Rh2(esp)2-catalyzed allylic and benzylic oxidations

Article abstract of DOI:10.1039/c4cc10336j

The dirhodium(ii) catalyst Rh₂(esp)₂ allows direct solvent-free allylic and benzylic oxidations by T-HYDRO with a remarkably low catalyst loading. This method is operationally simple and scalable at ambient temperature without the use of any additives. The high catalyst stability in these reactions may be attributed to a dirhodium(ii,ii) catalyst resting state, which is less prone to decomposition.

Full text of DOI:10.1039/c4cc10336j

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Rh2(esp)2-catalyzed allylic and benzylic
oxidations†
Cite this: Chem. Commun., 2015,
51, 5852
Yi Wang, Yi Kuang and Yuanhua Wang*
Received 7th January 2015,
Accepted 19th February 2015
DOI: 10.1039/c4cc10336j
www.rsc.org/chemcomm
The dirhodium(II) catalyst Rh2(esp)2 allows direct solvent-free allylic
and benzylic oxidations by T-HYDRO with a remarkably low catalyst
loading. This method is operationally simple and scalable at ambient
temperature without the use of any additives. The high catalyst
stability in these reactions may be attributed to a dirhodium(II,II)
catalyst resting state, which is less prone to decomposition.
In the past decade the scientific world witnessed a dramatic
improvement in the methods used for selective C–H function-
alization.1 Selective oxidation of C–H bonds is attractive synthetic
transformation and can convert relatively cheap hydrocarbons
into value-added oxyfunctionalized products.2 For example, a
Scheme 1 Half-wave potential of dirhodium(II) catalysts.
desirable synthetic goal would be the direct transformation of a This catalyst’s success is due to it readily engaging using a one-
methylene group into a ketone.3 Among these transformations, electron oxidation mechanism (E1/2 = 11 mV, in CH3CN, vs.
benzylic and allylic oxidations are fundamental functional trans- Ag/AgCl).6f In contrast to Rh2(cap)4, the common Rh2(OAc)4
formations in synthetic chemistry.2a,4 Although organocatalytic displays a higher oxidation potential E1/2 of 1170 mV and for
benzylic and allylic oxidations have been reported,5 traditionally that reason it has limited activity in the allylic oxidation of
such oxidation reactions have relied upon metal complexes that olefins (Scheme 1).8 During our study of the application of
are capable of carrying out one-electron redox processes mediated selective allylic oxidations of cyclic olefin 3,5,5-trimethyl-2-
by oxidant tert-butyl hydroperoxide (TBHP),6 such as Fe2+/Fe3+,6m–o cyclohexen-1-one (a-isophorone, a-1) to 3,5,5-trimethyl-2-cyclohexen-
Cu+/Cu2+,6p,q Rh24+/Rh25+ 6f–k and Co2+/Co3+.6l,m These procedures 1,4-dione (ketoisophorone, 2), we were surprised to find that
,
provided a feasible approach for benzylic and allylic oxidations even dirhodium(II) complexes with higher oxidation potentials
that showed moderate to good reactivities. However, despite still occur in allylic oxidation. These unexpected results appear
the advances, drawbacks still exist in this transformation, such to indicate that the requirement of dirhodium(II) catalysts with
as the decomposition of the metal catalysts during the process, lower oxidation potentials in allylic and benzylic oxidations
the lack of regio- and stereoselectivity, exothermic reactions, can be lifted.6k In order to clarify the possible reaction pathway,
etc.6,7 Thus, there is ongoing interest in finding more potent we therefore directed our efforts towards the investigation of
catalysts involving new mechanisms that enable this catalytic this interesting phenomenon.
transformation to take place under more practical, environ-
mentally friendly conditions.
Selective oxidation of isophorone (1) to 2 is an important
reaction since product 2 is not only an important substance in
Doyle and coworkers have demonstrated the bimetallic paddle- flavoring, but a key intermediate in the synthesis of pharma-
wheel catalyst dirhodium(II) caprolactamate Rh2(cap)4 as an excellent ceuticals, especially for vitamin E.9 The direct oxidation of readily
catalyst in activating TBHP in allylic and benzylic oxidation.6f–j available a-1 to 2, avoiding the intermediate isomerization of a-1 to
b-1 (3,5,5-trimethyl-3-cyclohexen-1-one), is particularly attractive.10
However, a-1, with the conjugated system of a double bond and a
carbonyl group is the more stable isomer thermodynamically,
College of Chemistry, Sichuan University, Chengdu, 610064, P. R. China.
E-mail: yhwang@scu.edu.cn
which makes the current oxidation method from a-1 to 2 suffer
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc10336j
drawbacks such as low conversions, long reaction times, harsh
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Table 1 Optimization of reaction conditionsa
focused our attention on the efficiency of the different dirhodium(II)
catalysts without solvents. By replacing the Rh2(esp)2 with
Rh2(OAc)4, which had exhibited a similar oxidation potential
(E1/2 = 1170 mV),13 the reaction occurred with only 31% conversion
to the desired product (Table 1, entry 12). When the Rh2(OAc)4(IMes)
catalyst combination was employed,8 the reaction was still less
6f–j
efficient (Table 1, entry 13). Doyle’s catalyst Rh2(cap)4 dissolved
in the a-1 substrate but provided only 49% conversion after 24 h
(Table 1, entry 14). Poor results were obtained when the reaction was
carried out with other structurally chelating dirhodium(II) catalysts
Deviation from standard reaction
conditions
Conversionb
(%)
(Table 1, entries 15–18).14 Notably, as the prototype of Rh2(esp)2,
Entry
14b
1
2
3
4
None
Without Rh2(esp)2
Without T-HYDRO
DCE added
TBHP instead of T-HYDRO
TBHP instead of T-HYDRO and DCE added
1 equiv. of T-HYDRO
3 equiv. of T-HYDRO
40 1C instead of r.t.
91
Taber’s catalyst Rh2(MPDP)2
gave only a trace amount of the
NR
NR
86
72
39
17
36
89
67
91
31
9
49
Trace
NR
Trace
8
desired enedione 2 (Table 1, entry 15). Interestingly, the color of the
reaction mixture stayed green until the end of the reaction under
standard conditions, while it was previously reported to have a
brilliant red color due to one-electron oxidized dirhodium(II,III).12d,e
In addition, the Rh2(esp)2 catalyst did not degrade and was recovered
from the reaction mixture at the end of this reaction.
Next, in order to explore the possible mechanism, we used
UV/visible spectroscopy to allow us for the assessment of metal
oxidation states in dirhodium(II) chemistry, so that we could
monitor the change in the Rh2(esp)2 catalyst during the oxidation
reaction (Fig. 1).15
5c
6c,d
7
8
9
10
11
12
13
14
15
16
17
18
60 1C instead of r.t.
N2 instead of air
Rh2(OAc)4 instead of Rh2(esp)2
Rh2(OAc)4(IMes) instead of Rh2(esp)2
Rh2(cap)4 instead of Rh2(esp)2
Rh2(MPDP)2 instead of Rh2(esp)2
Dirhodium(II) catalyst I instead of Rh2(esp)2
Dirhodium(II) catalyst II instead of Rh2(esp)2
Rh2(MPDP)(CO2CF3)2 instead of Rh2(esp)2
Initially, the Rh2(esp)2 catalyst directly dissolved in the
substrate, a-1, which may also have functioned as the ‘‘solvent’’
(see ESI†). The UV/visible spectrum of the obtained green clean
solution revealed a representative adsorption of dirhodium(II,II)
carboxylates. After T-HYDRO was added, no drastic change
was observed in the electronic spectrum until the end of the
reaction (Fig. 1). As previously reported, the adsorption spectro-
a
Unless otherwise noted, all reactions were performed by using
a-1 (4.00 mmol), T-HYDRO (20 mmol), and dirhodium catalysts
b
c
(0.1 mol%). Determined by GC (gas chromatography). 5–6
M
d
solution in decane. 2 mL DCE was added. cap = caprolactamate,
esp = a,a,a0,a0-tetramethyl-l,3-benzenedipropionate, NR = no reaction.
scopy of the Rh2(esp)2+ catalyst showed the characteristic band
conditions, and low chemoselectivity.11 After considerable experi- of one-electron oxidized Rh2
species at B800 nm.12d,e,15 The
II,III
mentation, we found that the chelating Du Bois’ catalyst Rh2(esp)2,12 unchanged UV/visible spectroscopy indicated that the Rh2(esp)2,
+
with a higher oxidation potential (E1/2 = 1130 mV),12d allowed for the not the Rh2(esp)2 , is the catalyst resting state during the reaction.
direct solvent-free oxidation of a-1 into the corresponding 2. This
Given the nearly identical potentials of one-electron oxidation of
had a 91% conversion at room temperature, with catalyst loadings Rh2(esp)2 and Rh2(OAc)4, the low conversion of the reaction catalyzed
as low as 0.1 mol% without detecting any other unwanted side by Rh2(OAc)4 (Table 1, entry 12) clearly indicated that the chelating
products, and yielded TON up to 910 (Table 1, entry 1). Solvent-free ligand of Rh2(esp)2 was relevant to the oxidation mechanism and
green oxidation reactions were achieved through dissolving catalysts also excluded the possible mechanism proposed by Ren and
directly into substrate a-1 with T-HYDRO (70% tert-butyl hydro- coworkers (their diruthenium(II,III) catalyzed the oxygenation of
peroxide in water) as an oxidant, without any additives neces- organic sulfide, suggesting that a binuclear catalyst would act as
sary. The results obtained during the optimization process are oxygen transfer).16 In addition, none of product 2 was obtained
summarized in Table 1.
when the H2esp ligand alone was used in an attempt to catalyze
There was no conversion to the desired product 2 in the absence this reaction. Based on our experimental results, we concluded that
of a metal catalyst or a T-HYDRO oxidant (Table 1, entries 2 and 3). Rh2(esp)2 was the catalyst resting state in this green oxidation, which
If organic solvents like DCE (1,2-dichloroethane) were added, a was different from the well-documented Doyle’s catalyst Rh2(cap)4.6f–j
lower conversion resulted (Table 1, entry 4). When TBHP (5–6 M
solution in decane) was used, the results showed less promise than
the T-HYDRO oxidant (Table 1, entries 5 and 6), which indicated
that water was crucial for this reaction. Reducing the amount of
T-HYDRO from 5 eq. to 1 or 3 eq. gave a significantly lower
conversion (Table 1, entries 7 and 8). We also screened the reaction
temperature and found that the conversion rate dropped signifi-
cantly when the temperature was raised (Table 1, entries 9 and 10).
Subsequent experimentation proved that dioxygen did not play a
Fig. 1 UV/vis spectrum of the Rh2(esp)2 catalyst during oxidation.
Chem. Commun., 2015, 51, 5852--5855 | 5853
role in the reaction system (Table 1, entry 11). Hence, we next
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Table 2 Oxidation by Rh2(esp)2/T-HYDRO under standard conditionsa
Entry
1
Substrate
Products
Yieldb (%)
78
Scheme 2 Different catalyst resting states.
2
3
71
54
4c
5c
43
Fig. 2 Evaluation of catalyst kinetic performance.
60
99
In Rh2(cap)4 catalyzed oxidation, the oxidized catalyst Rh2(cap)4+ was
the resting state (Scheme 2).
Subsequently, we used gas chromatographic studies in order
to investigate the different performances of these two catalysts
in the oxidation reaction. The plots in Fig. 2 revealed that the
6d
7d
8 (14) + 19 (15) +
40 (16)
oxidation reaction catalyzed by Rh2(esp)2 was more reactive 8d,e
than reaction catalyzed by Rh2(cap)4. We observed that oxida-
tive Rh2(cap)4 resulted in 49% conversion, as opposed to the
25 (17) + 44 (14)
48 (19) + 30 (20)
9d
91% conversion that resulted from using Rh2(esp)2 under the
same experimental conditions, showing clearly the advantage
of Rh2(esp)2 in driving the a-1 oxidation. Further study revealed
that even 0.01 mol% Rh2(esp)2, in the presence of 5 eq.
T-HYDRO, lives for 10 days and catalyzes the reaction with
10d
46 (22) + 19 (23)
65 (25) + 7 (26)
11d
12d
impressive 88% conversion (see ESI†). Given the fact that
oxidative dirhodium(II,III) complexes have been shown to slowly
degrade as the reaction proceeds,6i,12d,e,17 the dirhodium(II,II)
catalyst resting state may maintain the catalyst stability
throughout the reaction, thus avoiding metal catalyst decom-
position in the reaction.18 Next, we attempted catalyst recycling
experiments in an operationally simple procedure.19 The Rh2(esp)2
catalyst can be recovered through chromatographic purification.
94% conversion and 77% yield of 2 resulted after two cycles and
the NMR study confirmed that the recovered Rh2(esp)2 catalyst
remained unchanged (see ESI†).
78
13d
11 (28) + 45 (29)
a
Unless otherwise noted, all reactions were performed by using a-1
b
(4.00 mmol), T-HYDRO (20 mmol), and Rh2(esp)2 (0.1 mol%). Isolated
yield. 1 mol% Rh2(esp)2 and 1 mL H2O were added. The reaction
temperature was 40 1C. 0.1 mol% Rh2(cap)4 instead.
c
d
e
With the optimized conditions in hand, we then turned our
attention toward the scope and limitations of this unexpected successfully applied to a variety of representative benzylic sub-
process. The good solubility of the Rh2(esp)2 catalyst in the selected strates (Table 2, entries 6–13). Notably, a different reactivity was
substrates and the T-HYDRO mixture rendered the possibility of observed when 1,2,3,4-tetrahydronaphthalene (13) was used as the
performing the catalytic reactions without solvents (Table 2). In substrate (Table 2, entries 7 and 8). Under standard conditions, 13
general, this solvent-free catalytic reaction was compatible with a oxidized to give enedione 16 as the major product in moderate
wide range of allylic substrates, and excellent regioselectivity in the yield, along with a-tetralone (14) and an intermediate tert-butyl
oxidation at the secondary over primary carbon was achieved peroxy ether 15 that corresponded to enedione (Table 2, entry 7).
(Table 2, entries 1–5). The starting material in the fragrance and Meanwhile, this reaction resulted in 14 as a major product, with
flavoring industry, like a-1, valencene (3), b-ionone (5), a-pinene (7), tert-butyl peroxy ether intermediate 17 when Rh2(cap)4 was applied
was transformed into the corresponding oxidized product with a (Table 2, entry 8). Further experimentation confirmed that 14 would
good yield (Table 2, entries 1–4). These results indicate the potential transform into enedione 16 in good yield when catalyzed by
industrial application of this process. Interestingly, (+)-3-carene (9) Rh2(esp)2 (Table 2, entry 12). We then focused on the substrates
was transformed into enedione 10 under the reaction conditions that possessed two benzylic positions in this reaction system. We
(Table 2, entry 5). In addition, the present protocol was also also observed the formation of benzyl tert-butyl peroxides for
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M. Toyonari, T. Sueda, S. Goto and M. Shiro, J. Am. Chem. Soc.,
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enediones in the oxidation of isochroman (18) and indane (21)
(Table 2, entries 9 and 10). Oxidation of N-tosyltetrahydro-
isoquinoline (24) afforded small amounts of desired amide pro-
duct 26, and due to the decomposition of the peroxide being very
slow in this case peroxide compound 25 was the major product
(Table 2, entry 11). Though we did not detect a peroxide inter-
mediate in the a-1 oxidation reaction, based on the above results
from benzylic oxidation, we concluded that it is likely that t-BuOOꢀ
(t-butylperoxy radical) still forms and acts as an oxidant in this
reaction system. The successful tracing of the tert-butylperoxy
radical revealed that the Rh2(esp)2 catalyzed green process is a
radical reaction. According to the allylic and benzylic oxidation
mechanism in dirhodium(II) chemistry outlined by Doyle and
co-workers,6f–j the most probable pathway was believed to be as
follows: after T-HYDRO was added to the reaction mixture, the
one-electron oxidation of Rh2(esp)2 to Rh2(esp)2+ occurred. Due to
the high oxidation potential of the Rh2(esp)2 (E1/2 = 1130 mV), this
oxidation is also a rate-limiting step and the very reactive
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species then underwent a much faster chemical transformation
with the substrate to the desired product. This would explain why
the Rh2(esp)2 is the catalyst resting state during the reaction.20
Futhermore, because of the low conversion of the reaction cata-
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Rh2(esp)2 affects the catalyst remains to be investigated.
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In summary, we have reported the low loading chelating
dirhodium(II) complex Rh2(esp)2 catalyzing allylic and benzylic
oxidations with good conversion under operationally simple,
solvent-free, mild reaction conditions, a report which has
potential applications in both the fine chemical and pharma-
ceutical industries. These reactions generally exhibit high
catalyst stability because they have a dirhodium(II,II) catalyst
resting state, which is less prone to decomposition. Further
experiments will be used to elucidate the mechanism in detail
and to broaden the scope of the reaction.
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We are grateful for financial support from National Science
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