cis-Dihydroxylation and epoxidation of alkenes by [Mn2O(RCO 2)2(tmtacn)2]: Tailoring the selectivity of a highly H2O2-efficient catalyst

Article abstract of DOI:10.1021/ja050990u

The carboxylic acid promoted cis-dihydroxylation and epoxidation of alkenes catalyzed by [Mn^IV₂O₃(tmtacn)₂]²⁺ 1 employing H₂O₂ as oxidant is described. The use of carboxylic acids at cocatalytic levels not only is effective in suppressing the inherent catalase activity of 1, but also enables the tuning of the catalyst's selectivity. Spectroscopic studies and X-ray analysis confirm that the control arises from the in situ formation of carboxylate-bridged dinuclear complexes, for example, 2 {[Mn^III₂O(CCl₃CO₂)₂(tmtacn)₂]²⁺} and 3 {[Mn^II₂(OH)(CCl₃CO₂)₂(tmtacn)₂]⁺}, during catalysis. For the first time, the possibility to tune, through the carboxylate ligands employed, both the selectivity and activity of dinuclear Mn-based catalysts is demonstrated. To our knowledge, the system 1/2,6-dichlorobenzoic acid (up to 2000 turnover numbers for cis-cyclooctanediol) is the most active Os-free cis-dihydroxylation catalyst reported to date. Copyright

Full text of DOI:10.1021/ja050990u

Published on Web 05/11/2005
cis-Dihydroxylation and Epoxidation of Alkenes by [Mn2O(RCO2)2(tmtacn)2]:
Tailoring the Selectivity of a Highly H2O2-Efficient Catalyst
Johannes W. de Boer,† Jelle Brinksma,†,‡ Wesley R. Browne,† Auke Meetsma,† Paul L. Alsters,§
Ronald Hage, and Ben L. Feringa*,†
Department of Organic and Molecular Inorganic Chemistry, Stratingh Institute, UniVersity of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands, Kiadis B.V., Zernikepark 6-8,
9747 AN Groningen, The Netherlands, DSM Pharma Chemicals, AdVanced Synthesis, Catalysis and DeVelopment,
P.O. Box 18, 6160 MD Geleen, The Netherlands, and UnileVer R&D Vlaardingen, P.O. Box 114,
3130 AC Vlaardingen, The Netherlands
Received February 16, 2005; E-mail: b.l.feringa@ rug.nl
cis-Dihydroxylation and epoxidation of alkenes are key chemical
transformations in synthetic organic chemistry,1 for which both
stoichiometric oxidants (e.g., peracids, MnO4-, OsO4)2 and second-
and third-row transition metal-based oxidation catalysts3 have
proven to be very effective. In recent years, considerable advances
have been made in the development of atom-efficient and envi-
Figure 1. Structures of complexes 1, 2, and 3.
ronmentally friendly catalytic methods employing H2O2,1 most
notably in the use of MnII salts4 and FeII complexes5,6 in the catalytic
epoxidation of alkenes. In contrast, first-row transition metal-
catalyzed cis-dihydroxylation of alkenes remains a considerable
challenge. Nevertheless, the recent report by De Vos et al. with
heterogenized Mn-tmtacn,7 Que et al. with FeII pyridyl-amine-based
complexes,5 and our results8 on the use of the oxidation catalyst
[MnIV2O3(tmtacn)2]2+ (1),9,10 in the presence of electron deficient
aldehydes, demonstrate the potential of these metals toward cis-
dihydroxylation.
We describe here the carboxylic acid-promoted cis-dihydrox-
ylation and epoxidation of alkenes catalyzed by 1 employing H2O2
as oxidant. The use of carboxylic acids at cocatalytic levels (Table
1, entries 1-3 and 7-10) not only is effective in suppressing the
inherent catalase activity of 1,11 but also enables the tuning of the
catalyst’s selectivity toward cis-dihydroxylation and epoxidation.
Spectroscopic studies and X-ray analysis12 confirm that the control
arises from the in situ formation of carboxylate-bridged dinuclear
complexes, for example, complex 2 {[MnIII2O(CCl3CO2)2(tmtacn)2]2+}
Figure 2. Product formation (0-1.5 h) for reaction with 1, 2, and 3 (1.0
mol % CCl3CO2H; see Table 1, entries 1, 7, and 8) with cyclooctene. See
Figure S1 for complete (7 h) time trace.
Table 1. Oxidation of Cyclooctene to cis-Diol and Epoxidea
catalyst/cocatalyst
(mol %)
T.O.N.c cis-diol
epoxide
entry
conv %b
1
2
3
4
1/CCl3CO2H (1.0)
1/CCl3CO2H (0.2)
1/CCl3CO2H (0.1)
1/-
91
59
9
3
3
440
340
65
10
10
245
145
25
5
20
and complex 3 {[MnII (OH)(CCl3CO2)2(tmtacn)2]+}, during ca-
2
5
1/HPF6 (1.0)
talysis (Figure 1).
6
7
8
9
10
11
12
13
2/-
44
93
90
67d
82
3
270
380
380
525
60
20
325
370
110
260
280
75
695
15
Preliminary experiments employing 1 (0.1 mol %) and CCl3-
CO2H demonstrated improved activity toward the oxidation of
alkenes compared with the electron-deficient aldehydes reported
earlier,8 albeit with slightly reduced selectivity toward the cis-diol
(entries 12 and 13). In stark contrast, however, the use of carboxylic
acids at lower (cocatalytic) concentrations (2-10 equiv wrt 1,
entries 1 and 2) resulted in an increase in selectivity toward cis-
dihydroxylation.13 A significant decrease in activity was observed
only at <0.2 mol % CCl3CO2H (less than two carboxylate ligands
wrt 1, entries 2-5). This acid concentration dependence suggests
that the active catalyst contains two CCl3CO2- ligands. The activity
of 2 in the absence of CCl3CO2H (entries 2-6) supports this
conclusion.
2/CCl3CO2H (1.0)
3/CCl3CO2H (1.0)
1/2,6-Cl2PhCO2H (3.0)
1/salicylic acid (1.0)
1/chloral hydrate (1.0)
1/CCl3CO2H (25)
1/chloral hydrate (25)e
96
88
405
310
a Reaction conditions: 1/cyclooctene/H2O2 1/1000/1300, H2O2 added
over 6 h, reported data after 7 h, general procedure A. All values within
10%. b wrt substrate consumed. c For details of other oxidation products,
see ref 12. d Isolated yield cis-cyclooctane-1,2-diol: 46%. e Reference 8.
tively during the lag phase via a net two-electron reduction of 1 by
H2O2 and exchange14 of two µ-O ligands with two carboxylates.
This confirms that 1 is not the active catalyst and that 2 is either
the immediate precursor of the catalytically active species and acts
as a “catalyst reservoir” during the reaction or is the resting state
of the catalyst during the catalytic cycle. Indeed, the spectroscopic
properties and activity observed during the cyclooctene oxidation
A lag time (∼1 h) is observed in the oxidation of cyclooctene
catalyzed by 1/CCl3CO2H (entry 1 and Figure 2). Spectroscopic
analysis (UV-vis and ESI-MS)12 confirms that 2 forms quantita-
† University of Groningen.
‡ Kiadis B.V.
§ DSM Pharma Chemicals.
Unilever R&D Vlaardingen.
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J. AM. CHEM. SOC. 2005, 127, 7990-7991
10.1021/ja050990u CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
activation and/or allylic oxidation).15 For cyclooctene, in the later
stages of the reaction, the low alkene concentration results in
oxidation of the cis-diol as a competitive process, and continued
addition of excess H2O2 leads to complete oxidation of the cis-diol
formed initially. Maintaining cyclooctene concentration at pseudo-
steady-state levels, however, suppresses overoxidation (Figure S5)
and allows for up to 2000 turnover numbers (TONs) for cis-diol.12
For other substrates, this approach also resulted in an increased
yield of cis-diol and epoxide.12
Scheme 1
In summary, carboxylic acids as cocatalysts allow for both the
suppression of catalase activity and, through the formation of
carboxylate-bridged dinuclear Mn complexes, control over the
selectivity and activity toward cis-dihydroxylation and epoxidation.
To the best of our knowledge, the system 1/2,6-dichlorobenzoic
acid (2000 TONs for cis-cyclooctanediol) is the most active Os-
free cis-dihydroxylation catalyst reported to date.
catalyzed by complex 2 (MnIII2) or 3 (MnII ) (entries 7 and 8) were
remarkably similar to that of the parent “1/CCl3CO2H” system
(entry 1).12
2
Overall, only a very modest effect of the catalyst employed (i.e.,
1, 2, or 3) on reactivity and selectivity was observed (entries 1, 7,
and 8). In contrast, the effect observed on the lag time and selectivity
during the initial phase of the reaction is striking (Figure 2). For
both 2 and 3, a significant reduction in the lag time is observed
together with a reduced selectivity toward cis-dihydroxylation (until
∼90 min) compared with 1. Moreover, a distinct difference in the
reactivity of 2 and 3 during the first ∼90 min of the reaction was
observed, after which all three complexes exhibit similar kinetics
and almost identical spectroscopic features.12 The results indicate
that the reaction involves low-valent dinuclear Mn complexes (e.g.,
2 and 3).
Acknowledgment. We thank Ms. T. D. Tiemersma-Wegman,
Mr. C. Smit, and Ms. A. van Dam for assistance with GC and ESI-
MS analysis, Prof. J. Reedijk for useful discussion, and the Dutch
Economy, Ecology, Technology (EET) program for financial
support.
Supporting Information Available: Analytical, experimental, and
X-ray data for 2 and 3. Mass and UV-vis spectral data. Catalytic
protocols (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
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