Dirhodium(II)-Catalyzed Carbonylation Peroxidation of α,β-Unsaturated Esters: Mechanistic Insight into the Role of Aryl Aldehydes

Article abstract of DOI:10.1021/acs.inorgchem.7b00888

Peroxidation has received considerable attention as a synthetically useful method used to prepare organic peroxides, which are useful synthetic building blocks in synthetic chemistry. The difunctionalization of alkenes to introduce a peroxide and another functional group has become a useful tool for quickly increasing molecular complexity in synthesis. In this work, a three-component oxidative coupling of aryl aldehydes with α,β-unsaturated esters and tert-butyl hydroperoxide catalyzed by dirhodium(II) catalyst Rh₂(esp)₂ (esp = α,α,α′,α′-tetramethyl-1,3-benzenedipropanoate) under mild conditions is developed. The synthesized carbonylation peroxidation products (β-peroxyketones) are stable enough to be isolated by silica gel column and characterized. The β-peroxyketones used as reactants have been applied to the synthesis of the epoxides, polysubstituted furans, carbazole alkaloids, and biologically important natural products. Interestingly, besides being a reactant, aryl aldehydes also play an important role in avoiding the catalyst deactivation during the reaction as shown by ultraviolet/visible analysis. The excess amount of aldehydes was used to ensure the stability of the Rh₂(esp)₂ catalyst in the reaction by forming the monoaldehyde ligated dirhodium(II) complex. It is important to note that the aldehydes were also found to reduce the inactive Rh₂(esp)₂Cl species generated in the reaction.

Full text of DOI:10.1021/acs.inorgchem.7b00888

Article
pubs.acs.org/IC
Dirhodium(II)-Catalyzed Carbonylation Peroxidation of
α,β-Unsaturated Esters: Mechanistic Insight into the Role of Aryl
Aldehydes
Lili Zhao, Yi Wang, Ziling Ma, and Yuanhua Wang*
College of Chemistry, Sichuan University, Chengdu 610064, P. R. China
S
* Supporting Information
ABSTRACT: Peroxidation has received considerable atten-
tion as a synthetically useful method used to prepare organic
peroxides, which are useful synthetic building blocks in
synthetic chemistry. The difunctionalization of alkenes to
introduce a peroxide and another functional group has become
a useful tool for quickly increasing molecular complexity in
synthesis. In this work, a three-component oxidative coupling
of aryl aldehydes with α,β-unsaturated esters and tert-butyl
hydroperoxide catalyzed by dirhodium(II) catalyst Rh2(esp)2
(esp = α,α,α′,α′-tetramethyl-1,3-benzenedipropanoate) under
mild conditions is developed. The synthesized carbonylation
peroxidation products (β-peroxyketones) are stable enough to
be isolated by silica gel column and characterized. The β-peroxyketones used as reactants have been applied to the synthesis of
the epoxides, polysubstituted furans, carbazole alkaloids, and biologically important natural products. Interestingly, besides being
a reactant, aryl aldehydes also play an important role in avoiding the catalyst deactivation during the reaction as shown by
ultraviolet/visible analysis. The excess amount of aldehydes was used to ensure the stability of the Rh2(esp)2 catalyst in the
reaction by forming the monoaldehyde ligated dirhodium(II) complex. It is important to note that the aldehydes were also found
to reduce the inactive Rh2(esp)2Cl species generated in the reaction.
Scheme 1. One electron Oxidative Procedure of
Dirhodium(II) Catalyst Rh2(esp)2
INTRODUCTION
■
Catalyst deactivation is a fundamental problem in catalytic reac-
tions and is thus a key issue in industrial catalytic processes.1−3
Therefore, it is imperative to avoid catalyst deactivation by
elucidating catalyst behavior during the reaction. Previously, we
found that dirhodium(II) compound Rh2(esp)2 can catalyze
allylic oxidation and sulfide oxygenations efficiently using 70%
tert-butyl hydroperoxide in water (T-HYDRO) as an oxidant
(Scheme 1).4,5 The stable Rh2(II,II) species rather than an
unstable Rh2(II,III) species was demonstrated as catalyst rest-
ing state, which could help in avoiding catalyst oxidative
degradation. During the course of our studies, we found the
axial ligands of Rh2(esp)2 can determine the predominant Rh2
catalyst species in the oxidation reaction due to the competitive
coordination of reactants, solvents, and products to the axial
sites, which may result in either catalyst inhibition or arrest.5
This was because the Rh2(II,II) compounds possess a high
degree of potential cooperative and electronic communication
between the two rhodium metal sites across Rh−Rh metal
bond and, hence, provide a suitable molecular platform for axial
site chemistry (Figure 1).6−12
developed a novel method for synthesis of densely function-
alized carbonyl peroxides based on iron or cobalt-catalyzed
three-component reactions of radical precursors (such as
Organic peroxides contain the bivalent −O−O− structure
and play a significant role in organic chemistry.13−15 A number
of reactions introducing peroxy groups onto the organic
molecules have been developed and have received considerable
attention as synthetically useful tools.16 Recently, Li et al.
Received: April 6, 2017
© XXXX American Chemical Society
A
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orbital (LUMO) transition was strongly affected by axially coor-
dinated ligands as recorded by UV/vis spectrum. Thus, the
dirhodium(II) compounds provide a special opportunity to
understand the behavior of the catalyst during the reac-
tion.30−32 While keeping this in mind, we began our inves-
tigation by exploring the axially coordinated ability of each
reactant and selected solvent with Rh2(esp)2 using UV/vis
spectrophotometer (Figure 2).
Figure 2-(I) shows the UV/vis spectrum of the Rh2(esp)2
with different solvents. A green solution appeared when
dissolving the Rh2(esp)2 in 1,2-dichloroethane (DCE) and
was analyzed through UV/vis spectra to produce the char-
acteristic band of Rh2 π* to Rh2 σ* at 656 nm. It was reported
that the coordination with more strongly σ-donating ligands
create a larger HOMO/LUMO gap by raising the energy of the
σ* LUMO, which shifts the Rh−Rh band to a higher energy.33
The results of Figure 2-(I) showed that the coordinating sol-
vents such as ethyl acetate (EA), dimethoxyl ether (DME), and
tetrahydrofuran (THF) efficiently shift the band to a higher
energy, indicating the strong binding of these solvents to the
rhodium atom. Since the Rh2(II) complexes acting as one-
electron reductants are thought to homolytically cleave the
oxygen−oxygen bond of TBHP at the axial coordinating
site,34−38 these coordinating solvents may inhibit the reaction
and, thus, were not considered appropriate for Rh2(II)-cata-
lyzed oxidative coupling reactions. Noncoordinating solvents
were shown as suitable for performing the further reactions.
Hence, DCE was selected as the most appropriate solvent for
this reaction.
As shown in Figure 2-(II), it was observed that the added 1a
did not change the band value. It clearly shows that 1a did not
coordinate to Rh2(esp)2 in the presence of DCE. However, the
2 as weak Lewis bases was already reported to axially coor-
dinate with the Rh2 complexes in a dynamic equilibrium
composed of the free, mono-, and bis-aldehyde adducts with
Rh2(II) (Scheme 2).39−43
This prompted us to examine the interaction behavior of 2a
with Rh2(esp)2 in the presence of DCE. On the basis of the
protocol reported by Doyle et al.42,43 and Lebel et al.,44 UV/vis
spectra were applied to study the coordination of the aldehyde
with Rh2(esp)2. As seen in the Figure 3, by simulating the
reaction conditions, the 0.5 mmol % of the Rh2(esp)2 in 2 mL
of DCE resulted in the band of Rh2 π* to Rh2 σ* at 656 nm,
and the 1 equiv of 2a shifted the band to 643 nm. Increasing
the concentration of the 2a caused the band to move toward
Figure 1. Structure of dirhodium(II) carboxylate catalysts.
aldehydes,17,18 1,3-dicarbonyl compounds,19 and carbazates20)
with alkenes and hydroperoxide. Notably, the generated
multifunctional carbonyl peroxides act as very useful synthetic
building blocks and have been further applied to the syn-
thesis of the epoxides,21,22 polysubstituted furans,23 carbazole
alkaloids,24 and biologically important natural products.25,26
Subsequently, Wang et al. reported visible-light-enabled
photocatalytic generations of acyl radicals as key intermediates
to form β-peroxy ketone, which further underwent elimination
of t-BuOH to prepare α,β-epoxy ketones under alkaline
condition.27 Klussmann et al. later demonstrated the synthesis
of γ-peroxyketones from alkenes with unactivated ketones
and 5.5 M tert-butyl hydroperoxide in decane (TBHP) by acid
catalysis.28,29
Herein, to extend the scope of the Rh2(esp)2-catalyzed
oxidation reactions with tert-butyl hydroperoxide and to gain a
better understanding of the underlying mechanism behind how
the axial ligands with different coordinating abilities affect the
catalyst behavior, we investigated the Rh2(esp)2-catalyzed oxi-
dative coupling of α,β-unsaturated esters 1 with aryl aldehydes
2 and TBHP under mild conditions (Scheme 1). The catalytic
mechanism was studied by monitoring the interactions between
the Rh2(esp)2 and reactants along with noticeable changes
of oxidative state of Rh2(esp)2 during the reaction using
ultraviolet/visible (UV/vis) spectral analysis. On the basis of
the results of UV/vis spectra, a successful strategy was applied
to avoid catalyst deactivation.
RESULTS AND DISCUSSION
■
Initial studies were performed using isobutyl acrylate 1a,
benzaldehyde 2a ,and TBHP as reactants. It is known that the
Rh2(II,II) complexes could exhibit Lewis acid character and
then coordinate with Lewis bases at open axial sites.7−12
Because of the energy of Rh2 π* to Rh2 σ*, a highest occupied
molecular orbital (HOMO) to lowest unoccupied molecular
Figure 2. UV/vis spectrum of Rh2(esp)2 with reactants and solvents. (I) Rh2(esp)2 (3 mg, 0.004 mmol) in different solvents (2 mL). (II) Rh2(esp)2
(3 mg, 0.004 mmol) with 1a (1 equiv) or TBHP (5 equiv) in DCE (2 mL).
B
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the challenge that persists in this reaction system is how to
avoid the inhibition or deactivation of the Rh2(esp)2 catalyst.
Next, adding to the results of the UV/vis, the reaction studies
began with 0.5 mmol % of Rh2(esp)2 as a catalyst with 1 equiv
of 1a, 1 equiv of 2a, and 5 equiv of TBHP in DCE at room
temperature under nitrogen conditions. The color of the reac-
tion mixture turned to red, as all the ingredients were added
together. In addition to these results as well as the UV/vis
spectra that were performed, the formation of Rh2(esp)2Cl
species in the solution (Figure 4-(I)), along with trace amount
of desired product 3aa (Table 1, entry 2) was confirmed.
Indeed, the reaction efficiency was enhanced when the amount
of 2a was increased to 2 equiv, which resulted in a 42% yield of
the desired product 3aa (Table 1, entry 3), though the final
color of the reaction solution was red. As mentioned in the
above studies, the free ligated Rh2(esp)2 was dominated when
1 equiv of 2a was added. Thus, on the basis of the above
investigation of interaction between the 2a and Rh2(esp)2, we
propose that the use of the excess amount of the 2a inhibited
the free ligated Rh2(esp)2 to be the predominant species in the
reaction, which would ultimately lead to catalyst deactiva-
tion. At the same time, the benzoic acid 4 and tert-butyl
benzoperoxoate 5 as byproducts were also separated from the
reaction mixture, which indicated the possible pathway of the
aldehyde consumption. Further, the UV/vis spectrum indicated
that the product 3aa and the byproducts 4 and 5 could also
coordinate with Rh2(esp)2 (Figure 5). Among them, the 3aa
and 5 bearing the peroxide functional group had a similar effect
with TBHP, coordinating initially with Rh2(esp)2 and then
oxidizing the catalyst. All of these factors prompted us to find
the optimal amount of the 2a to ensure the preferential coor-
dination with Rh2(esp)2 during the reaction.
A series of experiments was performed, and it was observed
that 3 equiv of 2a ensures the stability of Rh2(esp)2 in the
reaction, and the yield of the reaction was increased to 60%
(Table 1, entry 1). On the basis of the results of UV/vis
spectrum, it can be inferred that 3 equiv of 2a caused the
monoligated Rh2(esp)2(PhCHO) species to dominate during
the reaction (Figure 4-(II)). In the monoligated species, only
one rhodium atom is bound to 2a, which influences the
electrophilicity of the other rhodium across Rh−Rh bond.49
Compared to the data of free ligated Rh2(esp)2 with TBHP at
the beginning, the band of Rh2 π* to Rh2 σ* was shifted to
632 nm (Figure 4-(II), D) from 639 nm (Figure 2-(II), C),
thus suggesting the stronger coordination between TBHP and
Rh2(esp)2, though the effect was subtle. However, when 4 or
5 equiv of 2a was used, the yield slightly decreased (Table 1,
entries 4 and 5). This is because that bis-adduct species
dominated while in the equilibrium at high concentrations of
2a, which resulted in a decreased reaction rate.
Scheme 2. Dynamic Equilibrium Composed of Aldehyde
Adducts with Rh2(II)
Figure 3. UV/vis spectrum of Rh2(esp)2 with different amounts of 2a.
high energy. It was observed that the addition of 3 equiv of 2a
shifted the band to 629 nm and that 5 equiv of 2a resulted
in a maximum absorption value of 616 nm, as the similar value
(613 nm) of band was measured on the addition of 10 equiv of
2a. These data suggested that the concentration of 2a is the
determining factor for the production of dominating Rh2
species at the equilibrium of reaction. Note that the 5 equiv
of 2a maintains the maximum concentration of the bis-aldehyde
adduct with Rh2(esp)2, whereas free ligated Rh2(esp)2 was
predominated when 1 equiv of 2a was used.
Similarly, the scope of TBHP was further examined. As
evidenced by the color change of the solution during the
UV/vis studies, the free ligated Rh2(esp)2 was susceptible to
deactivate under TBHP oxidation conditions, since a typical
band for a one-electron oxidized Rh2(II,III) species at 841 nm
was observed in 1 h after the addition of 5 equiv of TBHP
(Figure 2-(II), D).45−48 The reasoning for this is that DCE
+
captured the single electron oxidation products Rh2(esp)2 to
generate Rh2(esp)2Cl, which resulted in the inactivation of
catalyst (Scheme 3).45−48 It is noteworthy that the band was
shifted to 639 nm when 5 equiv of TBHP was initially added
(Figure 2-(II), C), suggesting that the TBHP does coordinate
to the rhodium but with weaker interactions as compared to
aldehyde. On the basis of the results shown by UV/vis studies,
Though the 3 equiv of 2a resulted in prevention of the deac-
tivation of Rh2(esp)2 catalyst, a small amount of Rh2(esp)2Cl
species may still persist in the reaction. During the inves-
tigation, we found that varying the concentration of 2a could
reduce the Rh2(esp)2Cl species in the reaction (Figure 6).
When the color of the reaction solution turned to red, the
continued addition of 2a to the solution resulted in the gradual
change of color from red to green. The characteristic Rh2(II,II)
band appeared again in the UV/vis spectrum, while the reaction
initiated again (Figure 6B). This implies that the 2a can reduce
the Rh2(esp)2Cl to Rh2(esp)2 even if the Rh2(esp)2Cl was
formed during the reaction. Though the acids are reported
Scheme 3. Formation of the Rh2(esp)2Cl
+
in the literature to reduce the Rh2(esp)2 in C−H amination
C
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Figure 4. Reaction monitoring though UV/vis spectrum. Condition: Rh2(esp)2 (3 mg, 0.5 mmol %) with 1a (1 equiv), 2a (1 or 3 equiv), TBHP
(5 equiv) in DCE (2 mL).
a
Table 1. Optimization of Reaction Conditions
b
entry
deviation from standard reaction conditions
3aa yield (%)
1
mone
60
2
1 equiv of 2a
trace
42
3
2 equiv of 2a
4
4 equiv of 2a
49
5
5 equiv of 2a
47
6
1 mol % Rh2(esp)2
60
7
40 °C instead of room temp
air instead of N2
57
8
49
9
di-tert-butyl peroxide instead of TBHP
tert-butyl peroxybenzoate instead of TBHP
tert-butyl peroxyacetate instead of TBHP
T-HYDRO instead of TBHP
3 equiv of TBHP
NR
NR
NR
53
10
11
12
13
14
15
16
17
18
19
20
35
THF instead of DCE
NR
trace
8
DME instead of DCE
EA instead of DCE
heptane instead of DCE
benzene instead of DCE
Rh2(MPDP)2 instead of Rh2(esp)2
Rh2(esp)2(OAc)2 instead of Rh2(esp)2
36
40
16
24
a
Reaction conditions: 1a (0.79 mmol), 2a (2.37 mmol), Rh2(esp)2 (0.004 mmol), TBHP (3.95 mmol), DCE (2 mL), rt, 6 h, nitrogen atmosphere,
b
unless otherwise noted. Yield after column chromatography. NR indicates no reaction.
reaction,50 our UV/vis studies confirmed that the Rh2(esp)2Cl
species produced during the reaction was not reduced by the
byproduct benzoic acid 4 formed in the reaction (Figure 6C).
It is well-known that the kinetic stability of the metal catalyst
determines the overall success of the reaction.1−3 In the present
case, the generated Rh2(esp)2Cl species was stable for 24 h at
room temperature (see the Supporting Information). Because
of the stability and reducibility of Rh2(esp)2Cl, the loss of
Rh2(esp)2 catalyst was minimized, and the higher possible
catalyst turnover numbers were resulted. We propose that the
chelate ligand of the Rh2(esp)2 is related to the desired kinetic
stability. To demonstrate this conclusion, the designed non-
chelated Rh2(II,II) compound 6, which possesses the structural
characteristics of the Rh2(esp)2, was synthesized. The rapid
catalyst degradation to rhodium(III) was clearly observed after
2 h, when the nonchelated compound 6 was used in this
D
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Figure 5. UV/vis spectra of Rh2(esp)2 (3 mg, 5 mmol %) with 4 (1 equiv), 5 (1 equiv), 3aa (1 equiv) in DCE (2 mL).
Scheme 4. UV/Vis Study of the Stability of the
Dirhoiudum(II) Compound 6
Figure 6. UV/vis study of the stability of Rh2(esp)2Cl. (A) Rh2(esp)2
(3 mg, 5 mmol %) with 1a (1 equiv) and TBHP (5 equiv) in DCE
(2 mL) (λmax = 843 nm). (B) Another 2 equiv of 2a was added (λmax
=
620 nm). (C) Then 3 equiv of 4 was added. (λmax = 844 nm).
oxidation experiment, suggesting the robustness of the Rh2(esp)2
catalyst related to the ligand structure (Scheme 4).45,46
likely reasons of this observation could be the fact that the
kinetic stability of one-electron oxidized species of these
Rh2(II,II) catalysts is less than that of Rh2(esp)2 species
(see the Supporting Information). Therefore, the optimized
conditions were concluded as follows: 0.5 mmol % of Rh2(esp)2
as catalyst, and a 1:3:5 mol ratio of acrylate, aldehydes,
and TBHP in DCE at room temperature under a nitrogen
atmosphere.
Subsequently, having optimized the reaction condition, we
then investigated the scope of the reaction with respect to
different α,β-unsaturated esters 1a−i with 2a and TBHP
(Table 2). Accordingly, the series of reactions involving 1
bearing different ester substitutions was performed to produce
the corresponding products 3aa−ea in 51−60% yields. It was
observed that the ester substituent of 1f bearing dimethylamino
substituent was not compatible with reaction conditions
because of a strong axial inhibitory effect of nitrogen atom on
the Rh2(II) catalyst. Because of the stability of secondary free
radicals, the α-substituted 1g−j were found to be effective
substrates for the reaction, and the desired products 3ga−ja
were obtained in good yields (70−78%) under optimized con-
ditions. Notably, in contrast to the nitrogen substitute (1f),
when oxygen atoms were present on the substrates (3h−j), the
reactions proceeded well, giving the coupling products 3ha−ja
Further exploration of the reaction conditions showed that
increasing either the catalyst loadings or temperature did not
improve the reaction yields (Table 1, entries 6 and 7). When
the reaction was run in the presence of the air, the yields of the
product decreased (Table 1, entry 8). Also, it was observed
during the screening of peroxides that the peroxides without a
hydroxy group were not able to initiate the coupling reaction
(Table 1, entries 9−11). The product 3aa was obtained in
lower yield, when T-HYDRO was used in the reaction (Table 1,
entry 12). When the amount of TBHP was reduced to 3 equiv, a
significantly lower conversion was observed (Table 1, entry 13).
To verify the solvents’ effect, various solvents were tested in the
reaction. When the coordinating solvents were used (Table 1,
entries 14−16), either a trace amount or none of 3aa was
obtained. Another factor contributing to the lack of reaction
when THF and DME were used (Table 1, entries 14 and 15) is
the formation of the α-alkoxyalkyl radicals, which are known to
suppress the reactions.51−53 Because of solubility issues, further
lower yields were obtained when benzene and hexane were
applied as solvents (Table 1, entries 17 and 18). Also, it is
worth noting that the low yields were obtained when the
reaction was performed with structurally similar chelating
Rh2(II,II) catalysts (Table 1, entries 19 and 20).54,55 The most
E
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ab
,
Table 2. Scope of Esters Compounds
a
Reaction conditions: 1a−i (0.79 mmol), 2a (2.37 mmol), Rh2(esp)2 (0.004 mmol), TBHP (3.95 mmol), DCE (2 mL), room temperature, 6 h,
b
c
1
nitrogen atmosphere, unless otherwise noted. Yield after column chromatography. Determined by H NMR.
a b
,
Table 3. Scope of Aldehyde Compounds
a
Reaction conditions: 1h (0.79 mmol), 2a (2.37 mmol), Rh2(esp)2 (0.004 mmol), TBHP (3.95 mmol), DCE (2 mL), room temperature, 6 h,
b
nitrogen atmosphere, unless otherwise noted. Yield after column chromatography.
in 70−78% yields. A 39% yield of 3ka was obtained when the
trisubstituted α,β-unsaturated ester 1k was used. The poor
yield obtained in this case may be due to steric hindrance.
In particular, the reaction of dimethyl fumarate 1l produced
epoxy ester 3la in 86% yield upon isolation, with diastereomeric
ratio (trans/cis = 1/2).
F
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Scheme 5. Proposed Mechanism
Furthermore, we probed the reaction generality of aldehyde
substrates 2a−m using (tetrahydrofuran-2-yl)methyl 2-methyl-
but-3-enoate (1h) as the coupling partner (Table 3). Gen-
erally, different aromatic aldehydes 2, with either electron-rich
or electron-poor substituents on aromatic ring, proceeded
smoothly to give the desired products 3ha−hl in moderate to
good yields (40−74%), though the substituents of the 2 might
affect the interaction between the 2 and Rh2(esp)2.42 Electron-
donating aldehydes (3b−c) displayed higher reactivity than
those bearing the electron-withdrawing substituted aldehydes
(3f−h). Not surprisingly, no product was formed when the
pyridine-2-aldehyde 2m was employed as a substrate. This could
be explained as the strong binding to Rh2(esp)2 through nitrogen
inhibiting the catalysis. But, the 5-methylfuran-2-carbaldehyde 2l
could serve as a suitable substrate and give the product 3hl in
61% yield. It is worth noting that the halo substituents including
F, Cl, and Br were proven to be tolerated under the optimized
conditions, affording the corresponding products (3he−hg) in
moderate yield. These compounds were therefore found attrac-
tive for further synthetic elaboration. Obviously, the position of
substituents of 2 impacted the reaction. ortho-Methoxy
substituted phenyl aldehyde 3d delivered the product 3hd in
47% yield, whereas the para-methoxy substituted aldehyde 3b
gave the product 3hb in 74% yield. The similar situation
occurred when the naphthalene-1-yl aldehyde 3i and naph-
thalene-2-yl aldehyde 3j were used as substrates in the reaction.
According to previous studies,17,18,21,22 subsequent studies
on the further conversion of peroxy products 3aa have led to
the trans epoxide 7 on the addition of the bases (eq 1).
On the basis of the results described above and previous
studies,34−38 a plausible mechanism for the reaction is shown in
the Scheme 5. The axial coordination of the aldehyde to the
Rh2(II,II) catalyst activated the TBHP in the other axial
position across the Rh−Rh bond, generating the tert-butoxy
radical t-BuO· and Rh2(II,III) species. Meanwhile, t-BuO· could
abstract a hydrogen from another TBHP to generate free tert-
butyl peroxide radical t-BuOO·. The generated Rh2(II,III)
species tentatively assigned as A would react with TBHP to
form dirhodium tert-butyl peroxyether complex B.19,35
Subsequently, t-BuO· abstracted hydrogen from 2 to produce
the acyl radical, which was then added to the double bond of 1.
This was followed by radical−radical coupling with t-BuOO· or
ligand transfer of the dirhodium-bound peroxide B to the
carbon-centered radical to form the desired product 3.19,35,56
Similarly, the formation of byproduct 5 resulted through the
above two coupling pathway. The aldehyde 2 was found to help
in avoiding catalyst oxidative degradation during the reaction.
CONCLUSION
■
In summary, we have developed the novel strategy of the
Rh2(II,II) catalyst, Rh2(esp)2-catalyzed three-component oxi-
dative coupling of aldehyde with α,β-unsaturated esters and
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TBHP under mild conditions. This reaction exhibits a wide
range of tolerance of functional groups. Notably, the aryl
aldehydes are found to play an important role in the reactions
as analyzed through UV/vis spectrum. The aldehydes as axial
ligands can coordinate axially to one of the rhodium atoms
in the Rh2(esp)2 and would not only avoid the catalyst
deactivation but also slightly enhance the interaction between
the Rh2(esp)2 and TBHP. The aldehydes were also found to
reduce the inactive Rh2(esp)2Cl species generated in the
reaction. As a result, the excess amount of aldehyde applied
could ensure the stability of the Rh2(esp)2 catalyst in the
reaction.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.7b00888.
■
S
The stability of one-electron oxidized species of
Rh2(II,II) catalysts, experimental characterization data
for products, and copies of 1H and 13C NMR spectra for
all compounds (PDF) Crystallographic data for com-
pound 6 (CIF) (PDF)
Accession Codes
CCDC 1535135 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
EXPERIMENTAL SECTION
■
General. All reactions were performed under an atmosphere of
nitrogen. All reagents were obtained from commercial suppliers and
used without further purification unless otherwise noted. Dichloro-
ethane (DCE), dimethoxyl ether (DME), and tetrahydrofuran (THF)
were distilled from CaH2 prior to use. Benzene was distilled
from sodium benzophenone ketyl prior to use. Benzaldehyde,
4-methoxybenzaldehyde, 4-fluorobenzaldehyde, and 5-methylfurfural
were distilled under reduced pressure prior to use. 1-Naphthaldehyde,
2-phenylbenzaldehyde, and 2-pyridinecarboxaldehyde were purified by
flash column chromatography. Rh2(esp)2 was prepared according to
the literature.45,46 Rh2(MPDP)2 and Rh2(esp)(OAc)2 preparations
have been previously reported.54,55 1H NMR and 13C NMR spectra
were measured on a 300 or 400 MHz Bruker spectrometer using
CDCl3 as the solvent at room temperature. UV/Visible spectroscopy
spectra were recorded on TU-1901. The high-resolution mass spectra
(HRMS) were obtained using a mass spectrometer with a TOF
(for ESI). Solid-state structure of dirhodium(II) complex 6 (CCDC
1535135) was determined by X-ray single-crystal diffractometry and
solved using SHELX program. Additional crystallographic information
is available in the Supporting Information.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yhwang@scu.edu.cn.
ORCID
Yuanhua Wang: 0000-0003-0514-6976
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We are grateful for financial support from National Science
Foundation of China (grant no. 21272162).
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REFERENCES
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1
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