, indicating that the phosphine substituted at an
D
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Summary. The two new dirhodium(II) compounds 3a,b
with metalation at the axial position have been synthesized by
the reaction of [Rh2(OAc)4] with the designed ligands 1a,b.
Further treatment of 3a,b with triphenylphosphine led to
metalation of the equatorial phosphine, providing the
corresponding dirhodium complexes 5a,b. Both 3a and 5a
appear to be good catalysts in the allylic oxidation, presumably
due to the influence of the axial ligand in the dirhodium
complexes. Studies on the catalytic reactivity and ligand effects
on these complexes are ongoing in our laboratory.
Table 5. Results of Oxidation of 1-Phenylcyclohexene with
TBHP
a
entry
catalyst
3a
base
oxidant
solvent
yield, %
1
Na2CO3
K3PO4
DBU
TBHP
TBHP
TBHP
TBHP
H2O2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
ether
71
2
3a
50
3
3a
58
4
3a
Et3N
14
5
3a
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
trace
39
6
3a
O2
7
3a
K2S2O8
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
13
8
3a
25
EXPERIMENTAL SECTION
■
9
3a
32
General Information. Nuclear magnetic resonance spectra were
recorded in CDCl3 on a Bruker AVANCE 400 or Bruker AVIII
800 MHz spectrometer. Chemical shifts are given in parts per million
relative to Me4Si for 1H and 13C NMR and relative to 85% H3PO4 for
31P NMR. All reaction and manipulation steps were performed under a
dry nitrogen atmosphere. Tetrahydrofuran was distilled under nitrogen
from sodium benzophenone ketyl. Dichloromethane was dried over
CaH2 and distilled under nitrogen. Other chemicals and solvents
were of analytical grade and were used after a degassing process.
2,6-Diamino-4-phenylpyridine-3,5-dicarbaldehyde (2) was prepared
according to the literature method.12
10
11
12
13
14
15
3a
EtOAc
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
35
3a
75
3b
52
4a
15
5a
77
Rh2(OAc)4
trace
a
Reaction conditions: catalyst (3.2 × 10−4 mmol), 1-phenylcyclohex-
ene (0.32 mmol), base (0.16 mmol), and oxidant (1.58 mmol) in
solvent (1.2 mL) at ambient temperature with stirring for 10 h.
5-Phenyl-2,8-di-2-thienylanthyridine (1a). A mixture of 2,6-
diamino-4-phenylpyridine-3,5-dicarbaldehyde (0.1 g, 0.415 mmol) and
1-(thiophen-2-yl)ethanone (0.15 mL, 174 mg, 1.38 mmol) in ethanol
(4 mL) was heated to reflux under a nitrogen atmosphere. After
30 min, a 10% NaOH aqueous solution (0.25 mL) was added. The
mixture turned dark green immediately, which then slowly changed to
brown upon heating for another 12 h. After removal of solvents under
reduced pressure, the residue was passed through a flash column with
DCM as the eluent. The filtrate was reprecipitated with DCM/diethyl
ether three times. The desired product was obtained as a light yellow
solid (163 mg, 93%): mp 291−291.5 °C; 1H NMR (400 MHz,
CDCl3) δ 7.99 (d, J = 8 Hz, 2 H), 7.86 (d, J = 4 Hz, 2 H), 7.71 (d, J =
8 Hz, 2 H), 7.63 (m, 3 H), 7.57 (d, J = 8 Hz, 2 H), 7.46 (m, 2 H), 7.16
(m, 2 H); 13C NMR (100 MHz, CDCl3) δ 157.5, 156.3, 150.3, 145.0,
136.3, 134.1, 131.4, 130.5, 129.1, 128.8, 128.2, 119.2, 118.3. ESI-
HRMS m/z for [M + H]+: calcd 422.0786 (C25H16N3S2); found
422.0785.
equatorial position does not affect the catalytic activity. A
comparison of entries 11, 13, and 14 shows that the coordination
of an axial ligand on the dirhodium core causes a major difference
in activity: i.e., the catalytic activities of 3a and 5a are better than
that of 4a. The catalytic activity of complex 3b, in which the axial
ligand is a chlorophenyl group, is slightly less active than that of 3a
(Table 5, entry 12). Nevertheless, the catalytic activities of these
complexes are much better than that of Rh2(OAc)4 (Table 5,
entry 15), showing the ligand effect on the metal complexes.11b
With optimized conditions, we examined a series of
cyclohexene derivatives containing various functional groups.
Table 6 summarizes the results of the allylic oxidation. We were
a
Table 6. Results of Allylic Oxidation of Cyclohexenes
2,8-Bis(2-chlorophenyl)-5-phenyl-1,9,10-anthyridine (1b). A
mixture of 2 (0.1 g, 0.42 mmol) and 2′-chloroacetophenone (0.16 g,
1.1 mmol) in ethanol (4 mL) was heated at 60 °C for 2 h. Then, a
solution of 10% KOH in EtOH (0.2 mL) was added to the above
reaction mixture. The solution turned brown immediately. The
resulting mixture was heated to reflux for 12 h. Upon cooling, water
(20 mL) was added and the solution was extracted with CH2Cl2
(25 mL × 2). The extracts were dried, concentrated, and
reprecipitated from hexane/CH2Cl2 to give 1b as a yellow solid
(0.14 g, 70%): mp 294−295 °C; 1H NMR (400 MHz, CDCl3) δ 8.16
(d, J = 8.0 Hz, 2 H), 7.97 (d, J = 4.0 Hz, 2 H), 7.87 (d, J = 8.0 Hz,
2 H), 7.62 (m, 3 H), 7.47 (m, 4 H), 7.42 (m, 4 H); 13C NMR
(100 MHz, CDCl3) δ 163.6, 155.96, 151.3, 138.6, 135.2, 134.0, 132.6,
132.3, 130.8, 130.5, 130.2, 129.3, 128.9, 127.3, 123.8, 119.7.
Preparation of Complex 3a. A mixture of Rh2(O2CCH3)4
(52.4 mg, 0.12 mmol) and 1a (50 mg, 0.12 mmol) in chloroform
(1 mL) was heated to reflux for 12 h under a nitrogen atmosphere.
The mixture turned dark green immediately and then slowly changed
to dark red during heating. After removal of solvents under reduced
pressure, the residue was passed through a flash column with CH2Cl2
as the eluent. The filtrate was reprecipitated with CH2Cl2/hexane
three times. The desired product was obtained as a dark red crystalline
solid (78 mg, 82%). Further recrystallization from CHCl3/hexane
provided single crystals suitable for X-ray analysis: mp 226−229 °C
dec; 1H NMR (400 MHz, CDCl3) δ 8.46 (d, J = 4 Hz, 1H, H-4), 8.29
(d, J = 4 Hz, 1H, H-2), 8.14 (d, J = 4 Hz, 1H, H-5), 7.92 (d, J = 8 Hz,
1H, H-7), 7.79 (m, 1H, Ph-H), 7.78 (d, J = 8 Hz, 1H, H-6), 7.63 (d,
J = 4 Hz, 1H, H-8), 7.62 (t, J = 4 Hz, 1H, H-9), 7.61 (m, 2H, Ph-H),
c
entry
1
substrate R
Ph−
yield, %
75
83
87
79
0
b
2
HCC−
3
4
5
6
7.
MeO−
CH3CO−
CH3C(NOH)−
CH3C(NOMe)−
NO2−
83
0
a
Reaction conditions: 3a (3.2 × 10−4 mmol), cyclohexene (0.32 mmol),
K2CO3 (0.16 mmol), and TBHP (1.58 mmol) in CH2Cl2 (1.2 mL) at
b
c
ambient temperature for 10 h. At 35 °C. Isolated yield.
pleased to find that various 1-substituted cyclohexenes do undergo
allylic oxidation to give the corresponding cyclohexenone except
for those with nitro and oxime groups. In terms of catalytic
activity, the turnover frequency (TOF) of 3a in the production of
3-phenylcyclohexenone (Table 6, entry 1) reaches 75 mol of
product (mol of catalyst)−1 h−1, which is slightly superior to that of
the known catalyst [Rh2(Cap)4] (TOF 65 mol of product (mol of
catalyst)−1 h−1), indicating that the axial ligand might affect the
activity of the metal complex.
E
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7.49 (d, J = 8 Hz, 1H, H-10), 7.32 (d, J = 4 Hz, 1H, H-3), 7.31 (m, 2H,
Ph-H), 2.30 (s, 3H), 1.64 (s, 6H); 13C NMR (200 MHz, CDCl3) δ
189.9, 189.3, 180.9 (d, J = 29.4 Hz), 176.8, 169.5, 163.8, 161.2, 159.2,
145.2, 144.0, 140.2, 137.2, 135.3, 135.2, 134.8, 132.9, 132.6, 129.9,
129.8, 129.4, 129.2, 129.1, 120.4, 119.7, 119.3, 119.0, 24.4, 23.7; ESI-
HRMS m/z for [M + H]+: calcd 803.9204 (C31H24N3O6S2103Rh2);
found 803.9202.
Preparation of Complex 3b. The procedure for the preparation
of 3b is similar to that for 3a. Complex 3b is a dark red crystalline solid
(78 mg, 86%): mp 221−224 °C dec; 1H NMR (400 MHz, CDCl3, 298 K)
δ 9.19 (d, J = 8 Hz, 1H, H-5), 8.82 (d, J = 8 Hz, 1H, H-4), 8.68 (d,
J = 8 Hz, 1H, H-12), 8.13 (d, J = 8 Hz, 1H, H-7), 8.10 (d, J = 8 Hz,
1H, H-8), 7.91 (d, J = 8 Hz, 1H, H-6), 7.73 (t, J = 8 Hz, 1H, H-3),
7.64 (m, 2H, Ph-H), 7.63 (m, 2H, Ph-H), 7.60 (t, J = 8 Hz, 1H, H-2),
7.56 (t, J = 8 Hz, 1H, H-11), 7.40 (t, J = 8 Hz, 1H, H-10), 7.39
(m, 1H, Ph-H), 7.36 (d, J = 8 Hz, 1H, H-9), 2.25 (s, 3H), 1.63
(s, 6H); 13C NMR (200 MHz, CDCl3, 298 K) δ 189.7, 188.6, 174.4
(d, J = 29.8 Hz), 174.3, 164.8, 163.6, 162.0, 144.4, 140.4, 137.5, 135.8,
134.7, 133.8, 133.7, 132.9, 132.5, 132.3, 132.0, 131.6, 130.8, 130.0,
130.0, 129.2, 128.3, 128.1, 125.6, 123.8, 122.5, 120.0, 24.3, 24.0. ESI-
HRMS (TOF) m/z for [M + H]+: calcd for C35H26N3O6Cl2103Rh2
859.9309; found 859.9308.
Preparation of Complex 4a. A mixture of 3a (20 mg, 0.025
mmol) and NH4PF6 (32.46 mg, 0.2 mmol) in chloroform was heated
to reflux for 24 h. The solvent was evaporated under reduced pressure,
and the residue was extracted by DCM/water (three times) to remove
the salt. After reprecipitation with ditheyl ether, the desired product
was obtained as a pure pale red solid (17.5 mg, 74%): mp 239−243 °C
dec; 1H NMR (400 MHz, CDCl3) δ 8.43 (d, J = 8 Hz, 1H, H-4), 8.31
(d, J = 8 Hz, 1H, H-5), 8.28 (d, J = 4 Hz, 1H, H-1), 8.26 (d, J = 8 Hz,
1H, H-6), 8.19 (d, J = 8 Hz, 1H, H-10), 8.17 (d, J = 4 Hz, 1H, H-3),
8.09 (d, J = 8 Hz, 1H, H-7), 7.87 (d, J = 4 Hz, 1H, H-8), 7.69 (m, 1H,
Ph-H), 7.68 (m, 2H, Ph-H), 7.63 (t, J = 4 Hz, 1H, H-2), 7.53 (d, J = 8
Hz, 2H, H-11), 7.35 (t, J = 4 Hz, 1H, H-9), 2.38 (s, 3H), 1.81 (s, 6H);
13C NMR (200 MHz, CDCl3, 298 K) δ 192.7, 190.7, 163.6, 161.3,
160.3, 160.1, 153.4, 142.4, 141.9, 141.3, 138.0, 137.2, 135.3, 133.8,
132.1, 131.0, 130.7, 130.6, 130.5, 130.1, 129.6, 122.3, 122.2, 122.1,
120.1, 24.2, 24.0; ESI-MS (TOF) m/z for M+: calcd for
C31H24N3O6S2103Rh2 803.92; found 803.88.
green. After removal of solvents, the residue was chromatographed to
give 5b as a dark green solid (33.8 mg, 91%): mp 233−237 °C dec; 1H
NMR (400 MHz, CDCl3) δ 9.13 (d, J = 8 Hz, 1 H), 8.49 (d, J = 8 Hz,
1 H), 8.44 (d, J = 8 Hz, 1 H), 8.14 (d, J = 8 Hz, 1 H), 8.03 (d, J = 8
Hz, 1 H), 7.71 (t, J = 8 Hz, 1 H), 7.62 (m, 1 H), 7.56 (d, J = 8 Hz,
1 H), 7.54(d, J = 8 Hz, 2 H), 7.52 (m, 2 H), 7.38 (t, J = 8 Hz, 2 H),
7.24 (d, J = 8 Hz, 1 H), 7.18 (d, J = 8 Hz, 2 H), 7.09 (m, 2 H), 7.02
(m, 3 H), 6.96 (d, J = 8 Hz, 2 H), 6.83 (t, J = 8 Hz, 1 H), 6.74 (m,
3 H), 6.70 (d, J = 8 Hz, 1 H), 6.57 (t, J = 8 Hz, 2 H), 2.25 (s, 3 H),
1.85 (s, 3 H); 13C NMR (125 MHz, CDCl3, 298 K) δ 188.4, 183.6,
179.2, 177.4 (JRh−C = 27 Hz), 175.5, 167.7, 163.3 (JRh−C = 24 Hz),
159.1, 147.7, 140.9, 138.0, 136.9 (JP−C =11.6 Hz), 135.2, 133.5, 133.0,
132.9, 132.6 (d, J = 9.8 Hz), 132.5, 132.2, 132.1, 132.0 (JP−C = 9.6 Hz),
131.2 (JP−C =13 Hz), 130.9, 130.5, 130.1, 129.9, 129.8, 129.1, 129.0, 128.8,
128.5, 128.4, 128.1, 127.8, 127.7, 127.4, 127.2 (JP−C = 9.8 Hz), 126.9
(JP−C = 9.5 Hz), 125.3, 122.9, 122.4, 119.0, 23.7, 23.0; 31P NMR (161
MHz, CDCl3) δ 34.6 (d, JRh−P = 162.6 Hz); ESI-MS m/z for [M + H]+:
calcd for C51H37Cl2N3O4PRh2 1062.00; found 1062.24.
General Procedure for Catalytic Oxidation. A mixture of
substrate (0.32 mmol), Rh complex (3.2 × 10−4mmol), K2CO3 (0.16
mmol), and TBHP (1.58 mmol) in CH2Cl2 (1.25 mL) was loaded in a
reaction tube. The reaction mixture was stirred at room temperature
for 10 h. The reaction mixture was poured into a saturated NaCl
solution, extracted with CH2Cl2 (3 mL × 2), and dried over anhydrous
MgSO4. After removal of solvents, the residue was chromatographed
on silica gel.
3-Phenylcyclohexenone:13 1H NMR (400 MHz, CDCl3) δ 7.53−
7.50 (m, 2H), 7.40−7.38 (m, 3H), 6.40 (s, 1H), 2.76 (m, 2H), 2.47
(t, J = 6.4 Hz), 2.14 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 199.9,
130.0, 128.7, 128.3, 126.1, 125.5, 125.1, 37.3, 28.1, 22.8; IR (KBr)
1663 cm−1 (νCO); ESI-HRMS m/z for [M + H]+: calcd for C12H13O
173.0961; found 173.0960.
3-Ethynylcyclohexenone:14 1H NMR (400 MHz, CDCl3) δ 6.25
(s, 1H), 3.51 (s, 1H), 2.47 (m, 2H), 2.43 (m, 2H), 2.05 (m, 2H); 13C
NMR (100 MHz, CDCl3, 298 K) δ 198.6, 142.3, 133.9, 87.2, 82.4,
37.2, 30.1, 22.4; IR (KBr) 2095, 1674 cm−1; ESI-HRMS m/z for
[M + H]+: calcd for C8H9O 121.0653; found 121.0658.
3-Methoxycyclohexenone:15 1H NMR (400 MHz, CDCl3) δ 5.36
(s, 1 H), 3.68 (s, 3 H), 2.38 (t, J = 6 Hz, 2 H), 2.32 (t, J = 6 Hz, 2 H),
1.96 (m, 2 H); 13C NMR (100 MHz, CDCl3, 298 K) δ 199.8, 176.7,
102.3, 55.6, 36.7, 28.8, 22.2; IR (KBr) 1597 cm−1 (νCO); ESI-HRMS
m/z for [M + H]+: calcd for C7H11O2 127.0754; found 127.0752.
1-(3-Oxocyclohexen-1-yl)ethanone O-methyloxime: 1H NMR
(400 MHz, CDCl3) δ 6.21 (s, 1 H), 3.96 (s, 3 H), 2.64 (m, 2 H),
2.40 (m, 2 H), 1.98 (m, 2 H). 1.92 (s, 3 H); IR (KBr) 1672 cm−1
(νCO); ESI-HRMS m/z for [M + Na]+: calcd for C9H13NO2Na
190.0838; found 190.0831.
Preparation of Complex 4b. The procedure for the preparation
of 4b is similar to that for 4a. However, we were not able to isolate this
1
complex in a pure form and it readily converted back into 3b: H
NMR (400 MHz, CDCl3, 298 K) δ 8.68 (m, 2 H), 8.46 (m, 3 H), 8.15
(m, 3 H), 7.97 (m, 2 H), 7.89 (m, 1 H), 7.69 (m, 1 H), 7.62 (m, 3 H),
7.45(m, 2 H), 2.09 (s, 6 H), 1.89 (s, 3 H).
Preparation of Complex 5a. Triphenylphosphine (6.6 mg, 0.025
mmol) was added to a solution of 3a (20 mg, 0.025 mmol) in CHCl3
(3 mL) under a nitrogen atmosphere. The resulting mixture was
stirred at room temperature for 2 h. The solution changed from red to
green. The solvent was removed under reduced pressure. The residue
was reprecipitated from dichlromethane/hexane/ether. The desired
complex 5a was obtained as a dark green solid (21.4 mg, 85%): mp
235−239 °C dec; 1H NMR (400 MHz, CDCl3, 298 K) δ 8.20 (d, J = 8
Hz, 1 H), 8.11 (m, 2 H), 7.85 (d, J = 4 Hz, 1 H), 7.75 (d, J = 4 Hz,
1 H), 7.59 (m, 2 H), 7.54 (d, J = 8 Hz, 1H), 7.48 (d, J = 8 Hz, 1 H),
7.41 (m, 3 H), 7.34 (d, J = 8 Hz, 2 H), 7.18 (d, J = 8 Hz, 2 H), 6.78
(m, 2 H), 6.70 (m, 3 H), 6.58 (t, J = 8 Hz, 2 H), 6.50 (t, J = 8 Hz,
2 H), 6.37 (t, J = 8 Hz, 3 H), 1.89 (s, 3 H), 1.26 (s, 3 H); 13C NMR
(125 MHz, CDCl3, 298 K) δ 191.3 (d, JRh−C = 37.5 Hz), 187.6, 181.3,
167.9, 163.1, 158.4, 157.6, 146.0, 144.0, 143.5, 143.3, 142.9, 142.4, 140.9,
3-Acetylcyclohexenone:13 1H NMR (400 MHz, CDCl3) δ 6.57
(s, 1 H), 2.52 (m, 2 H), 2.50 (m, 2 H), 2.46 (s, 3 H), 1.98 (m, 2 H);
13C NMR (100 MHz, CDCl3, 298 K) δ 201.3, 200.0, 154.6, 132.4,
37.8, 26.1, 23.3, 21.8; IR (KBr) 1681 cm−1 (νCO). ESI-HRMS m/z
for [M + H]+: calcd for C8H11O2 139.0759; found 139.0759.
Crystallography. Crystals suitable for X-ray determination were
obtained for 3a·2CHCl3·0.25C6H14, 3b·CHCl3, and 5a·4CH2Cl2 by
recrystallization from chloroform/hexane, chloroform, and dichloro-
methane, respectively. Cell parameters were determined with a
Siemens SMART CCD diffractometer. The structure was solved
using the SHELXS-97 program16 and refined using the SHELXL-97
program17 by full-matrix least squares on F2 values. Crystal data of
these complexes are given in Table S0 (Supporting Information).
Other crystallographic data are deposited as Supporting Information.
137.7, 136.4, 136.2, 135.8, 135.5, 135.0 (JP−C = 10 Hz), 134.2 (d, JRh−C
=
23.5 Hz), 133.3, 132.3, 131.9, 131.3, 130.7 (JP−C = 9 Hz), 129.9, 129.7,
129.5, 129.4, 129.1, 128.9, 128.6, 127.5 (JP−C =10 Hz), 126.6 (JP−C = 9.5
Hz), 120.5 (JP−C = 8 Hz), 120.3, 119.3, 117.8, 24.2, 23.8; 31P NMR (161
MHz, CDCl3, 298 K) δ 30.2 (d, JRh−P = 164 Hz); ESIMS (TOF) m/z for
[M + H]+: calcd for C47H35N3O4PRh2S2 1005.99; found 1006.08.
Preparation of Complex 5b. To a solution of complex 3b
(30 mg, 0.0349 mmol) in chloroform (1 mL) was added PPh3 (9.2 mg,
0.0349 mmol). The resulting mixture was stirred at room temperature
under a nitrogen atmosphere for 3 h. The solution turned from red to
ASSOCIATED CONTENT
* Supporting Information
Tables and CIF files providing crystal data, atomic positional
parameters, bond distances and angles, anisotropic thermal
parameters, and calculated hydrogen atom positions for
complexes 3a,b and 5a. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
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(14) Uyanik, M.; Fukatsu, R.; Ishihara, K. Org. Lett. 2009, 11, 3470−
3473.
(15) Winkler, C. K.; Stueckler, C.; Mueller, N. J.; Pressnitz, D.; Faber,
K. Eur. J. Org. Chem. 2010, 6354−6358.
(16) SHELXS-97: Sheldrick, G. M. Acta Crystallogr., Sect. A: Found.
Crystallogr. 1990, 46, 67.
AUTHOR INFORMATION
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(17) Sheldrick, G. M. SHELXL-97; University of Gottingen,
̈
We thank the National Science Council for financial support for
their financial support (NSC100-2113-M-002-001-MY3). We
also thank Miss Yu Chang Chao for her generous support for
ESI mass data analysis.
Gottingen, Germany, 1997.
̈
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