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Synthesis and X-ray crystal structure of a chiral molybdenum
porphyrin and its catalytic behaviour toward asymmetric
epoxidation of aromatic alkenes
Wei-Sheng Liu
a, Rui Zhang
a, Jie-Sheng Huang
a, Chi-Ming Che
a,*, Shie-Ming Peng
baDepartment of Chemistry, The Uni6ersity of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong bDepartment of Chemistry, National Taiwan Uni6ersity, Taipei, Taiwan, ROC
Received 3 April 2001; accepted 15 June 2001
Abstract
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 [MoVO(P*)(OMe)] (1) in80% 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
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 (MO) 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 that molybdenum-catalysed epoxidations with alkyl
hy-droperoxides usually involve [Mo(h2-O
2)] or [MoOOR] active intermediates [4].
Molybdenum porphyrins were first prepared by Sri-vastava and Fleischer in 1970 [5]. Since then numerous molybdenum complexes with porphyrin ligands have been known [6]. However, to our knowledge, all the
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-ide by employing catalysts [MoV(O)(tpp)(X)] (tpp =
meso-tetraphenylporphyrinato dianion, X = Cl, OH,
OMe, OAc, SCN) have failed [8].
Herein we report on the synthesis and structural characterisation of [MoVO(P*)(OMe)] (1) bearing a chi-ral, D4-symmetric P* ligand (H2P* = 5,10,15,20-te- 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
(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: + 852-2857-1586.
E-mail address:[email protected] (C.-M. Che).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 2 8 X ( 0 1 ) 0 1 0 7 5 - 0
2. Experimental 2.1. General procedure
Mo(CO)6 (98%), 1,2,4-trichlorobenzene (99 + %),
and H2O2 (50 wt.% in water) were purchased from
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 porphyrin H2P* was synthesised according to the pro-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-dichloroethane (4.1 M, determined by 1H-NMR on a
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 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.
2.2. Preparation of[MoV
O(P*)(OMe)] (1)
To a suspension of H2P* (200 mg) in
1,2,4-trichlorobenzene (C6H3Cl3, 120 ml) was added
Mo(CO)6 (3.2 g) under Ar. The mixture was stirred at 100 °C for 2 h and then refluxed for 48 h. After removal of the solvent by distillation in vacuo, the residue was column-chromatographed on alumina. The first band containing unreacted H2P* was removed with dichloromethane. The desired product was eluted with CH2Cl2– MeOH (1:1 v/v). Yield of 1: 80%. IR (KBr): 909 cm− 1 (w
MoO). ESR (CHCl3, 298 K): g = 1.970. Anal. Found: C, 75.67; H, 6.22; N, 3.41. Calc. for C85H79N4MoO2·1.5MeOH·0.5C6H3Cl3: C, 75.53; H, 6.13; N, 3.94%.
2.3. Epoxidation of aromatic alkenes catalysed by
[MoV
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.
2.4. X-ray structure determination
A single crystal (1·1.5MeOH) of the dimensions 0.40 × 0.35 × 0.25 mm, obtained from slow evaporation of a solution of 1 in CH2Cl2– MeOH, was used for data
collection on a Siemens SMART diffractometer (Mo – Karadiation,u=0.71073 A,) at 295(2) K. The q range 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
least-squares on F2.
3. Results and discussion
3.1. Synthesis and characterisation of complex 1 Treatment of Mo(CO)6 with the chiral, sterically demanding porphyrin ligand H2P* in refluxing 1,2,4-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 the H2P* ligand renders the insertion of molybdenum 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-num(IV) complex [MoIVO(P*)] rather than 1, as
re-vealed by UV – visible spectroscopy and mass
spectrometry.
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 spectrum of 1 shows wMoO at 909 cm− 1. Both spectral features are similar to those reported for [MoV (O)(tpp)-(OMe)] (g = 1.9687, wMoO= 905 cm− 1) [13].
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
a distorted octahedral molybdenum centre. The
mean MoO bond length of 1 (1.697(8) A,) falls in
the range of 1.673(3) – 1.714(3) A, reported for those
of [MoVO(tpp)(X)] (X = Cl [14], F or SCN [15])
but is appreciably smaller than that of [MoV
O-(dptbtmp)(OMe)] (1.80(1) A, , dptbtmp=5,15-diphenyl-2,8,12,18 - tatra - n - butyl - 3,7,13,17 -
tetramethylporphyri-nato dianion) [16]. The mean MoOMe bond
Table 1
Crystal data and structure refinement for complex 1
C85H79N4O2Mo·1.5CH3OH Formula Formula weight 1332.53 Monoclinic Crystal system C2/c Space group 41.3605(2) a (A, ) b (A, ) 17.1725(2) 24.8731(2) c (A, ) 90 h (°) 109.281(1) i (°) 90 k (°) V (A,3) 16675.5(2) 8 Z Dcalc(g cm−3) 1.062 5616 F(000) T (K) 295(2) 2.03 v (Mo–Ka) (cm−1) Reflections collected 48 551 27 383 No. of unique reflections
No. of parameters 1610 0.1100 R1 wR2 0.2514 1.145 Goodness-of-fit
Absolute structure parameter 0.06(6)
1.015 and −0.548 Largest difference peak and hole
(e A,−3)
Table 2
Selected bond lengths (A, ) and angles (°) for complex 1 Bond lengths 1.682(8) MoO(2) MoO(1) 1.983(8) 2.108(9) MoN(2) MoN(1) 2.039(8) 2.049(9) MoN(4) MoN(3) 2.101(9) C(85)O(2) 1.32(2) Bond angles 168.8(4) MoO(2)C(85) 132(1) O(1)MoO(2) 97.2(4) O(1)MoN(2) O(1)MoN(1) 87.3(4) O(1)MoN(4) 99.6(4) 89.1(4) O(1)MoN(3) 89.2(4) O(2)MoN(2) 83.4(4) O(2)MoN(1) 84.6(3) O(2)MoN(4) O(2)MoN(3) 89.7(4) N(2)MoN(3) 90.1(3) 90.2(3) N(1)MoN(2) N(1)MoN(4) 89.1(3) N(3)MoN(4) 89.9(3)
3.2. Asymmetric epoxidation of aromatic alkenes with
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 3a was detected if the same reaction was carried out in the absence of complex 1. We found that the predomi-nant epredomi-nantiomer of 3a adopts a (R)-configuration, which is different from the (S)-configuration observed in the NaOCl epoxidation of the same aromatic alkene catalysed by [MnIII(P*)Cl] [10b].
[MoVO(dptbtmp)(OMe)] (1.89(1) A, ) [16]. The chiral porphyrin ligand P* in 1 adopts a conformation similar to those in the structurally characterised ruthenium complexes with the P* ligand such as [RuVI(P*)(O)
2] [2b].
Fig. 1.ORTEPdrawing of one of the two independent molecules of 1 in the crystal structure of 1·1.5MeOH with thermal ellipsoids drawn at the 30% probability level (hydrogen atoms are not shown).
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 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 [MoVO(tpp)X] (X = Cl, OMe) and cis-[MoVI(tpp)(O)
2] most probably proceed via a
MoOOBut(rather than MoO) 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 – OOBut
species from the reaction of 1 with TBHP were unsuccessful.
Inasmuch as molybdenum porphyrins can also catalyse epoxidation of alkenes with H2O2[8,9], we then examined the reaction between complex 1 and H2O2, attempting to isolate the porphyrin product in the reaction and examine its reactivity toward stoichiomet-ric epoxidation of alkenes. Treatment of 1 with excess H2O2 in dichloromethane – methanol (8:2 v/v) resulted in a slow colour change of the mixture from green to
red brown. Removal of unreacted H2O2 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 40% yield. The UV–visible spectrum of 6 in dichloromethane shows bands at 422 (Soret), 518, and 647 nm, which is different from those of trans-diper-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
2)]+, and [MoO(P*)(O2)]+, re-spectively. We speculate that complex 6 belongs to a [MoO(P*)(O2)] species, and efforts to obtain single 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
R
Entry Alkene Product Time (h) TONa cis:trans ee (%)b
2a 16
H 4
1 3a 16
Me cis-2b cis- +trans-3b
2 3 3 36:64 29c 3 Me 4 5 30:70 28c 15 11 17:83 4 Me 26c Me 30 27 9:91 24c 5 (CH2)2 5 16 5 10 6 4
a(Moles of epoxides)/(moles of complex 1).
bAbsolute configuration: (R) (3a), (1R, 2S) (cis-3b, 5), (1R, 2R) (trans-3b). cEe of cis-3b. The ee of trans-3b is1% in all cases.
4. Supplementary material
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 (Fax: + 44-1223-336033; e-mail: [email protected]. ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
This work was supported by The University of Hong Kong, the Hong Kong Research Grants Council (HKU 7092/98P and PolyU 1/97C), and the Hong Kong Uni-versity Foundation.
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