Catalytic epoxidation of enones mediated by zinc alkylperoxide/ tert- buooh systems

Article abstract of DOI:10.1021/om400830e

The epoxidation of enones by zinc alkylperoxides is a challenging task receiving considerable attention in contemporary research; however, until now no well-defined zinc alkylperoxide based systems have been described. Here, a new catalytic method of epoxidation of enones in the presence of zinc alkylperoxides supported by N,N-bidentate ligands and tert-butyl hydroperoxide is reported. A new dimeric zinc alkylperoxide complex supported by an aminotroponiminate ligand is also presented. The studied catalytic systems show high activity in the epoxidation of trans-chalcone, and in the case of a chiral catalyst with the (S,S)-N,N′-bis(1-phenylethyl)aminotroponiminate ligand a moderate enantioselectivity was achieved.

Full text of DOI:10.1021/om400830e

Communication
pubs.acs.org/Organometallics
Catalytic Epoxidation of Enones Mediated by Zinc Alkylperoxide/tert-
BuOOH Systems
Marcin Kubisiak,† Karolina Zelga,† Iwona Justyniak,‡ Ewa Tratkiewicz,† Tomasz Pietrzak,†
Abdul R. Keeri,†,§ Zbigniew Ochal,† Larissa Hartenstein,∥ Peter W. Roesky,∥ and Janusz Lewinski*,†,‡
́
†Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
‡Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
§University of Warsaw, Faculty of Chemistry, Pasteura 1, 02-093 Warsaw, Poland
∥Institute for Inorganic Chemistry, Karlsruhe Institute of Technology, Engesserstr. 15, Geb. 30.45, 76131 Karlsruhe, Germany
S
* Supporting Information
ABSTRACT: The epoxidation of enones by zinc alkylperoxides is a challenging task
receiving considerable attention in contemporary research; however, until now no well-
defined zinc alkylperoxide based systems have been described. Here, a new catalytic method
of epoxidation of enones in the presence of zinc alkylperoxides supported by N,N-bidentate
ligands and tert-butyl hydroperoxide is reported. A new dimeric zinc alkylperoxide complex
supported by an aminotroponiminate ligand is also presented. The studied catalytic systems
show high activity in the epoxidation of trans-chalcone, and in the case of a chiral catalyst
with the (S,S)-N,N′-bis(1-phenylethyl)aminotroponiminate ligand a moderate enantiose-
lectivity was achieved.
ver the last two decades, numerous efforts have been
devoted to the development of efficient catalysts for the
characterized the first examples of zinc alkylperoxides.6a,b The
resulting zinc alkylperoxide complex stabilized by a β-
diketimine, [(EtOO)Zn(BDI)]2 (1), showed very high activity
in the epoxidation of enones in stoichiometric reactions.6a We
envisioned that zinc alkylperoxides supported by unsaturated
N,N-bidentate ligands might serve as platforms for a catalytic
version of this reaction. The proof of this concept is reported
herein. A number of examples demonstrate well that the
structure8 and reactivity8e,9 of RZn(N,N′)-type complexes
depend on the identity of the N,N-bidentate ligands. In
particular, supporting ligands play a crucial role in oxygenation
processes.6a,d,f,g For the current study a β-diketiminate ligand
and achiral and chiral aminotroponiminate ligands were
selected: N,N′-pent-2-en-2-yl-4-ylidenobis(2,6-diisopropylani-
line) (BDI), N-isopropyl-2-(isopropylamino)troponiminate
( A T I - 1 ) , a n d ( S , S ) - N , N ′- d i - ( 1 - p h e n y l e t h y l ) -
aminotroponiminate (ATI-2), which are capable of providing
a range of electronic and steric environments (Scheme 1a). The
desired zinc tert-butylperoxide complexes were synthesized in a
two-step procedure according to Scheme 1b.
O
asymmetric epoxidation of electron-deficient olefins. A number
of catalytic systems, including chiral metal complexes1 and
organocatalysts,2 have been successfully established. In light of
the increasing demand for green and sustainable chemistry, the
development of environmentally friendly metal-based catalysts
with considerable activity is highly desirable. In this regard, zinc
complexes seem very promising reagents for this process.
Nevertheless, the epoxidation of α,β-unsaturated ketones
mediated by zinc complexes remains an underdeveloped field
and only a few examples have been reported until now. In these
works alkylperoxide zinc compounds acted as efficient
epoxidizing reagents. Initially, Yamamoto and co-workers
used R2Zn/O2 systems as the stoichiometric oxidant for the
epoxidation of α,β-unsaturated ketones.3 In 1996, the first
approach of the enantioselective epoxidation of enone
derivatives that was based on stoichiometric quantities of
diethylzinc and a chiral alcohol in the presence of dioxygen was
reported.4 Then Pu and co-workers reported the use of a chiral
polybinaphthyl in similar stoichiometric reactions as well as the
development of a catalytic variant mediated by zinc species.5 In
this catalytic process, oxygen was replaced as the oxidant by
tert-butyl hydroperoxide (TBHP). It is worth stressing that all
of the mentioned reaction systems were conducted without the
use of well-defined zinc reagents, which were generated in situ.
We have been systematically exploring the chemistry of well-
defined organozinc complexes with a view toward the study of
fundamental transformations involving dioxygen.6,7 In the
course of these investigations we isolated and structurally
The syntheses and characterization of compound 1 have
been reported elsewhere.6a The zinc alkylperoxides stabilized
with ATI-1 and ATI-2 were prepared using a similar two-step
synthetic procedure involving equimolar amounts of t-Bu2Zn in
toluene and the selected aminotroponimine proligand, followed
by the addition of dioxygen at −20 °C. The resulting
Received: August 16, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om400830e | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
In the next step the resulting zinc alkylperoxides 1−3 were
tested with respect to their catalytic activity in the epoxidation
of enones with TBHP as an oxidant. trans-Chalcone was chosen
as a model substrate for epoxidation, as it is a commonly used
compound in similar reactions.2−5 Chalcone was dissolved in
toluene under a nitrogen atmosphere, and then a tert-BuOOH/
toluene solution and the corresponding catalyst were added at 0
°C and the reaction was conducted further at this temperature
(Scheme 2). The conversion to epoxide was monitored by gas
Scheme 1. a) Schematic Representation of Ligands
Investigated; and (b) General Synthetic Procedure for
Preparation of Zinc Alkylperoxides
Scheme 2. General Procedure for Epoxidation of trans-
Chalcone
alkylperoxides [(tBuOO)Zn(ATI-1)]2 (2) and [(tBuOO)Zn-
(ATI-2)]2 (3) were isolated in high yield as yellow crystals (2)
and an oily liquid (3) and fully characterized spectroscopically
in solution; additionally, the molecular structure of 2 was
determined by a single-crystal X-ray diffraction study. The
Table 1. Conversion of trans-Chalcone
ab
,
catalyst
time, h
conversion, %
ee, %
1
structures of 2 and 3 in solution were confirmed by H NMR
1
2
3
3
8
96
99
98
20
n.a.
n.a.
29
spectroscopy. The spectra contain signals characteristic for the
ligands and for the tert-butyl peroxide groups (for details, see
the Supporting Information). A control diffusion ordered
spectroscopy (DOSY) experiment was conducted for com-
pound 2, and its aggregation state was found to be 1.2, which
indicates that it most likely exists as a monomer in solution (for
details, see the Supporting Information). IR spectra in Nujol
were collected, and the characteristic O−O stretching vibration
band was observed at 841 cm−1 for 2, which is similar to that
observed for 16a and other alkylzinc peroxides6c,d (the IR
spectrum of 3 was not informative with respect to the νO−O
band).
4
c
3
24
17
a
b
Conversion determined by GC measurements. Conditions: 0.5
mmol of catalyst, 5.0 mmol of t-BuOOH, and 10.0 mmol of trans-
c
chalcone in 6 mL of toluene was used. The reaction was conducted at
−20 °C.
chromatography. The results are presented in Table 1. All of
the tested catalytic systems involving zinc alkylperoxides in
combination with TBHP allowed full conversion of trans-
chalcone to 2-benzoyl-3-phenyloxirane within no more than 8 h
at 0 °C.10 The most active proved to be compound 1, as it
allowed 96% conversion after 3 h. The chiral catalyst 3 gave a
similar result of 98% conversion after 4 h with 29% ee (the
2S,3R enantiomer was obtained).
Lowering the reaction temperature for 3 to −20 °C did not
lead to an improvement of enantiomeric excess; in fact, both
the conversion and ee after 24 h were decreased to only 20%
and 17%, respectively. Strikingly, the lowest activity at 0 °C
among the tested zinc alkyperoxides was exhibited by 2, as the
reaction reached full conversion only after 8 h. The obtained
reaction times for epoxidation of trans-chalcone place the
investigated systems among the most active known catalysts for
epoxidation of enones.2,4,11
The alkylperoxide 2 crystallizes in the space group P1 as a
̅
(centrosymmetric) dimer with a planar central Zn2−O2 core
with two μ2-bridging oxygen atoms of the tert-butylperoxide
groups (Figure 1). The four -coordinate zinc atoms adopt a
severely distorted tetrahedral geometry. The O−O bond length
in the bridging alkylperoxide groups is 1.475(2) Å and is similar
to those found in other zinc alkylperoxides.6a,b,d Zn−N
distances range from 1.982(2) to 1.994(2) Å, which is close
to the distances reported for 1.6a
It seems reasonable that the proposed catalytic reaction
proceeds according to the mechanism shown in Scheme 3. The
monomer−dimer equilibrium present in the solution of zinc
tert-butylperoxide allows the coordination of the enone to the
metal center of the monomeric complex, which is followed by
epoxidation of the enone by the alkylperoxide moiety and
formation of zinc tert-butoxide. The cycle is completed by
oxidation of alkoxide to peroxide by TBHP. The comparison of
reaction times needed to reach full conversion in each system
allows conclusions to be drawn concerning the relative activities
of the test systems. Catalytic systems involving compounds 1
and 3 as catalysts showed similar, high activity in the
epoxidation of trans-chalcone. This observation indicates that
the supporting ligand backbone does not essentially affect the
catalytic activity of zinc alkylperoxides. Remarkably, compound
2 showed much lower activity. This differentiation of activity
Figure 1. Crystal structure of 2. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): O1−O2 =
1.475(2), Zn1−O1 = 1.978(2), Zn1′−O1 = 2.021(2), Zn1−N1 =
1.982(2), Zn1−N2 = 1.994(2); Zn1−O1−Zn1′ = 100.6(7), Zn1−
O1−O2 = 115.0(1), N1−Zn1−O1 = 127.9(8), N2−Zn1−O1 =
124.2(8).
B
dx.doi.org/10.1021/om400830e | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
(4) Enders, D.; Zhu, J.; Raabe, G. Angew. Chem., Int. Ed. 1996, 35,
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Scheme 3. Proposed Catalytic Cycle for Zinc Alkylperoxide/
TBHP Epoxidation of trans-Chalcone
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supporting ligand and trans-chalcone. The presence of N-
bonded benzylic phenyl rings in the ATI-2 ligand is likely to
facilitate π−π interactions with phenyl rings of trans-chalcone,
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active site and the substrate. Such interaction is not possible in
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based on well-defined zinc alkylperoxides supported by
unsaturated N,N-bidentate ligands and tert-butyl hydroperoxide
as the oxidant. The studied system shows higher activity than
any organozinc-based system reported in the literature so far.
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the present system to other metal alkylperoxides as well as to
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ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and a CIF file giving synthetic procedures,
characterization data, and crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
́
■
S
Shi, J.; Han, H.; Guo, Z.; Tong, H.; Wei, X.; Liu, D.; Lappert, M. F.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for J.L.: lewin@ch.pw.edu.pl.
■
Lohnwitz, K.; Roesky, P. W. Chem. Commun. 2006, 3405−3407.
̈
(c) Gamer, M. T.; Roesky, P. W. Eur. J. Inorg. Chem. 2003, 2145−
2148.
Notes
(10) In the control reaction of trans-chalcone and TBHP in the
absence of catalyst unchanged substrate was recovered after 24 h.
(11) Nemoto, T.; Ohshima, T.; Yamaguchi, K.; Shibasaki, M. J. Am.
Chem. Soc. 2001, 123, 2725−2732.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the National Science Centre (Grant Maestro
DEC-2012/04/A/ST5/00595), European Union, Regional
Development Fund within a project of Foundation for Polish
Science (VENTURES/2011-7/6, M.K.; MPD/2010/13,
A.R.K.) and the Deutsche Forschungsgemeinschaft (P.W.R.)
for financial support. We thank MSc Dominik Kubicki for
performing DOSY experiments.
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dx.doi.org/10.1021/om400830e | Organometallics XXXX, XXX, XXX−XXX