Synthesis and X-ray crystal structure of a chiral molybdenum porphyrin and its catalytic behaviour toward asymmetric epoxidation of aromatic alkenes

Article abstract of DOI:10.1016/S0022-328X(01)01075-0

Reaction of M(CO)₆ with H₂P* (5,10,15,20-tetrakis-{(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1,4:5,8- dimethanoanthracene-9-yl}porphyrin) followed by treatment with methanol afforded [Mo^VO(P*)(OMe)] (1) in ~80% yield. Comple

Full text of DOI:10.1016/S0022-328X(01)01075-0

Journal of Organometallic Chemistry 634 (2001) 34–38
www.elsevier.com/locate/jorganchem
Synthesis and X-ray crystal structure of a chiral molybdenum
porphyrin and its catalytic behaviour toward asymmetric
epoxidation of aromatic alkenes
a
a
a
a,
b
Wei-Sheng Liu , Rui Zhang , Jie-Sheng Huang , Chi-Ming Che *, Shie-Ming Peng
a
Department of Chemistry, The Uni6ersity of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong
b
Department of Chemistry, National Taiwan Uni6ersity, Taipei, Taiwan, ROC
Received 3 April 2001; accepted 15 June 2001
Abstract
Reaction of M(CO)6 with H P* (5,10,15,20-tetrakis-{(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanoanthracene-9-
2
V
yl}porphyrin) followed by treatment with methanol afforded [Mo O(P*)(OMe)] (1) in ꢀ80% yield. Complex 1 was characterised
by IR and ESR spectroscopy and X-ray structure determination. This chiral molybdenum porphyrin was found to catalyse the
epoxidation of styrene, cis-b-methylstyrene, and 1,2-dihydronaphthalene with tert-butyl hydroperoxide in up to 29% ee. © 2001
Elsevier Science B.V. All rights reserved.
Keywords: Catalyst; Epoxidation; Molybdenum; Porphyrin; Structure
1
. Introduction
reported complexes bear non-chiral porphyrin macrocy-
cles, which prevents them from functioning as catalysts
in asymmetric catalysis. Indeed, while a few alkene
epoxidations catalysed by molybdenum porphyrins
have been reported [7–9], none of them are enantiose-
lective. We also note that these epoxidation studies are
focused on aliphatic or cyclic alkenes (such as hexenes
and cyclooctene) [7–9] and attempts to epoxidise an
aromatic alkene (such as styrene) with hydrogen perox-
Chiral metalloporphyrins constitute an important
class of catalysts for asymmetric epoxidation of alke-
nes. Previous studies on these catalysts are confined to
the porphyrin complexes of iron [1], manganese [1], and
ruthenium [2], in which cases the epoxidation reactions
are widely believed to occur via highly reactive ox-
ometal (MꢀO) intermediates [2,3]. To expand the scope
of metalloporphyrin-catalysed asymmetric alkene epox-
idations, we initiated investigations on chiral molybde-
num porphyrins, because it has been well documented
V
ide by employing catalysts [Mo (O)(tpp)(X)] (tpp=
meso-tetraphenylporphyrinato dianion, X=Cl, OH,
OMe, OAc, SCN) have failed [8].
Herein we report on the synthesis and structural
that molybdenum-catalysed epoxidations with alkyl hy-
V
2
characterisation of [Mo O(P*)(OMe)] (1) bearing a chi-
droperoxides
MoꢁOOR] active intermediates [4].
Molybdenum porphyrins were first prepared by Sri-
usually
involve
[Mo(h -O )]
or
2
ral, D -symmetric P* ligand (H P*=5,10,15,20-te-
[
4
2
trakis{(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1,4:5,8-
dimethanoanthracene-9-yl}porphyrin), along with the
catalytic behaviour of 1 toward the epoxidation of
aromatic alkenes with tert-butyl hydroperoxide
vastava and Fleischer in 1970 [5]. Since then numerous
molybdenum complexes with porphyrin ligands have
been known [6]. However, to our knowledge, all the
(
TBHP), which represents the first molybdenum por-
phyrin-catalysed asymmetric epoxidation of alkenes.
Our findings in this work highlight that chiral peroxo
or alkylperoxo complexes of metalloporphyrins, which
have not been studied before, are potentially useful for
asymmetric alkene epoxidation.
*
Corresponding author. Tel.: +852-2859-2154; fax: +852-2857-
586.
E-mail address: cmche@hku.hk (C.-M. Che).
1
0
022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01075-0

W.-S. Liu et al. / Journal of Organometallic Chemistry 634 (2001) 34–38
35
2
. Experimental
collection on a Siemens SMART diffractometer (Mo–
K radiation, u=0.71073 A) at 295(2) K. The q range
,
a
2.1. General procedure
for the data collection is 0.87–25.00° and the limiting
indices are −495h549, −205k519, and −295
l529. The structure was determined by employing
SHELXTL programme and refined by full-matrix-block
Mo(CO)6 (98%), 1,2,4-trichlorobenzene (99+%),
and H O (50 wt.% in water) were purchased from
2
2
2
Aldrich and were used as received. Styrene (99+%,
Aldrich) was purified by vacuum distillation, and 1,2-
dihydronaphthalene (99+%, Aldrich) by passing
through a dry column of activated alumina. The chiral
least-squares on F .
3
. Results and discussion
porphyrin H P* was synthesised according to the pro-
2
cedure of Halterman and co-workers [10]. cis-b-Methyl-
styrene was prepared by the general method of
preparing cis-alkenes [11]. TBHP solution in 1,2-
3.1. Synthesis and characterisation of complex 1
1
dichloroethane (4.1 M, determined by H-NMR on a
Treatment of Mo(CO)6 with the chiral, sterically
Bruker DPX-300 FT-NMR spectrometer) was obtained
from an aqueous TBHP solution (70%, Aldrich) by
following the procedure described by Sharpless and
Verhoeven [12]. IR spectrum was recorded on a Nicolet
demanding porphyrin ligand H P* in refluxing 1,2,4-
2
trichlorobenzene under an inert atmosphere for ꢀ48 h
followed by treating the reaction product with
methanol led to isolation of the chiral molybdenum
porphyrin complex 1 in ꢀ80% yield. This is similar to
the literature method of preparing non-chiral oxoalkox-
omolybdenum(V) complexes with simple porphyrins
(such as tpp) [13], except for the use of high-boiling
1,2,4-trichlorobenzene instead of decalin–octane as the
reaction solvent and the requirement of a longer reac-
tion time. Probably, the sterically demanding nature of
20 FXC FT-IR spectrometer (KBr pellet), UV–visible
spectrum on a HP 8453 diode array spectrophotometer,
ESR spectrum on a Bruker EMX100 ESR spectrome-
ter. GC measurements were performed on a HP 5890
Series II system equipped with a HP 5890A flame
ionisation detector and a HP 3395 integrator.
V
2
.2. Preparation of [Mo O(P*)(OMe)] (1)
the H P* ligand renders the insertion of molybdenum
2
into this macrocycle more difficult. Note that when the
reaction was carried out on a small scale, such as a
tenth of the scale described in the Section 2, the main
metalloporphyrin product isolated was an oxomolybde-
To a suspension of H P* (200 mg) in 1,2,4-
2
trichlorobenzene (C H Cl , 120 ml) was added
6
3
3
Mo(CO) (3.2 g) under Ar. The mixture was stirred at
6
IV
1
00 °C for 2 h and then refluxed for 48 h. After
num(IV) complex [Mo O(P*)] rather than 1, as re-
removal of the solvent by distillation in vacuo, the
residue was column-chromatographed on alumina. The
vealed by UV–visible spectroscopy and mass
spectrometry.
first band containing unreacted H P* was removed with
dichloromethane. The desired product was eluted with
Complex 1 exhibits an ESR signal with g=1.970 in
chloroform at room temperature, consistent with the
+5 oxidation state of its molybdenum centre. The IR
2
CH Cl –MeOH (1:1 v/v). Yield of 1: ꢀ80%. IR (KBr):
2
2
−
1
−1
9
09 cm
(wMoꢀO). ESR (CHCl , 298 K): g=1.970.
spectrum of 1 shows wMoꢀO at 909 cm . Both spectral
3
V
Anal. Found: C, 75.67; H, 6.22; N, 3.41. Calc. for
features are similar to those reported for [Mo (O)(tpp)-
−
1
C H N MoO ·1.5MeOH·0.5C H Cl : C, 75.53; H,
(OMe)] (g=1.9687, wMoꢀO=905 cm ) [13].
8
5
79
4
2
6
3
3
6
.13; N, 3.94%.
X-ray structure determination on 1·1.5MeOH (see
Table 1) reveals that the crystal unit cell contains two
independent molecules of 1 with similar metrical
parameters. The ORTEP drawing of one of the two
molecules is depicted in Fig. 1. Selected bond lengths
and angles of the molecule are listed in Table 2.
As is evident from Fig. 1 and Table 2, complex 1 has
2.3. Epoxidation of aromatic alkenes catalysed by
V
[Mo O(P*)(OMe)] (1)
To a mixture of alkene (1 mmol) and complex 1 (0.01
mmol) in benzene (2 ml) was added TBHP (1.1 mmol).
The solution was stirred at room temperature and
aliquots were analysed by GC with G-TA chiral
column.
a
distorted octahedral molybdenum centre. The
mean MoꢀO bond length of 1 (1.697(8) A) falls in
the range of 1.673(3)–1.714(3) A reported for those
,
,
V
of [Mo O(tpp)(X)] (X=Cl [14], F or SCN [15])
V
2.4. X-ray structure determination
but is appreciably smaller than that of [Mo O-
(
dptbtmp)(OMe)] (1.80(1) A, dptbtmp=5,15-diphenyl-
,
A single crystal (1·1.5MeOH) of the dimensions
.40×0.35×0.25 mm, obtained from slow evaporation
2,8,12,18-tatra-n-butyl-3,7,13,17-tetramethylporphyri-
nato dianion) [16]. The mean MoꢁOMe bond
of 1.964(8)
0
of a solution of 1 in CH Cl –MeOH, was used for data
A
,
in
1
is longer than that in
2
2

36
W.-S. Liu et al. / Journal of Organometallic Chemistry 634 (2001) 34–38
Table 1
Crystal data and structure refinement for complex 1
Table 2
Selected bond lengths (A) and angles (°) for complex 1
,
Formula
C85H79N O Mo·1.5CH OH
1332.53
Bond lengths
MoꢁO(1)
MoꢁN(1)
MoꢁN(3)
C(85)ꢁO(2)
Bond angles
O(1)ꢁMoꢁO(2)
O(1)ꢁMoꢁN(1)
O(1)ꢁMoꢁN(3)
O(2)ꢁMoꢁN(1)
O(2)ꢁMoꢁN(3)
N(1)ꢁMoꢁN(2)
N(3)ꢁMoꢁN(4)
4
2
3
Formula weight
Crystal system
Space group
1.682(8)
MoꢁO(2)
MoꢁN(2)
MoꢁN(4)
1.983(8)
2.108(9)
2.049(9)
Monoclinic
C2/c
41.3605(2)
17.1725(2)
24.8731(2)
90
109.281(1)
90
16675.5(2)
8
2.039(8)
2.101(9)
1.32(2)
a (A
,
)
)
)
b (A
,
c (A
,
168.8(4)
97.2(4)
89.1(4)
89.2(4)
84.6(3)
90.2(3)
89.9(3)
MoꢁO(2)ꢁC(85)
O(1)ꢁMoꢁN(2)
O(1)ꢁMoꢁN(4)
O(2)ꢁMoꢁN(2)
O(2)ꢁMoꢁN(4)
N(2)ꢁMoꢁN(3)
N(1)ꢁMoꢁN(4)
132(1)
87.3(4)
99.6(4)
83.4(4)
89.7(4)
90.1(3)
89.1(3)
h (°)
i (°)
k (°)
V (A
,
3)
Z
Dcalc (g cm−3)
F(000)
1.062
5616
T (K)
295(2)
2.03
v (Mo–K ) (cm−1)
a
3
.2. Asymmetric epoxidation of aromatic alkenes with
Reflections collected
No. of unique reflections
No. of parameters
R1
wR2
Goodness-of-fit
48 551
27 383
1610
0.1100
0.2514
1.145
TBHP catalysed by complex 1
The catalytic behaviour of 1 toward asymmetric
epoxidation of aromatic alkenes with TBHP was exam-
ined first with styrene (2a) as a substrate. When a
mixture of styrene and TBHP in benzene in the pres-
ence of 1 mol% complex 1 was stirred at room temper-
ature for 16 h, styrene epoxide (3a) was formed in 16%
ee with catalyst turnovers of 4 (entry 1 in Table 3); no
Absolute structure parameter
Largest difference peak and hole
0.06(6)
1.015 and −0.548
−
3
(
e A
,
)
3
a was detected if the same reaction was carried out in
V
[
Mo O(dptbtmp)(OMe)] (1.89(1) A) [16]. The chiral
,
the absence of complex 1. We found that the predomi-
nant enantiomer of 3a adopts a (R)-configuration,
which is different from the (S)-configuration observed
in the NaOCl epoxidation of the same aromatic alkene
porphyrin ligand P* in 1 adopts a conformation similar
to those in the structurally characterised ruthenium
complexes with the P* ligand such as [Ru (P*)(O) ]
2b].
VI
2
III
[
catalysed by [Mn (P*)Cl] [10b].
Fig. 1. ORTEP drawing of one of the two independent molecules of 1 in the crystal structure of 1·1.5MeOH with thermal ellipsoids drawn at the
0% probability level (hydrogen atoms are not shown).
3

W.-S. Liu et al. / Journal of Organometallic Chemistry 634 (2001) 34–38
37
V
Extension of the complex 1–TBHP system to other
aromatic alkenes, including cis-b-methylstyrene (cis-2b)
and 1,2-dihydronaphthalene (4), led to higher catalyst
turnovers or enantioselectivity (entries 2–6 in Table 3).
The epoxidation of cis-2b afforded a mixture of cis-
and trans-3b with the cis epoxide formed in much
higher ee (up to 29%, entries 2–5). Of interest is the
large dependence of the cis:trans ratio of 3b on the
reaction time. For example, the cis:trans ratio de-
creased from 36:64 to 9:91 as the reaction time in-
creased from 3 to 30 h. Such a time dependence of
cis:trans ratio is unlikely to arise from an isomerisation
of cis- to trans-3b catalysed by complex 1, since no
trans-3b was observed after a mixture of cis-3b and
complex 1 had been stirred for 24 h under the same
conditions as those employed for the catalytic reaction.
The mechanism of the foregoing complex 1-catalysed
epoxidations is not yet clear. Since the closely related
TBHP epoxidations of alkenes catalysed by molybde-
num porphyrins [Mo O(tpp)X] (X=Cl, OMe) and cis-
VI
[Mo (tpp)(O) ] most probably proceed via
a
2
t
MoꢁOOBu (rather than MoꢀO) active intermediate [7],
we propose that a similar active intermediate may be
involved in the complex 1–TBHP system. However,
our attempts to isolate or characterise any Mo–OOBu
species from the reaction of 1 with TBHP were
unsuccessful.
Inasmuch as molybdenum porphyrins can also
catalyse epoxidation of alkenes with H O [8,9], we then
examined the reaction between complex 1 and H O ,
attempting to isolate the porphyrin product in the
reaction and examine its reactivity toward stoichiomet-
ric epoxidation of alkenes. Treatment of 1 with excess
H O in dichloromethane–methanol (8:2 v/v) resulted
t
2
2
2
2
2
2
in a slow colour change of the mixture from green to
red brown. Removal of unreacted H O2 and chro-
matography on a short basic alumina column followed
by crystallisation from pentane afforded a chiral
molybdenum porphyrin (6) as a red–brown solid in
2
ꢀ
40% yield. The UV–visible spectrum of 6 in
dichloromethane shows bands at 422 (Soret), 518, and
47 nm, which is different from those of trans-diper-
6
oxo- [17] or cis-dioxomolybdenum(VI) [18] porphyrins.
In the positive-ion FAB mass spectrum of 6, there are
cluster peaks at m/z 1253, 1269 and 1285 ascribable to
+
+
+
[
MoO(P*)] , [Mo(P*)(O )] , and [MoO(P*)(O )] , re-
2
2
spectively. We speculate that complex 6 belongs to a
MoO(P*)(O )] species, and efforts to obtain single
[
2
crystals of 6 are under way.
Interestingly, complex 6 is reactive toward aromatic
alkenes such as cis-2b. When a solution of 6 and cis-2b
in 1,2-dichloroethane was stirred at room temperature
for 23 h, a mixture of cis- and trans-3b was formed in
a 58:42 ratio, with 31 and 11% ee obtained for the cis
and trans epoxide, respectively.
Table 3
Epoxidation of aromatic alkenes with TBHP catalysed by complex 1
Entry
R
Alkene
Product
Time (h)
TON a
cis:trans
ee (%) b
16
1
2
3
4
5
6
H
Me
Me
Me
Me
(CH2)2
2a
cis-2b
3a
16
3
4
15
30
16
4
3
5
11
27
5
c
cis- +trans-3b
36:64
30:70
17:83
9:91
29
28
26
24
c
c
c
4
5
10
a
(
Moles of epoxides)/(moles of complex 1).
b
c
Absolute configuration: (R) (3a), (1R, 2S) (cis-3b, 5), (1R, 2R) (trans-3b).
Ee of cis-3b. The ee of trans-3b is ꢀ1% in all cases.

38
W.-S. Liu et al. / Journal of Organometallic Chemistry 634 (2001) 34–38
[
[
3] B. Meunier, Chem. Rev. 92 (1992) 1411.
4
. Supplementary material
4] See for example: (a) J. Sundermeyer, Angew. Chem. Int. Ed.
Engl. 32 (1993) 1144. (b) E.N. Jacobsen, in: E.W. Abel, F.G.A.
Stone, G. Wilkinson (Eds.), Comprehensive Organometallic
Chemistry II, vol. 12, Pergamon, Oxford, 1995, chapter 11.1.
5] T.S. Srivastava, E.B. Fleischer, J. Am. Chem. Soc. 92 (1970)
5518.
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC no. 161122. Copies of this infor-
mation may be obtained free of charge from the Direc-
tor, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
[
[
6] (a) Y. Matsuda, Y. Murakami, Coord. Chem. Rev. 92 (1988)
1
57;
b) H. Brand, J. Arnold, Coord. Chem. Rev. 140 (1995) 137.
7] H.J. Ledon, P. Durbut, F. Varescon, J. Am. Chem. Soc. 103
1981) 3601.
(
Fax: +44-1223-336033; e-mail: deposit@ccdc.cam.
(
ac.uk or www: http://www.ccdc.cam.ac.uk).
[
[
[
(
8] G. Legemaat, W. Drenth, M. Schmidt, G. Prescher, G. Goor, J.
Mol. Catal. 62 (1990) 119.
9] P. Hoffmann, B. Meunier, New J. Chem. 16 (1992) 559.
Acknowledgements
[
10] (a) R.L. Halterman, S.-T. Jan, J. Org. Chem. 56 (1991) 5253;
b) R.L. Halterman, S.-T. Jan, H.L. Nimmons, D.J. Standlee,
This work was supported by The University of Hong
Kong, the Hong Kong Research Grants Council (HKU
(
M.A. Khan, Tetrahedron 53 (1997) 11257.
7092/98P and PolyU 1/97C), and the Hong Kong Uni-
[
[
[
11] H. Lindlar, R. Dubuis, Org. Synth. Collect. 5 (1973) 880.
12] K.B. Sharpless, T.R. Verhoeven, Aldrichim. Acta 12 (1979) 63.
13] H.J. Ledon, M.C. Bonnet, Y. Brigandat, F. Varescon, Inorg.
Chem. 19 (1980) 3488.
versity Foundation.
References
[14] H. Ledon, B. Mentzen, Inorg. Chim. Acta 31 (1978) L393.
[
[
15] T. Imamura, A. Furusaki, Bull. Chem. Soc. Jpn 63 (1990) 2726.
16] M. van Dijk, Y. Morita, S. Petrovic, G.M. Sanders, H.C. van
der Plas, C.H. Stam, Y. Wang, J. Heterocyclic Chem. 29 (1992)
81.
[
[
1] J.P. Collman, X.M. Zhang, V.J. Lee, E.S. Uffelman, J.I. Brau-
man, Science 261 (1993) 1404 and references therein.
2] (a) A. Berkessel, M. Frauenkron, J. Chem. Soc. Perkin Trans. 1
(
(
1997) 2265;
[17] B. Chevrier, Th. Diebold, R. Weiss, Inorg. Chim. Acta 19 (1976)
b) T.-S. Lai, R. Zhang, K.-K. Cheung, H.-L. Kwong, C.-M.
L57.
Che, Chem. Commun. (1998) 1583;
c) Z. Gross, S. Ini, Inorg. Chem. 38 (1999) 1446.
[18] T. Malinski, P.M. Hanley, K.M. Kadish, Inorg. Chem. 25 (1986)
3229.
(