Non-peripherally alkyl substituted ruthenium phthalocyanines as catalysts in the epoxidation of alkenes

Article abstract of DOI:10.1142/S108842461450103X

Non-peripherally alkyl substituted ruthenium phthalocyanines were demonstrated to be highly active epoxidation catalysts. It is compatible with pyridine N-oxides, and especially 2,6-dichloropyridine N-oxide. The catalytic activity towards a variety of alkenes was comparable to that published for other catalytic systems, but superior in the cases of 1,2-dihydronaphthalene and trans-stilbene. Linear substituents on the non-peripheral sites of the phthalocyanine were able to reduce aggregation and increase the solubility of the catalyst without compromising its activity by steric congestion as all substituted catalysts were more reactive than the unsubstituted phthalocyanine, whereas the bulky isopentyl and cyclohexyl substituted catalysts were less active than those with linear substituents. Although the epoxidation mechanism and the exact active intermediate is still ambigious, it likely involves the coordination of the N-oxide to ruthenium and subsequent transfer of the oxygen to the metal to form a high-valent oxo-ruthenium species. It is proposed that the alkene approaches this metal oxo moiety from the top and that oxygen transfer to the alkene is concerted with concomitant stereoretention.

Full text of DOI:10.1142/S108842461450103X

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2014; 18: 1–13
DOI: 10.1142/S108842461450103X
Published at http://www.worldscinet.com/jpp/
Non-peripherally alkyl substituted ruthenium
phthalocyanines as catalysts in the epoxidation of alkenes
Charles A. Enow, Charlene Marais and Barend C.B. Bezuidenhoudt*◊
Department of Chemistry, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
Received 23 September 2014
Accepted 11 October 2014
ABSTRACT: Non-peripherally alkyl substituted ruthenium phthalocyanines were demonstrated
to be highly active epoxidation catalysts. It is compatible with pyridine N-oxides, and especially
2,6-dichloropyridine N-oxide. The catalytic activity towards a variety of alkenes was comparable to
that published for other catalytic systems, but superior in the cases of 1,2-dihydronaphthalene and
trans-stilbene. Linear substituents on the non-peripheral sites of the phthalocyanine were able to
reduce aggregation and increase the solubility of the catalyst without compromising its activity by
steric congestion as all substituted catalysts were more reactive than the unsubstituted phthalocyanine,
whereas the bulky isopentyl and cyclohexyl substituted catalysts were less active than those with linear
substituents. Although the epoxidation mechanism and the exact active intermediate is still ambigious,
it likely involves the coordination of the N-oxide to ruthenium and subsequent transfer of the oxygen
to the metal to form a high-valent oxo-ruthenium species. It is proposed that the alkene approaches this
metal oxo moiety from the top and that oxygen transfer to the alkene is concerted with concomitant
stereoretention.
KEYWORDS: epoxidation, ruthenium, phthalocyanine, non-peripheral, pyridine N-oxide.
as epoxidation catalysts with various oxidants. Catalyst
INTRODUCTION
decomposition is a serious concern for most of these
Given the importance of epoxides in many naturally
systems, though, and product yields (based on substrate)
occurring molecules as well as industrial starting materials,
are usually low and reaction times extensive [2–5].
the development of more efficient catalytic systems for
Since the demonstration by Groves and Quinn [6] that
epoxidation is important for both industry and academia
ruthenium porphyrins could catalyze the direct transfer
[1–4]. Transition metals catalysts are of particular import-
of oxygen to an alkene in the absence of a co-reductant, a
ance in this respect as the versatile transition metals not
variety of catalytic oxidative systems based on ruthenium
only may exist in several oxidation states, but also have a
porphyrin complexes has emerged. Amongst them,
number of possible coordination numbers, thus enhancing
the possibility of successfully designing a catalyst suitable
the ruthenium porphyrin/2,6-dichloropyridine N-oxide
oxidation system developed by Hirobe et al. is quite
to the purpose. As compatibility between the type of
efficient with high turnover numbers (TON, stability) and
catalyst and the oxidant is important, an assortment of
selectivity being reported [7–10].
metal complexes of porphyrins, phenanthrolines, salens,
Despite the structural similarity between porphyrin
phthalocyanines (Pcs) etc. have been developed and studied
and phthalocyanine, research into the use of ruthenium
phthalocyanines as oxidative catalysts is surprisingly
limited, with the current understanding based almost
◊SPP full member in good standing
exclusively on the works of Capobianchi [11], Balkus [12],
*Correspondence to: Barend C.B. Bezuidenhoudt, email:
bezuidbc@ufs.ac.za, tel: +27 51-401-9021, fax: +27 51-444-6384
Murahashi [13] and their co-workers. Capobianchi et al.
Copyright © 2014 World Scientific Publishing Company

2
C.A. ENOW ET AL.
the compatibility of these catalysts with various oxidants
and alkenes.
R
R
R
R
R
R
N
N
N
OC
N
N
Ru
RESULTS AND DISCUSSION
N
N
N
Effect of oxidant
R
R
Since pyridine N-oxides, and particularly 2,6-DCPNO
2a, are commonly employed as oxidants in ruthenium
porphyrin catalyzed epoxidations, trans-stilbene 3 was
exposed to a series of pyridine N-oxides 2a–2e, and
also other common oxidants 2f–2i, in the presence of
the phthalocyanine catalysts 1a and 1d (toluene, 80°C,
0.23 mol.% of catalyst, catalyst/substrate/oxidant
molar ratio = 1:500:750 for 24 h). Similar to results
encountered for porphyrin-catalyzed epoxidations,
2,6-dichloropyridineN-oxide2aprovedtobesuperiortoall
other oxidants in reactions catalyzed by Ru(II)
1a R = n-C6H13 1b R = n-C8H15
1c R = n-C12H25
1e R =
1d R =
1f R = H
Scheme 1. Non-peripherally alkyl substituted carbonyl ruthenium
phthalocyanines
O
O
2a, cat.
Pc catalysts 1a and 1d, giving conversions of
100% and 51%, respectively (Table 1, entries 1
+
Ar, toluene
and 10). Whereas unsubstituted pyridine
N-oxide 2b was inactive under the conditions
3 trans
4 cis
5
6
used, electron-withdrawing chloro groups in
the positions ortho to the nitrogen rendered
Scheme 2. Epoxidation of trans- (3) and cis-stilbene (4)
N-oxide 2a highly active (Table 1, entry 2 vs. 1;
entry 11 vs. 10). When, in an attempt to fine-
[11] reported that unsubstituted ruthenium phthalocyanine,
which exists as a dimer, catalyzes the aerobic oxidation
of 1-octene to 2-octanone. Balkus et al. [12] reported the
oxidation of alkanes by zeolite-encapsulated perfluori-
nated ruthenium phthalocyanine (RuF16Pc) with tert-
butyl hydroperoxide as oxidant, while Murahashi et al.
[13] reported high TON in the aerobic alkane oxidation
with perhalogenated ruthenium porphyrins (RuTPFPP)
CO [TPFPP = 5,10,15,20-tetrakis(pentafluorophenyl)
porphyrinato].
Although some research has been carried out on Fe,
Mn, and Co phthalocyanines as epoxidation catalysts
[14, 15], most of the studies on phthalocyanines
had the objective of comparing the activity and
stability of these complexes with that of the porphyrin
equivalents. Development of the oxidation chemistry
of phthalocyanines has also been hampered by the
poor solubility of these compounds in common organic
solvents due to aggregation, but since examples with
alkyl substituents at the non-peripheral sites (1, 4, 8, 11,
15, 18, 22, 25) have shown much improved solubility [16,
17], these compounds were considered to have potential
as catalysts in the epoxidation of alkenes.
tune the coordination and oxidation potential of the
pyridine N-oxide and the non-coordinating characteristics
of the resulting amine [19], the electron density of the
oxidant was decreased by the addition of another electron-
withdrawing group (nitro group) in the para position 2c,
the activity of the oxidant decreased (Table 1, entry 3 vs.
1; entry 12 vs. 10). A para electron donating OMe group
2d increased the activity of the oxidant compared to 2c,
though it still performed poorer than 2a (Table 1, entries
4, 3 and 1; entries 13, 12 and 10). The oxidant with chloro
groups (known to be ortho and para deactivating) in the
meta (2e) rather than the ortho positions also performed
poorer than 2a (Table 1, entry 5 vs. 1; entry 14 vs. 10),
though the fact that the catalyst wasn’t deactivated
indicates that steric hindrance at the ortho positions is not
a prerequisite for non-coordination of the formed amine
to the metal as was reported for ruthenium porphyrin
catalysts [9].
R2
O
R
R
N
O
N
O
2f
In a recent work, we demonstrated for the first time that
alkyl substituted and specifically non-peripherally alkyl
substituted carbonyl[1,4,8,11,15,18,22,25-octa(alkyl)
phthalocyaninato] ruthenium(II) Pc complexes 1a–1e
(Scheme 1) are efficient catalysts in the epoxidation of
trans- 3 and cis-stilbene 4 with 2,6-dichloropyridine
N-oxide (2,6-DCPNO) 2a as terminal oxidant (Scheme 2)
[18]. In this paper, we would like to report in detail on
R1
N
O
R1
2e R = Cl
2a R1 = Cl, R2 = H
2b R1 = R2 = H
2c R1 = Cl, R2 = NO2
2d R1 = Cl, R2 = OMe
I
AcO
OAc
2g
Scheme 3. Oxidants
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 2–13

NON-PERIPHERALLY ALKYL SUBSTITUTED RUTHENIUM PHTHALOCYANINES AS CATALYSTS
3
Table 1. Epoxidation of trans-stilbene 3 catalyzed by 1a and 1d
(0.23 mol.%) with different oxidantsa
toluene (22%, 16%) (Table 2, entries 1–5). As was
previously encountered for trans-stilbene 3, an increase
in temperature from 45 to 80°C resulted in a drastic
increase in 1,2-dihydronaphthalene 7 conversion (100%
vs. 22%, Table 2, entries 5 and 7) and epoxide yield
(82% vs. 16%) in reactions catalyzed by 1a. A further
increase in temperature from 80 to 110°C confirmed
previous results [18] indicating enhanced reaction rates
(reaction time of 3 h vs. 6 h and TOF of 1120 vs. 618),
but lower yield (74% vs. 82%) and selectivity (76% vs.
82%) (Table 2, entries 7 and 9). Also, as was the case for
trans-stilbene 3, the sterically more demanding catalyst
1d gave slightly lower yields and selectivity compared to
1a (77% vs. 82% for both selectivity and yield, Table 2,
entries 7 and 11).
Entry
Cat
Oxidant
Conversion, %
Epoxide 4
(% yield)
1
2
1a
1a
1a
1a
1a
1a
1a
1a
1a
1d
1d
1d
1d
1d
1d
1d
1d
2a
2b
2c
2d
2e
2f
100
—
36
45
52
—
—
—
—
51
—
18
32
35
—
—
—
95
—
32
37
42
—
—
—
—
30
—
14
27
32
—
—
—
3
4
5
6
7
2g
2h
2i
8
9
10
11
12
13
14
15
16
17
2a
2b
2c
2d
2e
2f
Epoxidation of 1,2-dihydronaphthalene 7
Since Berkessel and Frauenkron [21] and Che et al.
[20] reported TONs of 880 (48 h) and 890 (3 h) in the
epoxidation of 1,2-dihydronaphthalene 7 with homo-
geneous chiral ruthenium porphyrins and 870 (24 h) for
soluble polymer-supported ruthenium porphyrin catalysts
respectively, this substrate 7, was selected for further
evaluation of the catalytic capabilities of the prepared
RuPc catalysts. Thus 1,2-dihydronaphthalene 7 was
reacted with 2,6-dichloropyridine-N-oxide 2a in the
presence of 0.1 mol.% of the RuPcs (1a–1f) at 80°C
since the reaction with 1a and 1d showed 80°C to be the
optimum temperature. Complete substrate conversion to
the epoxide 8 was observed within 4 h and high epoxide
yields (77–83%) and selectivities (77–83%) were
obtained with all the substituted ruthenium Pcs 1a–1e
(Table 2). Turnover numbers of up to 830 (in 6 h) were
also comparable to those reported by Che et al. [20]. At
0.02 mol.% cat., turnovers (TON; product/catalyst molar
ratio) of 2300–2800 were achieved in 12 h (Table 2,
entries 17–21; 0.02 mol.% cat.) at a turnover frequency
(TOF) of 260–457 h-1.
2g
2h
18
1d
2i
—
—
aReaction conditions: toluene, Ar, 80°C, catalyst/substrate/
oxidant molar ratio = 1:500:750 for 24 h.
N-methylmorpholine N-oxide 2f, (diacetoxyiodo)
benzene 2g, t-butyl hydrogen peroxide 2h and hydrogen
peroxide 2i failed to give any conversion of the substrate.
In the absence of the ruthenium Pc catalyst, only trace
amounts of the epoxide (≤0.59%), could be detected
after two days of reaction.
In the presence of the catalysts with linear substituents
1a–1c, the reactivity of 1,2-dihydronaphthalene 7 was
similar to that of trans-stilbene 3 with regard to conver-
sion, yield and selectivity (vide infra), though the TONs
and TOFs were considerably higher for 1,2-dihydro-
naphthalene 7 (Table 2, entries 7, 13 and 14 and Table 3,
entries 1–3). In the case of the bulkier catalysts 1d and 1e,
1,2-dihydronaphthalene 7 was considerably more reactive
than trans-stilbene 3 (Table 2, entries 11 and 15, Table 3,
entries 4 and 5) and cis-stilbene (Table 2, entries 11 and
15, Table 3, entries 20 and 21).
Effect of solvent and temperature
Although preliminary studies (45°C, 0.1 mol.%
of catalyst, catalyst/substrate/oxidant molar ratio
1:1000:1500, 48 h) indicated little difference in the yields
obtainedforthe1d-catalyzedepoxidationoftrans-stilbene
3 by 2,6-DCPNO 2a in toluene or dichloromethane
[18], 1,2-dihydronaphthalene 7 is commonly used as
substrate in the evaluation of epoxidation catalysts [20,
21]. It was therefore decided to use this compound as
substrate in the optimization of reaction conditions
for this reaction. When 1,2-dihydronaphthalene 7 was
subjected to 2,6-DCPNO 2a epoxidation catalyzed
by 1a in different solvents, no reaction was, however,
observed in the coordinating solvents THF or CH3CN,
whereas both conversion and epoxide yield increased
with decreasing polarity of the other solvents, i.e. ethyl
acetate (17%, 12%), dichloromethane (17%, 13%) and
Epoxidation of trans- (3) and cis- (4) stilbene
As was previously reported, the epoxidation of trans-
stilbene 3 with 2,6-DCPNO 2a proceeded optimally at a
catalyst concentration of 0.45 mol.% [18].At this concen-
tration, conversions above 95% and excellent selecti-
vities (>75%) could be obtained for all the evaluated
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 3–13

4
C.A. ENOW ET AL.
Table 2. Catalytic epoxidation of 1,2-dihydronaphthalene 7 by 2,6-DCPNO 2a with Ru(II)-Pc-complexes (0.1 mol.%)a
O
2a, cat.
Ar, toluene
7
8
Entry
Cat
Cat., mol.%
Solvent
T, °C
t, h
Conversion, % Epoxide Selectivity, %
yield, %
TON
TOF, h-1
1
2
1a
1a
1a
1a
1a
1a
1a
1a
1a
1d
1d
1d
1b
1c
1e
1f
0.1
CH3CN
EtOAc
THF
45
45
45
45
45
60
80
90
100
60
80
90
80
80
80
80
80
80
80
80
80
48
48
48
48
48
48
6
—
17
—
12
—
13
16
68
82
80
74
57
77
77
80
80
83
35
0.1
3
0.1
—
4
0.1
CH2Cl2
17
5
0.1
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
22
6
0.1
84
81
82
80
76
68
77
77
80
80
83
83
46
618
7
0.1
100
100
>98
84
820
8
0.1
6
600
9
0.1
3
1120
138
10
11
12
13
14
15
16
17
18
19
20
21
0.1
48
6
0.1
100
100
100
100
100
42
770
770
0.1
6
0.1
6
800
0.1
6
800
0.1
6
830
0.45
0.02
0.02
0.02
0.02
0.02
6
77
1a
1b
1c
1d
1e
12
12
12
12
12
53
2700
2500
2800
2500
2300
286
295
300
457
260
49
57
51
46
aCatalyst/substrate/oxidant molar ratio = 1:1000:1500 for 0.1 mol.% cat., 1:5000:7500 for 0.02 mol.% cat. and 1:220:330 for
0.45 mol.% cat. Product was identified by GC-MS.
catalysts, the only exception being the unsubstituted
RuPc complex 1f with which little epoxidation was
observed.
1a–1e. As indicated in Table 3 (entries 6–10), all of
the catalysts, except 1e, proved to be quite active with
turnover frequencies (TOF) of up to ca. 170 h-1. Although
yields and conversions dropped quite dramatically when
compared to the reactions at 0.45 mole % and 0.1 mol.%
(Table 3 and Enow, Marais and Bezuidenhoudt [18]), the
increase in turnover numbers indicated that the catalysts
were still active up to the end of the reactions in the
previous runs at the higher concentrations.
Since the best TONs reported in literature for the
homogeneous epoxidation of trans-stilbene 3 was 243
(after 48 h) [22] and 270 (after 16 h) [20], it can be
concluded that 1c (Table 3, entry 8) with a turnover
number of 1200 (after 48 h) is superior to all existing
catalysts in the homogeneous epoxidation of trans-
stilbene 3.
To emphasize differences in activity between the
different catalysts, conversion, yields, selectivity and
turnover numbers were determined at 0.1 mol.% catalyst
loading (catalyst/alkene/2,6-DCPNO = 1:1000:1500).
The selectivity and conversions for the alkyl substituted
RuPc 1a–1e were still acceptable (77–87% and 68–100%,
respectively) (Table 3, entries 1–5), while turnover
numbers of up to 840 were obtained (Table 3, entry 3)
after 48 h of reaction.
In order to determine if some of the catalyst might
have been deactivated by aggregation or otherwise and
to determine if differences in catalyst activity could be
accentuated, the catalyst concentration was reduced to
0.02 mol.% (catalyst:alkene:2,6-DCPNO = 1:5000:7500)
and the reaction repeated with all the prepared catalysts
This facile epoxidation of trans-stilbene 3 was
unexpected as Hirobe [7–9], Gross [22, 23], Berkesel [21],
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NON-PERIPHERALLY ALKYL SUBSTITUTED RUTHENIUM PHTHALOCYANINES AS CATALYSTS
5
and Zhang [20] reported poor or no ruthenium porphyrin
around the C–C bond [22, 30].* The stability and activity
of complexes 1a–1e in the epoxidation of cis-stilbene
4 was further evaluated through the determination
of turnover number (TON) and frequency (TOF) at
0.1 mol.% catalyst loading and 90°C for 48 h (Table 3,
entries 17–21). High TONs of up to 570 (after 48 h) at
TOFs of 25 to 45 h-1 (after 1 h) were obtained.
catalyzed epoxidation of this substrate, even with highly
electron deficient ruthenium porphyrin complexes. The
best conversion and turnover with homogeneous chiral
ruthenium porphyrins was reported to be ca. 21% and 270
(after 16 h) [22, 20], respectively, while heterogeneous
polymer-supported ruthenium porphyrin catalysts gave
88% conversion and 870 TON (after 24 h) [24]. The
poor reactivity of trans-stilbene 3 with these porphyrin
epoxidation systems was attributed to steric hindrance
during the “side-on” approach, i.e. when the substrate
approaches the metal oxo moiety (Ru=O) from the side
and at an angle relative to the plane of the porphyrin
(Fig. 1) [25]. According to this model, steric hindrance
between one of the phenyl groups of trans-stilbene 3
and the plane of the porphyrin can be invisaged due to
the angle of approach. Substituents on the porphyrin
periphery, and especially bulky substituents, decreased
the reactivity even further.
Within the assumption that a metal oxo species is
involved in the ruthenium phthalocyanine catalytic
cycle, the facile epoxidation of trans-stilbene 3 by the
alkylsubstituted ruthenium phthalocyanines in the
current investigation suggest either a difference in steric
hindrance around the metal oxo (Ru=O) moiety or an
approach to the reactive centre other than the “side-on”
one. Non-peripherally alkyl substituted Pcs tend to be
non-planar and to adopt saddle shaped structures [26].
The metal free isopentyl phthalocyanine, for example, is
inclined at an angle of 32° from the core of the complex
[26]. Such a distortion from planarity, if present in the
ruthenium phthalocyanine complexes prepared in this
study, will expose the reactive metal oxo (Ru=O) part of
the complex and therefore facilitate its interaction with
the substrate. With regards to approach, a mechanism
involving a “top-on” approach of the olefin to the Ru=O
(Fig. 1c), as suggested by Liu et al. [27], will sufficiently
account for the high activity despite the bulkiness of the
ligand substituents.
For both trans- 3 and cis-stilbene 4, superior epoxide
yields were obtained with the alkyl substituted RuPcs
1a–1e in comparison to the unsubstituted catalyst 1f
(Bezuidenhoudt and co-workers [18] and Table 3, entries
11–16). The activities and selectivities obtained with
the alkyl substituted catalysts 1a–1d were the same
within experimental error and these catalysts were
furthermore more active than their counterpart with
the bulkier cyclohexyl 1e substituent (Bezuidenhoudt
and co-workers [18] and Table 3, entries 11–16). This
observation is in agreement with Groves’ report that
oxygen transfer is sensitive to the steric bulk in the vicinity
of the active metal centre [25] and is corroborated by the
near planar trans-stilbene 3 (phenyl-vinyl torsions of 2.2
and 5.4°) [31] being more reactive than the non-planar
cis-isomer 4 (with phenyl pyrimidalization towards the
olefinic carbons and the phenyl rings twisted to alleviate
van der Waals repulsion by each other; phenyl-vinyl
torsions of 43°) [32, 33]. During a “top-on” approach,
the twisted phenyl groups in the cis-isomer 4 would
thus experience more steric interaction with the non-
peripheral substituents of the catalyst four bondlengths
away from the Ru=O moiety than the phenyl groups of
the trans-isomer (3).
Competitive epoxidation of cis- 4 and
trans-stilbene 3
Since it is claimed in literature that the homogeneous
[7, 9] and heterogeneous [34] ruthenium porphyrin-N-
oxide systems show preference towards the epoxidation
of cis-stilbene 4 over the trans-isomer 3, this aspect of
the current RuPc catalysts was subsequently evaluated,
especially given that our results with RuPcs 1a–1e
indicated trans-stilbene 3 to be more reactive than cis-
stilbene 4 (vide supra). A 1:1 mixture of cis- 4 and
trans-stilbene 3 was thus subjected to the epoxidation
conditions (catalyst concentration of 0.23 mol.%, 24 h
reaction period).
Both the trans-epoxide 5: cis-epoxide 6 ratio obtained
after 24 h (Table 4) and the kinetic profiles (Fig. 2)
indicated the trans-epoxide 5 to be forming 1.14–1.35
timesfasterthanthecis-epoxide6(53–57%trans-epoxide
5 after 24 h). The cis-trans ratio furthermore remained
essentially constant during the reaction catalyzed by 1a
The use of cis-stilbene 4 in the epoxidation reactions
to obtain information on the mechanistic pathway is
quite prevalent [20, 25, 28, 29]. Extension of the study
to include Pcs 1a–1f (0.45 mol.% cat., 90°C) confirmed
epoxidation with stereoretention (Table 3, entries 11–16).
This almost stereospecific formation of cis-stilbene oxide
6 suggests that oxygen transfer from the ruthenium Pc to
the alkene occurs via a concerted oxene insertion rather
than a one-electron oxidation which would allow rotation
O
M
O
M
O
M
a
a
*The small amounts of trans-epoxide 5 (ca. 4–5%) observed in
all of the reactions might be the result of a competing radical
mechanism [27] and/or of a trans-stilbene 3 impurity in the
commercial samples of cis-stilbene 4 (sold with >96% purity).
(a)
(b)
(c)
Fig. 1. The “side-on” approach of cis- (a) and trans-alkenes
(b) to the metal oxo moiety; and (c) the “top-on” approach
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C.A. ENOW ET AL.
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NON-PERIPHERALLY ALKYL SUBSTITUTED RUTHENIUM PHTHALOCYANINES AS CATALYSTS
7
Table 4. Competitive cis/trans-stilbene 4/3 epoxidation by
product, phenylacetaldehyde 15, at 44% conversion after
24 h. When the reaction with 1b was left to run for 36 h,
benzaldehyde 9 became the main product with a product
distribution of 31:48:21 (25, 38 and 17% in terms of
yield) for 12, benzaldehyde 9 and phenylacetaldehyde
15 at 77% substrate conversion. Trace amounts of
benzaldehyde 9 was similarly only observed after ca. 15 h
reaction time in the cis-stilbene 4 reaction, whereas cis-
stilbene oxide 6 decomposed into benzaldehyde 9 when
subjected to the epoxidation reaction conditions for 15 h.
This confirms that decomposition of the epoxide, rather
than competitive oxidative alkene cleavage, occurs to
form the benzaldehyde 9 [27, 36–38]. Similar variations
in the product ratios have been observed in ruthenium
porphyrin catalyzed epoxidation of styrene with 2a,
PhIO or TBHP (t-butylhydroperoxide) as oxidants [39].
(Ep)oxidation of the more electron rich 4-methoxy-
styrene 11 (Table 5, entries 7–11) catalyzed by 0.45 mol.
% of the RuPc complexes 1a–1e in toluene resulted in
increased substrate conversions (51–92% within 24 h),
though selectivities towards the desired epoxide product
13 were still moderate (32–64%). While the epoxide 13
remained the major product and the electron donating
properties of the 4-methoxy group had a beneficial effect
on the production of the desired epoxide (17–44 vs.
51–92%), it also enhanced decomposition of the epoxide
to the benzaldehyde 14 and rearrangement towards the
phenylacetaldehyde 16.
2,6-DCPNO 2a with Ru(II)-Pc-complexes at 0.23 mol.%
catalyst concentrationa
Entry
Cat.
Trans-epoxide Cis-epoxide
Ratio (5:6)
(5) yield, %
(6) yield, %
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
91
89
98
84
37
16
70
74
73
74
31
12
1.3:1
1.2:1
1.34:1
1.14:1
1.19:1
1.35:1
a Reaction conditions: Toluene (2 mL), 90°C, 0.23 mol.%
catalyst, 1:1 mixture of cis- (4) and trans-stilbene (3) for 24 h.
(Ep)oxidation of trans-b- (17), cis-b- (18) and
a-methylstyrene 19 catalyzed by complexes 1a–1d
(0.1 mol.% cat., for 48 h at 90°C) gave the desired
epoxides in acceptable yields. Moderate to high
conversions (74–90%), good selectivities (66–91%), and
moderate turnovers (640–750, 48 h) were obtained for
all substituted RuPcs, except for 1e (Table 6). As for all
the other substrates, the unsubstituted Pc 1f gave low
conversions (<20%) for all the substituted styrenes tested
(Table 6).
The alkyl substituted ruthenium Pcs tested during the
current investigation are considerably more reactive than
both the Jorgenson system (Fe(II)Pc/PhIO) [16], which
gave only 63% conversion with a turnover number of <50
in the epoxidation of trans-b-methylstyrene 17, and the
homogeneous ruthenium porphyrin/N-oxide system used
by Che [20] with a substrate conversion of only 24% at
the same catalyst loading, whereas it is comparable to
the heterogeneous ruthenium porphyrin/N-oxide system
reported on by Zhang and Che [24], which gave TON of
up to 890 (24 h).
Fig. 2. Time course plot for the competitive epoxidation of
cis-stilbene (4) and trans-stilbene (3) catalyzed by RuPc 1a
(0.23 mol.% cat., toluene, 90°C)
(ca. 1.3:1, Fig. 2). The higher reactivity of the trans-
alkene represents the opposite of what was found for
the porphyrin system (vide supra) and resembles the
m-chloroperoxybenzoic acid epoxidation of cis-4 and
trans-stilbene 3 in methylene chloride [21].
Epoxidation of styrenes
Treatment of planar styrene 10 (phenyl-vinyl dihedral
angle of 0°) [35] with 2a (1.5 eq.) and 1a–1e (0.45 mol.%)
in toluene at 90°C for 24 h, gave the styrene oxide 12
in moderate selectivity (39–57%) with 17–44% substrate
conversions (Table 5, entries 1–5), while the unsubstituted
Pc 1f again led to only 7% conversion (entry 6).
While no clear trend or influence on selectivity could
be identified originating from the alkyl substituents
attached to the Pcs, it was noticed that reaction time
played a crucial role in the product distribution. For the
RuPc 1b reaction, for example, a product distribution of
47:33:20 (42, 29 and 18% in terms of yield) were obtained
for epoxide 12, benzaldehyde 9 and the rearrangement
Substituting one of the phenyl groups of stilbene
with a methyl group resulted in decreased conversions
and yields for the trans-isomer 17 (with the exception
of the reaction catalyzed by RuPc 1d) (Table 6, entries
1–5, Table 3, entries 1–5), even though the phenyl-vinyl
dihedral angles are close to 0° for both isomers (0° for
17 [35] and 2.2 and 5.4° for 3 [31]). This difference in
activity might therefore be ascribed to increased steric
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 7–13

8
C.A. ENOW ET AL.
Table 5. Epoxidation of styrene 10 and 4-methoxystyrene 11 with 2,6-DCPNO 2a catalyzed by ruthenium phthalocyanines
1a–1f at 0.45 mol.% catalyst concentrationa
O
CHO
2a, cat.
CHO
+
+
Ar, toluene
R
R
R
R
10 R = H
11 R = OMe
12 R = H
13 R = OMe
9 R = H
14 R = OMe
15 R = H
16 R = OMe
Entry
Substrate
Cat.
Conversion, %
Selectivity, % epoxide Benzaldehyde
Phenylacetaldehyde
1
2
10
10
10
10
10
10
11
11
11
11
11
1a
1b
1c
1d
1e
1f
39
44
28
35
17
7
12 (39)
12 (42)
12 (57)
12 (51)
12 (41)
12 (35)
13 (51)
13 (44)
13 (32)
13 (64)
13 (41)
9 (33)
9 (29)
15 (15)
15 (18)
15 (15)
15 (17)
—
3
9 (36)
4
9 (31)
5
9 (35)
6
9 (20)
15 (19)
16 (14)
16 (14)
16 (7)
7
1a
1b
1c
1d
1e
90
92
69
67
51
14 (29)
14 (25)
14 (25)
14 (21)
14 (35)
8
9
10
11
16 (13)
16 (12)
a Reaction conditions: toluene, 90°C, catalyst/substrate/oxidant molar ratio = 1:220:330 for 24 h. Products were identified
by GC-MS.
interaction between the methyl group and the catalyst
compared to the phenyl group being co-planar to the
double bond in trans-stilbene 3. In the case of the cis-
isomer 18, conversion and epoxide yields drastically
increased when the twisted phenyl of cis-stilbene 4 was
replaced by a methyl group (18) (Table 3, entries 17–21,
Table 6, entries 7–11), to render a phenyl-vinyl moiety
with a smaller dihedral angle (43° for 4 [32, 33] and 35°
for 18 [35]). The importance of a planar phenyl-vinyl
moiety is further corroborated by cis-b-methylstyrene 18
also being less reactive than 1,2-dihydronaphthalene 7
(Table 6, entries 7–11 vs. Table 2, entries 11–15°; phenyl-
vinyl dihedral angles of 35° for 18 and 15° for 7 [7]).
For a-methylstyrene 19, conversions and yields
were remarkably comparable to those obtained for the
epoxidation of cis-b-methylstyrene 18 in the presence of
all of the catalysts except for RuPc 1c, which performed
considerably poorer with this substrate [79% conversion,
52% yield, 66% selectivity and 640 TON, Table 6, entry
15 vs. 90% conversion, 72% yield, 80% selectivity and
720 TON in 48 h for cis-b-methylstyrene 18, Table 6,
entry 9]. The reason for this outlier result is currently
not clearly understood, but the similarities point to the
similar steric interference between these substrates and
the catalyst in the transition state. The phenyl-vinyl
dihedral angle is 35° for both a-methylstyrene 19 and
cis-b-methylstyrene 18 [35].
better conversion than both cis-b-methylstyrene 18 and
a-methylstyrene 19 (dihedral angles of 35°) [35] (Table 6,
entries 4 and 5 vs. 10 and 11; entries 16 and 17), whereas
the non-conjugated [32] more nucleophilic 18 and
19 performed slightly better with the linear catalysts
(Table 6, entries 1–3 vs. 7–9 and 13–15).
Epoxidation of cyclooctene 24
Moving away from conjugated double bonds,
cyclooctene 24 was chosen as the next substrate. With 0.45
mol.% of the alkyl substituted ruthenium pthalocyanine
complexes 1a–1e, the catalytic epoxidation of cyclooctene
24 by 2,6-dichloropyridine N-oxide 2a proceeded
efficiently to afford cyclooctene oxide 25 as the only
identifiable product (Table 7, entries 1–5) in high yields
(68–86%) and selectivity (80–87%) within only 15 h.
Epoxidation of 1,5-cyclooctadiene 26
and limonene 29
After establishing that cyclooctene 24 could indeed
be epoxidized in good yield and selectivity by the
substituted ruthenium phthalocyanine catalysts, it was
decided to study the selectivity of these catalysts towards
the double bonds of dienes. 1,5-Cyclooctadiene 26,
which can be epoxidized to the mono-epoxide 27 or bis-
epoxide 28 (Table 8), was thus selected as the next model
compound.
In the case of the bulkier catalysts (1d and 1e), trans-
b-methylstyrene 17 (dihedral angle of 0°) [35] showed
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 8–13

NON-PERIPHERALLY ALKYL SUBSTITUTED RUTHENIUM PHTHALOCYANINES AS CATALYSTS
9
Table 6. Epoxidation of trans-b- (17), cis-b- (18) and a-methylstyrene 19 with 2,6-DCPNO 2a catalyzed by ruthenium
phthalocyanines 1a–1f at 0.1 mol.% catalyst concentrationa
R1
R1
O
R2
CHO
R2
2a, cat.
CHO
+
+
Toluene
R3
R3
17 R1 = R3 = H, R2 = CH3
18 R1 = R2 = H, R3 = CH3
19 R2 = R3 = H, R1 = CH3
20 R1 = R3 = H, R2 = CH3
21 R1 = R2 = H, R3 = CH3
22 R2 = R3 = H, R1 = CH3
9
23
Entry
Cat.
Substrate
Conv., %
Epoxideb yield, %
Aldehyde yield, %
Epoxide selectivity, %
TON
1
2
1a
1b
1c
1d
1e
1f
17
17
17
17
17
17
18
18
18
18
18
18
19
19
19
19
19
19
84
82
85
84
62
10
88
87
90
74
53
10
88
87
79
74
53
18
70
68
75
67
49
4
9 (3)
9 (4)
83
83
88
80
79
40
82
81
80
91
87
70
82
80
66
86
87
67
700
680
750
670
490
50
3
9 (6)
4
9 (3)
5
9 (3)
6
9 (1)
7
1a
1b
1c
1d
1e
1f
72
70
72
67
46
7
9 (4)
720
700
720
670
460
70
8
9 (4)
9
9 (3)
10
11
12
13
14
15
16
17
18
9 (3)
9 (3)
9 (1)
1a
1b
1c
1d
1e
1f
72
70
52
64
46
12
23 (3)
23 (2)
23 (2)
23 (2)
23 (2)
23 (<1)
750
720
640
660
480
120
aReaction conditions: toluene, 90°C catalyst/substrate/oxidant molar ratio = 1:1000:1500 for 48 h. Products were identified by
GC-MS. b 13 and 14 could not be distinguished from each other as the retention times were identical (23.04 min).
Employing 0.45 mole % catalysts, GC-MS analysis
showed 1,5-cyclooctadiene 26 to be converted to the mono-
epoxide 27 only. Bhattacharjee andAnderson [40] reported
similar observations in the manganese-salen/O2-aldehyde
system and explained that 27, once formed, is less prone to
oxidation than 26. Previous work by Larsen and Jorgensen
[16] showed that the epoxidation of this substrate with the
iron(II) Pc/iodosylbenzene system afforded a mixture of
both epoxides [mono-epoxide 27 (25%), bis-epoxide 28
(5%)], whereas only the bis-epoxide 28 was selectively
formed in 76% yield by a polymer supported ruthenium
porphyrin/N-oxide system [41].
Conversion, yield and selectivity were drastically
lower for this substrate compared to that of cyclooctene
24 (Table 8 vs. Table 7). This might be ascribed to
differences in the allylic bond angles, 1,5-cyclooctadiene
26 being in a boat conformation and cyclooctene 24 in a
chair conformation [40, 42].
When (+)-limonene 29 was exposed to the
epoxidation conditions with 0.45 mol.% of the ruthenium
phthalocyanine complexes for 24 h, the electron-rich
trisubstituted double bond was favored over the terminal
double bond, affording epoxide 30 as only identifiable
product (Table 9).
Comparable results were obtained for all the alkyl
substituted ruthenium phthalocyanine 1a–1e catalyzed
reactions with conversions above 92% (Table 9, entries
1–5). Yields (40–47%) and epoxide selectivity (40–51%)
were however low, which indicates that side reactions
and/or decomposition of the epoxide is prevalent. This
deduction is supported by the increase in epoxide yield
and selectivity when bulkier catalysts were used (Table 9,
entries 4 and 5 vs. 1–3). The low yields and selectivities
found are similar to that reported for the ruthenium-
bisoxazole catalyzed epoxidation of this substrate by the
molecular oxygen/isobutylaldehyde system [39]. Contrary
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 9–13

10
C.A. ENOW ET AL.
Table 7. Epoxidation of cyclooctene 24 with 2,6-DCPNO 2a catalyzed by ruthenium phthalocyanines 1a–1fa
2a, cat.
O
Ar, toluene
25
24
Entry
Cat.
Cat., mol.%
Conversion, %
Yield, %
Selectivity, %
TON after 48 h
TOF, h-1 b
1
2
1a
1b
1c
1d
1e
1f
0.45
0.45
0.45
0.45
0.45
0.45
0.23
0.23
0.23
0.23
0.23
100
100
100
93
82
84
86
74
68
10
61
73
73
62
44
82
84
86
80
87
67
77
73
84
76
67
3
4
5
78
6
15
7
1a
1b
1c
1d
1e
79
305
362
365
310
220
43
41
40
42
40
8
93
9
87
10
11
82
66
aReaction conditions: toluene, 90°C, catalyst/substrate/oxidant molar ratio = 1:220:330 for 20 h for 0.45 mol.% catalyst and
1:500:750 for 0.23 mol.% catalyst. b TOF after 2 h.
Table 8. Epoxidation of 1,5-cyclooctadiene 26 by 2,6-DCPNO 2a
(oxidant:substrate 1.5:1) catalyzed by ruthenium phthalocyanines
1a–1f at 0.45 mol.% catalyst concentrationa
Table 9. Epoxidation of (+)-limonene 29 by 2,6-DCPNO 2a
catalyzed by ruthenium phthalocyanines 1a–1f at 0.45 mol.%
catalyst concentrationa
2a, cat.
O
O
O
O
2a, cat.
+
Ar, toluene
+
Ar, toluene
O
26
27
28
Entry Cat. Conversion, Yield, Selectivity TON after
29
31
30
%
%
to 27, %
24 h
Entry Cat. Conversion, Yield, Selectivity TON after
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
52
48
62
50
53
11
24
31
34
24
28
4
46
65
55
48
53
36
53
68
75
53
62
11
%
%
to 30, %
24 h
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
100
100
100
92
40
40
40
40
42
51
48
—
88
88
42
92
47
103
101
—
96
46
a Reaction conditions: toluene, 90°C, catalyst/substrate/oxidant
molar ratio = 1:220:330 for 24 h. Product identified by GC-MS.
13
trace
a Reaction conditions: toluene, 90°C, catalyst/substrate/oxidant
molar ratio = 1:220:330 for 24 h.
to the current results, the epoxidation of limonene 29 with
the homogeneous ruthenium porphyrin/N-oxide system
previously reported, resulted in mixtures of 30 and 31 in
varying ratios [43].
of the epoxides corresponding to 1-octene 32 and trans-
2-octene 33 could be detected after 24 h in the presence
of 0.45 mol.% catalyst and 2a (5 eq.). Increasing the
catalyst load to up to 3 mol.% or raising the temperature
to 100°C did not increase the yield of the epoxide at all.
The conversion (up to ca. 5%) obtained for both 1-octene
Epoxidation of aliphatic acyclic alkenes
Aliphatic acyclic substrates are amongst the most
difficult to oxidize substrates. Only trace amounts (~2%)
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 10–13

NON-PERIPHERALLY ALKYL SUBSTITUTED RUTHENIUM PHTHALOCYANINES AS CATALYSTS
11
32 and trans-2-octene 33, was comparable to that
obtained with the chiral ruthenium porphyrin catalyzed
epoxidation of 1-octene [21].
cis- 18 and trans-b-methylstyrene 17, a-methylstyrene
19, styrene 10, 4-methoxystyrene 11, cyclooctene 24,
1,5-cyclooctadiene 26, limonene 29, 1-octene 31 and
trans-2-octene 32, were comparable to those reported for
other catalyst systems.
EXPERIMENTAL
For all the substrates tested, all of the substituted
ruthenium phthalocyanines 1a–1e performed markedly
better as epoxidation catalysts than the unsubstituted
equivalent (1f), most probably because of reduced
levels of aggregation in solution due to the acquired
“saddle shape” of substituted ruthenium phthalocyanines
with non-peripheral substituents. Increasing the steric
bulk of the substituents attached to the phthalocyanine
lowered the catalytic activity with a general order of
reactivity 1a ≈ 1b ≈ 1c > 1d > 1e. RuPc 1e had the lowest
activity towards all substrates evaluated, which might
be ascribed to steric congestion. Linear substituents on
the nonperipheral sites of the phthalocyanine were thus
able to reduce aggregation and increase the solubility of
the catalyst without compromising its activity by steric
congestion.
These non-peripherally substituted ruthenium
phthalocyanines proved to be highly effective towards
the epoxidation of conjugated and cyclic alkenes and,
in the presence of non-equivalent double bonds, showed
selectivity towards the more substituted double bond
above the terminal double bond. 1-Octene 32 and trans-
2-octene 33 could not be epoxidized in the presence of
these catalyzts, which correlates with the known low
reactivity of these substrates.
Though the mechanism is still ambigious and the exact
active intermediate not established (several high-valent
oxo-ruthenium species with oxidation numbers ranging
from IV to VIII are possible, for example)[36], the
epoxidation mechanism simplistically presented likely
involves the coordination of the N-oxide to ruthenium
and subsequent transfer of the oxygen to the metal to
form a high-valent oxo-ruthenium species [14, 46–49].
The approach of the alkene to the metal oxo moiety
seems to be different from the “side-on” approach
proposed for alkene epoxidation by porphyrins as trans-
stilbene 3 and trans-b-methylstyrene 17 were highly
reactive. A step-wise mechanism with intermediate
radical formation (which allows for rotation around
the C–C· bond) was furthermore ruled out by the
stereospecific epoxidation of cis-stilbene 4. A “top-on”
approach and concerted oxygen transfer with concomitant
stereoretention is thus proposed.
The reactivity of the substrates depended on the degree
of planarity around the double bond (phenyl-vinyl dihedral
angle), nucleophilicity of the double bond (degree of
conjugation and the presence of electron-withdrawing or
-donating substituents), the steric bulk of the remainder
of the molecule and the allylic angle. For phenyl-vinyl
systems, reactivity decreased in the order (phenyl-vinyl
dihedral angle given in brackets): 1,2-dihydronaphthalene
7 (15°) [35] > trans-stilbene 3 (2.2 and 5.4°) [31] >
All chemical reagents were obtained from Aldrich,
Fluka or Merck and used without further purification.
Solvents were freshly distilled using standard methods.
Carbonyl ruthenium phthalocyanines 1a–1f were synthe-
sized as reported earlier [18, 44, 45]. The oxidant 2,6-
dichloro-4-methoxypyridine-N-oxide 2a was prepared
according to a published procedure [21].
GC analyses were performed on a Shimadzu GC-2010
fitted with a PONA column (50.0 m × 0.20 mm × 0.50 mm)
and FID detector. The N2/Air (carrier gas) linear velocity
was1.07mL/minandtheinjectoranddetectortemperatures
200°C and 290°C, respectively. Injections were made in
the split mode. The initial column temperature of 60°C
was kept for 5 min, whereafter it was increased to 250°C
at 5°C/min and kept at this temperature for the rest of
the analysis. Retention times were compared to those
of commercially available samples. Where indicated
in the discussion, products were identified by GC-MS
analyses (electron impact ionization) on a Shimadzu
GC-MS Qp-2010 fitted with a column and operated under
conditions similar to that of the GC, but with helium as
carrier gas. Conversions and yields were determined by
GC using dodecane as internal standard.
Catalytic Reactions: A 15 mL Schlenk flask was
charged with the catalyst (0.5 mmol, 1 eq.), the olefin,
dodecane (internal standard) and dry toluene (2 mL)
under an argon atmosphere. 2,6-Dichloropyridine-N-
oxide 2a was added and the solution stirred at 90°C.
The reactions were followed by gas chromatographic
analysis.
CONCLUSION
In this study it was demonstrated for the first time
that ruthenium phthalocyanines can be used in the
epoxidation of a variety of alkenes and that non-
peripherally alkyl substituted ruthenium phthalocyanines
in particular are highly active catalysts with true catalytic
activities at very low concentrations (<0.45 mole %).
Complete conversion and high turnovers (>800 in 48 h
for 0.1% catalyst loading) comparable to or better
than those published for other catalytic systems could
be obtained for 1,2-dihydronaphthalene 7 and trans-
stilbene 3. At low catalyst loading (0.02 mole %),
TONs larger than 2000 in 12 h and TOFs above 260 h-1
were obtained for 1,2-dihydronaphthalene 7. The same
catalyst concentration gave TONs above 1000 in 48 h
and TOFs above 90 h-1 for trans-stilbene 3. Results
for the other substrates commonly used to evaluate
the activity of epoxidation catalysts, i.e. cis-stilbene 4,
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 11–13

12
C.A. ENOW ET AL.
trans-a-methylstyrene 17 (0°) [35] > cis-b-methylstyrene
18(35°)[35]≈a-methylstyrene19(35°)[35]>cis-stilbene
4 (43°) [32, 33]. Reactivity decreased in the following
order for cyclic alkenes (allylic angle in brackets):
cyclooctene 24 (124.7, 126.4°, chair) [42] > (+)-limonene
29 (ca. 123.5° for the more substitued double bond in
analogy to cyclohexene) [40] > 1,5-cyclooctadiene 26
(122.7, 122.8; 137.4, 137.6°, boat) [42, 40]. The aliphatic
acyclic alkenes 1-octene 32 and trans-2-octene 33 gave
only trace amounts of the epoxides.
17. McKeown NB, Chambrier I and Cook MJ. J. Chem.
Soc., Perkin Trans. 1 1990: 1169.
18. Enow CA, Marais C and Bezuidenhoudt BCB.
J. Porphyrins Phthalocyanines 2012; 16: 404–412.
19. Ohtake H, Higuchi T and Hirobe M. Heterocycles
1995; 40: 867–903.
20. Zhang R.,Yu W-Y, Wong K-Y and Che C-M. J. Org.
Chem. 2001; 66: 8145–8153.
21. Berkessel A and Frauenkron M. J. Chem. Soc.
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24. Zhang J-L and Che C-M. Org. Lett. 2002; 4:
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Acknowledgements
Financial support from Sasol Ltd is gratefully
acknowledged.
25. Groves JT and Nemo TE. J. Am. Chem. Soc. 1983;
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Supporting information
26. (a) Chambrier I, Cook MJ and Wood PT. Chem.
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Tseng A-H, Chambrier I, Hughes DL and Cook MJ.
Tetrahedron Lett. 2009; 50: 5254–5256.
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A list of GC retention times are given in the
supplementary material. This material is available free of
charge via the Internet at http://www.worldscinet.com/
jpp/jpp.shtml.
28. Groves IT, Kruper WJ Jr. and Haushalter RC. J. Am.
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