Olefin Epoxidation
FULL PAPER
was collected by filtration and washed with cold ethanol. The reaction
was performed under N2. 1H NMR (CDCl3): d=12.97 (2H), 8.65 (2H),
7.41–6.93 ppm (12H).
OOC(O)R and the ratio of heterolysis versus homoly-
sis.[2,4,16a,26,28,29] Therefore, these factors may influence the re-
action pathway shown in Scheme 2, resulting in the various
product partitions, cis-epoxide versus trans-epoxide, epoxide
versus aldehyde, and so on.[27] More detailed mechanistic
studies on the factors that influence heterolysis versus ho-
Preparation of MnIII
cording to the previously reported literature method[7] with a small modi-
fication. Mn(OAc)2·4H2O (1 mmol) was added to a stirred solution of
ACHTUNGTREN(NUNG saloph) complex: The complex was prepared ac-
AHCTUNGTRENNUNG
H2saloph (1 mmol) in ethanol (20 mL). After refluxing for several mi-
nutes and then stirring overnight at room temperature, a dark-brown
solid was collected by filtration and washed with acetone.
molysis, the lifetime of MnIII OOC(O)R (3), and the vari-
À
ous product partitions, are in progress in our laboratory.
Preparation of [Mn
(1-Se) and Te (2-Te)): The cluster compounds were prepared by a direct
diffusion technique in which methanolic solution of [PPh4]4-
[Re4Q4(CN)12] (0.05 mmol; Q=Se or Te) containing a trace amount of
H2O was carefully layered with a methanolic solution of [MnIII
(saloph)]
CAHUTTGNERN(NUG saloph)H2O]4ACHTUNGTREN[NNGU Re4Q4(CN)12]·4CH3OH·8H2O (Q=Se
a
Conclusion
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
(0.25 mmol). Dark-violet crystals of 2-Te suitable for X-ray analysis were
obtained after three weeks. The powder X-ray pattern of 1-Se was found
to be the same as that of compound 2-Te, indicating that they share a
We have successfully synthesized discrete molecules of rhe-
nium cluster-supported [MnIII
ACTHNUTRGNE(UNG saloph)] complexes, which
catalyze a wide range of olefin epoxidations with excellent
efficiencies, even with terminal olefins, while retaining their
integrity under mild experimental conditions. Reactivity and
Hammett studies, H218O exchange experiments, and the use
of PPAA as a mechanistic probe suggest that the multiple
common
C96H88Mn4N20O24Re4Te4 (2-Te) (3380.82): C 34.11, H 2.62, N 8.29; found:
34.06, 2.65, 8.34; elemental analysis calcd (%) for
structure.
Elemental
analysis
calcd
(%)
for
C
H
N
C96H88Mn4N20O24Re4Se4 (1-Se) (3186.26): C 36.19, H 2.79, N 8.79; found:
C 36.12, H 2.73, N 8.81.
Olefin epoxidations by the cluster-supported Mn complexes (1-Se and 2-
Te) with mCPBA: a) Aerobic conditions: mCPBA (0.05 mmol) was
added to a mixture of substrate (0.035 mmol), the cluster-supported Mn
complex (2.96ꢂ10À5 mmol), and solvent (CH3CN/CH2Cl2 1:1; 1 mL). The
mixture was stirred for 10 min at room temperature. Each reaction mix-
ture was applied to a column of silica gel and the organic products were
eluted; the eluent was monitored by subjecting periodically withdrawn
20 mL aliquots to GC/MS analysis. Dodecane as an internal standard was
used to quantify the products and conversions of substrates. All reactions
were run at least in triplicate and the average product yields are present-
ed. The product yields are based on substrate consumed. In the competi-
tive reaction of cis-2-octene and trans-2-octene, the amount of each sub-
strate was 0.035 mmol. b) Anaerobic conditions: Substrate (0.035 mmol),
the cluster-supported Mn complex (2.96ꢂ10À5 mmol), and solvent
(CH3CN/CH2Cl2 1:1; 1 mL) were placed in a vial that could be sealed
with a rubber septum. N2 was then bubbled into the vial containing the
reaction solution for 10 min. Thereafter, mCPBA (0.05 mmol) was added
to the solution. The rest of the procedure was the same as above.
V
IV
oxidants MnIII OOC(O)R, Mn =O, and Mn =O operate si-
multaneously as the key active intermediates in the epoxida-
tion reactions. However, an attempt to observe the proposed
À
reactive intermediates MnIII OOC(O)R, MnV=O, and
À
MnIV=O by UV/Vis spectrophotometry at low temperatures
(À408C and À708C) proved unsuccessful. Nonetheless, the
present results provide important clues for a resolution of
the long-standing dichotomy of views pertaining to the
nature of the active oxidants in [Mn
tions, and even in those catalyzed by [Mn(porphyrin)] and
[Mn
(nonheme)] complexes.[1–4]
ACHTUNGERTN(NUNG salen)]-catalyzed reac-
ACHTUNGTRENNUNG
Experimental Section
Competitive reactions of styrene and para-substituted styrenes for the
construction of a Hammett plot: mCPBA (0.018 mmol) was added to a
mixture of styrene (0.01 mmol) and para-X-substituted styrene
(0.01 mmol, X=-OCH3, -CH3, -Cl, and -CN), the cluster-supported Mn
complex (2.96ꢂ10À5 mmol), and solvent (CH2Cl2/CH3CN 1:1; 1 mL). The
mixture was stirred for 10 min at room temperature. The amounts of styr-
enes before and after reactions were determined by GC. The relative re-
General: Olefins, epoxides, cyclohexenol, cyclohexenone, ethanol, metha-
nol, acetone, o-phenylenediamine, salicylaldehyde, dichloromethane, ace-
tonitrile, Mn
ment) were purchased from Aldrich Chemical Co. and were used without
further purification. [PPh4]4A
[Re4Se4(CN)12][30] and peroxyphenylacetic
ACHTUNGTRENNUNG
(OAc)2·4H2O, mCPBA (65%), and H218O (95% 18O enrich-
CTHUNGTRENNUNG
acid (PPAA)[10a] were synthesized according to literature methods. Prod-
uct analyses following olefin epoxidations, partition reactions of PPAA,
and 18O incorporation reactions of cyclohexene oxide were performed on
either a Hewlett–Packard 5890 II Plus gas chromatograph interfaced with
a Hewlett–Packard model 5989B mass spectrometer or a Donam Sys-
tems 6200 gas chromatograph equipped with an FID detector using a
30 m capillary column (Hewlett–Packard DB-5 or HP-FFAP). XRD data
were obtained on a Rigaku X-ray diffractometer using CuKa radiation
(l=1.5418 ꢁ). 1H NMR spectra were recorded on a Bruker 250 MHz
spectrometer from samples in CDCl3 solution, with TMS as an internal
standard. Elemental analyses for C, H, and N were performed on a
Perkin–Elmer 240C instrument. Magnetic susceptibility measurements
on powder samples were carried out in the temperature range from 5 to
300 K in an applied magnetic field of 1000 G using a Quantum Design
MPMS-5 SQUID magnetometer.
activities were determined using the equation kx/ky =logACTHNUGTRENNUG(Xf/Xi)/logACHTUNGTRENNUNG(Yf/Yi),
where Xi and Xf are the initial and final concentrations of substituted
styrenes and Yi and Yf are the initial and final concentrations of styre-
ne.[22a]
H218O experiments: mCPBA (0.005 or 0.01 mmol) was added to a mix-
ture of cyclohexene (0.005–0.02 mmol), the cluster-supported Mn com-
plex (2.96ꢂ10À5 mmol), and H218O (10–40 mL, 95% 18O-enriched, Aldrich
Chemical Co.) in a dried solvent mixture (CH3CN/CH2Cl2 1:1; 1 mL).
The reaction mixture was stirred for 3 min at room temperature and then
directly analyzed by GC/MS. The 16O and 18O compositions were deter-
mined from the relative abundances of the mass peaks at m/z 99 for
[16O]-cyclohexene oxide and m/z 101 for [18O]-cyclohexene oxide. All re-
actions were run at least in triplicate and the average values are present-
ed.
À
Analysis of the O O bond cleavage products from the reactions of the
Preparation of H2saloph: The ligand N,N’-disalicylidene-1,2-phenylenedi-
amine (H2saloph) was prepared by the condensation reaction of o-phe-
nylenediamine and salicylaldehyde according to the previously published
procedures[7b,31–34] with some modification. A solution of salicylaldehyde
(10 mmol) in ethanol (10 mL) was added to a stirred solution of o-phe-
nylenediamine (5 mmol) in ethanol (15 mL). After refluxing for 2.5 h and
then leaving the mixture to stand overnight, the orange-yellow product
cluster-supported Mn complexes with PPAA: PPAA (0.05 mmol) was
added to a mixture of cyclohexene (0–0.14 mmol), the cluster-supported
Mn complex (2.96ꢂ10À5 mmol), and solvent (CH3CN/CH2Cl2 1:1; 1 mL).
The mixture was stirred for 10 min at room temperature. Each reaction
was monitored by GC/MS analysis of 20 mL aliquots withdrawn periodi-
cally from the reaction mixture. All reactions were run at least in tripli-
Chem. Eur. J. 2010, 16, 4678 – 4685
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4683
Lee, Sun Hwa
