Z.-M. Wang et al. / Tetrahedron Letters 55 (2014) 1736–1739
1737
Table 2
a challenge. Our previous studies revealed the epoxidation–isom-
erization reaction (E–I) of alkenes catalyzed by [RuIV(TDCPP)Cl2]
(TDCPP = meso-tetrakis(2,6-dichlorophenyl)porphyrin) with 2,6-dichlo-
ropyridine N-oxide (Scheme 1).
Oxidation of internal styrene compounds with Cl2pyNO catalyzed by [RuIV(TDCPP)Cl2]
R2
R2
2.0 mol% [RuIV(TDCPP)Cl2]
1.03 eq Cl2pyNO, CDCl3, 60 o
C
O
R1
R1
(Cl2pyNO) or air as a terminal oxidant gave aldehydes as the
major product.7 The main objective of this work was to prepare
ketones using the ruthenium porphyrin catalyzed E–I reaction of
alkenes under optimized conditions, which we conceive to be a
supplement of Wacker oxidation.
R1= OMe, R2= Me: 1a, 2a; R1= Me, R2= Me: 1b, 2b; R1= H, R2= Me:
1c 2c
1e 2e
1f 2f
,
,
; R1=OMe, R2= n-propyl:
,
; R1= OMe, R2= n-pentyl:
At the outset, compared to the previous work7 on the
ruthenium porphyrin catalyzed E–I reaction, a solution of alkene
1-phenyl-1,3-heptdiene which was a mixture of cis and trans-
isomers (about 1:1 ratio), Cl2pyNO (1.03 equiv) and catalyst
[RuIV(TDCPP)Cl2] (2.0 mol %) in CDCl3 was stirred for 5 h at 60 °C.
O
MeO
NO2
NO2
1d
2d
1h
O
CHO
The b,c-unsaturated ketone 1-phenyl-1-heptene-4-one was
formed in 87% yield. A trace amount of aldehyde product was
detected in the reaction mixture.
O
MeO
MeO
2h
2h'
1g
2g
Subsequently, the effects of solvent, catalyst, temperature, and
equivalent of oxidant were examined and the results are depicted
in Table 1. Similar results were obtained with CHCl3, CH2Cl2, or
ClCH2CH2Cl as the solvent. Other solvents, such as toluene, aceto-
nitrile, and ethyl acetate, were inferior (entries 1–7). Probably,
the ruthenium porphyrin catalyst is more soluble in CHCl3, CH2Cl2,
or ClCH2CH2Cl, thereby such a halide solvent is advantageous for
the catalysis. CDCl3 was chosen as the solvent and the catalysis
was monitored/followed by 1H NMR. The reaction of 1-phenyl-
1,3-heptdiene with Cl2pyNO was more efficiently catalyzed by
[RuIV(TDCPP)Cl2] than that by other porphyrin catalysts, one of
which, the [RuII(TDCPP)(CO)] complex, was a relatively inactive
catalyst in the reaction (entries 8–12). This finding is in a good
agreement with our previous work reported in the literature.7
The use of 1.03 equiv of Cl2pyNO gave the best results in terms
of the yield of ketone product (87%). The conversion of 1-phenyl-
1,3-heptdiene became incomplete if insufficient Cl2pyNO was
used. If excess oxidant was used, no ketone product could be
observed at all (entries 12–14). Changing the temperature to
25 °C resulted in a lower yield of the ketone (entry 1).
Entry
Substratea
Time (h)
Product
Yieldb (%)
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
cis-1f
trans-1f
1h
2
24
24
24
2
5
6
2
2
2a
2b
2c
2d
2e
2f
2g
2f
2f
85
52
15
0c
76
67
0c
71
33
23
9
10
2
2h
2h0
a
0.1 mmol alkene.
Yield determined by 1H NMR with PhTMS as the internal standard.
Only epoxide has been obtained.
b
c
product yield dramatically. For example, with substrate 1a having
a strong electron rich substituent, the reaction time was shortened
to 2 h and the ketone was obtained in 85% yield, while for 1b, 1c hav-
ing weak electron rich substituents, a longer reaction time was re-
quired and the product was obtained in lower yield (entries 1–3).
When a strong electron-withdrawing group nitro was used, epoxide
was obtained without the ketone product observed even an increase
in catalyst loading and a longer reaction time (entry 4) were used.
The substrate with a longer carbon chain gave a lower product yield
(entries 5 and 6) whereas alkyl alkene, which is not a conjugated
olefin, could not be converted into ketone (entry 7).
With the optimized conditions in hand, we examined the scope
of substrates for the [RuIV(TDCPP)Cl2] catalyzed oxidation of styrene
compounds to ketones. As depicted in Table 2, various aryl alkenes
underwent the catalytic reaction to give the corresponding ketones
with most of them obtained in good or moderate yields. The substi-
tuent on the phenyl ring was found to affect the reaction time and
Table 1
Oxidation of 1-phenyl-1,3-heptdiene under various conditionsa
2.0 mol% [RuIV(TDCPP)Cl2]
1.03 eq Cl2pyNO, CDCl3, 60 oC
O
Entry
Solvent
Catalyst
Cl2pyNO (eq)
Temperature (°C)
Conversion ratio (%)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
CDCl3
CH3CN
PhCH3
CH3COOCH2CH3
ClCH2CH2Cl
CH2Cl2
CHCl3
CDCl3
CDCl3
CDCl3
CDCl3
[RuIV(TDCPP)Cl2]
[RuIV(TDCPP)Cl2]
[RuIV(TDCPP)Cl2]
[RuIV(TDCPP)Cl2]
[RuIV(TDCPP)Cl2]
[RuIV(TDCPP)Cl2]
[RuIV(TDCPP)Cl2]
[RuIV(TPP)Cl2]
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.03
0.5
25
60
60
60
60
60
60
60
60
60
60
60
60
60
100
37
93
85
91
92
93
57
100
46
73
31
66
71
83
85
85
19
71
0c
[RuIV(TMP)Cl2]
[RuII(TDCPP)(CO)]
[RuVI(TDCPP)O2]
[RuIV(TDCPP)Cl2]
[RuIV(TDCPP)Cl2]
[RuIV(TDCPP)Cl2]
100
100
61
68
87
81
0c
CDCl3
CDCl3
CDCl3
2.0
100
a
b
c
0.1 mmol substrate and catalyst(2 mol %).
Yield determined by 1H NMR with PhTMS as the internal standard.
Only epoxide has been obtained.