A synthetically useful, self-assembling MMO mimic system for catalytic alkene epoxidation with aqueous H2O2 [18]

Full text of DOI:10.1021/ja015884g

7194
J. Am. Chem. Soc. 2001, 123, 7194-7195
A Synthetically Useful, Self-Assembling MMO Mimic
System for Catalytic Alkene Epoxidation with
Aqueous H2O2
Scheme 1. Epoxidation of 1-Decene by the 1/Acetic Acid
System
M. Christina White, Abigail G. Doyle, and Eric N. Jacobsen*
Department of Chemistry and Chemical Biology
HarVard UniVersity, Cambridge, Massachusetts 02138
ReceiVed March 26, 2001
ReVised Manuscript ReceiVed May 31, 2001
perchlorate complexes of the mep ligand (mep ) N,N′-dimethyl-
9
N,N′-bis(2-pyridylmethyl)-ethane, 1,2-diamine) effect hydroxy-
The discovery of efficient and practical epoxidation methods
that utilize aqueous H O as terminal oxidant stands as an
2 2
2 2
lation of cyclohexane in the presence of aqueous H O with
modest catalytic activity (2-5 turnovers). Thus far, however, the
application of functional MMO model systems to preparative
oxidation chemistry has been prevented by low catalyst turnover
numbers, poor selectivity for product formation (often attributed
to generation of free hydroxyl radicals), and the requirement for
large excesses of substrate relative to oxidant. Nonetheless, the
important objective in synthetic chemistry. While significant
advances have been made in identifying catalysts for peroxide-
based epoxidations, important challenges remain; these include
1
the development of highly reactive systems that effect rapid
substrate conversion with high selectivity, and the use of
inexpensive, environmentally friendly metals in coordination
environments that can be adapted to sterically and electronically
(mep)iron system appeared to hold special promise for epoxidation
2
catalysis because it was demonstrated to effect oxidation without
the participation of free hydroxyl radicals.8 This feature, com-
bined with the synthetically accessible and tunable nature of the
mep ligand, prompted us to investigate its potential utility in
epoxidation catalysis.
tunable chiral ligands. Nature has evolved a variety of remarkable
e
oxidative enzymes that may point to a solution. For example, in
addition to its well-known biological role in the selective
hydroxylation of small hydrocarbons, methane monooxygenase
(
MMO) is an efficient and selective catalyst for epoxidation of
3
3 2 4 2
We evaluated mononuclear [Fe(II)(mep)(CH CN) ](ClO )
small terminal olefins (i.e. ethylene, propylene, 1-butene).
complex (5 mol %) for epoxidation of 1-decene in the presence
of varying amounts of H O . With 4 equiv of oxidant, complete
Moreover, oxidized MMO (diiron(III)) displays this oxidation
4
activity with H
2
O
2
. In this paper we describe a new protocol
2
2
conversion of alkene was achieved; however, epoxide was
produced in only 40% yield and a variety of over-oxidized
that employs low loadings of an inexpensive, easily prepared iron-
5
2 2
tetradentate ligand complex and 50% aqueous H O to effect
byproducts were detected. Use of the corresponding SbF
1
6
complex
led to a substantially more efficient reaction, with only 1.5
equiv of H required to achieve complete conversion of alkene,
epoxidation of a wide variety of aliphatic olefinssincluding
terminal olefinsswithin 5 min in 60-90% isolated yields (e.g.
Scheme 1). On the basis of spectroscopic and crystallographic
data, it is shown that this catalyst system self-assembles under
the reaction conditions to form a µ-oxo, carboxylate-bridged
diiron(III) complex reminiscent of the µ-hydroxo, carboxylate-
10
2 2
O
and with selectivity for formation of 1,2-epoxydecane improved
to 71%. A screen of additives and solvents revealed that
improvement to 82% selectivity for epoxide formation was
possible with the addition of as little as 1 equiv of acetic acid
with respect to catalyst. Moreover, the acetic acid-containing
system was very well-behaved, with reductions in catalyst loading
bridged diiron(III) core found in the hydroxylase active site of
oxidized methane monooxygenase (MMO).6
A variety of interesting synthetic non-heme iron complexes
7,8
2 2
(S/C up to 100) and increases in H O addition rates (from
have been identified as functional mimics of MMO. For
example, both mononuclear8 and pre-assembled binuclear iron
e
8d
dropwise to rapid addition) resulting in no change in selectivity
for epoxide formation. In contrast, the system lacking acetic acid
as additive displayed significant decreases in epoxide yield both
at lower catalyst loadings and with increased addition rates of
(
1) For examples of catalytic systems for alkene epoxidation employing
2 2
aqueous H O
: Methyl trioxorhenium: (a) Rudolph, J.; Reddy, K. L.; Chiang,
J. P.; Sharpless, K. B. J. Am. Chem. Soc. 1997, 119, 6189-6190. (b) Herrmann,
W. A.; Fischer, R. W.; Marz, D. W. Angew. Chem., Int. Ed. Engl. 1991, 30,
2 2
H O .
1
638-1641. Heteropolyoxotungstates: (c) Zuwei, X.; Ning, Z.; Yu, S.;
Kunlan, L. Science 2001, 292, 1139-1141. (d) Sato, K.; Aoki, M.; Ogawa,
M.; Hashimoto, T.; Noyori, R. J. Org. Chem. 1996, 61, 8310-8311. (e)
Venturello, C.; Alneri, E.; Ricci, M. J. Org. Chem. 1983, 48, 3831-3833.
(7) For a review on MMO model systems, see: Hu, Z.; Gorun, S. M. In
Biomimetic Oxidations Catalyzed by Transition Metal Complexes; Meunier,
B., Ed.; Imperial College Press: London, 2000; pp 269-307.
(
7
TMTACN)Mn: (f) Berkessel, A.; Sklorz, C. A. Tetrahedron Lett. 1999, 40,
(8) Hydroxylations with MMO model systems: (a) Leising, R. A.; Kim,
J.; Perez, M. A.; Que, L., Jr. J. Am. Chem. Soc. 1993, 115, 9524-9530. (b)
Nishada, Y.; Okuno, T.; Ito, S.; Harada, A.; Ohba, S.; Matsushima, H.; Tokii,
T. Chem. Lett. 1995, 885-886. (c) Ito, S.; Okuno, T.; Matsushima, H.; Tokii,
T.; Nishada, Y. J. Chem. Soc., Dalton Trans. 1996, 4479-4484. (d) Okuno,
T.; Ito, S.; Ohba, S.; Nishida, Y. J. Chem. Soc., Dalton Trans. 1997, 3547-
3551. (e) Chen, K.; Que, L., Jr. Chem. Commun. 1999, 1375-1376.
965-7968. (g) De Vos, D.; Bein, T. Chem. Commun. 1996, 917-918.
MnSO : (h) Lane, B. S.; Burgess, K. J. Am. Chem. Soc. 2001, 123, 2933-
934.
2) To date, efforts directed toward the development of asymmetric catalytic
systems for epoxidations with H have met with limited success. See, for
example: (a) Francis, M. B.; Jacobsen, E. N. Angew. Chem., Int. Ed. 1999,
8, 937-941. (b) Pietikainen, P. Tetrahedron 1998, 54, 4319-4326. (c) Bolm,
C.; Kadereit, D.; Valacchi, M. Synlett 1997, 6, 697-698.
4
2
(
2 2
O
3
2 2
Epoxidations with MMO model systems: (f) H O as oxidant: Duboc-Toia,
C.; Menage, S.; Lambeaux, C.; Fontecave, M. Tetrahedron Lett. 1997, 38,
3727-3730. (g) TBHP as oxidant: Menage, S.; Vincent, J. M.; Lambeaux,
C.; Chottard, G.; Grand, A.; Fontecave, M. Inorg. Chem. 1993, 32, 4766-
4773. (h) PhIO as oxidant: Stassinopoulos, A.; Caradonna, J. P. J. Am. Chem.
(
3) (a) Ono, M.; Okura, I. J. Mol. Catal. 1990, 61, 113-122. (b) Higgins,
I. J.; Best, D. J.; Hammond, R. C. Nature 1980, 286, 561-564. (c) Colby, J.;
Stirling, D. I.; Dalton, H. Biochem. J. 1977, 165, 395-402.
(4) Anderson, K. K.; Froland, W. A.; Lee, S.-K.; Lipscomb, J. D. New J.
2
Soc. 1990, 112, 7071-7073. (i) O as oxidant: Kitajima, N.; Fukui, H.; Moro-
Chem. 1991, 15, 411-415.
5) CAUTION: Hydrogen peroxide solutions are strongly oxidizing and
should be handled with appropriate precautions. Use of commercial 30% and
0% solutions could be used interchangeably providing similar results in all
cases examined.
6) (a) Waller, B. J.; Lipscomb, J. D. Chem. ReV. 1996, 96, 2625-2657.
b) Elango, N.; Radhakrishna, R.; Froland, W. A.; Wallar, B. J.; Earhart, C.
oka, Y. J. Chem. Soc., Chem. Commun. 1988, 485-486. (j) Murch, B. P.;
Bradley, F. C.; Que, L., Jr. J. Am. Chem. Soc. 1986, 108, 5027-5028.
(9) Toftlund, H.; Pedersen, E.; Yde-Andersen, S. Acta Chem. Scand. A
1984, 38, 693-697.
(
5
(10) Preparation of 1: Complexation of mep with Fe(II)Cl
effected in CH
yellow complex was precipitated by addition of ether and washed with ether.
Addition of AgSbF in CH CN (2 equiv) followed by stirring for 24 h, filtration
through Celite to remove silver salts, and solvent removal afforded
2
‚xH
2
O was
(
3
CN by stirring at room temperature over 24 h. The resulting
(
A.; Lipscomb, J. D.; Ohlendorf, D. H. Protein Sci. 1997, 6, 556-568. (c)
Rosenzweig, A. C.; Norlund, P.; Takahara, P. M.; Frederick, C. A.; Lippard,
S. J. Chem. Biol. 1995, 2, 409-418. (d) Rosenzweig, A. C.; Frederick, C. A.;
Lippard, S. J.; Norlund, P. Nature 1993, 366, 537-543.
6
3
-
3 2 6 2
CN) ](SbF ) (1) as a purple solid that can be stored
[Fe(II)(mep)(CH
indefinitely in the solid state (see Supporting Information).
1
0.1021/ja015884g CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/29/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 7195
Table 1. Epoxidations Catalyzed by the 1/Acetic Acid System (2)a
2 3 2 2
Figure 1. ORTEP diagram of [Fe (µ-O)(µ-CH CO )(mep) ] (cation of
2
).
a
Reaction conditions (see ref 12 and Supporting Information): olefin
(
2.0 mmol, 0.16 M in CH
3
CN), 1 (3.0 mol %), CH
3
CO
2
H (10 equiv
CN,
relative to 1), H (aqueous 50 wt %, 3.0 mmol, 1.5 M in CH
O
2 2
3
b
added dropwise over 2 min), 4 °C, 5 min. Isolated yields based on
an average of 3 runs. c GC yields were determined using nitrobenzene
d
as an internal standard for especially volatile substrates. Reaction
carried out at [olefin]
mol % 1. 1.5 mol % 1/1.5 mol % CH CO
e
0
) 0.13 M due to poor solubility in CH
3
CN. 5
f
H. g 6 mol % CH
3
2
3
CO
2
H.
of MMO, it is likely that a “carboxylate shift” is responsible for
1
3
generating the coordination site required for catalysis.
3
Figure 2. Electronic spectra of CH CN solutions of monomer 1 (6.5
mM), dimer 2 (0.81 mM) obtained by crystallization of 1/acetic acid under
air, and dimer 2 (0.81 mM) generated in situ by addition of 0.5 equiv. of
Further optimization of reaction conditions and a preliminary
investigation of substrate scope led to the results shown in Table
14
H
O
2 2
to a solution of 1/acetic acid.
1. A wide range of aliphatically substituted alkenes proved to
15
be excellent substrates for epoxidation with catalyst 2. Terminal
olefins (entries 1-5), which are normally least reactive to
electrophilic oxidants and typically require long reaction times
with any of the known epoxidation methods (e.g. 12 h to several
days with peracid reagents), were found to undergo epoxidation
within 5 min and in 60-90% isolated yields. The reaction is
operationally simple in that it is insensitive to air and moisture
and product isolation is effected simply by distillation or filtration
through a short plug of silica. To our knowledge, this is the first
example of an MMO model system that is useful for preparative
oxidation chemistry. The mep ligand framework is modular in
nature and promises to be a very useful template for chiral ligand
design. Experiments aimed toward developing enantioselective
variants of this epoxidation catalyst are underway.
Closer examination of the effect of acetic acid on the
epoxidation system revealed dramatic effects on catalyst structure.
X-ray crystallographic analyses were carried out on single-crystals
grown by slow diffusion of ether into concentrated CH
solutions of 1 both in the presence and absence of added acetic
acid. In the latter case, an octahedral, mononuclear (mep)Fe(CH
CN) (SbF complex was identified, whereas in the presence of
3
CN
3
-
2
6 2
)
acetic acid a µ-oxo, carboxylate bridged diiron(III) complex 2
was generated (Figure 1). The presence of characteristic absor-
bances in the UV-vis spectrum of 2 suggests that the binuclear
structure is maintained in solution.11 Complex 2 exhibited the
same selectivity in the epoxidation of 1-decene as displayed by
monomeric complex 1 in the presence of added acetic acid.
Formation of the carboxylate-bridged (µ-oxo) diiron(III) dimer
from 1 and acetic acid could be monitored readily by UV-vis
spectroscopy. Under aerobic conditions, oxidation was found to
require several days, indicating that generation of 2 under the
conditions of epoxidation (<5 min) cannot be attributed to air
Acknowledgment. This work was supported by the NIH (GM-43214)
and a postdoctoral fellowship to M.C.W. from the NIH (1 F32 GM20077-
01). The crystal structure analysis was performed by Dr. Richard Staples
(Harvard University).
oxidation. Addition of H
CH CN solution of 1 and acetic acid resulted in immediate
2 2
O (0.5 equiv with respect to 1) to a
3
conversion to a species with an electronic spectrum that was
Supporting Information Available: Complete experimental details
identical to that of complex 2 (Figure 2).12 Addition of 1-decene
of catalyst synthesis, epoxidation reactions, crystallization, UV-Vis
studies(PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
2 2
to this complex followed by H O (1.4 equiv with respect to
olefin) led to formation of 1,2-epoxydecane in the same yield as
observed with isolated complex 2 and with 1/acetic acid. This
appears to be the first example of a catalytically active µ-oxo,
carboxylate-bridged diiron(III) complex 2 that is generated by
self-assembly under the conditions of epoxidation. As in the case
JA015884G
(13) Whittington, D. A.; Lippard, S. J. J. Am. Chem. Soc. 2001, 123, 827-
8
38.
(
11) The following spectroscopic features are characteristic of bent (µ-
(14) General procedure: A CH
M), 1 (0.007 mmol, 3 mol %, S/C ) 33), and CH
cooled to 4 °C with rapid stirring in a screw cap glass vial. Nitrobenzene (40
mol %, 0.097 mmol) was added as internal standard. A CH CN solution of
50 wt % H (1.5 equiv, 0.36 mmol, 0.36-5.0 M depending on scale) was
3
CN solution of olefin (0.24 mmol, 0.16
oxo) diiron(III) compounds: (1) strong absorbance near 330 nm (see
Supporting Information), (2) three features from 400 to 500 nm, (3) shoulder
near 525 nm, and (4) broad band near 700 nm: (a) Norman, R. E.; Yan, S.;
Que, L. Jr., Backes, G.; Ling, J.; Sanders-Loehr, J.; Zhang, J. H.; O’Connor,
C. J. J. Am. Chem. Soc. 1990, 112, 1554-1562. (b) Reem, R. C.; McCormick,
J. M.; Richardson, D. E.; Devlin, F. J.; Stephens, P. J.; Musselman, R. L.;
Solomon, E. I. J. Am. Chem. Soc. 1989, 111, 4688-4704. (c) Reference 8d.
3
CO H (30 mol %) was
2
3
2 2
O
precooled to 4 °C and added dropwise over <2 min. Reaction progress was
monitored by GC, and in all cases complete conversion of olefin was observed
within 5 min with epoxide as the only detectable product. However, small
amounts of polar, carboxylic acid-containing byproducts were detected by
NMR and IR analysis. Epoxide was isolated by extraction of the crude reaction
mixture into pentane and purification through a short plug of silica.
(15) In contrast, substrates bearing aromatic substituents underwent
decomposition to unidentified products.
2 2
(12) Moist air or H O have been shown to effect oxidation of iron(II)
complexes of salen or porphyrin to the corresponding µ-oxo diiron(III)
compounds: (a) Murray, K. S. Coord. Chem. ReV. 1974, 12, 1-35. (b)
Sadasivan, N.; Eberspaecher, H. I.; Fushsman, W. H.; Caughey, W. S.
Biochemistry 1969, 8, 534-541.