Selectivity Trends in Olefin Epoxidations Catalyzed by (NNNN)Manganese(+II) Complexes using Trifluoroethanol as the Solvent

Article abstract of DOI:10.1002/cctc.201600755

The epoxidation of olefins is certainly one of the most important oxygen-transfer reactions in organic synthesis. The development of both regio- and stereoselective protocols and the search for catalysts that combine high selectivity with abundancy is a topic of current interest. Herein, we report an unexpected odd–even-type selectivity trend in the diastereoselective epoxidation of unfunctionalized aliphatic alkenes by using a non-hem-type Mn catalyst in trifluoroethanol as the solvent.

Full text of DOI:10.1002/cctc.201600755

DOI: 10.1002/cctc.201600755
Communications
Selectivity Trends in Olefin Epoxidations Catalyzed by
(NNNN)Manganese(+II) Complexes using Trifluoroethanol
as the Solvent
Samuel Lorenz and Bernd Plietker*[a]
The epoxidation of olefins is certainly one of the most impor-
tant oxygen-transfer reactions in organic synthesis. The devel-
opment of both regio- and stereoselective protocols and the
search for catalysts that combine high selectivity with abund-
ancy is a topic of current interest. Herein, we report an unex-
pected odd–even-type selectivity trend in the diastereoselec-
tive epoxidation of unfunctionalized aliphatic alkenes by using
a non-hem-type Mn catalyst in trifluoroethanol as the solvent.
the ligand, profound effects on the catalytic turnover in this
type of multistep catalysis were observed. Having these li-
gands in hand and with regard to the seminal contributions
on the use of (NNNN)Mn complexes bearing related ligands by
the groups of Que, Jr.,[5] White,[6] Costas,[7] Sun,[8] and Brylia-
kov,[9] we prepared a set of Mn complexes with different N-
substitution patterns at the ethylenediamine backbone of the
ligand. The ligands were accessible in a straightforward
manner and were subsequently transferred into the corre-
sponding paramagnetic Mn2+ complexes (Scheme 1). The
structures of complexes 5–7 were unequivocally assigned by
X-ray crystallography.).
The identification of parameters that allow selectivities in
chemical transformations to be influenced is one of the major
challenges in chemistry. Systematic variation of the solvent,
temperature, and, if required, pressure often affects the course
of a chemical reaction. Furthermore, significant changes in se-
lectivities can be obtained by changing the nature of the spe-
cific reagent. Within the past 40 years, the latter strategy has
built the basis for the success of transition-metal catalysis in
organic chemistry.[1] The interplay of ligand structure and elec-
tronic properties of the metal center has been used to direct
stereo- but also regioselectivities mostly by using favorable
steric interactions. However, in the most successful cases
strong and sterically defined coordination of the substrate to
the catalytic center is necessary.[2] The selective transformation
of unfunctionalized organic molecules such as simple aliphatic
olefins lacking any polar coordinating functional group is
somewhat less elaborated than the number of successful selec-
tive transformations of functionalized organic molecules.[3] The
electronic properties of the different isomeric olefins are similar
and, hence, not suitable for differentiation. Steric arguments
are not as valid as for heavily functionalized starting materials.
In this manuscript, we report an unexpected selectivity trend
for the oxidation of olefins by using a (NNNN)Mn2+ complex
and H2O2 as a stoichiometric oxidant. First indications for an
unusual odd–even-type selectivity were obtained.
Subsequently, the catalytic activity of [Mn(bep)(OTf)2] (6) in
the epoxidation of cis-cyclooctene (8) was tested. The litera-
ture-reported conditions (CH3CN/HOAc, H2O2) were chosen as
a starting point; however, only a slight excess amount of H2O2
was used (Table 1). Initial test experiments indicated that fast
catalytic decomposition of H2O2 took place. Hence, we decided
to add both a catalyst and a H2O2 solution simultaneously by
using a syringe pump. Moderate conversion was observed at
À208C. Upon increasing the reaction temperature to 08C,
Recently, we reported the synthesis and evaluation of a set
of (NNNN,P)Ru complexes [NNNN=N,N’-bisbenzyl-N,N-bis(2-pyr-
diylmethyl)ethylenediamine (bep)] as catalysts in hydrogen au-
totransfer catalysis.[4] Depending on the substitution pattern of
[a] S. Lorenz, Prof. Dr. B. Plietker
Institut fꢀr Organische Chemie
Universitꢁt Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
E-mail: bernd.plietker@oc.uni-stuttgart.de
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cctc.201600755.
Scheme 1. Synthesis of complexes 5–7. napht=naphthyl, OTf=trifluorome-
thanesulfonate.
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Table 1. Optimization table.[a]
Table 2. Epoxidation of alkenes/cycloalkenes–Effect of the N substituent
of the catalyst.[a]
Entry
Product
Yield [%]
6
Entry
T
[8C]
Additive
Solvent
Yield[b]
[%]
5
7
1
2
3
4
5
6
7
8
cyclopentene oxide (10)
cyclohexene oxide (11)
cycloheptene oxide (12)
cyclooctene oxide (9)
1-hexene oxide (13)
71
74
84
94
74
74
71
72
65
71
83
85
89
91
86
71
70
70
66
66
73
70
47
71
72
76
86
95
92
84
88
84
67
62
69
1[c]
2
3
4
5
6
7
8
À20
À20
0
HOAc[d]
HOAc[d]
HOAc[d]
HOAc[d]
none
HOAc[d]
HOAc[d]
none
MeCN
MeCN
MeCN
MeCN
59
83
92
84
–
91
91
29
26
r.t.
1-heptene oxide (14)
1-octene oxide (15)
À20
À20
À20
À20
À20
MeCN
TFE/EA (7:3)
TFE/EA (9:1)
TFE/EA (9:1)
TFE
trans-2-octene oxide (16)
trans-3-octene oxide (17)
trans-4-octene oxide (18)
cis-4-octene oxide (19)
1-nonene oxide (20)
trans-5-decene oxide (21)
norbornene oxide (22)
chalcone oxide (23)
9
10
11
11
12
13
14
9
none
[a] Reaction conditions: all reactions were performed on a 1 mmol scale
by using the appropriate solvent (2 mL) at the given temperature for 3 h.
Both the catalyst (0.01 mmol, dissolved in 1 mL of the appropriate sol-
vent) and the aqueous H2O2 solution (1.3 mmol, 30 wt%) were added si-
multaneously by means of a syringe pump (0.5 mLhÀ1). [b] Determined
by GC integration by using n-dodecane as an internal standard. [c] Only
H2O2 was added by syringe pump. [d] HOAc (14 equiv.) was employed.
76
53
58
70
74
53
72
71
[a] Reaction conditions: all reactions were performed on a 1 mmol scale
by using TFE/EA (9:1, 2 mL) as solvent and HOAc (14 mmol) as additive at
À208C for 3 h. Both the catalyst (0.01 mmol, dissolved in 1 mL TFE) and
the aqueous H2O2 solution (1.3 mmol, 30 wt%) were added simultaneous-
ly by means of a syringe pump (0.5 mLhÀ1). Yields were determined by
GC integration by using n-dodecane as an internal standard. [b] Catalyst
was dissolved in MeCN.
almost quantitative conversion and a yield of 91% were ob-
tained (Table 1, entry 2). A further increase in the reaction tem-
perature, however, led to a decrease in the yield (Table 1, en-
tries 3 and 4). At that point, we became interested in the role
of the additive, acetic acid (HOAc); by using acetonitrile as the
solvent, no conversion was observed in the absence of this ad-
ditive (Table 1, entry 5). In an attempt to increase the acidity of
acetic acid, we replaced acetonitrile with trifluoroethanol (TFE)
as the solvent. Given that cyclooctene and TFE are not misci-
ble, a small amount of ethyl acetate (EA) was added. We were
delighted to find that the reaction proceeded equally well as it
did in acetonitrile (Table 1, entries 6 and 7), but more impor-
tantly, the product was isolated in 29% yield in the absence of
acetic acid (Table 1, entry 8). To exclude the possible formation
of acetic acid through hydrolysis of ethyl acetate that was
added as a co-solvent, the reaction was performed in the ab-
sence of ethyl acetate. Interestingly, almost identical yields
were observed (Table 1, entry 9).
Figure 1.
&
^
Epoxidation of cycloalkenes (yield refers to GC yield); =mep, =bep,
~
=nep.
With the optimized conditions in hand, we decided to per-
form a first comparative screening of the application range.
The catalytic system showed good substrate scope in the ep-
oxidation of unfunctionalized olefins (Table 2). Interestingly, al-
though identical reaction conditions were used for all catalysts,
the N substituent within the Mn complexes had a strong
impact on the yield of this reaction. In general, the reactivities
of the complexes with the N,N’-phenylmethylene (bep) and
N,N’-(2-naphthyl)methylene (nep) ligands were significantly
higher than that of complex 5 with the N,N’-methyl (mep)
ligand for small ring sizes (Figure 1). Moreover, [Mn(nep)(OTf)2]
(7) showed a conversion maximum in the epoxidation of cyclo-
heptene, and the product was isolated in 95% yield by using
a substrate/H2O2 stoichiometry of 1: 1.3 (Table 2, entry 3;
Figure 1). Subsequently, we investigated the reactivity of vari-
ous 1-alkenes under the standard conditions (Table 2, en-
tries 5–14; Figure 2) and were surprised to find that the effect
of the N substituent on the conversion was similar to that ob-
served in the oxidation of cycloalkenes. [Mn(nep)(OTf)2] (7)
again showed preference for C7 olefins and gave 1-heptene
oxide in 88% yield (Figure 2). In sharp contrast, the corre-
sponding bep and mep complexes gave the product in yields
of 74 and 71%, respectively. Furthermore, the use of [Mn(be-
p)(OTf)2] (6) resulted in a significant drop in reactivity upon
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responsible for this wavelike conversion trend. With regard to
substitution of the p bond, it appears as if the length of the
substituent chain is somewhat responsible for this effect.
A time–conversion analysis for the oxidation of all four
octane isomers in one pot with continuous addition of the cat-
alyst and oxidant was performed to check the course of the se-
lectivity over time (Table 4). As can be seen from the time–
yield course, the selectivity trends that were observed after full
consumption of H2O2 are the result of the fact that oxidation
of 2-octene is faster than oxidation of 1-octene. The product
ratio over time did not change.
Figure 2.
Table 4. Time–yield curve for the oxidation of octenes.[a,b]
&
^
Epoxidation of 1-alkenes (yield refers to GC yield); =mep, =bep,
~
=nep.
moving from 1-hexene to 1-heptene. The mep complex
showed significantly lower, but almost constant, reactivity, in-
dependent of the chain length of the employed olefin.
As mentioned before, chemoselectivity in the epoxidation of
isomeric unfunctionalized olefins is one of the challenges in
catalysis. The unexpected “C7” selectivity in the oxidation of al-
kenes and cycloalkenes by using catalyst 7 attracted our inter-
est, and we decided to set up competition experiments by
treating a 1:1:1:1 mixture of 1-octene/2-octene/3-octene/4-
octene with H2O2 (2.6 equiv.) under the established conditions
in the presence of Mn catalysts 5–7 (Table 3).
Olefin
Yield [%]
0 min 15 min 30 min 45 min 60 min 90 min 180 min
Again, catalyst 7 showed the most significant discrimination
between the four olefins employed. In general, the yield in-
creased upon shifting the p bond from C1 to C2 but dropped
significantly for 3-octene. A further shift in the double bond
led to a moderate increase in reactivity. Similar, yet not as pro-
nounced, tendencies were observed for catalysts 5 and 6, and
the latter showed sharper discrimination of the isomers than
the former. Apparently, the aromatic substituent is somewhat
1-octene
2-octene
3-octene
4-octene
0
0
0
0
3.3
3.9
3.0
2.8
8.6
10.3
6.5
15.2
18.0
14.0
13.7
20.4
24.0
18.8
18.2
32.6
37.3
29.7
28.9
51.4
56.4
46.5
45.3
6.4
[a] Reaction conditions: all reactions were performed with each octane
isomer (0.5 mmol) by using TFE/EA (9:1, 2 mL) as solvent and HOAc
(14 mmol) as additive at À208C for 3 h. Both catalyst 7 (0.01 mmol, dis-
solved in 1 mL MeCN) and the aqueous H2O2 solution (1.3 mmol, 30 wt%)
were added simultaneously by means of a syringe pump (0.5 mLhÀ1).
Yields were determined by GC integration by using nitrobenzene as an
internal standard. [b] The presented data are the average of two inde-
pendent competition experiments. Only minor deviations were observed.
Table 3. Selectivities in olefin epoxidations in competition experiments.[a]
Olefin
Selectivity [%]
6
5
7[b]
7[b,c]
46
Subsequently, we performed similar experiments by using
equimolar mixtures of isomeric heptenes and nonenes. The re-
sults are shown in Figure 3. To compare the reactivities, we
normalized the yields relative to that of 2-heptene as the most
reactive substrate. As can be seen from this figure, the reactivi-
ty decreases with increasing chain length; however, we attri-
bute this effect mainly to the lower miscibility of nonenes rela-
tive to that of heptenes. In each case, “wavetype” conversion
was observed with a clear maximum for the epoxidation of 2-
alkenes and, less pronounced, for 4-alkenes. Interestingly, it ap-
pears as if the chain length of the shorter side chain is some-
what responsible. If the side chain has an odd number, the re-
activity increases. In 2-octene and 4-octene, both side chains
have an odd number of C atoms, and these olefins show sig-
nificantly higher selectivity preferences than 2-nonene and 4-
nonene, for which only one side chain has an odd number.
47
52
44
41
47
31
37
50
57
40
45
57
40
45
46
[a] Reaction conditions: all reactions were performed with each octane
isomer (0.5 mmol) by using TFE/EA (9:1, 2 mL) as solvent and HOAc as ad-
ditive (14 mmol) at À208C for 3 h. Both the catalyst (0.01 mmol, dissolved
in 1 mL TFE) and the aqueous H2O2 solution (1.3 mmol, 30 wt%) were
added simultaneously by means of a syringe pump (0.5 mLhÀ1). Yields
were determined by GC integration by using n-dodecane as an internal
standard. [b] Catalyst was dissolved in MeCN. [c] The reaction was per-
formed in MeCN instead of a TFE/EA mixture.
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Keywords: alkenes
· diastereoselectivity · epoxidation ·
manganese · oxidation
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Figure 3.
Structure–reactivity relationship in the oxidation of heptenes versus octenes
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~
&
^
Herein, we reported the synthesis of (NNNN)manganese(+II)
complexes and some unexpected selectivity trends in the ep-
oxidation of linear alkenes. All tested Mn complexes showed
good to excellent epoxidation activity. Complex 7 possessing
a naphthylmethylene substituent at the ethylenediamine back-
bone showed excellent performance. First indications for an
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Experimental Section
General procedure for catalytic epoxidation
The alkene (1 mmol, 1 equiv.) and HOAc (0.8 mL, 14 mmol) were
dissolved in a mixture of 2,2,2-trilfuoroethanol (1.8 mL) and ethyl
acetate (0.2 mL). The catalyst (0.01 mmol, 1 mol%) and H2O2 (30%
in water, 1.3 mmol, 1.3 equiv.) were diluted to 1 mL with 2,2,2-tri-
fluoroethanol and simultaneously added at À208C over a period of
2 h by using a syringe pump. The mixture was then stirred for 1 h
before dodecane (57 mL, 0.25 mmol) was added as an internal stan-
dard. After filtration over a short plug of silica gel (Et2O), the crude
product was analyzed by gas chromatography. For isolation of the
product, the mixture was diluted with Et2O (20 mL) and washed
with a saturated aqueous solution of Na2SO3 (10 mL) and a saturat-
ed aqueous solution of Na2CO3 (10 mL). The solution was then
dried with Na2SO4, and the solvent was removed under reduced
pressure. The crude product was filtered over silica gel (Et2O) and
finally purified by flash column chromatography (silica gel).
Received: June 22, 2016
Revised: July 14, 2016
Published online on && &&, 0000
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S. Lorenz, B. Plietker*
&& – &&
Selectivity Trends in Olefin
Epoxidations Catalyzed by
(NNNN)Manganese(+II) Complexes
using Trifluoroethanol as the Solvent
Evening out the odds: An unexpected
odd–even-type selectivity trend in the
diastereoselective epoxidation of un-
functionalized aliphatic alkenes by using
a non-heme-type Mn catalyst in trifluor-
oethanol as the solvent is observed.
~
&
^
(
=octene, =heptene, =nonene)
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